Digital Photography For What It's Worth

Infrared (IR) basics for digital photographers—capturing the unseen

Digital cameras make it easy to explore a world of invisible light just beyond red. 

In this topic...

• Why Infrared?

  • Extra-Sensory Perception

  • A Fresh View Within Easy Reach

  • The Digital Advantage

  • Sidebar: Jay Scott's Excellent IR Adventures

  • Near, Not Far IR

  • That IR Look

• IR Performance in Digital Cameras

  • Can Your Camera Handle IR?

  • What Makes a Good IR Camera?

  • Internal IR Cut Filters

  • Honey, Where's The Remote?

  • IR-Sensitive Cameras

  • Less Sensitive Cameras

• IR Filter Choices

  • First, Some Filter Terminology

  • NIR Transmission Spectra For Several Common IR Filters

  • The Ever-Popular Hoya R72

  • The Wratten 87 and 87c

  • The Wratten 88a

  • What Do IR Filter Numbers Mean, Anyway?

• Basic IR Techniques

  • The Short Version

  • Apply Liberally

  • Know Your Sources

  • Sidebar: Black Body Radiation

  • Exposure And Camera Support

  • Supplemental Filters

  • Recording: Color vs. Grayscale

  • Focusing

• Whence the Digital IR Look?

  • Source Spectra

  • Relative NIR Reflectivities

  • Camera Variables

  • False Colors and Monochromes

  • Sidebar: Forbidden Absorptions

  • R72 False Colors

  • Wratten 87 Grays

  • Wratten 87c Blues

  • White Balance Caught Blue-Handed

  • The Real Skinny?

• IR Contamination—the Other Side of the IR Coin

  • If You Need Something To Worry About, Find Something Else

  • What Would IR Contamination Look Like?

  • Hot Mirror Filters — A Cure Worse Than The Disease

  • The Heliopan 8125 "Digital" IR/UV-Cut Filter

• References and Links

  • IR Galleries

  • IR Information

  • Suppliers

See also the IR/UV Checklist

Last updated October 30, 2004

Why Infrared?

Conventional visible light photography is challenging enough. Why bother with infrared? Because it opens up an otherwise unseen corner of the world — one of serene beauty and never-ending surprise. Digital cameras make this peek around the red end of the visible spectrum easier than ever before.

On this page...

• Extra-Sensory Perception

• A Fresh View Within Easy Reach

• The Digital Advantage

• Sidebar: Jay Scott's Excellent IR Adventures

• Near, Not Far IR

• That IR Look

Topic Index

Last updated October 30, 2004

Extra-Sensory Perception

Denver's Washington Park and the Rockies beyond

Curiosity about the world beyond natural perception motivated some of our greatest inventions and scientific advances. Since the 17th century days of Galileo and Leeuvenhoek, telescopes and microscopes working with visible light have extended the reach of human vision to ever larger and smaller scales — today by many, many orders of magnitude. To say that these instruments have revolutionized all of science and much of Western philosophy and even religion in the process would not overstate the case.

In the last 2 centuries, visual observation escaped not only the human scale but also the visible spectrum — that narrow band of electromagnetic (EM) radiation where the solar power spectrum and the sensitivity of the human eye both peak. (Certainly no coincidence there.) Cameras and films sensitive to infrared and ultraviolet light gave us our first glimpses of a world awash in invisible light. Imaging devices based on more exotic forms of light (X-rays, radio waves, etc.) soon followed and continue to proliferate.

Today, as ever more sophisticated observing devices open up new segments of the EM spectrum to our view and analysis, astronomers and cosmologists find it necessary to revise their understandings of the cosmos — and even of our own solar system — on an almost continuous basis. And along with these views of the world beyond the senses have come many scenes of unimaginable beauty. Any image at Hubble Space Telescope Images by Subject or in the W. M. Keck Observatory gallery will attest to that.

Saturn's moon Titan at 0.8-5.1 microns (near to far IR) as captured by Cassin on 10/26/2004 from an altitude of ~450,000 kilometers (280,000 miles).

Out-of-spectrum experiences have generally been beyond the reach of the average photographer, but today's silicon-based consumer-grade digital cameras make it easy to explore the strange and serene corner of the invisible world found just beyond visible red in the near infrared (NIR) band of the EM spectrum at 700-1200 nm (0.7-1.2µ) wavelengths. Throughout this article, the terms infrared, IR, near IR and NIR will refer to the 700-1200 nm band of interest to digital photographers unless otherwise noted.

Twin Keck telescopes atop Mauna Kea

To this day, the NIR remains one of the most useful extra-visible bands in the EM spectrum. Aerial photographers have long relied on NIR imagery to capture the landscape with the greatest possible clarity over a wide range of atmospheric conditions — including some quite unsuitable for visible light photography. For much the same reasons, the world-class Keck telescopes atop Mauna Kea (right) spend much of their precious observing time with sophisticated digital NIR detectors mounted. Abundant interstellar NIR radiation conveniently passes through dust, gas and our own atmosphere to allow glimpses into otherwise hopelessly obscured regions like the Milky Way's galactic center.

At left is one of humankind's first-ever looks at the surface of Titan, one of Jupiter's four large Gallilean moons. The 0.8-5.1 micron infrared wavelengths were chosen specifically for the Cassini flyby in order to cut through the haze that completely obscured the surface to the Galileo flyby a decade earlier. Titan is in many ways a frozen version of Earth. 

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A Fresh View Within Easy Reach

NIR reflectance patterns
Visible Light	Near Infrared

The false color schemes seen in digital IR images like the park scene at right and the leaf still-life at top are another matter entirely. The colors are nothing more than artifacts deeply rooted in camera hardware and firmware, with no direct connection to the objects imaged. Colors aren't even defined in the NIR, of course, but the false colors can add their own mystique to digital IR photographs, and some digital IR photographers like Chris Miekus work hard to manipulate them to their own ends.

In all fairness, NIR images aren't for everyone. As accomplished film IR photographer Josh Putnam once put it on RPD, "... people either love [IR photography] or just don't get it, but the ones who love it really love it." That seems to be more true of photographers than of viewers, however. IR photographers commonly find that their IR images generate more interest than their visible light images. Many people appreciate IR's fresh view of things, but it's not just a matter of novelty. IR images have a rich beauty all their own.

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The Digital Advantage

Unlike ordinary films, silicon-based CCDs and CMOS sensors turn out to be quite sensitive to the near infrared (NIR) in the  700-1200 nm (0.7-1.2µ) range — so much so, in fact, that some of the incoming NIR has to be filtered out in order to reduce IR contamination artifacts to acceptable levels in the visible light images most buyers aim to take. The usual solution is to fit digital camera sensors with special internal IR cut filters (IICFs). These sensor-mounted filters vary in their IR transmission spectra, but most consumer-grade digital cameras let enough NIR through to allow some IR photography. Despite a clear trend toward ever-lower IR sensitivities in higher-end cameras, that's still true in 2004, but it gets harder with every passing year. If you get hooked on digital IR, you may end up searching high and low for an Oly C-2020Z or Nikon Coolpix 900, or for the more recent 5MP Minolta Dimage 7. These discontinued cameras are all still quite competent by any standard, but the high prices they command largely reflect their extraordinary IR capabilities. No, my C-2020Z isn't for sale. 

Special external filters passing NIR while blocking most or preferably all visible light make infrared photography possible. Film-based IR photographers have been using such filters for decades, but daunting technical and financial challenges continue to keep IR film photography well out of the photographic mainstream.

Luckily, digital cameras have changed all that. Armed with even an inexpensive IR pass filter, a sufficiently IR-sensitive digital camera makes IR photography

• Eminently affordable — no need for expensive IR film and developing,

• Enjoyably impromptu — no need to switch back and forth between regular and IR-sensitive films,

• Easily learnable — via immediate feedback in the field,

• Immensely fun — for the many surprises the NIR world holds (and for all the reasons above), and

• Intensely satisfying — for the serene beauty you'll discover all around you.

With quality IR pass filters like the Hoya R72 going for as little as $24, there's hardly a good reason not to try IR if your camera's up to it.

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Sidebar: Jay Scott's Excellent IR Adventures

In late 1999, dpFWIW contributor Jay Scott shared with me his first forays into the IR realm with an Oly C-2020Z:

I got a Hoya R72 IR filter, and I love it. Taking infrared pictures could not be easier. Put the filter on the lens, optionally set the camera to black-and-white, and go. The camera has enough sensitivity to IR that daylight IR photos can be taken handheld, and I was even able to get indoor IR photos under incandescent light. White plants, dark sky—I love it!

A few months later, Jay was still at it, more enthusiastic than ever.

IR photography with a digital camera is almost like having an IR eye. You point the camera and look at the display, and you see what it sees. It's so much fun to peer at this surreal invisible world that I wonder why everyone doesn't!

Most infrared photos are made either outdoors or in a studio. But incandescent lights are IR-bright, and I found it easy to make IR photos in ordinary indoor lighting. You have to be prepared for exposure times up to one second, depending on how bright the room is, but it's not a problem if you have camera support. Because incandescent lights are redder than daylight, the "infrared effect" is stronger indoors, at least with my Hoya R72; you are effectively photographing using longer wavelengths than you would outdoors. Household objects can surprise you with their weird appearance in IR; my blue dishwashing soap turned out to be IR-transparent.

Plants are white in IR. Flowers are bright, but seed-heads are often dark. The sky is dark, but clouds are bright. Skin looks strangely smooth, which could be an advantage for some portraits. Any object which is hot enough to glow red is more than hot enough to glow near-infrared. I've photographed gas flames and hot coals.

The camera's flash is bright enough for IR macro photography with the R72. A decent external flash should be bright enough to take pictures at portrait ranges. Someday I hope to get an IR flash head so I can take IR photos at night without being noticed.

Check out Jay's IR photos and commentary.

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Near, Not Far IR

Let me emphasize here that digital IR photography typically relies on reflected NIR from sources like the sun and incandescent lamps. Digital camera sensors based on silicon are not sensitive to the far (thermal) IR wavelengths (typically 3.0µ and longer) emitted by objects at room to body temperatures. Heat leaks from houses aren't visible in the NIR, and people, animals and other objects at room to body temperatures don't glow in the NIR any more than they do in visible light. To photograph them in the dark, you have to provide proper NIR illumination using a suitably equipped camera like the Sony DSC-F7x7 or an external NIR-only flash with no filter.

This article doesn't have much to offer on NIR-illuminated night photography, but many other web sites only a Google search away address this rich and useful field.

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The IR Look

Digital and film IR photographs have a look many describe as surreal. Clear, serene, bold and tonal are additional words that come to mind, at least for landscapes. Physical and firmware-related factors contributing to the IR look are discussed below. Aerial and reconnaissance photographers have long valued the often stunning clarity characteristic of IR photographs, and it tops my list of IR virtues as well.

Clarity

Clarity comparison
Visible light	Near infrared

IR images owe their great clarity to the atmosphere's exceptional transparency in the NIR. Scattering by air molecules is much less efficient at NIR than at most visible wavelengths. As a result, NIR photons take on average a much straighter path from object to CCD.

In visible light (left), scattering severely limits detail on the more distant portions of the far hillside in this hazy afternoon scene. Removing visible light with a Hoya R72 IR filter takes out much of the detail-scrambling scatter. An impressive amount of detail shines through the haze in the IR image on the right, despite the odd false-color scheme.

Usually monochrome or nearly so, IR images also partake of the deeply tonal beauty typical of black-and-white photographs. In combination, these visual charms make for some truly stunning IR images. To see for yourself, take a moment now to browse the galleries in Beyond Red..., a site created by talented landscape photographer and dpFWIW contributor Carl Schofield.

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IR Performance in Digital Cameras

Before rushing out to buy an IR filter, test your camera to make sure it can do its part.

On this page

• Can Your Camera Handle IR?

• What Makes a Good IR Camera?

• Internal IR Cut Filters

• Honey, Where's The Remote?

• IR-Sensitive Cameras

• Less Sensitive Cameras

Topic Index

Last updated October 30, 2004

Can Your Camera Handle IR?

Warning! Some digital cameras are better suited to IR work than others, and a few are downright hopeless.

Ever since 3 MP CCDs hit the consumer scene in late 1999, digital cameras have varied widely in their IR performance, but the overall trend has been toward lower and lower IR sensitivities ever since. As of 4Q2004, quality handheld IR photos are iffy with most new cameras — even through an IR filter with minimum light loss like the popular R72. But if you come prepared for long exposures with good in-camera or post-processing noise reduction and rock-solid camera support (read "tripod"), even relatively insensitive cameras can produce satisfying IR photos.

If you're looking for a digital camera specifically for IR work, you'll probably have to buy used. Legendary IR cameras like the Oly C-2020Z and the Nikon CoolPix 950 were discontinued ca. 2000. The 2001-vintatage Minolta Dimage 7 and the 1999-vintage Nikon D1 SLR haven't officially been discontinued, but I doubt that they're widely stocked now.

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What Makes a Good IR Camera?

With few exceptions, the very best IR cameras ever — most notably, the Oly C-2020Z, the Nikon CoolPix 950 and D1, and the Canon Pro70 — came out ca. 1999, and most were built around Sony's then-popular 2.11 MP CCD. The 5MP Minolta Dimage 7 (D7) was a surprise late-comer to the club. All had some combination of the following features:

• CCD or CMOS sensors with large sensels — through either large sensor diameters (the D1 and D7) or fewer sensels crammed onto average-sized chips (all the others)

• Less restrictive internal IR cut filters (IIRCF) — presumably through less manufacturer paranoia about IR contamination in visible light photos (the C-2020Z)

• Fast lenses (especially the D1 and C-2020Z)

• Flexible exposure, including manual

• Effective in-camera noise reduction at high ISO (the D1)

• Firmware that happened to render IR images in pleasing false color schemes (the C-2020Z and Nikon 950)

The Gold Standard: The Oly C-2020Z

Handheld C-2020Z/R72 in the camera's native false color scheme

Oly's 2nd generation digital rangefinder, the C-2020Z, achieves its legendary IR performance through a happy combination of large 0.0039 mm sensels, a permissive IIRCF and a fast f/2.0 lens. With an R72 IR filter, it suffers only 4-6 stops of light loss and easily produces crisp handheld IR shots on sunny days at ISO 100, as at right. The C-2020Z also renders R72s in a tolerable to handsome false-color scheme, depending on taste. And for all that IR sensitivity, I have yet to detect convincing IR contamination in its visible light photos.

The C-2020Z has become the de facto gold standard for IR sensitivity among consumer-grade digital cameras, and it's the standard I'll use when comparing all other cameras mentioned here.

A Typical Recent Camera: The Oly C-5050Z

Handheld C-5050Z/R72 in the camera's native false color scheme

No digital camera sold new or even factory-refurbished in 2003-2004 can match the C-2020Z's legendary IR performance. Consider the C-2020Z's 5th generation descendent, the Oly C-5050Z, which I purchased as a factory refurb in 4Q2003. The C-5050Z is a highly regarded rangefinder in its own right, but it's IR performance is so-so at best. Smaller 0.0028 mm sensels and a much more restrictive IIRCF render the C-5050Z a whopping 5-6 stops less IR-sensitive than the C-2020Z. It manages to stay in the IR game with an even faster f/1.8 lens and reduced noise at maximum ISO (400), but only with the R72; the deeper 87-series filters are out of the question. Handholding is possible if you're good at holding still. At right is a handheld C5050Z/R72 taken at maximum ISO and aperture with shutter speed fixed close to my handholding limit at 1/20 sec. I had to work at maximum wide-angle zoom to keep the aperture at f/1.8.

In theory, the C-5050Z's less IR-friendly IIRCF reduces IR contamination in visible light work, but in practice, I can't say that I see the benefit, even against the C-2020Z. The table below details the differences that contribute to their respective suitabilities as IR cameras.

IR Performance Comparison: Oly C-2020Z vs. C-5050Z
Property	C-2020Z	C-5050Z	Comments
Sensel size (mm)	0.0039	0.0028	These sensel diameters translate into 94% more sensel area in the C-2020Z.
Widest aperture	f/2.0	f/1.8	The fast C-5050Z lens gives it a 0.3 stop edge, but that doesn't offset its smaller sensel area.
Noise at ISO 400	Moderate to severe	Mild to moderate	Increased quantum noise due to smaller sensels partially offsets the reduced noise of the C-5050Z's electronics.
Remote  test			A remote test result as dim as the C-5050Z's doesn't necessarily put it out of the IR game, but it does mean that IR work will generally require solid camera support and long exposure times far beyond the handheld range. Technical Note: For a straight-across comparison, both cameras were fixed at  f/2.8 @ 1/20 sec, ISO 100 in manual exposure mode at full zoom (105 mm EFL). The full-sized test images were cropped without further post-processing.
Handheld sample : Visible light			In visible light, the two cameras are equally sensitive, as one can see from the nearly equal meteredl EVs, which have been normalized to ISO 100. Technical Note: For the handheld samples in this row and the next, both cameras were set for maximum zoom (105 mm EFL), auto-exposure and auto-WB with in-camera sharpening disabled. Post-processing of the full-size sample images consisted only of resampling to 800x600 and unsharp masking (USM) applied the brightness channel in Lab mode with identical USM parameters. All images were saved with identical JPEG compression settings.
	f/6.3 @ 1/500 sec, ISO 100	f/4.5 @ 1/800 sec, ISO 64
	EV 14.3	EV 14.6
Handheld sample:  IR with R72 filter			The C-2020Z's faster shutter speed eliminates camera shake and partially stops the motion of the passing cyclist. It is just this kind of IR performance that makes the C-2020Z a perennial favorite among IR enthusiasts. Note also the strikingly different false-color schemes these cameras assign to identical R72 scenes. Differing auto-WB algorithms probably account for much of the disparity. One would normally apply an IR-specific WB setting to improve the C-5050Z's garish rendering. Luckily, the C-5050Z has that capability. The C-2020Z doesn't, but many find its false color scheme rather pleasant.
	f/2.8 @ 1/40 sec, ISO 100	f/2.6 @ 1/5 sec, ISO 400
	EV 8.3	EV 3.1
Sensitivity loss in the NIR band	-6.0 stops	-11.5 stops	Bottom line: The C-2020Z is 5.5 stops more IR-sensitive than the C-5050Z under these test conditions.

You'll find many additional IR sensitivity comparisons involving a number of different 4-5 MP cameras among the excellent online photography articles posted by Andrzej Wrotniak, Jens Roesner and Alfred Molon.

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Internal IR Cut Filters (IIRCFs)

Silicon-based (CCD and CMOS) image sensors are equally sensitive to visible and NIR wavelengths out to about 1200 nm. To fashion a peak sensitivity in the visible band and to minimize IR contamination of visible light images, most if not all digital camera manufacturers cover their silicon sensors with an internal IR cut filter (IIRCF). Little is known about them outside the camera manufacturers and their suppliers, but some IIRCFs are clearly more restrictive than others. It's often stated, quite incorrectly, that professional cameras usually have very restrictive IIRCFs and consumer models usually don't, but the empirical Nikon D1 vs. 990 IR sensitivities noted below tell a very different story. The pattern — if there is one — remains unclear. 

Look Ma, No IIRCF

Some hardcore digital IR enthusiasts have gone so far as to disassemble their cameras and remove the IIRCF, a move guaranteed to increase IR sensitivity and void the warranty. James Wooten's Removing the IR Blocking Filter in the Nikon CoolPix 990 and 995 nicely illustrates the results and the surgery, which involves replacing the IIRCF with a plain piece of glass to preserve the camera's auto-focus ability. The IR galleries at Don Ellis' www.kleptography.com include many shots captured with a similarly modified Canon G1. Few have Don's eye for IR images. 

The Sony DSC-7x7's Nightshot Mode

With built-in NIR illuminators and a "Nightshot" mode that removes the IIRCF from the lightpath to the CCD, Sony DSC-F7x7 digital still cameras excel at IR-enhanced low-light work. They would seem to be naturals for daylight IR work as well, but Sony felt a need to force auto-exposure metering and to restrict Nightshot exposures (f/2 at 1/60 sec or longer) to keep voyeurs from subverting Nightshot to see through clothing during the day. (Some fabrics are apparently transparent enough in the NIR to reveal what's underneath in bright sunlight.)

These firmware restrictions pose challenges for legitimate daylight IR work with F7x7 cameras, to be sure, but with ISO locked at 100, a deep IR filter (e.g., the Wratten 87c or the Hoya RM90 or RM100) and one or more ND filters to cut daylight NIR input, the F7x7s can rise to the occasion, as Paul Cordes recently detailed on RPD:

I'm using a B+W 58 093IR, equivalent to an 87c. The filter is absolutely black and admits no light to visual inspection. The pictures are great.... You'll also need some Neutral Density filters, as the 707 is limited to f2 and 1/60th sec as the fastest shutter speed (Nightshot Mode). I'd suggest 0.3, 0.6 and 0.9 ND filters to give you flexibility for exposure control. If you don't mind a 3-filter stack, you can skip the 0.9 ND. Don't forget to block the IR emitters with something IR-opaque, or you'll see reflections from the back side of the filters.

Stacking filters of course results in vignetting. If you think that you will be stacking, a step-up ring and IR and ND filters of, say, 67mm might be a good starting choice. Wish I had thought of that before I bought mine!

Paul recommends the "Sony Talk" forum at www.dpreview.com as a good resource for F7x7 IR photographers.

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Honey, Where's the Remote?

As usual, testing is better than assuming.  Here's a very crude test of IR sensitivity for digital cameras:

• Put your camera in program mode at ISO 100.

• Point a TV, camera or other IR remote into the lens from no more than 12" away.

• Press any button on the remote.

• Look for the IR beam in the camera's through-the-lens (TTL) LCD or EVF (electronic viewfinder).

Remote Test Comparison
C-2020Z	C-5050Z

If the beam is as dim as the one captured at left with my Oly C-5050Z, you can still get IR images with an R72, but handholding will be iffy at best. You'll generally need solid camera support and long exposure times.

Technical Note: For a straight-across comparison, both cameras were fixed at  f/2.8 @ 1/20 sec, ISO 100 in manual exposure mode at full zoom (105 mm EFL). The full-sized test images were cropped without further post-processing. 

But Will an 87 Fly?

If your camera passed the remote test, it's almost certainly good to go with a Hoya R72 filter or Wratten 89b equivalent. But can it also handle the more expensive black IR filters like the Wratten 87 and 87c? The C-5050Z certainly can't. Compatibility with 87-series filters is hard to predict reliably with the remote test alone, but you can at least get an idea if you happen to have access to another camera of known 87 compatibility. 

If you can't compare with a known reference, consider starting out with a Hoya R72 or equivalent Wratten 89b filter. (FWIW, the R72 is still my favorite IR filter overall because it can be handheld most of the time on my C-2020Z.) If your camera sustains less than 7-8 stops of light loss in bright sunlit scenes with an R72/89b, it has a very good chance with an 88a and at least a fighting chance with an 87 filter. If it loses more than 7-8 stops, it's not likely to fare well with a black 87, but it still has a chance with the shallower 88a.

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IR-Sensitive Cameras

Included in this category are cameras that come within 3 stops of the Oly C-2020Z when used with an R72 filter. As noted above, the most IR-sensitive higher-end consumer-grade digital cameras were built around the large-sensel original Sony 2.11MP CCD ca. 1999. The Oly C-2020Z, Oly C-2000Z and the Nikon CoolPix 950 all used this CCD, and all do well with deeper IR filters like the Wratten 87 series. The red-filtered sensels in this now-obsolete CCD approached the 700 nm visible-IR boundary with a whopping ~70% residual sensitivity and a relatively shallow drop-off into the NIR. That left a generous window for NIR recording, particularly for shortwave (700-770 nm) NIR. The updated 2.11MP CCD found in the Oly C-2040Z is much less IR-sensitive, as noted below.

I've also seen excellent IR images taken with the 2MP Canon Pro70 using an R72-equivalent Kodak Wratten 89b gel, but I don't know what CCD it used. By all accounts, the IR sensitivity of the large-sensel 2.5MP Nikon D1 also seems to fall in this category, but I know little about D1 internals. So much for the theory that professional digital cameras have more restrictive IIRCFs.

Minolta Dimage 7

The 5MP Minolta Dimage 7 (D7) bucks the generally valid inverse relationship between pixel count and IR sensitivity, thanks in part to the large sensels on its 2/3" type CCD. Based on exposure data provided with a number of online D7 IR images taken with an R72 or equivalent filter, the D7 appears to run only 3 stops or so behind the C-2020Z in IR sensitivity. Soon after the D7 came out, Minolta released Dimage 7i ahd 7hi successor models, both with much more restrictive IIRCFs and correspondingly much weaker IR performance. If you're interested in high-resolution IR work using a camera with a more current feature set, consider the D7 but stay away from the D7i and D7hi. You can learn more about the D7 as an IR camera by visiting Jens Roesner's well-illustrated IR sensitivity and IR white balance pages.

Acknowledgements: Thanks to Michelle Cox for providing the D-490Z data points and to Jens for a wealth of D7 intelligence.

Surprises

IR performance can be hard to predict. When physicist David Brown conducted an informal IR comparison pitting an Oly C-2040Z, C-4040Z and E-10 against each other using an R72 filter and auto-ISO, the C-4040Z produced the best IR images by a wide margin, not by virtue of greater IR-sensitivity, but via lens speed and superior performance at high ISO. At an automatic ISO of 337, his handheld C-4040Z turned in shorter exposures and crisper R72 images. He also found the C-2040Z to considerably less IR-capable than his old C-2000Z. These findings jibe with Oly's claim that the C-2040Z's 2.11MP CCD had been redesigned since the C-2020Z and C-2000Z (and apparently since the C-2100UZ), but the difference could have been nothing more than a more restrictive internal IR cut filter.

Also much to my surprise, Roland Karlsson reported on RPD that his Heliopan RG 780 (Wratten 87 equivalent) and Heliopan RG 850 (Wratten 87b equivalent) IR pass filters both work well with his 3.34MP Sony S70. Since the RG 850 has an even deeper NIR window than the Wratten 87c, the entire 87 series should work with the S70. The S70's substantially greater IR sensitivity relative to most other cameras (the Oly C-30x0Z, Nikon 990, Canon G1, etc.) with the Sony 3.34MP CCD points to a less restrictive IIRCF.

In Nightshot mode, Sony's DSC-7x7 cameras remove their IIRCFs from the lightpath to the CCD in order to capture low-light scenes illuminated with NIR. If it weren't for firmware restrictions designed to thwart voyeurs, their 7x7 cameras would be the most IR-sensitive around by a wide margin.

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Less IR-Sensitive Cameras

Included in this category are cameras more than 3 stops less sensitive than the Oly C-2020Z when used with an R72 filter. 

For better or for worse, higher-end consumer-grade digital cameras have been getting progressively less IR-sensitive since 3MP CCDs hit the market in 1999. If you lay awake at night worrying about IR contamination in your visible light images, that's a feature, but if you hope to get in some digital IR photography with one of these cameras, it's a bug.

Early 3MP Cameras

Many of the early 3MP cameras introduced in 1999-2000 — the Nikon CoolPix 990 and 995; Oly C3030-Z, c-3020Z and C-3000Z; Canon PowerShot G1 and IS Pro90 — were based variations of the Sony 3.34MP CCD. All ran at least 3-4 stops less IR-sensitive than the Oly C-2020Z, as you can see from R72 exposure data gleaned from Don Ellis' IR gallery (Canon G1) and Todd Walker's R72 samples (Canon Pro90). IR buffs who tried them with deeper IR filters like the Wratten 87 series and the Hoya RM1000 eventually gave up, but those willing to put up with tripods and long exposures had considerable success with the more forgiving R72/89b filters. Don certainly didn't let the challenges get in his way.

4MP and Later Cameras

The Minolta Dimage 7 aside, subsequent cameras with 4MP and higher resolutions have on average been even less IR-sensitive than their 3MP forebears. My 5MP Oly C-5050Z, a typical example, runs 5-6 stops behind the C-2020Z through an R72. To understand why and what that means for use in IR work, see the detailed IR-oriented comparison of the C-5050Z and C-2020Z above. You'll find many additional IR sensitivity comparisons involving a number of different 4-5 MP cameras among the excellent online photography articles posted by Andrzej Wrotniak, Jens Roesner and Alfred Molon.

Canon D30 DSLR

D30/R72 false color scheme

Chris Miekus, an RPD regular with an enviable knack for non-landscape IR work, took the featured photo at the top of this article with a Hoya R72 mounted on a Canon D30 digital SLR with a Canon EF 28-135 mm, f/3.5-5.6 IS lens at f/5.6, 8 sec and ISO 100 (EV = 2.0). A common "heat lamp" provided the delicate lighting for this indoor subject. Chris reports that with auto white balance, the D30 produces a subdued magenta R72 false color scheme similar to that of the Canon G1, as shown here. For the photo at top, however, he applied a custom white balance to the RAW D30 recording after the fact. 

The D30 appears to run 4-5 stops less IR-sensitive than the Oly C-2020Z. According to Chris, the D30 is more IR-sensitive than its immediate successor, the D60, and about 3 stops more sensitive than the more recent D100 offering.

Chris cautions that lens choice is critical in IR work with Canon DSLRs. The popular Canon EOS 50 mm f/1.4, EF 16-35mm f/2.8 L and EF 28-70 mm f/2.8 L lenses all have anti-IR coatings that create bright central artifacts. As of 3Q2003, I know of no other IR-incompatible Canon lenses.

Making Less Sensitive Cameras Work

Many newer digital cameras still retain enough IR sensitivity for patient tripod-based IR work with a dark red R72 filter or equivalent Wratten 89B, as Don Ellis' IR gallery attests. Some even appear able to venture a bit deeper into the NIR. Danny Gossens reported on RPD that his G1 works well with a Heliopan RG715 filter — a Wratten 88A equivalent with a ~750 nm 50% cut-off falling between the Hoya R72 (Wratten 89B) and the Wratten 87 series, according to the spectral data published here. Note that Danny's posted G1 IR samples all required tripod support and shutter speeds of 1/3 sec or longer, but his results are more than acceptable.

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IR Filter Choices

Specially-designed filters that block everything but IR make IR photography possible. Here we discuss the main types and some of the properties that distinguish them.

On this page...

• First, Some Filter Terminology

• NIR Transmission Spectra For Several Common IR Filters

• The Ever-Popular Hoya R72

• The Wratten 87 and 87c

• The Wratten 88a

• What Do IR Filter Numbers Mean, Anyway?

Topic Index

Last updated October 30, 2004

First, Some Filter Terminology

Mount Tamalpais from the Berkeley-Oakland Ridge, Northern California

Let's pause to clarify some potentially confusing filter terminology. The filters used for IR photography are commonly referred to as "IR filters". To the extent that the much more common filters blocking UV light are properly called "UV filters", that may seem something of a misnomer, but like it or not, this usage is firmly entrenched in the IR band.

In this article, I'll refer to filters that block IR and pass visible light as "IR cut filters", as opposed to the "IR filters" or "IR pass filters" that enable IR photography by doing just the opposite.

Among the many IR filters available, two are particularly noteworthy:

• The affordable and forgiving very dark red Hoya R72, and
• The pricey and more restrictive black Tiffen 87.

In glass in the 49 mm size, these filters went for $25 and $83, respectively, at The Filter Connection as of April, 2000.

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NIR Transmission Spectra For Several Common IR Filters

Data from Clive Warren's Infrared Photography FAQ and the Heliopan No. 8125 Digital filter page.

The graphs at right show transmission spectra for several popular filters — the Wratten 89b (R72), 87 and 87c (B+W 093) IR pass filters and the Heliopan 8125 UV/IR cut filter — based on data from Clive Warren's Infrared Photography FAQ and the Heliopan No. 8125 Digital filter page.

Note that the 89b (R72) has a sliver of transmissivity in the deep visible red just below the 700 nm visible-infrared boundary. Some refer to it as a "dark red" filter because you can see through it to a very limited extent, but that appellation doesn't give the R72 proper credit for blocking nearly all visible light. The "black" 87-series filters, on the other hand, pass no visible light at all.

The R72, 87 and 87c pass progressively less light to the camera's sensor, which itself loses IR sensitivity at ~1200 nm. For all three IR pass filters, peak transmissivities of only 80-90% also contribute to unwelcome light loss in IR work. No wonder, then, that some cameras can handle an R72 but not an 87 or 87c.

The Heliopan 8125 "digital" IR/UV cut filter claims to block unwanted NIR and near UV as well, but the spectrum here shows that most of the longwave NIR still gets through. (That's the kind that theoretically reduces saturation.) I haven't been able to detect any benefit from this filter under normal shooting conditions with several different digital cameras.

The Ever-Popular Hoya R72 Filter (Wratten 89b, B+W 092 equivalent)

Looking north from Carson Spur, Sierra Nevada crest, California; handheld C-2020Z withR72

On an IR-sensitive camera like my Oly C-2020Z, the R72 typically takes a 5- to 6-stop exposure correction. Handheld shots are often possible in bright sunlight, but solid camera support is still a good idea. With the R72/C-2020Z combination, I've had very good luck with a monopod. The clarity samples shown above were taken by bracing against a post. The upper visible light sample was metered at EV 13.3 and exposed at EV 14.0 to darken the bright haze; the lower R72 version was metered at EV 9.7 and exposed at EV 9.0. 

The feature photo at the top of this page and the photo just above were both taken with a Hoya R72, as were many of the stunning IR images dpFWIW contributor Carl Schofield's displays at his Beyond Red... gallery. As illustrated in Carl's much-appreciated samples below, R72 images recorded in color are typically rendered brick red and pale cyan tones. That's as true of my Oly C-20x0Zs as it is of Carl's Nikon CoolPix 950. Truth be told, I've come to like R72 false color scheme.

My experiences with the Hoya R72 match Jay Scott's and Carl Schofield's: It's truly a joy. (If only my photographs matched Carl's!) To my mind, the R72 is a great value, a flexible tool, and a forgiving entry into the fascinating infrared world, and just plain fun. Most digital cameras seem to be able to handle the R72, but test your camera before you buy.

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The Wratten 87 and 87c Filters

These black "deep IR" filters pass no visible light to speak of. You can examine the transmission spectra of the original Wratten 87 and 87C gel filters in the chart above.  

Many digital cameras lack the IR sensitivity to handle 87-series IR filters, especially newer ones.

That can be true even for cameras that work well enough with an R72 or 89b filter.

The Wratten 87

Autumn sky; C-2020Z with 87 on braced monopod	Looking south from Grizzly Peak to Round Top, Berkeley-Oakland hills, northern California; C-2020Z with 87 on monopod

The Wratten 87 offers greater contrast and yields nearly pure grayscale output on most digital cameras, even with color recording, as you can see at right and in Carl Schofield's IR filter samples below. Unfortunately, the Wratten 87's additional 2-stop bite out of exposure precludes handholding under most conditions, but I've gotten away with it on very bright days with lots of bracketing for camera shake.

The Heliopan RG780 is an 87 equivalent. I'm unaware of a B+W equivalent for the Wratten 87. 

The Wratten 87c (B+W 093 equivalent)

With a 50% cut-off at 850 nm, the pricey black Wratten 87c (B+W 093) operates deeper yet in the NIR. The 87c typically yields blue monochromes like the one shown in the sample table below. (This was not always the case with the 87c on Carl's CoolPix 950, however, as discussed below.) The 87c presumably requires an even greater exposure correction than the 87.

Common IR Filter Comparison Samples
Kodak Wratten Filter	89b	87	87c
Hoya Equivalent	R72, RM72	n/a	n/a
B+W Equivalent	092	n/a	093
50% cut-off mark	720 nm	800 nm	850 nm
Nikon CoolPix 950 IR Images, all with auto white balance and color recording, firmware v. 1.3. Courtesy Carl Schofield. All rights reserved.

I have no personal experience with the Wratten 87C, but it's got to be an even tougher challenge than the 87 on all fronts.

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The Wratten 88a

With a 50% transmissivity at ~750 nm, this less common IR filter falls between the 89b/R72 and the 87 series with regard to total light loss. By the numbers, at least, the 88a runs closer to the R72. Some cameras (like the Canon G1) can't handle an 87 but do well with an 88a. The Heliopan RG715 filter is an 88a equivalent.

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What Do IR Filter Numbers Mean, Anyway?

The short answer is, not much for non-Hoya filters. Kodak Wratten IR filter numbers (89b, 88a, 87, 87c, etc.) tend to go down and Heliopan RG numbers tend to go up with increasing 50% transmissivity wavelengths, but if they mean more than that, it's not at all obvious to me. B+W IR filter numbers (092, 093, etc.) are pretty much meaningless.

The naming system Hoya uses for its IR pass filters is refreshingly rational. The R72 hits 50% transmissivity at 720 nm, just inside the NIR. The RM90 hits 50% transmissivity at 900 nm, and so on. How Hoya R and RM series IR filters differ, I have no idea.

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Basic IR Techniques

IR/UV Checklist

Digital cameras make IR photography easy, fun and affordable, but there are some tricks and pitfalls worth knowing.

On this page...

• The Short Version

• Apply Liberally

• Know Your Sources

• Sidebar: Black Body Radiation

• Exposure And Camera Support

• Supplemental Filters

• Recording: Color vs. Grayscale

• Focusing

Topic Index

You can pick up useful technical info on IR photography in Jay Scott's IR observations or in Carl Schofield's Beyond Red... information page. The references listed below include many other helpful IR resources, and a web search will turn up many more. 

Last updated October 30, 2004

The Short Version

We'll flesh them out below, but here are the basics of digital IR photography up front:

• Carry your IR filters and use them liberally

• Know your NIR light sources

• Know how to play your camera's ISO vs. noise and resolving power vs. shutter speed trade-offs. 

• Arrange for adequate camera support

• Record in color

• Check your work at the scene

• When in doubt, bracket like crazy for exposure and camera shake

But first, a very important safety reminder:

Never, ever view the sun directly through an IR filter, however black it may appear.

The transmitted near IR can permanently damage your eyes in a matter of seconds before you know it!

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Apply Liberally

Carry one or more IR filters (at least an R72) at all times and use them liberally — even when you don't "see" any promising subjects. The IR realm is full of surprises, and good IR shots can be impossible to predict from the visible light version. If you limit yourself to landscapes, the traditional IR fare, you'll miss half the fun — and a lot of good IR material.

To see what I mean, take a moment to browse Don Ellis' IR gallery. His striking R72 images of Hong Kong underscore the versatility of the most popular IR filter around. As Don puts it, "... the real image isn't always the obvious one". I'd say that goes double for IR.

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Know Your Sources

Digital IR photos typically record reflected NIR. Emitted NIR is much less commonly encountered but is by no means rare. Modern flash tubes emit enough NIR to be useful with IR filters, but fluorescent lights emit very little. Objects hot enough to glow visibly emit lots of NIR, too, but 

Objects at room to body temperatures don't glow in the NIR any more than they do in the visible band.

Reflected Near IR

The sun provides most of the NIR captured in IR photography

Objects that appear bright in IR images almost always do so because they have high NIR reflectivities. Objects that appear dark in IR images reflect little NIR, and they're more likely to transmit than absorb it. Generally speaking, NIR reflectivity does not follow reflectivity in the visible band. Thanks to the "forbidden" molecular transitions that correspond to NIR wavelengths, NIR emission and absorption are both relatively uncommon.

At this point, you can skip directly to Exposure and Camera Support or read on for more background on IR emissions.

Emitted Near IR

Emitted NIR comes primarily from objects at temperatures of many hundreds to thousands of Kelvins (°K) — far above body and room temperatures. One way to spot NIR-rich sources is to look for incandescence, the thermal emission of visible light. After a feeble start at temperatures around 500°C (773°K; 932°F), incandescence becomes conspicuous at around 627°C (900°K; 1,160°F), regardless of the material being heated. Since most of the energy radiated at such temperatures falls squarely in the NIR, 

Anything incandescent will also glow brightly at NIR wavelengths.

That includes glowing coals, electric heater coils, molten metal and glowing lava. Objects heated to 300°C (573°K; 480°F) or above can also glow substantially in the NIR, even when they're not yet incandescent. For example, just before its first dusky red tones appear, an electric heating coil coming up to temperature radiates strongly in the near IR and continues to do so throughout its working temperature range.

To gain a direct understanding of the relationship between temperature and emitted wavelength, try out the interactive Java tutorial in this superb color temperature tutorial. 

Thermal Radiation

The thermal radiation emitted by bodies at room to body temperatures lies in the far IR at wavelengths of 3µ (3,000 nm) or longer — well beyond the reach of the silicon-based digital cameras discussed on this site. (Silicon loses all IR sensitivity at 1.2µ.) Another major hurdle to thermal IR  imaging is our atmosphere. While highly transparent at visible wavelengths (0.3-0.7µ) and gloriously transparent at NIR wavelengths (0.7-1.4µ), air is quite opaque at 5-8µ, as shown here. Above 5µ, glass also becomes opaque, so you'll have to resort to mineral lenses that make high-end 35 mm glass lenses look cheap.

So, to "see" body heat in complete darkness, you'll have to give up your silicon-based CCD or CMOS sensor for something truly exotic that operates in the 3-5µ band where air and glass are still reasonably clear. If you have tens of thousands of dollars to spend, you might consider something like the indium antimonide (InSb) focal plane array IR sensor in the military night vision FLIR MilCAM. I don't think we're in Kansas anymore, Toto.

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Sidebar: Black Body Radiation

Wein's Law states that a black body at temperature T radiates with peak intensity at wavelength

w₀ (nm) = 2.898 x 10^6 / T (°K)

 The table below was constructed using Wein's Law. Note that the human body peaks far beyond the 5µ cut-off for atmospheric and glass transparency.

Acknowledgments: Information on Wein's Law came from the rather technical but very informative Electro Optical Industries' black body radiation tutorial. The solar data was found here.

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Exposure and Camera Support

With most if not all of the visible light cut out of the picture, exposures with IR filters require substantial compensations. On my highly IR-sensitive Oly C-2020Z, a Hoya R72 typically produces 4-6 stops of light loss. I can usually handhold C-2020Z/R72 shots on bright sunny days or in other settings with lots of NIR illumination, but I have even better luck with monopod support in such situations.

Support, ISO, Resolving Power and Noise

Some recent cameras (like my newer Oly C-5050Z) are still IR-sensitive enough for handheld R72 shots at maximum aperture and ISO, particularly if you bracket for shake. You may avoid camera shake that way, but you may also end up with suboptimal resolving power, unacceptable noise levels or both. If your camera has an action-shot program mode favoring short exposures at the expense of wide-open apertures and high ISO, give it a try with handholding and see what you get. You may be able to clean up a good bit of the noise in post-processing with a program like Neat Image, but don't count on that out until you've tried it.

Otherwise, bring along a tripod or some other rock-steady camera support. Remote control triggering would also help. Remember, 

    With adequate support comes more freedom to choose shutter speed, aperture and ISO according to scene qualities.

Admittedly, adequate camera support can be hard to come by on impromptu IR outings, but the R72 is fairly forgiving, and there's almost always something around to brace against — a tree, a rock, a lamp post, a steady companion.  When available support has been less than optimal, bracketing for camera shake can be very effective. Just take 2-4 exposures of everything you shoot. You can't shake all the time.

Unexpected Brightness Shifts

NIR reflectance patterns
Visible Light	Near Infrared

Keep in mind that objects that appear quite dark at visible wavelengths may be very bright in the near IR. Foliage is the classic example. The opposite may also obtain. Clear skies that look bright blue to you will usually photograph quite dark with an IR filter in place because the atmosphere scatters little NIR. These intensity shifts can lead to unexpected exposure variations in IR shots, especially with spot metering.

For all those reasons, I generally prefer to shoot IR with matrix metering in a program or priority mode, depending on the challenges at hand. I also spend a lot of time previewing with my LCD.

Accentuate the Digital

Shoot everything in sight and ask questions later. Digital cameras excel at the instant feedback needed to work through unfamiliar photographic situations. To be safe, load a big memory card and bracket heavily for both exposure and camera shake, especially if handholding. Before shooting in a priority mode, check the exposure settings via your LCD to make sure you haven't exceeded your camera's capabilities.

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Supplemental Filters

IR glare off San Francisco Bay

Atmospheric scatter is seldom a concern in IR work, but unwanted IR reflections may still warrant a polarizer. I should have used one to suppress the bright sunlight reflecting off the bay in the R72 photo of San Francisco at right. I also can think of a few snowy IR scenes where an upside-down GND might have helped, but the naturally dark skies in IR photos generally make life easier on the excess contrast front. At altitude, your IR shots might benefit from a UV-cutting haze filter. 

As always, keep an eye out for flare and vignetting when stacking filters.

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Recording: Color vs. Grayscale

Here's a counter-intuitive one for you: Color isn't even a meaningful concept in the NIR band, but color recording is still your best bet for high-quality IR work. As in B&W work, and for all the same reasons, you'll have more control and more options in post-processing with color recording, and you won't have burned bridges with a simplistic in-camera grayscale conversion algorithm.

With color recording, early digital cameras like the Oly C-2020Z and the Nikon CoolPix 950 rendered R72 images in a characteristic and I think rather pleasing brick-and-cyan false color scheme. Wratten 87 series images usually came out as pure grayscales. These false color schemes are illustrated above and discussed in more detail both above and below. Later cameras with more sophisticated Bayer color interpolation and white balance algorithms tended to render Wratten 87c color recordings as blue monochromes and R72s as magenta monochromes garish enough to make anyone wince — at which point the hunt was on for ways to manipulate the false colors, both in-camera and at post-processing.

If your camera's IR false colors don't suit, you have several alternatives, depending on available camera features, time, skill and software tools:

1. Color recording with grayscale conversion in post-processing

2. Color recording with color manipulation (red-blue channel swapping, admixture of channels from IR and visible light versions of the same image, and so on) in post-processing

3. Color recording with manipulated in-camera white balance (keep showing cards of different colors to your camera's "show me something neutral" custom white balance function until you get a false-color scheme you like)

4. RAW color recording with white balance manipulation at conversion to an RGB image.

5. Grayscale (B&W) recording mode

Many experienced digital IR photographers greatly prefer the power and flexibility of the first approach, but some assembly is required. B&W recording is certainly more convenient, but you'll need to stay on top of the recording mode in effect to avoid mishaps between IR sessions. 

Personally, I've come to like the look of R72 color images. My cameras offer B&W recording, but I always record IR images in color.

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Focusing

If you stick with auto-focus (AF), you won't have to worry about wavelength-related focus shifts. If you focus manually, be sure to check focus in your LCD or EVF. Digital zoom isn't good for much, but it's great for confirming good focus.

Depending on the spectral characteristics of your lens (achromat, apochromat, etc.), IR images are theoretically subject to a wavelength-related focus shift. Typically, IR light comes to a focus just past the focal plane, which has of course been positioned for visible light. If NIR is the only light coming in, AF should be able to adjust accordingly — provided it has enough incoming light to do its magic. (Cameras with NIR AF illuminators will do a better job here.) IR filters don't seem to interfere with AF accuracy on most digital cameras. 

Focus shift can be problematic in IR film work, even with AF, but on the digital side, any residual shift is generally absorbed within the generous depth of field typical of consumer-grade digital cameras. If you have trouble focusing with an IR filter in place, try increasing depth of field by stopping down the aperture and reducing subject magnification as best you can. 

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Whence the Digital IR Look?

The distinctive digital IR look is a many-layered thing. This page will explore its origins, which will in turn suggest ways to manipulate it. 

On this page...

• Source Spectra

• Relative NIR Reflectivities

• Camera Variables

• False Colors and Monochromes

• Sidebar: Forbidden Absorptions

• R72 False Colors

• Wratten 87 Grays

• Wratten 87c Blues

• White Balance Caught Blue-Handed

• The Real Skinny?

Topic Index

Last updated October 30, 2004

If you're not in the mood for speculation, skip ahead to a discussion of IR contamination.

Source Spectra

The top layer is the source of near IR (NIR) illumination, most often the sun. The intensity of sunlight may peak at green wavelengths, but sunlight is loaded with NIR as well — particularly at the shortest NIR wavelengths. The same is true of incandescent illumination. Conventional flash units produce a different spectrum, as do IR flashes and the LED-based IR illuminators available today. Whatever the source, the balance between longwave and shortwave components will have an impact on the way your IR photos look.

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Relative NIR Reflectivities

Next come the NIR reflectivities of the elements in the scene. In the bulleted paragraphs below, the links lead to thumbnails of images that illustrate the brightness relationships discussed. In each case, the source is assumed to be solar NIR.

• Deciduous tree leaves and grasses are almost always very bright in the NIR. Although made predominantly of NIR-transparent materials, leaves reflect NIR very efficiently because their complex internal air spaces offer many opportunities for shallow-angle internal reflections that eventually bounce the NIR out again. Leaves lose their NIR reflectivity when soaked in water and pressed to drive out the air. In the visible band, tiny airspaces work the same magic in fallen snow and fluffy soap suds, both of which start out bright white but approach transparency as trapped air escapes. Plant temperatures and pigments like chlorophyll have nothing to do with leaf reflectivity at NIR wavelengths.

• Leafless deciduous branches are also quite bright.

• Conifer needles tend to be less bright than deciduous leaves for reasons I have yet to divine, but internal structure differences are the prime suspect. 

• Clouds are very bright and the clear sky dark because condensed water droplets scatter all NIR wavelengths very efficiently, but air molecules hardly scatter them at all (and then only at the shortest NIR wavelengths). 

• Water surfaces tend to be dark in the NIR, mainly for want of bright skylight to reflect.

An IR Stroll Through the Park

Visible	NIR (R72)

I had a lot of fun on this IR stroll through Denver's Washington Park, the location for many of the images in this article. Checking out the NIR world though the LCD of a digital camera with an IR filter mounted is a good way to learn to pre-visualize IR shots. It's also full of surprises.

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Camera Variables 

Next in line among the many layers contributing to IR look come several layers related to the camera itself:

• the external IR filter used,

• the Bayer pattern color filters applied to the CCD,

• the internal IR cut filter applied to the CCD,

• the CCD itself, and

• the camera's firmware—particularly the color interpolation and white balance algorithms in effect.

Each camera layer puts its own stamp on the final IR image.

Black Boxes

Solid information regarding the last 4 camera layers listed above is hard to come by. Silicon-based CCD and CMOS sensors are equally sensitive at visible and NIR wavelengths but abruptly lose all sensitivity at around 1,200 nm. The CCD thus puts a fixed limit on NIR input to the image at the long end of the spectrum, while the external IR filter imposes a variable limit on NIR input at the short end. 

The proprietary internal IR cut filters sensor manufacturers apply to their digital camera products remain shrouded in mystery. I have yet to see a transmission spectrum for one, but they clearly vary widely and seemingly arbitrarily from camera to camera, as chronicled above. The inner workings of camera firmwares also seem to be hush-hush, but we manage to peek under the hood below. 

The sections that follow focus on 3 critical camera-related layers, 

• the external IR filter used,

• the Bayer pattern color filters applied to the CCD,

• the white balance algorithm used,

and what we can surmise of their intertwined contributions to the final IR image.

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False Colors and Monochromes

Color-mode digital IR images have a look and feel that varies with the IR filter and to a lesser extent with the camera used. The popular dark red R72 filter typically produces images with either a brick red and pale cyan or a red and magenta color scheme, depending on the camera, while images made with the deeper 87 filter tend to come out as grayscales or nearly so, regardless of the camera used.  Representative color-mode digital IR samples are shown again in the table below.

False Colors Rendered with Common IR Filters
Kodak Wratten Filter	89b	87	87c
Hoya Equivalent	R72, RM72	n/a	n/a
50% cut-off mark	720 nm	800 nm	850 nm
Transmissivity at 700-770 nm	Moderate	Low	Negligible
Shortwave NIR at 700-770 nm wavelengths appears to drive most of the false color seen in these digital IR samples.

Table Note: Images courtesy Carl Schofield, all rights reserved; Nikon CoolPix 950 with auto white balance and v. 1.3 firmware.

Color is, of course, an attribute of visible light alone. Since color is meaningless at NIR wavelengths, any color found in an NIR image is by definition false color, gray included. A digital camera is nevertheless compelled by its firmware to do something with the sensor data an IR filter generates. The samples above are representative of what camera firmwares tend to come up with when confronted with NIR scenes recorded in color.

How do these false colors arise? All available clues point to white balance algorithms and the NIR spectral properties of the Bayer pattern color filters covering the CCD sensels in single-CCD color digital cameras. The shortest NIR wavelengths (in the 700-770 nm band) appear to drive most of the false color rendering, as we'll see.

The Short Answer

Based on several different lines of evidence, I've concluded that one must simultaneously consider what goes on in 2 functionally different NIR bands in order to understand the false colors seen in digital RGB images taken with IR filters:

• 700-770 nm, hereafter referred to as shortwave NIR

• 770-1100 nm, hereafter referred to as longwave NIR

Longwave NIR (770-1100 nm) stimulates all sensels equally because Bayer pattern filter dyes are uniformly transparent at those wavelengths for quantum mechanical reasons elaborated in the sidebar. Thus, longwave IR primarily affects false-color saturations.

Shortwave NIR (700-770 nm), on the other hand, stimulates red sensels quite a bit, green sensels a little and blue sensels minimally if at all. Shortwave NIR thus drives the false colors, especially with shallow filters like the R72 (50% cut-off at 720 nm). The red tinge typical of R72 skies bears this out: Shortwave NIR is heavily over-represented in what little NIR the atmosphere manages to scatter because scattering efficiency by air molecules falls off inversely with the 4th power of wavelength.

If you block most of the shortwave NIR with a deeper Wratten 87 IR filter (50% cut-off at 800 nm), you get a near-perfect grayscale image, as you'd expect with equal stimulation of all 3 sensel types by the remaining longwave NIR.

Things get even more interesting with the Wratten 87c (50% cut-off at 850 nm), which in many digital cameras produces blue monochromes rather than the grayscales one might expect. I believe that this reflects a slight blue bias built into most auto white balance algorithms to counter the anti-blue bias carried by NIR contamination in visible light digital images.

At this point, you can skip directly to a discussion of IR contamination in visible light images, or you can brace yourself for a slog through the arguments supporting the theory above.

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Sidebar: Forbidden Absorptions

In an earlier version of this section, I wondered out loud about why digital IR images tend to be monochromatic, especially with deeper IR filters like the 87 series. Andrew Fong, a Ph.D. analytical chemist whose dissertation dealt with the use of NIR radiation in chemical analysis, e-mailed me this compelling answer: 

In my work, we generally use wavelengths greater than about 1200 nm to around 3000 nm, but I have some knowledge of the NIR band around 1100-770 nm.

The short answer is that the CCD mask filters are all nearly equally transparent to short NIR radiation [at 770-1100 nm]. The blue filters absorb some of the NIR radiation as a spill-over of their red-absorbing properties.

Basically, most substances are fairly transparent to this short-wavelength NIR radiation [at 770-1100 nm]. Some larger dye molecules can be made to absorb there (an electronic type absorption). However, this energy region does not happen to correspond with most molecular vibrations or electronic absorptions. Most molecular vibrations correspond with the Mid-IR [> 3000 nm]. Overtones and combinations of these Mid-IR vibrations occur between 3000-1200 nm. By the time you get to 1100-770 nm, these overtones and combinations of molecular vibrations become very weak (they are quantum-mechanically forbidden). Therefore, there aren't many compounds which absorb any light at all between 1100-770 nm unless they are dyes specifically designed to absorb there. (Water vapor does have a very weak absorption band somewhere around 900 nm).

Therefore, all of the color filter types on the CCD probably pass almost all of the short NIR radiation [at 770-1100 nm]. It is possible that the blue CCD mask filters absorb some of the light at the shortest end of the NIR band (an electronic absorption tail).

    —Andrew Fong, Memphis, TN USA

[Text in square brackets added by editor for clarity.]

With no suitable molecular energy gaps to fall into, longwave NIR photons at 770-1100 nm can be transmitted or reflected, but they're seldom absorbed. Shallow angles of incidence will promote reflection off NIR-transparent materials, just as they produce strong reflections off clear glass in the visible band. The complex air spaces found within deciduous leaves and fallen snow set up strong NIR reflections in just this manner. In the absence of reflection, longwave NIR transmission will dominate. Note that the forbidden NIR absorptions start at 770 nm, not at the visible-IR boundary at 700 nm. Allowed shortwave NIR absorptions in the 700-770 nm band will become important below. 

Andrew's explanation jibes with dpFWIW contributor Jay Scott's early observation that the red CCD filters in his Oly C-2020Z appear to pass shortwave NIR preferentially, while the blue CCD filters preferentially pass longwave NIR. It also jibes with Oly C-2020Z CCD spectral response plots showing ~70%, ~10% and ~0% respective sensitivities for the red, green and blue sensors at 700 nm, the visible-IR boundary. I have no hard data for shortwave NIR absorptions at 700-770 nm, but these plots are also consistent with Jay's observations. 

Now we're ready tackle the false colors produced by some of the more popular IR filters. What goes on in the undocumented 700-770 nm seems to be the key.

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R72 False Colors

The R72 produces images rendered in brick red and pale cyan tones by some cameras and in bright red to magenta colors by others. To my mind, the brick and cyan R72 pattern suggests an overall excess of red sensel stimulation and a blue sensel stimulation deficit, presumably due to the red filter's relatively high transmissivity and the blue filter's electronic absorption tail at the shortest NIR wavelengths. 

I don't have a ready explanation for the red and magenta R72 color scheme, which has become more and more common in recent cameras. At one time I wondered if it might occur with CCDs sporting CYGM (cyan, yellow, green, magenta) rather than GRGB (green, red, green, blue) Bayer pattern filters. However, the CYGM Canon G1 renders R72s in a purple-and-cyan color scheme similar to the brick-and-cyan scheme my GRGB Oly C-2020Z produces. See Don Ellis' IR gallery for some G1/R72 samples. Todd Walker's Canon IS Pro90 R72 samples resemble Don's G1 R72s very closely, as you might expect from 2 cameras built around the same 3.34MP CYMG CCD.

Since Bayer pattern filters appear to have some differential effect on shortwave NIR (the 700-770 nm band), those must be the wavelengths primarily responsible for the false colors typical of images taken with shallower IR filters (like the R72 and the 88a) with 50% transmissivities in the 720-750 nm range. I believe that the sky takes on a reddish tone in the R72/89b sample above because the atmosphere scatters less longwave than shortwave NIR, which in turn preferentially stimulates the red sensels. Foliage and clouds reflect all NIR wavelengths equally but acquire a cyan cast from a white balance algorithm trying to compensate for an overall excess of red sensel stimulation.

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Wratten 87 Grays

Beyond 770 nm, forbidden absorptions imply grayscale output from equally transparent red, green and blue filters, and grayscale is precisely what the Wratten 87 filter delivers, regardless of the camera. Why? Because the 87's 50% cutoff at 800 nm blocks both visible light and most of the color-generating shortwave NIR below 770 nm. Mild red and green sensel stimulation by the small amount of shortwave NIR transmitted by the 87 offsets an apparent blue bias built into typical auto white balance algorithms for reasons I'm about to propose.

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Wratten 87c Blues

The blue monochromes typical of 87c filters provide important clues to the IR false color puzzle.

With a 50% transmissivity deep in the NIR at 850 nm, the 87c absorbs virtually all of the color-generating shortwave NIR. These are the same NIR wavelengths blocked by the blue filter's electronic absorption tail in visible light work. The 87c thus deprives the red and green sensels of the shortwave NIR photons normally unavailable only to the blue sensels. A white balance algorithm conscious of NIR contamination and preprogrammed to boost blue sensels a bit to compensate for their normally reduced NIR stimulation would automatically add a touch of blue to the pure grayscale the 87c should otherwise have produced.

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White Balance Caught Blue-Handed

If you're still skeptical of the role white balance plays in IR false colors, consider this data point: Under the original v. 1.1 firmware in his Nikon CoolPix 950, Carl Schofield routinely got pure grayscale images from his color Wratten 87 and B+W 093 (Wratten 87c equivalent) recordings. But when his camera came back from a factory repair with an unexpected upgrade to v. 1.3 firmware, his 87c-equivalent images came out as blue monochromes instead, as shown above. The firmware change had no visible effect on his visible light, R72 or Wratten 87 images. Whatever else Nikon might have updated between v. 1.1 and v. 1.3, there was an acknowledged white balance handling adjustment which clearly pumped up the blue.

My Oly C-2020Z handles R72 and Wratten 87 color images just like Carl's CoolPix 950. I have no Wratten 87c to test, but when I stack a hot mirror (IR cut) filter on top of my Wratten 87 to block out all remaining shortwave NIR, I get blue monochromes, too. Note that the 87 had already removed all visible light from the mix.

These observations point to a strong white balance input. So does the ease with which you can manipulate false-color schemes via the custom white balance features common in recent higher-end cameras. Just show your "show me something neutral" custom white balance input a white or colored card and watch the false colors react.  

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The Real Skinny?

Given the camera manufacturers' reticence regarding firmware and internal IR cut filter details, we may never know for sure, but the evidence pieced together so far is fairly compelling: White balance and color filter spectral properties in the NIR both play key roles in the false colors and monochromes that appear in digital IR images recorded in color. 

If anyone out there has solid information along these lines, please drop me an e-mail at dpfwiw@cliffshade.com.

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IR Contamination—the Other Side of the IR Coin

Unfortunately, IR sensitivity isn't all good news for digital photographers. For those with more IR-sensitive cameras, the otherworldliness of many IR images raises a disturbing question:

If my camera's IR-sensitive enough to take decent IR photographs, and if my NIR and visible light photos differ that much, shouldn't I be worried about IR contamination in my visible light work?

The welcome short answer seems to be "seldom if ever", but just how we've managed to get off the hook on this score isn't all that clear. 

On this page...

• If You Need Something To Worry About, Find Something Else

• What Would IR Contamination Look Like?

• Hot Mirror Filters — A Cure Worse Than The Disease