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SATURDAY 26 OCTOBER, 2013 | RSS Feed

Digital Color Infrared Techniques: A Brief Catalogue

by UVIRCUT | post a comment

Note: This thread is intended as a summary and review of the topic of digital color infrared photography. Comments on how to make this catalogue more accurate or inclusive are welcome, but research-and-development type posts are probably better made on one of the other threads.

“Color infrared” (as opposed to tinted monochrome images, which are a separate topic) refers to types of color photographic image where at least one of the three image channels contains infrared information. Unlike black-and-white infrared images, most color infrared images are not pure infrared images; the vast majority of such photographs contain image information from other wavelength bands as well (most often visible light.) There are three general classes of such images. These images are all in some way cross-sampled—the colors that appear in the red, green, and blue channels of the image do not represent red, green and blue light, respectively, as they do in an ordinary color photo, but rather some other mapping. The particular mapping of the image gives each technique its characteristic look. Most of the images produced do have one thing in common: surfaces appearing bright green are rare or absent, because to appear this way a surface would have to reflect a specific, limited band of light without reflecting anything of longer wavelength; with infrared in the mix, few surfaces can meet this condition. 

Color infrared images fall into three broad categories:

1) IRG–>RGB images 

These are the oldest kind of color infrared image. They originated in several (now defunct) Eastman Kodak films which were developed for vegetation health monitoring and similar purposes. In this type of image, green light is rendered as blue, red as green, and infrared as red. In the films this was accomplished by changing the usual pairings of sensitizers and dye couplers in the different emulsion layers and including an infrared-sensitive bottom layer that coupled to the cyan dye. 

Lewis Memorial Cemetery

A color film infrared image, by oldoinyo


Such images (when acquired through the usual sharp-cutoff yellow filter) have several signature characteristics–skies appear a darker and more violet hue than on ordinary photos and healthy vegetation is either bright red or magenta. Stressed vegetation appears yellowed, brown or even faintly greenish and stands out much more in this type of image than in an ordinary photo, due to large changes in the infrared reflectance profile of the foliage. In general, red objects appear yellow; some dark dyed clothing such as blue jeans will appear red. Curiously, skin tones end up rendered in this image approximately as they are in ordinary color photographs, although lips are yellow rather than red and dark hair often picks up a distinctive reddish tinge. Bright green is almost unheard-of in these images, for surfaces that absorb infrared while reflecting red are very rare; only a monochromatic light source such as a red laser or LED is likely to produce a vivid green color in this kind of image.

1A) Two-Image Techniques

These are the only digital techniques which can produce exact IRG images with ordinary consumer gear. Such images, if the proper infrared filter is chosen, will have the same scientific precision and usefulness as their film counterparts.

1A-1) Sequential two-image method

Two images are taken with the same camera on a tripod, one an ordinary photo and the other through a black-IR filter. If care is taken not to disturb the camera during the filter change, no registration of the images will be needed. In post-processing, the infrared image is subjected to initial clean-up and then written into the red channel of the final image; the red and green channels of the other image are written into the green and blue channels of the final image, respectively. 

Fog in the Firs

Honolulu Skyline from Diamond Head, Hawaii (2009)

Sequential IRG images by oldoinyo (top) and Infrachrome (bottom); the latter includes a reference image.

This method is only usable on completely static subject matter. If anything in the frame moves between or during exposures, the product will be marred by ghosting and fringing.

1A-2) Concurrent two-exposure method

Two complete, identical cameras are required for this technique, along with a device enabling simultaneous shutter release and, most often, a beam-splitter box device enabling the two cameras to share a single optical path. One of the two cameras is fitted with a black-IR filter, and the two exposures are acquired simultaneously. Some registration of the images is usually required with this method. The workup is otherwise the same as with the sequential method.

Rainbow (American) Falls, Niagara Falls, NY (November 2010)

Concurrent 2-exposure IRG, by Infrachrome 


If the shutter speed permits, moving subject matter can be photographed this way, and even hand-holding is possible. The set-up is, however, somewhat unwieldy and requires duplicate camera outfits to realize. The beam-splitter exacts about a stop of exposure penalty, and does not easily handle wide-angle lenses. A variation of the technique dispenses with the beam-splitter and mounts the cameras side-by-side; this accommodates wide-angle lenses better, but any nearby objects in the frame will show parallax fringing, which severely limits the usefulness of this approach.

1B) Single-exposure method

A digital camera with a Bayer-mask type sensor and with the infrared-blocking filter replaced by a clear blank is required, in addition to a sharp-cutoff yellow filter such as a #12 or 525LP to exclude all blue light. The camera’s blue channel then captures only infrared light, while the other channels capture mixtures of infrared with red and green light, respectively. Through one of several experimental mathematical procedures, the resulting image can be reprocessed into a good approximation of an IRG photo.



IRG

Single-exposure IRG, by H.H Pictures (top) and crankykoopa (bottom) using #12 filter.

Since there is only one exposure, the issue of registration does not arise, hand-holding is quite possible, and moving subject matter can be handled easily. There is, moreover, no need for duplicate camera gear. However, the spectral profile of the infrared signal captured by the blue channel is not identical with that of other channels (in particular, it contains much less short-wave IR than the red channel), so the approximation must remain such, and the image is unlikely to have the scientific utility of one obtained with a two-image method. This method cannot be used with laminar (Foveon) sensors, because these work in such a way that the infrared signal cannot be isolated in one channel.

2) Techniques which produce images somewhat reminiscent of IRG–>RGB images 

2A) Foveon color IR

Mapping: [IR][I’G][I”B]–>RGB

False color infrared

Foveon color IR, by spytzer

No filter is used for this technique (the camera’s hot mirror is also removed.) Due to the Foveon’s high infrared sensitivity, foliage is rendered magenta to red; post-processing can boost the effect for a brighter color. Skies remain blue.

2B) Dark, IR-pass neutral-density filter (usually unexposed slide film) with modified camera (balances visible and IR sensor response.)

Mapping: same as above. 

(unfortunately, the only known example of this has been deleted.)

Success of this technique is critically dependent on the ND filter’s cancelling out the hot mirror’s unbalancing effect, which depends in turn on the exact combination of filter, hot mirror, and sensor. This technique may be usable only for time exposures if the camera's hot mirror is strict. This is one of the few techniques in which rendition of surfaces as bright green or blue is possible.

2C) XNite XDP filter (Bayer-type sensor)

Mapping: unknown; one might guess something like [I][I’RB][I”B'U]–
>RGB

The XDP filter is of the Wood's-glass type (see the transmission spectrum here.) Some post-processing is evidently required to obtain the colors below.



XNite XDP filter, by H.H. Pictures

Foliage tends to be rendered red and skies dark blue to green, depending on the exact camera and post-processing used. Unconverted cameras are limited to time exposures.



2D) Miscellaneous orange filters (B+W 099, 040, 041) with full-spectrum camera (Bayer-type sensor)

Mapping: I[I’Y][I”R]–>RGB

20070805_DSIR0786

Varengeville Beach, Digital Infrared, B+W 099 Filter

Full-spectrum camera with 040 filter (above, by Joshua Putnam) and with 099 filter (below, by Jon S Page)

After the exposure is taken (isolating the infrared signal in the “blue” channel), the red and blue channels are swapped and final adjustments are made.

Appearance varies somewhat depending on the exact filter and camera used, but generally foliage is rendered coral-pink to orange-red and skies are turquoise to pale green.



3) Other Color Infrared Techniques (not producing IRG-like images)

3A) R72 color

Mapping: II’[I”R]–>RGB

Taken with the Hoya R72 filter or one of its many clones, this is by far the most widely practiced variety of digital color infrared, and trades on the residual differences in channel response in the near-infrared region as well as red leakage into the red channel. It is most easily achieved with a modified camera, although some older cameras have sufficiently permissive hot mirrors to make attempting this technique possible. In the most common procedure, camera white balance is usually preset to render foliage dead white, and it is customary in some outdoor shots to swap the red and blue channels of the finished image.

With Eyne

IRPortrait2

R72 color by zachstern (above) and Cameron Shaw (below)

Skies tend to be vivid blue (orange to brown if the channel swap is omitted,) but all other colors are very pale pastel; skin appears bluish, as does blond hair. There are many variations on this technique, some of which involve extensive post-processing to alter the colors.

3B) Trichrome Infrared

Mapping: II’I”–>RGB

These relatively rare images are acquired using a combination of two special bandpass (750-850 nanometers, #BPG and 850-950 nanometers, #BPR) filters from maxmax.com together with a deep cutoff (950+ nanometer) filter, and are the only pure infrared color images. They are acquired by a sequential three-image technique and thus can only depict static subject matter. It is possible that they could be acquired concurrently with three cameras, but no one is known to have attempted this.

Trichrome Infrared

Trichrome infrared sample, adapted from the maxmax.com website

Skies appear a striking deep, midnight blue in these pictures and vegetation a subtle off-white; some dark textile dyes are rendered faintly reddish or brownish, and almost all other surfaces seem virtually monochromatic. Most surfaces have very little differential reflectivity in the near-infrared range. Only a few things, such as special inks and paints, longer-cutoff IR filters, or reflections from IR lasers, could show any strong color when photographed this way; such images would be excellent tools for highlighting these items (an 093 filter would appear bright yellow, for example.) A variation of this technique dispenses with the long-cutoff filter and uses instead a shorter-wavelength (650-750) nanometer bandpass filter for the third channel; the resulting images are slightly more colorful but are not pure infrared, being more R72-like in appearance (but not needing the channel swap for the sky to be rendered blue.)

3C) Wideband Images

Mapping: I[RGB]U–>RGB

These images contain infrared, visible, and ultraviolet information in three successive channels, and are acquired by a sequential three-image technique with three filters (most often one of the Baader filters for the UV exposure) and thus can only depict static subject matter. It is possible that they could be acquired concurrently with three cameras, but no one is known to have attempted this. 

(unfortunately, the instream link to the image no longer works. If you wish to view the image click here.)

Skies appear paler and greener than to the naked eye; vegetation appears orange, and asphalt pavement often appears bluish, due to the anomalous high UV reflectivity of that surface. Most white paint will appear distinctly yellowish, due to the poor UV reflectivity of the primary pigment, titanium dioxide. Green on surfaces is seldom seen because it requires a surface more reflective in the visible than in the IR.

3D) 403 color

Mapping: [IR][R’B][B’U]–>RGB

Taken with the B+W 403 filter or a similar Wood’s-glass-based filter, these images are characterized by pale, straw-gold vegetation and striking violet skies, due to the UV component of the sky light (UV contributes very little to other image areas.) The type of filter used is highly opaque to green light, so the color green is never seen; what signal ends up in the green channel is residual deep red and deep blue leakage. 

Holy Trinity Collegiate Church

403 color, by publicenergy

3E) Other miscellaneous methods

Various other images have been made that include some infrared signal, including those made with full-spectrum cameras with no filter or with yellow or orange filters. The appearances of these photographs varies with the exact characteristics of sensor and filter, and thus they answer to no single description. I leave you with two random examples:

Velae False Color

Full-spectrum camera with no filter, by myogi

China @ Bukit Jalil Park

Full-spectrum camera with floppy disk filter, by christopher lezheng 






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