Colour infrared films such as Kodak Infrared Ektachrome (IE, EIR) and Aerochrome (now discontinued) use the Infrared-Red-Green (IRG) colour-mapping scheme, instead of Red-Green-Blue (RGB) as in regular colour film. Hence, images created with IRG-mapped channels will have the look of EIR film or "EIR-style". This is a not a "simulation" but a true reproduction of the spectral sensitivity and functionality of Infrared Ektachrome (EIR) film and thus may be used as a digital replacement for EIR film.
Kodak patented a rather clever way of generating EIR-style false colour infrared (CIR) images by using existing Bayer colour filter array CCD sensors. Kodak actually put into practice the principles of its patent by producing and marketing the DCS 420CIR and 460CIR colour infrared cameras in the mid 1990's as a digital replacement for EIR film.
The diagram describes the principles of operation of these digital CIR cameras.
1. The camera CCD module is modified by replacing its usual IR-blocking filter with a clear IR-transmitting cover glass. This is called a "full-spectrum conversion" when performed on a modern digital camera. A deep yellow filter is placed in front of the camera lens to block out all blue light. Optionally, a partial IR-blocking filter can also be added to reduce the IR intensity to better balance the relative proportions of [IR], [R] and [G] light in the image. The Tiffen 12 & KG3 filter combination passes wavelengths in the range of 500-800 nm, similar to the original 650BP300 band pass filter that came with the CIR cameras.
2. Since [B] is filtered out, and all channels of the CCD respond to [IR], the Red channel image is actually [R + IR], Green channel is [G + IR] and the Blue channel is [IR] only.
3. This is the clever part. The image of the Blue channel [IR] can now be substracted from the Red channel to derive the [R] image, and similarly from the Green channel to derive the [G] image.
[R] = [R + IR] - [IR]
[G] = [G + IR] - [IR]
4. The derived [IR], [R] and [G] images are white balanced by using a neutral object as a reference. The correction factors depend on the light source and the filters used.
5. The [IR], [R] and [G] images are mapped to the Red, Green, and Blue channels of the final false-colour CIR image.
Kodak patent US6292212
DCS 460CIR camera restoration story
More digital IRG/CIR images in these groups:
Digital IRG
WavelengthPro
Discussion group on creating digital IRG/CIR images.
