Selective masking using a digital imaging program provides great flexibility for spectral selectivity.
Pre exposure, spectral selectivity using color filters can be combined with post exposure, selective masking using a digital image editing program.
The 800 to 900 nanometer region of infrared can be isolated by subtracting an image shot on Konica 750 from an image of the same scene shot on Kodak HS Infrared film.
The Konica 750 extends to about 800 nm, while the Kodak HS Infrared extends to just past 900 nm.
The difference between them is the relatively narrow band of wavelengths from approximately 800 to 900 nanometers.
It is possible to create pseudo color images like Kodak Ektachrome Infrared produces, but using any set of three bands of wavelengths.
One possible example would be to shoot 3 exposures - one on panchromatic film, one on Konica 750 and one on Kodak HS Infrared.
This would allow generation of 3 separate bands: 400 to 700 nm (visual), 700 to 800 nm (near infrared) and 800 to 900 nm (extended infrared).
These three images could be used as blue (visual), green (near infrared) and red (extended infrared) channels of an RGB image.
Pseudo color images created from carefully chosen slices of spectrum offer an interesting way to examine objects.
One use that seems especially interesting is an RGB pseudo color image where red records infrared, green records visual spectrum and blue records ultraviolet.
The individual layers of tri-color films like Kodachrome 25 and Ektachrome Infrared can be used to selectively mask other layers from these color films or B&W films.
One simple way to isolate the infrared region beyond the visible spectrum is to subtract the yellow and magenta layers of Ektachrome Infrared film from the cyan, infrared recording layer.
This will result in a higher cut off wavelength than a Wratten #25 or #29 filter without the expense of an exotic infrared filter.
In cases of large diameter lens such as longer focal length zooms and telephotos, the extreme infrared range filters can be very expensive.
This post exposure, spectral selectivity method of combining digital images produces results similar to filtering but far less expensively.
A further advantage to post exposure selective masking is that it provides great flexibility with the option to preview the results while working.
Digital imaging programs like Adobe Photoshop, provide some new opportunities to explore the World using the digital imaging methods of spectral selectivity.
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Combining Images to Mask Spectrum
