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To minimize interference from room lighting, optical communications receivers and some infrared TV camera systems will often
place an infrared low-pass filter in front of the light detector. The filters are designed to block most of the visible light, allowing the near
infrared light to pass and reach the detector. However, glass filters, that are often used for such applications, are expensive.
A cheap alternative is ordinary 35 mm photographic film that has been exposed to fluorescent light and then developed. As shown
in the attached figure, the color negative produced after the photographic developing process has a sharp cut-off at about 830 nanometers and
completely blocks most of the visible spectrum. The filter's transmission is perfect for many near infrared LEDs and lasers with wavelengths
between 830 and 950 nanometers. Kodak Kodacolor film with an ASA rating of 100 seems to work the best. A 5 second exposure to "Cool White"
fluorescent light will work do the trick.
Have the film processor develop the film in the usual manner but not make any prints. The color negatives become the filter
material. A typical 36 exposure roll will cost only about $5.00. The film can be easily cut into any size or shape that may be needed. However,
the film is not recommended for applications where it can become scratched or exposed to moisture.
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