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your Hobby Projects"
Last Updated on:
07/27/2008 02:20:12 PM
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Color Film Negative Makes Visible Light Blocking Filter
designed by David Johnson, P.E. |
| This isn’t an electronic circuit but the drawing below shows the curve for an optical filter made from exposed color film. The
material blocks most visible light but is very transparent to 880nM infrared light. Kodak 100ASA is exposed to "cool white" fluorescent light for about 5 seconds. Then, the film is developed
in the usual way. The color film negatives form the filter material and does a fine job of blocking visible light wavelength and passing the longer near infrared wavelengths. |
| 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, which 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 Kodakcolor 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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