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DiscoverCircuits.com -- Hobby Corner
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Circuits for your Hobby Projects"
Last Updated on:
06/19/2009 02:52:09 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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