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More  Infrared Circuits

Color Film Negative Makes Visible Light Blocking Filter
 


This isnít an electronic circuit but the file 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 negatives form the filter material and does a fine job of blocking visible light wavelength and passing the longer near infrared wavelengths.
This isnít 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.

Click on Drawing Below to view PDF version of Schematic

Circuit Color Film Negative Makes Visible Light Blocking Filter

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