The design and construction of an exposure meter for use with infrared sensitized film

In this project, an exposure meter which is sensitive to, and calibrated for infrared radiation was designed and constructed. An EG&G SGD-100A silicon photodiode was chosen to use as the photodetector. This choice was based on cost, availability, ruggedness, stability, and sensitivity. A circuit using an operational amplifier to amplify the output of the photodetector was used. The circuit design allowed easy variation of the signal gain to give a detectable meter deflection under varying lighting conditions. Five different gains were used to allow use of the meter under conditions ranging from outside under direct sunlight, to inside with typical room lighting. The gains used were empirically established. The film used was Kodak HIE 135-20. This is a black-and-white panchromatic emulsion with special sensitizing agents added to extend the film\u27s sensitivity into the near infrared to about 900 nanometers. The spectral response of the photodetector was to be filtered to match the spectral response of this infrared sensitized film. This proved not to be possible, due to the unavailability of any regular, stock filter which cuts off wavelengths above 900 nanometers. Despite this problem, the meter is much more accurate for setting exposures for the infrared sensitized film used than a normal exposure meter. The designed meter typically indicates within one stop of the optimum exposure for this film. A normal exposure meter reading made using Kodak\u27s exposure recommendations was incorrect by as much as three stops