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There are many circumstances in which we would want to measure the amount of light at a given location. This is a common problem for engineers, scientists, optical designers, and photographers. In the case of photographers there is a need to determine how the effect of the camera’s f-stop and filters will combine with the exposure time to produce a desired image. An optical engineer might want to quantify how much light is lost in its passage through an optical system, or how effective an optical coating is performing. An astronomer or optical scientist might be interested in knowing the throughput of a telescope and its optical system, and how that converts to a recognized photometric scale. These examples motivate the need for devices that can accurately quantify how much light is in a given location or system. One of the more popular components for measuring the amount of light is the photodiode. Photodiodes are semiconductor devices whose primary role is to convert the energy contained in light into a current. These devices are rugged, reliable, and robust, and are used in a wide range of instrumentation that includes spectrometers, optical power meters, optical interlocks, encoders, and more. They are important in communications links such as fiber optics and free space optical transmission. Photodiodes are currently mass produced commercial items that are available at very reasonable costs to the consumer. In this chapter we will be concentrating on the electrical configuration, signal amplification, and filtering of the photodiode output and how to select and tune the performance of photodiodes. The operational amplifier plays a critical role in the operation and signal conversion from the photodiode, so we will look in some detail at the transconductance property of operational amplifiers.
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