Photovoltaic cells develop an emf in the presence of light. Copper oxide and
selenium are examples of photovoltaic materials. Amicroammeter calibrated in
lux (lm/m2) is connected across the cells and measures the current output.
Photoconductive devices change their resistance with light intensity. Such
materials are selenium, zirconium oxide, aluminum oxide, and cadmium sulfide.
Photoemissive materials, such as mixtures of rare earth elements (cesium
oxide), liberate electrons in the presence of light.
Semiconductors are photosensitive and are commercially available as photodiodes
and phototransistors. Light generates hole-electron pairs, which causes leakage
in reversed biased diodes and base current in phototransistors. Commercial
high-resolution optical sensors are available with the electronics integrated onto
a single die to give temperature compensation and a linear voltage output with incident
light intensity are also commercially available. Such a device is the TSL 250.
Also commercially available are infrared (IR) light-to-voltage converters (TSL 260)
and light-to-frequency converters (TSL 230). Note, the TSL family is manufactured
by Texas Instruments.