The photocell consists of a metal plate coated with a semiconductor. When the photocell is exposed to light, electrons are liberated from the metal surface and are trapped at the interface unless an external circuit is provided. For making interior measurements, the photocell should be cosine and color corrected. The cosine correction is achieved with a specially designed translucent diffusing plate that allows the photocell to correctly evaluate the illumination regardless of incident direction. V lambda .
Light is expensive to measure accurately.
More accurate measurement of light usually requires a laboratory quality photometer that will cost three or four times this amount.