This omnidirectional photometer is intended for the measurement of daylight under grass swards; but the principle used in the instrument can be applied to measure the mean illumination at a point in other cases, for example, interior illumination. It consists of two small circular barrier layer cells, insulated from one another and mounted back to back, each being surmounted by a translucent diffusing hemispherical shell, diameter 5/8 in. The cell sensitivities are adjusted electrically to equality, the combined output being read on an indicating meter. Even for collimated light the response is uniform with orientation to better than ±5%.
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R. G. Giovanelli (1953) studied this question.