A precise and feasible equipment for the measurement of light distribution is described. It consists of a light-source support, a revolving mirror, an optical system, servo systems for photometry and recording-pen speed control, and a polar coordinates recorder.
The revolution of the optical system on the light receiver is synchronized with the revolving mirror in order to keep the normal incidence to the detector.
The method of automatic balancing flicker photometry with a rotating chopper is newly adopted in the measurement of luminous intensity from a light source in order to eliminate the error caused by the fluctuation of luminous flux and by that of detector sensitivity.
The intensity of light as a relative value to a luminous flux emitted in a certain direction is, therefore, measured.
An automatic controller which keeps the speed of the recording pen approximately constant in a polarcoordinates recording by varying the rotating speed of the revolving mirror, is also newly introduced to perform the precise and efficient recording.
One complete measurement in a vertical plane takes about four minutes. Maximum weight of the luminaire mountable is 100 kg.

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Differential photovoltaic characteristics of PbS layer deposited chemically on frosted glass and oxidized by heating in an air atmosphere have been determined experimentally. The sign and the magnitude of photovoltage of PbS layer depend on the incident direction and the intensity of light falling on PbS layer.
Two tungsten lamps are used as light sources for measurement of differential photovoltage of PbS layer, and it is confirmed after several experiments that the photovoltage of PbS layer when two tungsten lamps are lighted simultaneously is an algebraical sum of each photovoltage due to the individual light source.
By regulating the incident direction and the intensity of the light of each tungsten lamp adequately, the photovoltage of PbS layer should be reduced to zero, and so the possibility ofnull method photometry is proposed.
In order to illustrate the mechanism of differential photovoltalic effect, a model of “secondary p-n boundary of PbS layer” is proposed.
Temperature dependence of the photovoltage of PbS layer and the value of activation energy are obtained experimentally, spectral response of the photovoltage shows no incident direction dependence.
When this PbS layer is used as photovoltaic cell, the photovoltage must be measured by vacuum tube voltmeter having sufficiently high input impedence because the resistance of the PbS layer used is 3 MΩ/cm²-15MΩ/cm².

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