PURPOSE: To equal the light quantities of a hollow cathode lamp and a deuterium lamp which are integrated with the time by changing the ratio of the lighting periods of the two lamps and the integrating periods of the light signals according to the brightness of the two lamps.

CONSTITUTION: The light rays radiated from the hollow cathode lamp 1 and deuterium lamp 2 are synthesized by a half mirror 3 and are converted to an electric signal by a photoelectron amplifier 4. The electric signal is passed through a preamplifier 5 and is integrated 6 in synchronization with the lighting clocks of the lamp 1 or 2. A lamp lighting clock generating circuit 8 generates the lighting clocks of the lamps 1 and 2 and the ratio of the lighting periods of the lamps 1 and 2 change according to the brightness of the respective lamps. The ratio of the lighting periods is previously determined for each of measuring elements or the lighting periods are determined by executing computation on the basis of the result obtd. by measuring the light quantities.