PURPOSE: To enable the measurement of a light source and a filter to be measured both in an individual manner or in a combining condition, by a method wherein a device for switching a reference light and a sample light, a filter housing chamber for holding a filter in a required temperature, a monochrometer, and a photometer are combined together.

CONSTITUTION: Light from a reference light source 3 is guided to a measuring light path through operation of a switching device 5, light of a reuired wavelength range is caused to enter a photometer 14 through rotation of a diffraction grid 15b to set the output to a reference value. If only a sample light source 1 is lighted, the luminous spectrum of the sample light source 1 is obtained in a similar procedure. If a plurality of filters 9 are attached to a holder 8d, the interior of a room is kept in a required temperature by a heating wire 8f, and light from the reference light source 3 is then caused to transmit through the filters 9 and enter a spectroscope 15, the spectrotransmission properties and temperature properties of the filters 9 can be obtaned. The entrance of a sample light instead of the reference light permits the measurement of spectrometric properties of a virtual marking light obtained through combination of the sample light source 1 and optical properties at the required temperature of the required filter 9.