A new precise comparator was manufactured on trial in order to compare the spectral radiances of two tungsten strip lamps which are used as pyrometric standards. The comparator is composed of two objective lenses, two right-angled prisms, a light chopper and a light detecting system.
The beam of light from the filament of one of the two lamps which are to be compared, being converged by the setup of a lens and a prism arranged on the one arm of the comparator, makes a real image of the filament on the chopper, which receiving also the other beam, changes the cross-sectional area of the two beams reciprocally. The beams, thus chopped reciprocally, enter the detecting system through an aperture. The system, composed of an interference filter (λ_max=0.648μm), a photomultiplier and a phase discriminator, detects the periodical change of the incident monochromatic radiant power at the wavelength of 0.648μm, discriminates the phase difference between the periodical change of the radiant power and that of the vibration of the chopper and judges whether the spectral radiances of the lamps are equal or not.
It is confirmed experimentally that in this comparator systematic errors and drift are negligibly small and the precision of comparison is about ±0.05°C at luminance temperature of 1065°C.