A precision, direct-reading, colour temperature meter

The measuring element in the instrument described is a photomultiplier tube operated in a logarithmic difference circuit which views, alternately through red and blue filters, a diffuser illuminated by the lamp which is being calibrated. The output from the circuit is linearly proportional to the reciprocal of the colour temperature of the lamp and is independent of its brightness. Readings can be made with an accuracy of approximately ±5° K over the colour temperature range from 2200 to 3200° K.