The instrument measures the intensity of radiant heat. It is used in testing the performance of commercial radiant heaters such as the domestic gas fire or overhead electric radiator. The detector element is a lead sulphide cell that is exposed for about 1/20 second by a shutter similar to one found in a simple camera. The indication is given immediately on the dial of a moving-coil meter, so individual measurements are very speedy. The instrument is much quicker and easier to use than the conventional thermopile.