An instrument for measuring the brightness of a star by means of the electric current produced when its light falls on a light-sensitive surface (the photoelectric effect). The light gathered by a telescope passes through a filter on to a light-sensitive surface known as the cathode. The cathode emits electrons which are multiplied within a photomultiplier so that the signal is easily measurable. Each photon detected by the cathode generates a pulse, and the number of pulses per second is directly proportional to the star’s brightness. This is known as pulse-counting photometry. In high-speed photometry the pulses are counted at intervals as short as 10 ms. The best photometers can yield magnitudes to accuracies approaching 0.001 mag.