Phototube

A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light.

Phototubes operate according to the photoelectric effect: Incoming photons strike a photocathode, knocking electrons out of its surface, which are attracted to an anode.

Unlike photomultiplier tubes, no amplification takes place, so the current through the device is typically of the order of a few microamperes.

[1] The light wavelength range over which the device is sensitive depends on the material used for the photoemissive cathode.

The frequency response of vacuum devices is generally limited by the transit time of the electrons from cathode to anode.