A photodetector for detecting photo intensity through a liquid comprises a body (15), which defines a chamber (17) that is open at one end, a photosensor (14) mounted to the body and arranged to sense light entering the chamber (17) through its open end, a layer of air being disposed in the chamber between the photosensor and the open end of the chamber. The layer of air forms a barrier between the photosensor (14) and the liquid and thus slime and other matter is unable to build up in front of the photosensor.
