Photosensitive: Sensitivity to photons of light. The human retina is such a device. Photographic film is another. Film is photosensitive because it has an electrochemical reaction when struck by light. Photosensitivity also occurs in an electronic device called a photocathode, which is a surface that emits electrons when struck by light. An electron tube called a photocell can be constructed with a cathode coated with light-sensitive coatings such as the alkali metals, an anode; both enclosed in a vacuum. When this is inserted in an electrical circuit it will allow current to flow when the cathode is illuminated with light.