- An electronic device which increases the brightness of images, and which can operate at very low levels of light. For example, the image whose brightness will be intensified is first focused onto a photocathode, from which electrons are ejected. These electrons are then accelerated and focused onto a phosphorescent screen, whose displayed image may be thousands of times brighter than the original. Used, for instance, to obtain clear X-ray images with markedly reduced exposure. Also called image inten-sifier.