An image intensifier is a device used to amplify the intensity (or brightness) of images at low light levels so that they can be readily viewed or recorded. The amplification is achieved by electronic means, and the image intensifier is a passive device that relies only on the very low levels of light coming from the object being viewed. Typical applications include the amplification of X-ray images in medical diagnosis, in astronomy for recording very faint telescopicimages, and for night viewing—this aspect being particularly .useful for military and security applications.
Light amplification
Light energy from a scene comes in the form of photons, which can be considered as packets of light energy. While the energy of these photons is sufficient to affect the retina of the eye, it is quite low in electronic terms. In addition, photons cannot be directly amplified or accelerated, and so to amplify light, it is first necessary to convert the photon energy into a form that can be amplified.
The normal way to accomplish this goal is to allow the photons to fall on a photoemissive surface that absorbs the light and gives out electrons. Different photoemissive materials are used to suit the frequency of the radiation involved, which can range from X rays through visible light to infrared. The materials normally include elements such as cesium, which has large atoms with a single electron in the outermost shells, these electrons being rapidly dislodged by a photon. A typical photoemitter material used for the visible-light range is a compound of sodium, potassium, cesium, and antimony. The electrons produced by the photoemitter can readily be accelerated and focused by electric and magnetic fields to give the required amplification. Following amplification, the "electron image" is converted back into a light image that can be viewed by the eye.
Image tube
In its simplest form, an image intensifier consists of a sealed glass cylindrical tube about 1 in. (2.5 cm) in diameter and length, evacuated of air and with electrodes at either end. A photoemitter layer (the photocathode) is deposited on one of the flat end plates (the faceplate). The other end plate (the anode) is coated with a fluorescent material. Between them is a photomultiplier tube that carries out the amplification.
Light from the scene to be viewed is focused onto the faceplate photoemitter by a conventional lens system. Around 10 percent of the photons in the incident image stimulate the emission of electrons from the photoemitter layer, with the intensity of the electron emission corresponding to the brightness of the optical image. The electrons from the photoemitter are accelerated down the photomultiplier tube by a potential difference of several thousand volts and focused onto a secondary emission electrode, or dynode. The dynode gives off a number of electrons for each one hitting it. These secondary electrons are in turn accelerated to a further dynode and the process is repeated. A series of dynodes can be used in a single photomultiplier tube, greatly increasing the amplification. The electrons resulting from this process are then focused on the fluorescent screen at the other end of the tube. At this screen, the accelerated electrons excite the fluorescent coating so that it glows to give an image. Around 30 percent of the energy in the electrons is converted into light, but since the energy of the electrons is considerably increased by the acceleration along the tube, there is a net amplification of the light.
