How did the sniperscopes used in World War II work? They seem to have used an invisible light to illuminate the target and the sniper then looked through the scope and was able to see the target. — ND, Florence, Montana
These sniperscopes used infrared light to illuminate their targets and then detected this infrared light with the help of an infrared-sensitive photocathode. Producing infrared light is easy; any incandescent bulb produces large amounts of it. The sniperscope simply filtered out the visible light from an incandescent bulb, leaving only the invisible infrared light to illuminate the target.
Understanding the photocathode system requires an examination of the interactions of light and metal. Whenever a particle of light—a photon—strikes a metal surface, there is the possibility that the photon will eject an electron from that metal surface. However, each type of metal requires a certain minimum photon energy before it will release an electron. Because infrared light photons carry very little energy, they can only eject electrons from very special metals. The sniperscope contained a very thin layer of one such infrared-sensitive metal.
Actually, this metal layer was deposited on a transparent glass window that formed the front end of a vacuum tube. Light from the scene in front of the sniper passed through a converging lens that formed a real image of the scene on the metal layer. The metal layer was so thin that light striking its front surface through the glass window caused electrons to emerge from its back surface. Electrons ejected from the back of the metal layer were accelerated by a high voltage that was applied between this metal photocathode layer and a phosphor-coated anode layer located very nearby. Each electron acquired so much energy during its brief flight that it caused the phosphors on the anode to glow brightly when it hit them. The electron flight path was short so that electrons emitted by a certain spot on the photocathode would hit a corresponding spot on the phosphor anode and the sniper would see a clear image of the scene in front of the sniperscope.
Because one infrared photon striking the photocathode could lead to the release of dozens of photons from the phosphors on the anode, this sniperscope provided a modest amount of “image intensification.” But modern starlight scopes go far beyond this level of amplification. Like the old sniperscope, these modern devices also use a photocathode to turn a pattern of light from the real image of a lens into a pattern of free electrons. But the starlight scope then amplifies these electrons by sending them through narrow channels that have highly charged walls. As the electrons bounce their ways through the channels, they knock out hundreds, then thousands, then even millions of other electrons so that each original photon can release more than a million electrons from the amplifying system. When these electrons strike the phosphor-coated anode, the image they produce is bright and visible, so that the person looking at the anode can effectively see when each photon of light strikes the photocathode and initiates one of these electron cascades. With such incredible light sensitivity, there is no longer any need to actively illuminate the target with infrared light—even starlight is enough illumination to make the target visible through the starlight scope’s image intensification system.