Image intensifiers are devices which enhance ambient light to assist vision in low-light situations. Image intensifiers are used in night-vision goggles, as well as a in number of scientific devices such as microscopes and telescopes. Image intensifiers work by using a photocathode to convert ambient light to electrons, then intensifying the signal when they are converted back to photons. They have been used since World War II, and have passed through four generations of technology.
The zero generation of image intensifiers bore very little resemblance to modern image intensifiers. They were simply a way of seeing the infrared spectrum, allowing snipers to sight with infrared beams that were invisible to their enemies.
First generation image intensifiers were deployed during the Vietnam War and provided substantial improvements over the zero generation. Though a fair amount of ambient light was still needed, making moonless nights still essentially dark, they were able to amplify that light dramatically. Though first-generation image intensifiers have since been more-or-less abandoned by the military, they are still widely used amongst amateurs because of their relatively low cost.
Second-generation image intensifiers offered a dramatic, but short lived, advance in night-vision technology. By introducing new components, second-generation intensifiers were able to achieve completely passive, highly enhanced night vision. And unlike earlier generations, the new technology was very energy efficient, allowing for upwards of 30 hours of use on a single battery charge.
Soon after the second-generation breakthrough, however, experiments with new materials for the photocathode tubes yielded a much better form of night vision. By using gallium arsenide instead of the S-25 in second-generation intensifiers, third-generation image intensifiers achieved a much higher enhancement, with greater clarity and the ability to utilize more starlight.
Third-generation image intensifiers are widely used by the US and other militaries around the world, and are also used by some astronomers for activities such as meteor spotting. First- and second-generation image intensifiers are still available for purchase, however, and are widely used within the consumer population because they are so much more affordable than their third-generation counterparts.
A handful of manufacturers have begun producing what they call fourth-generation image intensifiers. These new intensifiers use fundamentally the same technology as the third generation, with better resolution, less distortion from bright sources of light, and significantly reduced noise in the image.
It should be noted that image intensifiers are not the same as thermal imagers, which may be used in no-light situations. Image intensifiers take light from in and around the visible spectrum and amplify it. Heat is much, much lower on the light spectrum and requires entirely different technology.
Night-vision systems may range from US$300 to well in excess of US$50,000 for cutting-edge technology.