Final answer:
The square ratio of the input to output screen diameter in an image intensifier tube is termed minification gain, which increases the brightness of the image by shrinking it onto a smaller area while maintaining the same number of photons.
Step-by-step explanation:
The square ratio of the input to output screen diameter in an image intensifier tube is termed minification gain. Image intensifiers are used to increase the visibility of low-light images in various applications, including medical imaging and night vision. The minification gain is a result of the image being shrunk down from a larger input screen to a smaller output screen, effectively increasing the brightness of the image because the same number of photons is concentrated onto a smaller area.
The concept is similar to other types of optical gain, like flux gain, which refers to the increase in light intensity and brightness gain, which is the overall increase in image brightness including the effects of both flux and minification gains. Minification gain specifically describes the relationship between the areas of the input and output phosphor screens and does not account for photon gain or conversion efficiency.