In an image intensifier, what results from the reduction in image size at the output phosphor?

The correct answer is that the reduction in image size at the output phosphor leads to increased brightness. This phenomenon occurs due to the principle of "image inversion" and the conservation of light intensity. When the image produced at the input phosphor of an image intensifier is focused onto a smaller output phosphor, the same amount of light is concentrated into a smaller area.

This concentration of light results in a higher intensity of illumination at the output, leading to an increase in brightness. The output phosphor emits light in response to the electron bombardment it receives from the input, which has been amplified due to the image intensifier's functions. Thus, as the output size decreases while maintaining the same light intensity, the overall illumination appears brighter to the viewer.

This concept is crucial for understanding how image intensifiers enhance images in fluoroscopy, making it possible to obtain clearer and more detailed visuals during medical imaging procedures.