Why use a Gershun tube for my application?
Using a Gershun tube restricts the field of view of your sampling setup to a known value. Consider this application: You’re collecting light emitted from a large projection screen in a movie theater. With a bare optical fiber, which has a 25° FOV, you may collect light from the entire screen. With the Gershun tube, you can adjust that FOV to 6°, for example, and collect light from a smaller part of the screen. This would allow you to compare spot to spot uniformity in brightness or color.