Etendue

Etendue is a measure of how much light is captured by an object, like a window or a camera lens. It's basically a way of measuring how much light can pass through or get captured by something. Imagine a very wide window: by increasing how wide the window is, you get more light coming through, and that is etendue. Etendue is measured in square degrees, so the bigger the etendue, the more light is captured.