Suppose you have a projector that produces 100 nits at a throw of 25 feet. (A nit is a measure of brightness equal to one candela spread over one square meter.) Let’s not concern ourselves with the details of nits for the moment; let’s just accept that it’s some measure of brightness. If we now measure the brightness at a throw of 50 feet — twice the original throw distance — then how many nits would be expect to see? That’s where the inverse square law comes in. Since the throw distance has doubled, we can figure that the projection area is now four times bigger and brightness is one-fourth intense; therefore we might expect to measure 25 nits.
-From the December Video World column
