The lens coupling efficiency in megavoltage imaging

In TV‐based megavoltage imaging systems it is useful to be able to estimate the geometrical efficiency g 2 of the camera lens. It is shown that the appropriate expression is g 2 =(16 n 2 ) −1 ×[ F (1+1/ m )] −2 , where n is the refractive index of the scintillator, F is the F ‐number of the lens and m is the optical magnification. This expression yields estimates for g 2 that are 5 to 10 times smaller than the estimates that have appeared previously in the literature.