Light field is routinely used for matching electron fields with adjacent photon fields and also positioning an ion chamber for cutout output measurements. At extended distance, however, the peripheral aspects of a radiation field are shifted with respect to their light shadow by up to 10 mm at SSD of 115 cm. This is mostly due to a difference in positions of light and virtual electron sources; however loss of the lateral scatter equilibrium, electron scatter in air, and partial transmission through cutout edges can also contribute to the effect. For the aperture sizes large enough for the lateral scatter equilibrium to be achieved, a simple geometrical formula allows one to calculate the misalignment at off-axis locations if the virtual electron source position is known. If the source position is not known or the aperture is too small for the lateral scatter equilibrium to be established, then a light-radiation coincidence test using a ready-pack film at d(max) can be used to measure the light and electron radiation fields coincidence. (C) 2002 American Association of Physicists in Medicine.
