Additionally, the two file system required that we interpolate to derive the data for the more commonly used mid focus, and this proved to be very inneficient. The current system uses three files to describe a particular range of related photometry. Special cases requiring more than three files are described in the Elipsoidal Reflector Spotlight section. The simplist method is to pick the specific file which most closely matches the design requirement. Where fine tuning is required, the files on either side of the desired effect can be used to interpolate the new photometric file, specific to the application.
Related files use the same sample angles which simplifies the interpolation process.
An axample of these six files in IES format can be see in the text file for a 50 degree Source4 ERS . The lens tube or barrel is slid in and out until the shadow caused by the shutter edge falls abruptly to darkness over the shortest possible distance. This condition defines sharp focus. The shutters are then slid to their maximum test-out position, and the photometry of the sharp focus is measured. The shutters are returned to their test-in position and the barrel is slid towards the lamp.