AbstractID: 8114 Title: The characterization of a powerful light source for field positioning A previous publication1 proposed and demonstrated the feasibility of using a transmission LCD for the optical projection of complex treatment fields including those generated by a multileaf collimator. A practical difficulty encountered was the optical attenuation of the LCD device that resulted in a low illuminance of the planned treatment field at the patient. We report here the characterization of a commercially available light source, which not only facilitates the implementation of the previous proposal but may also enable both simulation and patient setup for treatment under ambient room lighting conditions. The light source consists of a 150W bulb coupled by a lens system to a liquid filled light-guide of 1m length. Our experiments show that the unattenuated illuminance at the patient surface from this source is 170lux on the central axis. With an LCD of attenuation 80% in place, the luminance will be 34lux, very close to current recommendations. Variation across a 40cm wide field is about 20% and attributed to inverse square law fall-off and light-guide propagation. This decrease is comparable with that from a conventional simulator. The effective light source position is at the exit of the light-guide. Source diameter is comparable with that of a conventional simulator and characterized by an LSF of 3mm FWHM at 100cm. This source coupled with the computer controlled LCD projection panel shows considerable promise as an aid to the accurate positioning of radiation fields for both simulation and treatment. 1 P. Dunscombe, et al. Brit. J. Rad., 67, (1994).