Veiling glare point-spread function of medical imaging monitors

When radiographic images are displayed using cathode-ray tubes (CRTs), the perception of low-contrast, clinically relevant details can be hindered by veiling glare. To reduce the effect of glare, high performance CRTs typically have an absorptive faceplate. Other approaches that have been implemented in color tubes include filtered and pigmented phosphor grains, and the use of a black absorptive matrix between phosphor dots. In this work, we present results on experimental measurements and computational predictions of the veiling glare for a medical imagin CRT of recent design. Experimental measurements were performed using a collimated conic probe with a high gain detector, and test patterns having a dark circular spot of varying diameter in a bright circular field. Computational and experimental results were obtained before and after application of an AR coating to the surface of the monitor. The glare ratio for a 1 cm diameter dark spot was measured to be 138 without AR coating, and 241 with coating. The results establish a bright ring in the PSF at about 45 mm distance. The predictions from the computational model agree well with the measured ring response functions for radii less than 50 mm. We speculate that the veiling glare response for large radii is greatly influenced by electron backscattering processes. While primarily designed to decrease specular reflections, the AR structure also affects the glare characteristics of an emissive display by increasing the probability that a light photon will exit the structure through the outer surface.