The influence of ambient and viewbox light upon visual detection of low-contrast targets in a radiograph

Thirty-one observers (five radiologists, five radiology residents, and 21 nonradiologists) examined low-contrast images of 13 polyvinylchloride cylinders. Image density ranged from 1.91 to 2.05 OD units with a background density of 2.09 OD units. The experiment tested visual performance under various clinically relevant lighting conditions by asking the observers to locate a groove in each cylinder. Use of low ambient light and restricting light from surrounding veiwboxes significantly improved performance (P less than 0.0001). In general, the less the amount of extraneous light, the higher was the detection rate. Some observers did significantly better than others. Reduced scattering of light within the eye is suggested as an explanation for their increased performance.