Object detectability at increased ambient lighting conditions

Under typical dark conditions encountered in diagnostic reading rooms, a reader’s pupils will contract and dilate as the visual focus intermittently shifts between the high luminance display and the darker background wall, resulting in increased visual fatigue and the degradation of diagnostic performance. A controlled increase of ambient lighting may, however, reduce the severity of these pupillary adjustments by minimizing the difference between the luminance level to which the eyes adapt while viewing an image ( L adp ) and the luminance level of diffusely reflected light from the area surrounding the display ( L s ) . Although ambient lighting in reading rooms has conventionally been kept at a minimum to maintain the perceived contrast of film images, proper Digital Imaging and Communications in Medicine (DICOM) calibration of modern medical-grade liquid crystal displays can compensate for minor lighting increases with very little loss of image contrast. This paper describes two psychophysical studies developed to evaluate and refine optimum reading room ambient lighting conditions through the use of observational tasks intended to simulate real clinical practices. The first study utilized the biologic contrast response of the human visual system to determine a range of representative L adp values for typical medical images. Readers identified low contrast horizontal objects in circular foregrounds of uniform luminance (5, 12, 20, and 30 cd ∕ m 2 ) embedded within digitized mammograms. The second study examined the effect of increased ambient lighting on the detection of subtle objects embedded in circular foregrounds of uniform luminance (5, 12, and 35 cd ∕ m 2 ) centered within a constant background of 12 cd ∕ m 2 luminance. The images were displayed under a dark room condition ( 1 lux ) and an increased ambient lighting level ( 50 lux ) such that the luminance level of the diffusely reflected light from the background wall was approximately equal to the image L adp value of 12 cd ∕ m 2 . Results from the first study demonstrated that observer true positive and false positive detection rates and true positive detection times were considerably better while viewing foregrounds at 12 and 20 cd ∕ m 2 than at the other foreground luminance levels. Results from the second study revealed that under increased room illuminance, the average true positive detection rate improved a statistically significant amount from 39.3% to 55.6% at 5 cd ∕ m 2 foreground lum