Ambient illumination revisited: a new adaptation-based approach for optimizing medical imaging reading environments.

Ambient lighting in soft-copy reading rooms is currently kept at low values to preserve contrast rendition in the dark regions of a medical image. Low illuminance levels, however, create inadequate viewing conditions and may also cause eye strain. This eye strain may be potentially attributed to notable variations in the luminance adaptation state of the reader's eyes when moving the gaze intermittently between the brighter display and darker surrounding surfaces. This paper presents a methodology to minimize this variation and optimize the lighting conditions of reading rooms by exploiting the properties of liquid crystal displays (LCDs) with low diffuse reflection coefficients and high luminance ratio. First, a computational model was developed to determine a global luminance adaptation value, Ladp, when viewing a medical image on display. The model is based on the diameter of the pupil size, which depends on the luminance of the observed object. Second, this value was compared with the luminance reflected off surrounding surfaces, Ls, under various conditions of room illuminance, E, different values of diffuse reflection coefficients of surrounding surfaces, Rs, and calibration settings of a typical LCD. The results suggest that for typical luminance settings of current LCDs, it is possible to raise ambient illumination to minimize differences in eye adaptation, potentially reducing visual fatigue while also complying with the TG18 specifications for controlled contrast rendition. Specifically, room illumination in the 75-150 lux range and surface diffuse reflection coefficients in the practical range of 0.13-0.22 sr(-1) provide an ideal setup for typical LCDs. Future LCDs with lower diffuse reflectivity and with higher inherent luminance ratios can provide further improvement of ergonomic viewing conditions in reading rooms.