Improper room luminance may hinder interpretation

Radiologists ignore the effects of room luminance in soft-copy reading environments at their peril. High ambient room light can degrade image contrast and can even lead to clinical misinterpretation, according to a recent study.

"It is paramount that viewing conditions are properly optimized and regulated," said Kish Chakrabarti, Ph.D., of the Radiological Devices Branch of the FDA.

Room luminance, coupled with monitor black luminance, significantly affects image contrast by enhancing the adverse effects of monitor veiling glare and monitor diffuse reflectivity, he said.

The study found that ambient room light greater than 15 lux degrades image contrast, thus affecting image interpretations (J Digit Imaging. 2003;16(4):350-355).

"This may cause misinterpretations of clinical images," he said.

With the increasing use of full-field digital mammography (FFDM) and soft-copy display systems, variables such as ambient room light, diffuse and specular reflections, and veiling glare must be carefully controlled for diagnostic accuracy, Chakrabarti said.

The researchers observed that a change in room light from 0 to 50 lux with a 0% to 5% surrounding monitor luminance has a much more drastic effect on image quality than the same change of room luminance when the surrounding luminance is between 15% and 20%.

In one evaluation, the effect of reflection on image luminance was tested using a monitor with a central black target square of up to 20% luminance. Investigators measured the effect using a photometer placed 1 m away, while room luminance was increased incrementally from 0 to 50 lux.

The researchers did not observe any significant changes in measured target luminance with varied room light luminance from 0 to 50 lux and surrounding monitor luminance from 0% to 20%. The black level luminance value as measured at eye level increased, however. This change was higher for lower values of surrounding monitor luminance.

"With the white level (maximum brightness) remaining the same, an observer loses contrast at higher ambient light because of an increase in black level," Chakrabarti said.

The federal government maintains an interest in the luminance issue. Under the Mammography Quality Standards Act of 1992, all FFDM manufacturers seeking approval for marketing their soft-copy image interpretation systems are required to provide display system quality control criteria and ambient room light requirements.

"It is very important that radiology facilities follow these quality control requirements, and once monitor calibrations are established, that the facilities maintain setup integrity," Chakrabarti said.

This work is currently being expanded to cover flat-panel display systems.