Study proposes quality index for active matrix crystal displays

With the introduction by several PACS vendors of active matrix crystal display (AMLCD)-based monitors, concern has risen over their increasing use in primary diagnoses.

"The characteristics of AMLCD monitors, especially contrast sensitivity, are not widely known," said Jihong Wang, Ph.D., an assistant professor of radiology at the University of Texas Southwestern Medical Center.

No simple metric can be used to compare the quality of software display systems. Many monitor measurements can be done in a laboratory, but no one comparable to the gradient of H&D curve used for screen-film exists for soft-copy displays.

Wang conducted a contrast sensitivity study (J Digit Imaging. 2003 Sep 11 [Epub ahead of print]) that resulted in a proposal of a quality index or metric for soft-copy display systems.

Wang's metric will make the assessment of the contrast resolution of soft-copy displays simple and easy, he said. It also takes into the consideration the effect of ambient light on contrast resolution.

Wang measured the contrast sensitivity of an AMLCD monitor and compared it with that of the conventional CRT monitor. Contrast sensitivities were measured with various display system configurations.

His results showed that contrast sensitivity depends on factors such as type of monitor, monitor brightness, and gamma settings of the graphics card. There was a clear correlation, however, between the measured contrast thresholds and the gradient of the display device's luminance response curve.

"Metrics that have served the industry for decades with CRT-based displays must now be redefined to encompass attributes such as off-axis viewing, latent image retention, and discontinuities in response inherent in the AMLCD," he said.

Wang said the contrast sensitivity of soft-copy display systems is a complex function of monitor characteristics and graphics card configuration as well as viewing conditions, although it can be measured with good repeatability and accuracy.

"The contrast sensitivity of the human observer correlates well with the gradient of the luminance response curve," he said.

Thus, the overall quality of a soft-copy display system can be assessed using the gradient curve of the luminance response curve. The gradient of luminance response curve can then be used as a quality index for soft-copy display systems.

Wang cautioned that the luminance response curve should be measured in the actual location of usage so the effects of viewing conditions on soft-copy display quality can be taken into account.

"We need to update the ACR standard on soft-copy display systems," he said.