Index helps to ensure quality when reading flat-panel displays

Researcher correlates CRT and AMLCD function

CRTs are going the route of other "tube"-based technologies, pushed from the mainstream by increasingly popular flat-panel displays. But now concerns are surfacing about the use of active matrix liquid crystal displays (AMLCDs) in primary diagnoses.

"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 AMLCDs," said Jihong Wang, Ph.D., an assistant professor of radiology at the University of Texas Southwestern Medical Center.

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. But key measurements of the performance of AMLCDs can be done accurately and with good reproducibility.

Research conducted by Wang has led to the development of a quality index for soft-copy display systems that takes into account the contrast sensitivity of an AMLCD monitor and compares it with that of the conventional CRT monitor. His results show that contrast sensitivity depends on variable factors such as type of monitor, monitor brightness, and gamma settings of the graphics card. There is, however, a clear correlation between the measured contrast thresholds and the gradient of the display device's luminance response curve.

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

The overall quality of a soft-copy display system, therefore, can be assessed using the gradient of the luminance response curve. The gradient of luminance response curve can then be used as a quality index for soft-copy display systems. Testing, Wang said, should be done where the monitor is being used, so the effects of viewing conditions on soft-copy display quality can be taken into account.

This metric will make the assessment of the contrast resolution of soft-copy displays simple and easy, according to Wang. In so doing, it could provide the means for benchmarking flat-panel displays, thereby reducing concerns over image quality that now accompany soft-copy reading.