Display performance measurement becomes faster, simpler

One common measure of CRT and LCD performance is the spatial resolution of the imaging system, usually described by its modulation transfer function, or MTF.

Typically, MTF of an imaging display is determined by a cumbersome method of measuring its response to bar patterns, white noise, and line stimuli. A new paper presents two less tedious methods of evaluating workstation MTF from edge-spread function (ESF).

"Unlike existing methods, which can be slow and may not report the response correctly, our methods are easy to implement and better in reporting the low-frequency response of the display," said Amarpreet Chawla, Ph.D., from the department of electrical and computer engineering at the University of Arizona.

Chawla said this is clinically important because if CRTs or LCDs blur images, diagnostic information such as subtle formations potentially indicative of breast or lung cancer may be lost or undetectable by radiologists.

MTF characteristics are potentially useful when shopping for reading room displays or as quality assurance tools, since CRTs and LCDs tend to degrade over time.

"MTFs can also be used to determine which displays in a reading room may potentially be showing degraded medical images due to degraded MTFs," Chawla said.

He determined the MTF of a high-resolution CRT from its edge response using two methods:

• reducing periodic raster noise by subtracting a shifted ESF from the ESF

• using a low-pass differentiator to combine the behavior of a differentiator and a low-pass filter

Using these methods, MTF can be accurately estimated over all frequencies up to the Nyquist frequency (the highest frequency that can be coded at a given sampling rate in order to fully reconstruct the signal), Chawla said.

"The MTF obtained is comparable to that obtained from the square wave response, but it is less labor-intensive," he said.

Although the two methods described in the paper apply to CRTs, they can also be used to determine the MTF of LCDs, Chawla said.

"The results indicate in what ways LCDs are better than CRTs and also how CRTs degrade rising vertical edges in an image more than the falling vertical edges," he said.