Temporal response measurements of medical liquid crystal displays

Liquid crystal displays (LCDs) are fast gaining ground over the cathode ray tube (CRT) displays in the medical display market. High performing LCDs are considered to have comparable or better performance than CRTs in displaying static images. However, LCDs are inferior to CRTs in displaying moving scenes due to their slow response. The response time provided by display manufacturers is typically measured while switching the LCD from black to white and white to black. This is usually not the longest response time. In reality, the transition time between different gray scales can be many times longer. In this paper we report preliminary work on measuring the gray level response time of LCDs and simulating luminance errors caused by slow transition between some gray levels. We first characterized the measuring system using a fast light-emitting diode (LED) to explore the accuracy and noise-filtering capability of the system. A 256x256 matrix of response time between different gray levels was then measured. Nearly half of the gray level transitions are much longer than the frame time (16.67~ms) of LCD displays. The longest response time was above 100~ms. When driving a display between these gray levels, the targeted gray level can't be achieved until many frame times. To understand how the slow response may affect the display's ability to render the desired image values, we calculated the achieved luminance based on the measured matrix. The results simulate the visual effect of displaying a moving object on the LCD monitors, and providing a reference for determining LCD performance.