Visual assessment of angular response in medical liquid crystal displays

Abstract

In spite of having non-Lambertian emission, displays based on liquid crystal technology are becoming popular for medical diagnostic work stations. For all liquid crystal displays (LCDs), the contrast performance varies with viewing direction. Accurate measurements of the angular distribution of light emission require expensive instrumentation and extensive expertise. We investigated the possibility of using a test pattern to visually assess the angular response performance of LCDs. We found that this procedure offers the end user of displays a simple, fast, and relatively consistent technique to verify that the viewing angle performance of the display device is within certain acceptable limits.

Fig 1

Test pattern for the visual assessment of viewing angle (a). In all display devices tested, the targets (b) were 2.5 cm in diameter and 13.2 cm apart from each other.

Fig 2

Test pattern results for the five displays, three eye-to-screen distances, and background gray levels. For clarity, an empty peripheral circle represents no visual change in the number of transition lines detected with respect to the center target. Otherwise, the number of lines represents the number of discernable transitions.

Fig 3

Luminance and contrast response of the five displays for the center, top-center, top-left, and left-center positions (top-left quadrant of the screen) The vertical lines indicate the gray levels of the three test patterns.

Fig 4

Numeric scores S for the six observers and their mean score. The error bars on the mean score are ±2 standard deviation based on the six individual scores. The x-axis is labeled with the positions on the targets with respect to the center target: C is the center target, TC is the top-center target, TL is the top-left target, LC is the left-center target. The left side of the graphs also displays the contrast scale.

Fig. 5

Luminance and contrast response of the five MP-mono display at 60° toward the top-left screen corner.

Fig 6

Visual results for the three observers in the prediction experiment at 60° for the 5 MP-mono display.

Fig 7

Test pattern readings of the upper left quadrant screen area encompassing the center target and three other targets, for the five displays and six observers participating in our study.