Goniometric and conoscopic measurements of angular display contrast for one-, three-, five-, and nine-million-pixel medical liquid crystal displays

Abstract

Active-matrix liquid crystal displays (AMLCDs) have become the preferred choice for displaying digital diagnostic images because of their low cost of ownership, high contrast, and small footprint. However, the contrast and luminance of AMLCDs changes significantly with viewing direction. In this paper, we use a goniometric and a conoscopic method to measure angular contrast and luminance in one-, three-, five-, and nine-million-pixel medical AMLCDs. The viewing angle characteristics of the displays are compared by evaluating the compliance with the desired grayscale and recommended tolerance limits. Using the measured contrast responses, we determined the angle along each of the orthogonal and diagonal axes for which the luminance ratio drops to 0.5, 0.2, and 0.1 of the maximum ratio, seen at perpendicular viewing. The results show a reduction as large as 54% in available JND levels between the perpendicular and off-normal viewing directions. All of the display systems proved to have better angular response in the horizontal and vertical direction as compared to the diagonal directions, and none were able to satisfy a 20% contrast tolerance limit for any viewing direction lying in a cone at an angle of 45 degrees from the normal. Additionally, we show that the measured contrast can be significantly affected by the measurement method used to record the angular change in luminance.