The effect of viewing angle response on DICOM compliance of liquid crystal displays
Cathode-ray tube (CRT) and liquid crystal display (LCD) are currently two main technologies for displaying medical images. LCDs possess a number of advantages, but their performance varies as a function of viewing angle. The purpose of this study was to delineate the impact of the angular response of LCDs on their DICOM grayscale display function (GSDF) compliance, and to develop a framework to define an angular acceptance for medical LCDs. Measurements were made on a calibrated dual-domain LCD (IBM T221). The on-axis luminance values were measured at all 8-bit driving levels three times using the TG18-LN test patterns and a baffled luminance meter and the results averaged. The luminance was also measured as a function of viewing angle at 17 discrete levels using a Fourier-optics-based luminance meter. The luminance data were analyzed according to the AAPM TG18 methodology. The on-axis results showed close conformance with the TG18 criteria with L min, L max, mean ΔJND/Δp, and maximum local deviation in ΔJND/Δp from GSDF, K 256, of 0.83 cd/m 2, 263 cd/m 2, 2.1, and 0.8, respectively. However, the values varied notably as a function of viewing angle. Overall, the luminance ratio remained greater than 175 within a ±20° viewing angle cone (βn5 = ±20°). Aiming to maintain K 17≤0.3, an acceptable viewing angle cone of ±35° was indicated (α 0.3 = ±35°). The findings demonstrate the significant impact of angular response on image contrast, and the utility of α 0.3 and β 175 quantities for defining the viewing angle cones within which a medical LCD device can be effectively utilized.
