A common view is that color displays cannot match the performance of monochrome displays, normally used for diagnostic x-ray imaging. This view is based largely on historical experience with cathode-ray tube (CRT) displays, and does not apply in the same way to liquid-crystal displays (LCDs). Recent advances in color LCD technology have considerably narrowed performance differences with monochrome LCDs for medical applications. The most significant performance advantage of monochrome LCDs is higher luminance, a concern for use under bright ambient conditions. LCD luminance is limited primarily by backlight design, yet to be optimized for color LCDs for medical applications. Monochrome LCDs have inherently higher contrast than color LCDs, but this is not a major advantage under most conditions. There is no practical difference in luminance precision between color and monochrome LCDs, with a slight theoretical advantage for color. Color LCDs can provide visualization and productivity enhancement for medical applications, using digital drive from standard commercial graphics cards. The desktop computer market for color LCDs far exceeds the medical monitor market, with an economy of scale. The performance-to-price ratio for color LCDs is much higher than monochrome, and warrants re-evaluation for medical applications.
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<sub>min</sub>, L<sub>max</sub>, mean ▵JND/▵p, and maximum local deviation in ▵JND/▵p from GSDF, &kappa<sup>256</sup>, of 0.83 cd/m<sup>2</sup>, 263 cd/m<sup>2</sup>, 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 (β<sub>175</sub> = ±20°). Aiming to maintain κ<sup>17</sup>≥0.3, an acceptable viewing angle cone of ±35° was indicated (α<sub>0.3</sub> = ±35°). The findings demonstrate the significant impact of angular response on image contrast, and the utility of α<sub>0.3</sub> and β<sub>175</sub> quantities for defining the viewing angle cones within which a medical LCD device can be effectively utilized.
Recently, IBM introduced a very high-resolution high-information content flat-panel display, which incorporates a number of technological advances. This display contains 9.2 million pixels at a density of 204 pixels per inch, in a 3840x2400 pixel format. This is the first large-area display with a pixel density which satisfies normal visual acuity at normal reading distances, and increases the screen information content by a factor of 5 to 7 times over conventional displays. This display is appropriate for a wide variety of applications in which visualization of a large amount of data is important. At the same time, this technology advance heralds a number of infrastructure changes in system architecture, digital data protocol, and user interface design for both operating system and software applications.
For more than a decade the Image Applications department at IBMs Watson Research Center has been involved in cultural and commercial imaging projects that demand high-fidelity color reproduction of precious objects like paintings, illuminated manuscripts or jewelry. Our primary display media have been high-resolution cathode ray tubes (CRT), but for the last three years our customers have been replacing them with liquid crystal displays (LCD). The color calibration model we have been using for the CRT is the one described in the literature. It assumes that the chromas of the primaries are independent of intensity, that the colors produced from them are additive and that the intensity of black is almost zero. We measured several models of LCDs and observed that they poorly satisfied these assumptions at medium to low intensities. This becomes noticeable if the image has dark areas or if the display is viewed under a weak ambient light. In this paper, we use a modified version of the CRT model to calibrate the LCD. First we measure four sets of red, green, blue and gray patches. THen we determine the correction factor needed to make, at each level,the sum of the primaries equal to the corresponding gray. Finally, we use these factors to modify the data of red, green and blue.
Capacitance crosstalk and TFT photoleakage affect the transmission-voltage array characteristics in high- resolution TFTLCDs. These effects depend upon the drive inversion scheme use, and are image-dependent. Photoleakage can also be a cause of flicker in TFTLCDs at low frame rates. One characterization method utilizes comparison of front-of-screen measurements with either measured test cells or theoretical cell characteristics. A second method utilizes digital crosstalk compensation, in which the image data provided to the panel is modified to offset the effects of crosstalk in various test image patterns.