SPIE Medical Imaging 1995: Image Display, San Diego, California, February 26-March 2, 1995.
The CRT is commonly used to display digital medical image data. However, the brightness L(), as a function of input signal s is different for each CRT. In general, L(s) is nonlinear and poorly matched to the human eye's perception of brightness change. Also, the maximum brightness of a CRT decreases over time and L(s) is easily changed with hardware contrast and brightness adjustments. Previous studies have suggested display brightness functions based on human visual experiments which produce the same contrast perception for a given Delta s independent of s. The best L(s) depends on CRT luminance and scene dependent variables. We have developed an X Window based client server approach to maintain perceptually equivalent display scales using unique transformation tables for many display devices.
Our research laboratory has approximately 30 image display stations used for medical image display using X Window based software. Each of the display applications use a monochromatic table obtained from an independent display dependent display scale application. The X11 R4 color specification is used for image intensity control, which allows 16 bits each for the red, blue, and green signal levels. However, all our systems are restricted to 256 simultaneously displayable colors, of which 200 are allocated for image display by our display scale manager application. This allows 56 free colors for other applications. An image is mapped onto the range of 200 gray levels by assigning each pixel value to the appropriate index in the color table. The color table controls the input signal to the CRT and can be dynamically updated. Luminance as a function of input signal is measured for each display device at 20 input signal levels, from zero to full signal, and stored in a central database. A luminance meter (International Light 1400A) with a cylindrical sensor (SEL 033) specifically designed for CRT luminance measures is used. A polynomial is fit to L(s) and transformation to a perceptually linear response is made, using a perceptually linear function family (Blume et al; SPIE vol. 1897,, 1993). The color table is then loaded with the function which produces the perceptual linear response for that particular CRT from a client accessible centrally stored database. Extensive use of our tone scale application has been made for display of images simultaneously from 8 software applications running on 15 hosts. Field measurement of L(s) has been performed and stored in the central database for the display stations. Preliminary results indicate that a network of perceptually linear display devices can be easily maintained. The attributes of the X Window System such as unique screen ID's and flexible color management provide excellent mechanisms for managing networked image display. Recalibration of display devices can be quickly and easily made in the field by persons with minimal training.