Characterization of Color CRT Display Systems for Monochrome Applications G. Spekowius Soft-copy presentation of medical images is becoming presented frequently on color CRT display systems. more and more important as medical imaging is Particularly, if general-purpose workstations or strongly moving toward digital technology, and health care facilities are converting to filmless hospital and PCs are used for medical viewing, color monitors radiological information management. Although most are more or less standard. These common computer medical images are monochrome, frequently they are graphic displays ate applied without any further displayed on color CRTs, particularly if general- modification. This is in contrast to the medical purpose workstations or PCs are used for medical monochrome monitors, which normally are devel- viewing. In the present report, general measurement oped especially to fulfill the high image quality and modeling procedures for the characterization of color CRT monitors for monochrome presentation are requirements of medical imaging. Because the total introduced. The contributions from the three color number of medical displays is small in comparison channels (red, green, and blue) are weighted accord- to consumer applications, there is little incentive ing to the spectral sensitivity of the human eye for for the consumer display industry to develop spe- photopic viewing. The luminance behavior and the cial color CRTs for medical imaging. Hence, the resolution capabilities of color CRT monitors are ana- lyzed with the help of photometer and charge-coupled limitations of the consumer monitor CRT also device (CCD) camera measurements. For the evalua- apply to medical usage and might limit the image tion of spatial resolution, a two-dimensional Fourier quality for some applications. analysis of special test images containing white noise The purpose of this report is to characterize the (broadband response) is employed. A stage model for special image quality aspects of medical image a color CRT monitor is developed to discuss the effects of scanning and dot sampling. Furthermore, display presentation on color monitors and to develop intrinsic veiling glare and reflectivity of typical color measures for display selection, display calibration, CRT monitors are measured and compared with those and display maintenance. of monochrome CRT monitors. The developed meth- ods and models allow one to describe the image COLOR CRT MONITOR TECHNOLOGY quality aspects of color monitors if they are applied for medical monochrome image presentation. Particu- Color viewing on monitors is realized by merg- larly, because of the reduced luminance and dynamic ing the three base cotors--red (R), green (G), and range of color monitors, the calibration and control of blue (B)-----emitted from a structured phosphor their luminance curves is a very important task. For screen that is excited by three individual electron present color CRT monitors, 1,280 x 1,024 turns out to be an intrinsic limit for the displayable matrix of beatos. This article provides a brief illustration of medical images. the current color CRT technology and discusses Copyright 9 1999 by W.B. Saunders Company some descriptors that are important for image quality (eg, dot pitch). The major components of a KEY WORDS: monitor characterization, color CRT standard color CRT are shown in Fig 1. Generally, monitors, soft-copy viewing, image quality. it consists of an RGB triple-electron gun, a shadow mask, anda structured phosphor screen with red, ODAY, more and more health care facilities green, and blue patches. Each of the three electron are converting to digital filmless hospital and T beams must pass the shadow mask before it hits the radiological information management. Within that phosphor dots of its color. The screen shadow scope, the soft-copy presentation of medical im- mask distance, of approximately 1 cm, provides the ages gains more and more relevance for an efficient spatial separation of the three beams on the screen. and cost-effective hospital organization. Although A sketch of the screen shadow mask region is most medical images are monochrome, they are provided in Fig 2. The light emitted by the indi- vidual phosphor dots is merged by the human eye, From Philips Research Laboratories, Aachen, Germany. forming a smooth colored image impression if the Address reprint requests to Dr Gerhard Spekowius, Philips viewing distance is sufficiently high. Research Laboratories, Weisshausstrasse 2, D-52066 Aachen, In Fig 3, the three major types of shadow masks Germany. Copyright 91999 by ~B. Saunders Company and the respective screen structures are indicated. 0897-1889/99/1203-000251 O. 00/0 Typically, we find the dot mask in computer mon- 102 Journal of Digital Irnaging, Vol 12, No 3 (August), 1999: pp 102-113 MONOCHROME APPLICATIONS OF COLOR CRT SYSTEMS 103 cover commonly used in-line gun. The horizontal and vertical dot pitches can be different. Usually they are chosen such that scan and video Moir› are adow mask minimized for the typical image matrix, which will be displayed on the screen. Present high-resolution monitor CRT technology : glass comes with dot pitches of 0.26 to 0.28 mm for ;phor screen screen diameters of about 20". The transmission of the shadow mask is only about 10% to 15%, and the major portion of the current in the electron beato does not contribute to the luminance. Typi- cally, color CRT monitors have a maximum lumi- nance of 80 to 100 cd]m2 only, whereas mono- Fig 1. The basic components of a color cathode-ray-tube chrome CRT monitors can achieve more than 800 (CRT). cd/m2.1-3 Table I provides some typical numbers and compares them with actual monochrome CRT itor tubes, whereas the slot mask is applied in monitors. television tubes. The Trinitron mask is applied with Modern digital monitors provide a number of different line pitches in TV as well as monitor automatic corrections to improve image quality. CRTs. The dot and slot masks provide a separation Examples ate controlling the uniformity of color in horizontal and vertical direction, whereas the and luminance, or adjusting dynamically the conver- Trinitron mask separates in the horizontal direction gence of the three beams. A digital control interface only. A shadow mask is characterized by its dot or allows for the establishment of interactive as well line pitch, giving the distance from hole to hole or as automatic procedures to optimize or recalibrate slot to slot. For the dot mask, we have a hexagonal the monitor in combination with the applied graph- structure, and the dot pitches (eg, given in technical ics card. These control opportunities are particu- data sheets) refer to the diagonal pitch of two dots larly important for medical image applications of the same color, as indicated in Fig 3. In the because they allow for the implementation of delta-gun arrangement, the three (RGB) guns form image quality maintenance procedures such as an equal triangle, and thus the phosphor dots do. control of the display function. 2 The three guns are horizontally in line for the most MONITOR CHARACTERIZATION blue, red, green In this section, measurement procedures are phosphor dots described for analyzing the (gray) display function, uniformity of luminance and color, veiling glare, reflectivity, resolution, and screen fixed pattern. black I matrix Before the detailed description is provided, it is ! important to consider some basic luminance as- red pects on the formation of monochrome images on a gun color monitor. Basically, color vision can be described by a set of three parameters. 4-6 Most common is the x, y, z green system of the CIE 1931 standard colorimetric gun observer. For this system, all three color matching functions [x(k), y(k), z(k)] have a positive sign, blue and the y(k) curve represents the standard spectral luminous sensitivity curve [V(k)] of the human eye gun I for photopic viewing. For the characterization of shadow mask luminance performance as well as spatial resolu- tion, the contributions of the three color channels of Fig 2. The shadow-mask screen region of a color CRT, a CRT monitor are weighted according to the V(k) 104 G. SPEKOWIUS dot mask slot mask Trinitron I I __ .J_ o~oiooo I R G B R G ,B Q~Q~OQ~ i i ,:i ~9169 , ,{I llll ! ! I Ir I ...... I I 'slot pitch' ~h line pitch Fig 3. The different screen structures of color CRTs (shadow masks) and their relevant dot pitches (P,, horizontal pitch; Pv, vertical pitch), curve. Although the dark part (<10 cd/m 2) of the allow selection of the color temperature; a typical luminance range of color monitors falls into the preset value is 9,300 K with x = 0.294 and y = mesotopic viewing range of the human eye, it 0.294. Television studio monitors often are standard- appears to be sufficient to apply the photopic ized to 6,500 K (D65) with x = 0.313 and y = sensitivity curve for the characterization of the 0.329. For an RGB monitor, all colors falling into displayed image quality. the phosphor triangle can be realized. A gray Figure 4 shows a CIE x, y chromaticity diagram impression on a color CRT monitor is achieved if overlaid by the black-body line (BBL) or Planckian the applied video signal for the RGB channel is locus anda color triangle formed by typical televi- almost equal. Important for the analysis of gray- sion, here EBU (European Broadcast Union) phos- phors. Color temperatures of correlated color tem- peratures 7 refer to coordinates falling on of close 0.9- A: Tun, sten, filam ~nt 28 50 K to the Planckian locus. Many natural light sources 521 D65 ph~ ~e of c aylig~ of 65 30 K have coordinates very close to the BBL and, o~/~~.~ roughly, a white color impression is perceived between 2,500 K (warm white) and 10,000 K 0.7- (bluish white).