How to choose a monitor for image-processing applications

Most display vendors such as CTX, Samtron, and Viewsonic and personal-computer (PC) makers such as Acer America, Apple Computer, and Compaq Computer offer a range of resolutions for color monitors extending from the 640 ¥ 480-pixel resolution of the IBM Video Graphics Adapter (VGA) to the 1600 ¥ 1200-pixel resolution of the Ultra Extended Graphics Adapter (UXGA) standard (see Fig. 1). Applications requiring other resolutions generally use more expensive, application-specific monitors.

Oct 1st, 1997
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How to choose a monitor for image-processing applications

By Rick Nelson, Contributing Editor

Most display vendors such as CTX, Samtron, and Viewsonic and personal-computer (PC) makers such as Acer America, Apple Computer, and Compaq Computer offer a range of resolutions for color monitors extending from the 640 ¥ 480-pixel resolution of the IBM Video Graphics Adapter (VGA) to the 1600 ¥ 1200-pixel resolution of the Ultra Extended Graphics Adapter (UXGA) standard (see Fig. 1). Applications requiring other resolutions generally use more expensive, application-specific monitors.

Standard monitor sizes range from 14- to 21-in. diagonal before the cathode-ray tube (CRT) is mounted in an enclosure. Diagonal viewing areas typically measure one inch less than this specified diagonal, and viewing area may be further reduced by an unusable black-band area that lies between the visible image and the perimeter of the enclosure`s CRT opening. Viewing area is rarely specified directly, and system integrators should pay close attention to this specification.

Pitch the specification

Another important monitor specification is dot pitch--the center-to-center spacing of same-color (red, blue, or green) dots within adjacent pixels--which has also become more complex. Typical values cover 0.24 to 0.28 mm, with the smaller number indicating closer pixel spacing and, therefore, a sharper picture. Standard industry practice is to specify the diagonal distance of adjacent dots for dot pitch. Horizontal measurements would provide smaller numbers and would represent an inflated indication of monitor resolution.

The dot-pitch specification evolved when most tubes used color dots in a dot-trio or delta configuration for each pixel. Today, because some tubes have three vertical color stripes per pixel, the appropriate specification is stripe pitch--the horizontal distance between like-colored stripes in adjacent pixels. This specification produces a lower number than the diagonal dot pitch for similar tubes. To add to the confusion, some manufacturers list stripe pitch as dot pitch.

NEC USA (Melville, NY) takes another approach. Its CromaClear CRT uses elliptical phosphor shapes instead of round phosphors or stripes. The company prefers a mask-pitch specification (0.25 mm for its CromaClear CRTs) to either stripe pitch or dot pitch.

Tube technology

Even with consistent measurements, specifications such as dot pitch can prove less useful than other aspects of tube technology. Factors such as shadow-mask or aperture-grill construction and dynamic-focusing capabilities also merit attention.

Low-end displays typically have shadow masks of a ferrous material. The mask, parallel to the phosphor screen, directs electrons from the scanning electron gun to the screen through an array of holes in the mask.

Shadow-mask technology exhibits two drawbacks. Because some electrons strike the mask, the intensity of the energy reaching the phosphor is reduced, limiting the brightness of the image. Also, electrons striking the mask produce heat and, consequently, mask deformations that lead to image distortion or doming.

Replacing a ferrous shadow mask with an Invar mask alleviates the doming problem, but brightness is still reduced. To tackle this problem, Sony Corp. (San Jose, CA) replaced the shadow mask in its Trinitron technology with an aperture grill of vertical wires that is less obtrusive to electrons than is the shadow mask.

But the aperture grill is not without its own problems. Vertical grill wires require one or two horizontal stabilizing wires to keep them in position. And, although normally invisible, under low brightness conditions these wires can be seen as horizontal gray lines on the display.

Grill wires also can be disturbed by monitor vibrations, causing the picture to waver. This vibration sensitivity can be troublesome in systems where speakers need to be mounted on the CRT enclosure. To minimize display distortion in such systems, Apple Computer (Cupertino, CA) designed its speakers to eliminate common resonance frequencies with the CRT and used a dampening suspension system for mounting the speakers to the CRT enclosure.

Multiple tube choices

Although the aperture grill remains the technology of choice for high-end CRT monitors, many vendors offer shadow-mask and aperture-grill technologies. For example, Digital Equipment Corp. (Maynard, MA) uses a standard shadow mask on its 14-in. ValueLine models, an Invar shadow mask on its 15-in. Value Line and Corporate Line units, and an aperture grill on its 17- and 21-in. Professional Line models. Similarly, Hewlett-Packard (Palo Alto, CA) makes use of both aperture-grill and shadow-mask technologies for its line of PCs and workstations. For high-end PC and workstation graphics, the company suggests aperture-grill designs, while for standard windowing applications, it indicates that a shadow mask should suffice.

Hewlett-Packard also recommends dynamic focus enhancement, especially for 15- to 21-in. monitors. Dynamic focus enhancement is becoming a more critical parameter as CRT displays become larger and flatter, because the electron-gun-to-screen-corner distances become much greater than the electron-gun-to-screen-center distances. To eliminate the distortion that the constantly varying gun-to-screen distance causes in a single-lens-focus approach, dynamic focusing alters the effective focal length depending on the instantaneous gun-to-screen distance.

Moreover, Viewsonic (Walnut, CA) also offers both shadow-mask and aperture-grill technologies. The company recommends an Invar shadow mask for general business use and CAD/CAM/CAE applications and an aperture-grill mask for desktop-publishing, prepress work, and graphic-design applications.

Apple Computer is also targeting desktop-publishing and prepress applications. Because of the popularity of its Macintosh computer line among graphic artists, the company has long concentrated on providing bright displays with accurate color representation. Its ColorSync technology is aimed at color matching across multiple devices, including printers, scanners, and monitors.

Recently, Apple unveiled its 17-in. (16.1-in. viewable) AppleVision 750 and 750 AV monitors. Both displays incorporate Sony Trinitron tubes with antireflective screen coatings and 1280 ¥ 1024-pixel resolution at a 75-Hz refresh rate. The monitors support the company`s ColorSync and DigitalColor technologies that adjust color over time to compensate for variations due to CRT aging.

To further control color for print applications, Apple displays use variable white-point control that allow monitors to simulate on-screen how specific pigments (a printer-ink color, for example) will look on paper under various lighting conditions. White-point is measured on the Kelvin scale; Apple monitors are adjustable from 4100 K to 9300 K (where 3300 K, 4100 K, and 6500 K correspond to viewing conditions under fluorescent light, halogen incandescent light, and daylight, respectively).

One way to increase resolution is to use more than one display, an approach that STB Systems (Richardson, TX) is taking with its MVP Workstation multimonitor graphics adapter. The company provides a virtual desktop, enabling a pointing device and data objects to move from monitor to monitor with as many as 16 displays connected to a single PC via four 4-port adapters. Using this approach, a two-display system has been configured for financial-trading applications (see Fig. 2).

Prices plummet

Monitor prices are falling, driven at the low end by competitive pricing in the home PC market. Packard Bell, for example, is offering a 120-MHz Pentium system with a modem and a 14-in. monitor for $999 (see Fig. 3). IBM Corp. (Somers, NY) introduced its 15-in. (13.6-in. viewable image) G51 color monitor. At $369, it is intended as a cost-effective alternative to a 14-in. monitor. It uses an Invar shadow mask with a 0.28-mm dot pitch, 800 ¥ 600-pixel resolution, and refresh rates to 85 Hz. In noninterlaced mode, the monitor supports resolutions to 1024 ¥ 768 pixels.

At the high end, NEC has cut the price of its 21-in. (19.6-in. viewable image) MultiSync P1150 monitor by 15%, to an estimated price of $1599. The monitor is aimed at architects and CAD/CAM engineers.

Beyond the extended VGA architectures available in off-the-shelf, top-of-the-line PCs, companies are making available specialized high-resolution monitors that are taking advantage of low-cost PC architectures. This trend is occurring because PCI-compliant controllers are emerging that support resolutions beyond 2k ¥ 2k pixels. The PCI-compatible Metheus P1540, for example, supports resolutions to 2048 ¥ 2560 pixels in 1024 shades of gray displayed simultaneously, while providing real-time windowing and leveling at a 72-Hz refresh rate. It supports displays from vendors such as Data Ray, Image Systems, MegaScan, Nortech Systems, and Orwin Associates.

For example, Data Ray`s Model DR80 is a 25-in. portrait monochrome monitor that provides 1536 ¥ 2304-, 1728 ¥ 2304-, and 2048 ¥ 2560-pixel resolutions (see Fig. 4). It includes a 90 CRT to provide a sharper focus than the more common 110 models. In addition, it achieves a 110-fL light output at 47% transmission with brightness uniformity within 10%.

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FIGURE 1. Personal-computer makers such as Compaq Computer are offering VGA through UXGA monitors to the price-sensitive home market. In the resulting economies of scale, image-system integrators can reduce costs if they can accommodate standard displays in their systems.

Click here to enlarge image

FIGURE 2. Multiple monitors can make up a single virtual desktop when connected via an STB Systems MVP Workstation graphics adapter. The company developed the two-monitor system for financial trading applications.

Click here to enlarge image

FIGURE 3. High volumes and low monitor prices are enabling Packard Bell to offer the Platinum Series multimedia computer system for less than $1000. It sports a 120-MHz Pentium, a modem, 14-in. monitors, speakers, and 2048 ¥ 2560-pixel resolution.

Click here to enlarge image

FIGURE 4. Data Ray`s Model DR80, aimed at medical applications, is a 25-in. portrait monochrome monitor that provides 1536 ¥ 2304-, 1728 ¥ 2304-, and 2048 ¥ 2560-pixel resolutions.

LCDs challenge CRTs on the desktop

Eventually, bulky, power-hungry desktop displays are expected to lose market share to compact, power-efficient, large-screen liquid-crystal displays (LCDs). And many PC, cathode-ray tube (CRT), and CRT-monitor makers are actively pursuing a piece of the emerging LCD market.

Compaq`s TFT500 15-in. (viewable) LCD display, for example, uses 65% less power and 60% less desk space than a 17-in. CRT with a similar viewing area. NEC, for its part, is developing its XtraView technology, which allows for a 160° horizontal and vertical (80° from center) viewing angle. That contrasts with a 40° viewing angle for traditional LCDs.

Philips, Sharp, and Sony have joined forces to develop large flat-panel displays by licensing plasma-addressed liquid-crystal (PALC) technology. The collaboration is expected to result in a 40-in., flat PALC display by year`s end. Sony, the originator of the Trinitron CRT, is slated to contribute its fundamental PALC technology; Sharp, its wide-viewing-angle liquid-crystal technology; and Philips, its high-aperture, high-resolution display technology.

Samsung, too, is looking at LCD technology. Now a supplier of CRTs, the company hopes to capture a share of the TFT (thin-film transistor) LCD market as TFT-LCDs eventually displace CRTs. It anticipates that TFT-LCDs will capture 10% of the world monitor market by the year 2000 and 60% by 2005.

To that end, Samsung has completed the development of a prototype 17-in. TFT-LCD with a 1.67 million-color, 1280 ¥ 1024-pixel, SXGA resolution. The 2.5-kg product provides a 0.26-mm dot pitch and offers an effective screen size of a 19-in. CRT while consuming one-fifth the power. It complements the company`s 21.3-in. UXGA and 14- and 15-in. XGA TFT-LCD models. Because Samsung can carve out four 17-in. displays from each of its 600 ¥ 720-mm LCD substrates, it expects to keep costs low when full production begins in the first quarter of 1998.

Designed as a direct replacement for CRT monitors in desktop PCs, desktop publishing systems, medical diagnostic equipment, and CAD/CAM systems, the TFT-LCD exhibits a response time of 40 ms, 25% to 50% faster than that for CRTs. The viewing angle is 120° horizontal and 100° vertical. R. N.

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