LCD versus CRT Displays: A Comparison of Visual Search Performance for Colored Symbols

Article excerpt

INTRODUCTION

This paper describes an experiment examining the relative effectiveness of two display technologies: a liquid-crystal display (LCD) and a conventional cathode-ray tube (CRT) of similar resolution and size. Within the naval community, there is interest in the use of LCDs on ships because of their lower weight and power consumption, smaller footprint, and lowered susceptibility to electromagnetic interference. This specific interest mirrors a more general consumer interest in LCD technology for desktop computers.

LCD and CRT Technologies

LCDs are composed of many tiny liquid crystals arranged in rows and columns. Liquid crystal molecules can be reoriented by an electric field. LCDs function by twisting the axis of polarization of the light as it passes through the liquid crystal such that when the light reaches the front polarizer, it is oriented correctly to pass through, allowing it to be seen by an observer.

When an electric field is applied, the structure is untwisted and no light is emitted (Asian Technology Information Program, 2002). The LCDs used for computer displays are most commonly active matrix, with an electronic switch at each pixel location (Luder, 1997).

In contrast, conventional CRT displays rely on an evacuated glass tube with a display screen at one end and electron guns at the other. The guns emit electron beams that are deflected to various screen locations by use of magnetic fields generated by a deflection yoke. The beams strike phosphors near the screen, converting the electrons to observable light energy. A particular pixel is illuminated or not by coordinating the timing of the gun output and the magnetic fields controlling the deflection yoke.

Given the vastly different technologies underlying each display type, it is no wonder that the images rendered with LCD and CRT displays differ. Among other differences, the pixel definition with LCDs is much sharper than for CRTs (Wright, Bailey, Tuan, & Wacker, 1999). CRTs tend to produce a blurred Gaussian distribution of light at each pixel, whereas LCDs produce a sharp edge to each pixel (Menozzi, Napflin, & Krueger, 1999). This sharp edge has clear advantages, but the edge may be problematic when rendering curves (leading to aliasing problems) and may introduce a high spatial frequency noise component into the display.

Viewing angle is an important factor in operational display performance. Photometric measures of angular luminance and chromaticity indicate reduced luminance and color distortion with off-axis viewing orientations on an LCD (Selhuber & Parker, 1997). When viewed off axis, the light takes a longer path through the liquid crystal material and the light receives a greater twist than that from an on-axis position. This results in a drop in luminance for an "on" pixel and an increase in luminance for an "off" pixel. Optical measurements with CRTs indicate that although off-axis luminance and color distortion problems occur, they are less of a problem. However, it is not known whether the off-axis problems with display optics are severe enough to affect human participants performing an operational task. To our knowledge no studies have investigated this question.

The question is important because LCDs have many advantages, as noted; those faced with the decision about whether to purchase LCDs may be interested in knowing about possible limitations of LCD technology. Off-axis viewing is a special problem for display viewing in the naval operations room because it is common for the same display to be viewed by multiple individuals, some of whom are standing to the left or right of the display screen. Business computer users are also accustomed to information sharing on monitors. Displays are getting larger, and the larger the display, the more likely it is that an observer will be off axis with respect to some part of the display. …