Contrast Ratio

Contrast ratio, most often talked about with LCD monitors, is the measure of the difference of brightness levels between the brightest white and the darkest black. A good contrast ratio of 120:1 easily displays vivid colors, while ratios as high as 300:1 support superior grayscaling.

In order to ensure a proper measurement of contrast ratios, it is recommended that any specification provided adhere to the VESA Flat Panel Display Measurement Standard for contrast ratio measurements. Additionally, as a more clear indication of overall monitor performance, contrast ratio should also be evaluated in conjunction with brightness, viewing angles and response time since all specifications are inter-related.


Aspect Ratio

Aspect ratio refers to the ratio of the display's width to the display's height. Most people think of computer monitors and TVs as having a square image or a 1:1 aspect ratio where the height is equal to the width. In actuality, most traditional computer monitors and TVs have a 4:3 aspect ratio where the width is slightly larger than the height. With the emergence of technologies such as HDTV, people are becoming more comfortable with alternate aspect ratio displays, which have a more rectangular look. Many of these type displays carry either a 16:9 or a 16:10 aspect ratio. HDTV has a 16:9 aspect ratio, but for computer displays, a 16:10 aspect ratio is optimal for rectangular displays because it allows for two full pages of text to be shown side-by-side on one screen. While rectangular computer displays are not currently popular, mostly due to compatibility issues between the computer interface card and the display, they are emerging as a potential display format for the future.

Since computer monitors are measured on the diagonal, it is important to understand the aspect ratio of a display in conjunction with the screen size to make proper comparisons (Fig. 8). As an example, while a 22" 16:10 display sounds large, since measuring a rectangle on a diagonal is a longer distance than measuring a square (or near square) on a diagonal, the height of a 22" 16:10 display looks small. In fact, the height of a 22" 16:10 display is actually smaller than the height of a 17" 4:3 display, so it actually looks like a 17" monitor stretched out to the sides.

Figure 8. Since all displays are measured on the diagonal, the diagonal measurement of a rectangle will always seem larger than the diagonal measurement of a square. In pixel count terms, a display with a resolution of 1600 x 1200 vs. a display with only 1600 x 1024 clearly shows that the former is capable of showing more information.

Viewing Angle

Intuitively, given that most viewing is done by a single person sitting in front of his or her monitor, how a picture looks from directly in front of the display should be of utmost concern and the primary way that the quality of a monitor should be evaluated. However, off-angle viewing can be critical for many reasons, including for color-intensive and multi-viewer applications, when the user needs to be able to see the screen from different positions, or when the monitor is pivoted for portrait usage.

Due to the emissive characteristics of phosphors located on the front face of the tube, off-angle viewing has never been an issue with CRT monitors as they naturally provide comfortable 180° views in every visible direction. This is not the case with LCD monitors, however, due to their inherent makeup. As light must pass through a series of materials prior to being emitted from the front face of the LCD, viewing angles can be narrowed, resulting in color shifts at the extremes and particularly in the vertical direction (up and down). LCD monitors have made vast improvements over the years in regards to their off-angle usability. As a point of reference, LCD viewing angle measurements are usually provided in a specification in both the horizontal (left and right) and vertical (up and down) direction.

There are two main ways that manufacturers can increase the viewing angles of their LCD products. One method involves the very design of the internal components of the monitor and requires the microscopic liquid crystals to be twisted in different ways (Fig. 9) so that light is emitted in different directions and in the widest angles possible. This method for improving viewing angles can be achieved by many types of technologies such as those generically referred to as inplane switching (IPS) or multi-vertical alignment (MVA), or by manufacturer-specific implementations such as NEC's XtraView® technology. These types of technologies generally deliver superior viewing angles in excess of 170° in both the horizontal and vertical planes.

Figure 9. In conventional LCD technology (top), liquid crystals stand on end, resulting in a loss of viewing angle. With wide-angle viewing technology (bottom), the crystals remain horizontal, which dramatically improves the viewing angle because the light passes through the crystals at their most efficient orientation.

The other, less costly, method for increasing viewing angles involves the adherence of a light dispersion or compensation film to the front of the LCD display. The inclusion of such an optical film generally allows for improvements in viewing angles primarily in the horizontal plane. This method results in typical viewing angles of 150° in the horizontal direction and 130-140° in the vertical direction.

Viewing angles can be easily evaluated by standing to the extreme sides of the monitor as well as viewing the monitor from both an upward (looking up from below) or downward (looking down from the top) direction. With larger displays, a component level technology like inplane switching is usually essential since without a wide-angle viewing technology present, users might be required to move their heads as they view the display to prevent noticing any shifts in colors. Additionally, if the monitor rotates to allow for both portrait and landscape viewing orientations, it should be noted that when in a portrait mode that the left-right viewing angle becomes the up-down viewing angle. Hence, both the left/right and the up/down viewing angles should both be taken into consideration (Fig. 10).

Figure 10. Wide-angle viewing technology allows users to view the monitor screen from angles up to 170..., both horizontally and vertically.
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