To evaluate a display's performance, CNET uses a variety of DisplayMate test screens in conjunction with diagnostic equipment, and our most important tool, the human eye. Each test screen we use is specifically designed to emphasize a particular area of performance, such as text readability, color accuracy, or screen uniformity.
CNET Labs' monitor test bed consists of a 3.2GHz Core i7 960 CPU, 4GB of DDR3 RAM running at 533MHz, an ATI Radeon HD 6850-based graphics card, centered around the 32-bit version of Windows 7. We test all monitors with Windows' Display Properties set to 32-bit color and use the display's native pixel resolution as specified by the manufacturer.
We are currently evaluating the possibility of upgrading our test bed, including a move to Windows 8.
Variables such as the light source, the viewing angle, and the system's graphics card can have a dramatic effect on a display's performance. To maintain consistent and ideal viewing conditions, all testing is performed in a controlled lighting environment using CinemaQuest Ideal-Lume lights, which help to preserve accurate color perception. Additionally, each display is adjusted to perform optimally, based on contrast setting recommendations from DisplayMate.
CNET Labs tests luminance levels using the Minolta CA-210 LCD color analyzer. Because displays typically vary in brightness across the entire screen, we take brightness readings from different sections of the display as laid out in the VESA Brightness Uniformity Test screen found in the DisplayMate test suite. The brightness number that CNET publishes represents an average of the luminance readings taken at nine specific points on the screen.
To test a monitor's contrast ratio, we use the display's factory default contrast and brightness settings, under its default preset. Using the Minolta CA-210 LCD color analyzer, we measure the brightness of both the light and dark squares found on the DisplayMate ANSI Checkerboard Contrast screen. We divide the average luminance of the white squares by the average luminance of the dark squares to yield the display's contrast ratio.
With guidance from DisplayMate Technologies, we created our own scripted selection of test screens in DisplayMate. These screens are designed to isolate common phenomena such as digital noise, streaking and ghosting, ringing and overshoot, and color-tracking errors.
We evaluate each display in four categories: sharpness, grayscale range, color quality, and image uniformity. We compile the product's performance scores from each category and distill them into a single performance rating: the CNET Labs DisplayMate test score.
Most of the screens in this test suite can be configured in a number of different ways, such as altering the background and foreground colors. Depending on the characteristics of an individual display, we might use several variations of these screens as well as additional DisplayMate screens not found in our custom script to conduct further testing. Check below for details on the actual tests we run.
In addition to our suite of DisplayMate test screens, we also use a number of tests designed to mirror real-world use, such as Blu-ray movie playback and games. For our Blu-ray test, we use Panasonic DMP-BDT220 Blu-ray player with an "Avatar" Blu-ray disc. We use this movie to evaluate a monitor's ability to display dark detail in dark scenes as well as color quality. For color quality we look at the display's ability to reproduce bright, vivid, accurate colors, including bright whites and solid, deep blacks.
We use several games for games testing, including, but not limited to Dragon Age II, Starcraft 2, and Crysis 2. With games we evaluate vibrancy and color quality. To test streaking, we use DisplayMate's motion graphics tests where we closely watch a number of colored blocks as they move around the screen at various speeds. Each block leaves an impression of its image behind as it moves around. The longer the impression, the worse the streaking.
CNET's DisplayMate tests
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These screens illustrate a display's ability to render text under a variety of conditions. We cycle through various text and background colors, view split screens with inverse text and background colors, and adjust the type and the size of a font.
Intensity and grayscale
Monitors often have trouble reproducing all of the levels of the grayscale (the range of grays between true black and true white). This screen helps to identify a display's ability to deliver seamless gradation across the full spectrum of grays, both horizontally and vertically across the screen.
Low saturation colors
When producing a bright white image, many monitors oversaturate the grayscale: the lightest grays of the scale are lost in the white background. Oversaturation can also lead to loss of color range; this screen is used to evaluate color reproduction at the brightest end of the scale, closest to white.
64-256 intensity color ramp
Similar to the grayscale tests, the color ramp illustrates a monitor's capacity to render gradations of primary colors smoothly, uniformly, and consistently. This screen is also used to check that the colors don't shift hue as the color levels increase or decrease.
A color-tracking error occurs when the intensity of red, green, and blue (RGB) do not adjust identically with signal-level changes. This lack of balance among the RGB channels affects color as well as grayscale, but it is most easily identified as a shift in color within shades of gray. We use this screen to look for grays that appear to be tinted with color.
256 intensity color ramp
Similar to the grayscale tests, the color ramp illustrates a monitor's capacity to render gradations of primary colors smoothly, uniformly, and consistently. This screen is also used to check that the colors don't shift hue as the color levels increase or decrease. This is a good screen to test for evidence of color banding.
Because this test screen is designed to appear uniformly black, it is useful for evaluating a monitor's black-level capabilities. Additionally, a dark screen is the easiest way to spot glare and reflection problems, both of which can have distracting effects when you're viewing an LCD.
Similar to the intensity color ramp, the color-scales screen helps us evaluate the smooth gradation of colors, expanding the palette to 10 principal colors.
Streaking and ghosting
This screen helps us detect streaking and ghosting -- light or dark shadows that trail an image in areas where large changes in contrast are present. This should not be confused with the streaking that is often found in moving images. This test deals only with problems that arise when a display renders large, chunky graphic elements, such as bar graphs or tiled arrangements of open windows.
Perfectly uniform backlighting across a monitor's entire display surface is difficult to achieve. It is not uncommon for a monitor to have bright or dim patches or subtler variations in color intensity, which give the appearance of shading across the screen, or variable color intensity on the display. We use this screen to check for irregularities caused by backlighting issues or other screen-uniformity factors, such as variations or reflections inside the glass panel.