The days of bulky, expensive business projectors are thankfully behind us. Today's business projector is smaller, lighter, and easier to use, making it ideal for anyone looking to put some punch in their pitch. CNET Labs begins the business-projector evaluation process with a number of quantitative and qualitative tests to help determine which models are best suited for particular applications and which best meet the needs of the user. We evaluate image quality with tests that are specifically designed to emphasize a particular area of performance, such as text readability, color accuracy, or image uniformity. We have also devised a suite of usability tests that cover practical issues, such as warm-up and cool-down times, noise level, temperature, and throw ratio.
CNET Labs' projector test bed consists of a 2GHz Intel Pentium M 755 processor-based Dell Inspiron 9200 notebook running Windows XP Professional SP2, using an Intel 855PM chipset with 1GB of 333MHz DDR SDRAM and an ATI Mobility Radeon 9700 graphics chip with 128MB DDR SDRAM. We test all projectors with Windows Display Properties set to 32-bit color, and we use the projector's native pixel resolution as specified by the manufacturer. We allow the projector to warm up for at least 15 minutes before calibration and testing.
Variables such as the light source, the distance from the projector to the screen, and the system's graphics engine can dramatically affect a projector's performance. To maintain consistent and ideal viewing conditions, all testing is performed in a controlled lighting environment, and each projector is adjusted to perform optimally, based on the manufacturer-recommended or default settings when possible. All tests are conducted with the projector's optical zoom set so as to result in the largest possible projected image, and the projector is positioned so that it produces a one-square-meter image (56.8 inches diagonally for a 4:3 image) on the screen.
One of the most important criteria for projector performance is brightness. CNET Labs uses a Minolta T-10 luminance meter in conjunction with the ANSI (American National Standards Institute) Brightness Test Screen, found in the DisplayMate test suite, to measure a projector's luminance levels. All readings are taken at the projector's default brightness setting. In accordance with ANSI specifications, we measure brightness at nine specific points on the screen, then average the readings and multiply the result by the screen area to yield a single brightness figure, reported in lumens. If the projector supports a low-power mode, the brightness is measured at that setting as well.
Some manufacturers use a full-screen contrast-test technique (full white/full black), which yields artificially inflated ratios that are much higher than those we obtain when using the checkerboard pattern. For example, one projector manufacturer claims a full-screen contrast ratio of 1,800:1, while our test methodology of that same projector yields a more conservative contrast ratio of 400:1.
Some projectors produce images that are much brighter in the center than in the corners. To determine how close a projector comes to uniform illumination, we test brightness in the center of the screen and in each of the image's four corners. The average brightness of the corners is then divided by the center to yield a uniformity rating, which is expressed as a percentage. A score of 100 percent indicates perfect uniformity.
A large throw ratio enables a projector to display a big image in a small room. The throw ratio is obtained by dividing the diagonal size of the projected image by the distance from the front surface of the lens to the screen.
In order to test color accuracy, we perform two separate tests. First, the color temperature of an all-white image is determined using a Minolta XY-1 colorimeter. The ideal white color temperature is between 5,500 and 6,500 degrees Kelvin, which corresponds to the color temperature of sunlight at noon. Higher color-temperature values tend to be bluer, and lower values redder.
Second, using the colorimeter, we measure how well a projector handles a full gamut of color by evaluating how it projects the three additive primary colors: red, green, and blue at the corners of the gamut. For each color, the colorimeter determines the red (x) and the green (y) values. These values are then plotted on a color gamut diagram and compared against a reference color monitor.
To determine how quickly a user can power up and shut down a projector, we test warm-up and cool-down time. The projector is connected to a notebook computer that is already running, the projector is then turned on, and we time how long it takes for an image to appear on the screen. We let the projector run for 60 minutes, then power it down, timing how long it takes for the projector's fan to shut off.
To determine the obtrusiveness of a projector's fan noise, we perform sound-level tests. The projector is placed in a soundproof room, with a background noise-level measured at 36 decibels, then it's powered on and warmed up for 15 minutes. Five measurements (front, rear, left, right, and top) are taken with a sound meter placed one meter from the projector. These readings are then averaged to yield the overall noise level in decibels.
To measure how hot a projector gets, a digital thermometer is held against the hottest part of a projector's external casing after the projector has warmed up for 30 minutes. Invariably, a projector's hottest part is its main fan exhaust grill.
|DisplayMate image-quality tests|
To test a projector's image quality, we use the projector script in DisplayMate Technologies' DisplayMate Multimedia Edition. These screens are designed to isolate common phenomena such as sharpness, digital noise, streaking, ghosting, and color accuracy deviations. Our projector test script includes more than 40 test screens. Many of these screens can be configured in a number of different ways; you can, for example, alter the background and foreground colors. Depending on the characteristics of an individual projector, we might use several variations of these screens, as well as additional DisplayMate screens not found in our custom script, to conduct further testing. The test screens described below represent key examples of our projector test script, but they represent only a small portion of the overall image-quality testing.
This screen resembles the test patterns used by television stations, but it is designed expressly for computer-based displays. Much like getting slices of multiple screens on one page, it includes grayscale bars, color bars, geometric shapes, and various phase patterns. This assortment provides an excellent starting point for testing a projector's image quality.
The graphic pattern on this screen reveals variations in focus across the display. This screen is used to adjust and to test image focus. It is also handy for checking geometric linearity, especially in the corners.
Pixel Tracking and Phase-Lock Test (DisplayMate screen 10:16)
This screen enhances noise fluctuations and shows how well a projector can match a computer's pixel tracking and timing lock, which is sometimes referred to as phase, timing, or horizontal time.
Black-Level Adjustment (DisplayMate screen 2:3)
This screen displays the dark end of the grayscale (the range of grays between true black and true white) and allows us to observe how well a projector resolves these dark shades.
Similar to the testing of the dark end of the grayscale, we use this screen to evaluate how well a projector resolves the bright end of the scale.
Pages of text (DisplayMate screens 7:1 and 7:2)
These screens allow us to examine a projector's ability to render text under a variety of conditions. By using these screens, we can 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.
64-256 Intensity color ramp (screen 9:32)
This screen illustrates a projector'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.
These screens help 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.
Contributions to CNET Labs' projector testing methodology were made by veteran computer journalist and consultant Rich Malloy. Rich currently writes for several magazines and Web sites and is considered an expert in the projector field. He can be reached at firstname.lastname@example.org.