Key parameters

What parameters should you pay attention to, when you choose a home theater projector? Along with price tags, consider the following four parameters:

• Light flux
• Contrast
• Noise of the cooling system
• Color rendition

Light flux (this parameter is sometimes addressed as brightness, which is not entirely correct) characterizes the amount of light emitted by a projector. Light flux is measured in lumens (lm). The more lumens, the brighter the image on the screen. The 196M SMPTE standard (Society of Motion Picture and Television Engineers) defines sufficient brightness within 41-75 cd/m². That is 55 cd/m² is a recommended value of image brightness without any outside light sources. Image brightness in a slightly dark room must be at least 75 cd/m². If the room is not dark (provided there is no direct sunlight), it must be 200 cd/m² and higher. For example, brightness of CRT TV sets is approximately 200 cd/m², and LCD TV sets provide 250-450 cd/m². Outdoor brightness in an overcast day is about 350-1000 cd/m². Brightness of the projected image equals light flux divided by screen area (which gives us illumination is lx) and multiplied by a coefficient, which depends on reflective properties of a screen. If the screen is made of regular matte fabric, this coefficient is about 1/3, that is brightness (cd/m²) of the white field on the screen is three times as low as illumination (lx). Recommended brightness values allow to calculate an approximate screen size for a given light flux and room type. The table below publishes maximum width of a 16:9 screen (m, the value in brackets is diagonal size).

We recommend to consult the subdued light column even for a dark room, so that the image on the screen would not seem too bright. Lots of measurements taken according to the ANSI procedure show that light flux is one of few characteristics, which can be taken directly from official specifications. However, you should take several issues into account. First of all, in case of a single-chip DLP projector with a transparent element in its color filter (a rare thing in theater projectors), you should halve the light flux from specs to evaluate the screen size. Secondly, light flux is usually published for modes without any color correction (software or with an optical filter), which takes up another 30% (or more). Thirdly, you may have to use a projector in a reduced power mode, if you want to prolong the service life of the lamp and reduce noise of the cooling system, that is minus another 30%. So our generic advice is as follows -- the higher the light flux, the better. The best choice for a home theater projector is a 1.5-m wide screen or larger, and these projectors are not very bright. Note that it's more comfortable to play games in a room with subdued light. In this case you should choose a projector with high brightness (image on the screen will not fade as much), paying a little less attention to contrast and noise of the cooling system.

Contrast is a very important parameter that determines image quality. According to the ANSI procedure, contrast is calculated as average illumination in the center of while and black fields on a 4x4 check pattern. ANSI contrast is a very good parameter that characterizes quality of the entire optical system of a projector. Unfortunately, manufacturers practically never publish ANSI contrast values in their specs. They calculate contrast as a relation of brightness in the middle of the screen for alternating white and black fields -- the so-called full on/full off contrast. Such contrast is usually several-fold higher than the ANSI value, and it looks good in specifications. Full on/full off contrast has little to do with real image contrast, because it does not characterize illumination of dark areas versus light areas, which appears inside the optical system of a projector. So it does not take into account nonuniformity of parameters across the projected area. So the only source of information about contrast is results of independent tests, which evaluate ANSI contrast. However, when you choose a projector, you may evaluate contrast of several projectors visually. The easiest way to do it is to use several projectors simultaneously in the same conditions, if their light flux values do not differ much (you must carefully tune projectors, because factory defaults are not always optimal).

Adaptive control of light flux has become a wide-spread tendency among LCD projectors -- with the help of adjusting the diaphragm and/or lamp brightness. Light flux is lowered for dark scenes and raised for light ones. It allows to obtain incredible full on/full off contrast (tens of thousands to one). And along with purely marketing issues, it improves perception of dark scenes. Projector settings usually allow to adjust response rate and/or light flux reduction range. You should keep in mind that the dynamic diaphragm drive sometimes acts as another source of acoustic noise.

Even good contrast is just a potential to be reached. That's why the room must be dark, with dark ceiling and walls -- they will eliminate any flare spots on the screen from outside light sources or from reflections. Besides, if you cannot eliminate flare spots, you may resort to a screen with high-contrast fabric.

What conditions are perfect? Theoretically, ANSI contrast of 200:1 is enough for the image to look sharp. In this case, the only irritant will be grey fields to appear, if the movie is smaller than the projected area. You can remove these fields, for example, by decreasing the screen height for a movie in 2.35:1 format. In this case the image will subjectively look contrast, even if its real contrast is low.

It goes without saying that contrast above 200:1 only improves the picture. When the contrast exceeds 350:1, grey fields stop to be a problem. Typical contrast of theater LCD projectors ranges from 200:1 to 400:1, DLP projectors -- from 450:1 to 650:1. Contrast practically does not change when the lamp brightness is decreased. It may grow a little, when the lens diaphragm is reduced, and color correction lowers contrast (insignificantly with optical filters, much stronger in case of software correction).

You'll all agree that noise from the cooling system is not welcome at all. All people have different sensitivity to this problem, of course. Some of you may ignore whirring of the office projector, the others will get irate about barely audible rustling of a theater projector in low brightness mode. Anyway, the quieter a projector, the better. Noise is characterized by sonic pressure, expressed in decibels (dB) versus auditory threshold. If spectral sensitivity of human ear is taken into account, the letter A is added to dB. Just as in case of contrast, you cannot judge how noisy a projector is proceeding from official specs. We cannot say that manufacturers deliberately lower these values, but they don't publish how they measure the noise. You can only compare noise values from specifications of the same manufacturer at best. In our lab we measure noise reproducing certain conditions, so our test results allow to compare projectors by their noise. In our subjective opinion, the noise level of 35 dBA (measured by our procedure!) and lower is an acceptable value for a home theater projector. It's a rough approximation, of course, because we don't take into account the real position of a viewer relative to a projector, room properties, individual perception peculiarities, etc. When you choose a projector, try to evaluate noise of several models in the same conditions, if possible.

Quality of color rendition determines how accurate a screen colors versus original values. It's an important parameter, out of doubt. However, if professional usage is not intended, it stops playing a relevant role when colors are approximately the same. The most reliable way to evaluate color rendition quality is to use a photocolorimeter/spectophotometer. Unfortunately, it takes much time, effort, and money. So the only affordable solution is visual evaluation with test materials, which helps judge various aspects of color rendition. Let's mention several points you should pay attention to.

Fine color scales should be rendered both in lights and in shadows. Ideally, computer image should differentiate between color transitions within 0-255 at 1 steps, at least for greyscale, if not for all three main colors. In case of video, the color range may be a bit narrower -- from 16 to 235, if an extended range is not used. (Let's confine ourselves to 8-bit colors only.) In order to display all color ranges, you should adjust brightness and contrast, white and black balance, etc. If you use DVI or HDMI connection, the video source and the projector must use the same mode -- extended or regular color ranges.

The gamma curve (numeric representation of colors versus their relative brightness) must be similar to the generally accepted value (exponential function with the value of 2.2). This condition includes and supplements the previous one. For example, a non-standard gamma-curve may raise contrast in shadows, which will reveal artifacts in dark areas even in high-quality movies. Projectors offer settings to change the flection of the gamma-curve and to edit check points on the gamma curve.

Grey color must have more or less the same color temperature across the entire range -- from black to white. Moreover, absolute color temperature is not as important as its match for white color and greyscale. If these values differ much, objects of neutral colors with have irritating tinge. Projectors usually allow to choose a profile for global color correction and/or color temperature profile, to adjust it step-by-step and/or to adjust color balance in other ways.

In the most general case colors on the screen must be close to real, the only exception is deliberate distortions introduced at the production stage. The easiest way to evaluate how true the colors are is to use familiar objects: company logos, human skin, etc. As projectors form colors using the additive principle, if the white and grey colors are displayed correctly, the other colors will also be more or less adequate. If we exclude the influence of video source, distortions in color rendition can be explained with image processing functions. And vice versa, you can try to use projector's settings to eliminate the disbalance of colors (regardless of its origins). You can use simple options, such as saturation of one of the main colors, as well as adjustment of a certain color (or several colors), selected with the cursor on the screen, etc.

Some High-End projectors support professional color calibration systems (for example, ISF, as in Epson EMP-TW2000).

An example of color disbalance is excessively high brightness of the white field versus brightness of colored areas. In this case colors on the screen will be dull and unsaturated. This effect appears in single-chip DLP projectors, which use a transparent segment in their color filters. In most cases you can use projector's settings to disable the transparent segment to normalize color brightness. It will reduce brightness of the white field, of course. That is the light flux will be lower. For example, disabling the transparent segment in InFocus X8 (more white = 0) reduces the light flux from 2280 lm to 940 lm. However, I'd like to note one more time that color filters of theater DLP projectors rarely contain transparent segments.

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