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Colour Science: Calibrating Front Projectors

 
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PostLink    Posted: Thu Jun 10, 2010 9:07 pm    Post subject: Colour Science: Calibrating Front Projectors Reply with quote

Colour Science: Calibrating Front Projectors

Many home theater enthusiasts have begun to recognize that one can effectively reproduce a real theater experience at a surprisingly low cost by using digital front projectors. Today's digital front projectors offer truly amazing performance at an entirely reasonable price point. The only limitation holding many back from moving towards front projection is the fact that—like a commercial theater—you really must watch in a darkened environment. Front projector systems also require a considerable amount of space, so they are best employed in dedicated home theaters, though many manage to integrate them into their living room.

Some History

I recall the first time I saw a front projector intended for home theater use. It was in 1986 at a local trade show. The image was from the original Star Wars film and was projected onto a 60" 4x3 screen. Coupled with the first affordable surround sound system developed for home use I had ever heard, I consider this experience to be the moment when I caught the home theater bug. I was left speechless by the experience.

However, in mid 1980s there were several limitations to front projectors that made them impractical. First, the only projection technology available was CRT. Although capable of producing stunning images, CRT front projectors were very dim, expensive, and quite difficult to setup and maintain. The other problem was the source material. The only widely-available source of the day was VHS tapes, which suffered from relatively low image quality. These deficiencies were especially evident when blown up to the 60"+ size provided by front projectors. There were also laser discs, which offered increased resolution, but they were bulky, expensive, hard to find (this is before the Internet), and tedious to use.

In the early 2000s two separate events set the stage for a revolution in home theater. The first was the introduction of the DVD format. It offered image quality that exceeded even laser discs and they were easier to use than VHS tapes to boot. With the introduction of DVD, VHS and laser discs quickly faded from sight. The second event was the refinement of digital projectors targeted for home theater use. Unlike DVD, this was not a new technology. LCD front projectors had been around for years as business presentation devices. However, they lacked the resolution, color fidelity, and smoothness necessary for home theater use. In time these projectors gradually got better. I think of the introduction of the Sony HS10 LCD projector as a milestone in this development. It offered 1366x768 resolution, reasonable contrast and brightness, good color, good video processing, and a substantial reduction of the dreaded screen door effect that had plagued so many LCD designs in the past. Mated with a good DVD source, these projectors were capable of very high quality video images. To top it off, they were affordable and extremely easy to setup and maintain. This development was the death nell for CRT projectors.

Today we have DLP and LCoS projectors, which offer even higher performance than their LCD cousins. We also have Blu-ray, which has raised the bar on performance to the point that for under $10,000 (projector, Blu-ray player, screen, and 5.1 surround system) anyone can own a home theater system that will routinely provide a superior cinematic experience than what you typically see at the local cinema.

Getting the most out of front projectors

So, once you have purchased a front projector, are their any special issues that arise when calibrating the device for best performance? In most respects, calibrating a front projector is no different than calibrating a direct view device, but there are a couple of areas that warrant special discussion.

Three variables to consider

First, the biggest difference between a front projector and a flat panel is that a front projector is a hybrid device. The image you see is the result of three separate variables: the projector itself, the screen on which you project the image, and the room in which the image is viewed (room affects flat panels as well, but not nearly to the same degree). The quality and size of the screen plus the room characteristics are enormously important in determining the quality of the image you will see.

Perhaps the biggest problem I see with front projector installations, even high-end ones, is that the screen and the projector are not well matched. Many people purchase a screen that is simply too large for the chosen projector. The issue here is image brightness. The larger the screen, the more lumens the projector must produce to provide adequate reflected luminance to the audience. 12-14 fL is a good standard for screen luminance. However, to get 14 fL on a 160" 16x9 screen you would need a projector capable of outputting over 1000 lumens. Most projectors will do this only in modes that sacrifice color accuracy. Furthermore, you have to consider that projector bulbs lose light output fairly rapidly, especially in the beginning. This means that the only projectors capable of providing high quality, color-accurate output for several hundred hours are 3-chip DLPs and a few multi-lamp projectors, which are VERY expensive devices.

For most installations I would not recommend going beyond 110" diagonal, unless you are willing to invest a lot of money in a projector with very high lumen output even in a theater mode.

Users can reduce the need for high output projectors by selecting a screen with some gain. Gain takes the light that it receives from the projector and, instead of reflecting it back in wide dispersion as would a pure lambertian source, reflects the light over a narrower viewing cone. Such screens can produce very bright images on axis, but brightness falls off rapidly when the viewer moves to the left or right. If you have a 160" screen with 1.5 gain, as in the example above, then you will need only a little over 700 lumens to achieve target luminance. Unfortunately, it is not quite this simple. Manufacturers routinely inflate the gain specifications of their screens, so rated gain and actual gain are not always the same. Stewart Filmscreen is one company whose gain ratings are reasonably accurate. Having some screen gain offers another benefit as well. Since the light it reflects off axis is less intense, the light striking the surfaces of the room is less intense. This is good because it means that less reflected light can make its way back to the screen, washing it out. Often the most cost effective way of improving image quality is to simply cover or coat the ceiling immediately in front of the screen with some non-reflective material. There are, however, some downsides to screen gain. The primary problem, which varies considerably depending on the quality of the screen, is hot-spotting. This is a phenomenon whereby the center of the screen appears visibly brighter than the sides.

One other issue beyond screen size and gain that consumers need to consider is the screen material. Most screens are white, but some screen materials have a gray coating. The main purpose of this is to lower the black level. In my opinion, gray screens only made sense several years ago when front projectors were hobbled by poor black levels. This is no longer the case. Many projectors from well-known manufacturers offer excellent black levels. In these cases, a gray screen offers little of value. I would stick with traditional white, just as commercial theaters do.

Finally, there is the room itself. It needs to be large enough to accommodate the distance between the projector and screen without obstructing the view of the audience. These variables are a function of the screen's size and the projector's throw distance and offset. Throw distance is the distance between the lens and the screen required to produce an image of the desired size. Offset is the vertical placement of the projector relative to the screen. Most projectors have an image zoom, which offers some flexibility with respect to throw distance, and lens shift, which offers some flexibility with respect to vertical offset, though to maximize image quality you should use as little lens shift as possible. There should be no horizontal offset. The projector should be installed at the absolute center of the screen. The room and the seating positions must be able to accommodate all of this. Finally, the ceiling and walls should be as non-reflective as possible and should be free of any direct light. The image from a front projector will degrade very rapidly with only a little ambient light in the room. Just like a commercial theater, it should be as dark as possible during viewing sessions. If you can read your watch, then there is too much light in the room.

Now it is time to calibrate

Once you have resolved all of the issues with the screen and the room, all that's left is calibration. As I indicated earlier, there are just a couple of special issues here. First, many people want to know how they should measure the image. Should readings be taken directly from the lens or off the screen? SMPTE guidelines are very clear on this point. All readings should be taken off the screen from the middle of the viewing area. Despite this, many calibrators prefer to take readings directly from the projector's lens. The reason for this is that given the limitations of most colorimeters, a direct-from-lens reading is the only way to read low light levels. The black level in particular from many projectors is simply too low to be read off the screen.

If you are going to take readings from the lens, you must do two things to ensure good results. First, you must use a diffuser. Colorimeters are designed to read luminance only, the diffuse light reflected from a surface. If you attempt to read illuminance, the direct light from the lens, you will get inaccurate and/or unstable readings. Second, you must correct your results with an offset obtained by taking off-of-the-screen readings. The color characteristics of the light coming directly from the lens will almost never be the same as what your eyes see when you view reflected light off the screen. Generally, it is much better to simply take readings from the screen to begin with and then supplement this with some other technique to measure the black level of the projector.

To order ChromaPure software & meter packages at CurtPalme.com exclusive prices, see our ChromaPure order page. Same product, same support - just more money in your pocket at the end of the day!


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