Making a home theater around a projector is a very attractive proposition. After all, with a projector you can make very large images, such as 100 inches and larger, for a very reasonable price. A comparable television costs a small fortune. Buying a projector or beamer? We will guide you through the basic knowledge of projectors.
Technology: LCD? DLP? LCOS?
If you want to make a good choice, it is important that you know the three important technologies of projection, and of course the advantages and disadvantages of each.
LCD (Liquid Crystal Display).
An LCD projector uses three LCD screens (one each for red, green and blue, hence the name 3LCD) to compose the image. The light from the lamp is split into red, green and blue by dichroic mirrors and is directed through the three corresponding LCD screens. Each LCD screen determines which pixels are on or off, and the resulting image is combined in a prism into an overall image and sent through the lens.
LCD projectors were initially known for their weak contrast and visible screen grille, but those days are over (unless you choose a very low resolution projector). Now they deliver very good contrast, with excellent black values and light output. LCD projectors are usually quite quiet too.
The main drawback of LCD: the three LCD grilles must be perfectly aligned. Many projectors offer the option to adjust the convergence manually. But if it doesn’t and your projector doesn’t converge properly, the image may be a bit fuzzy and objects may have a colored border. LCD projectors can also suffer from dust that travels internally to the light path (especially on the LCD panels). In that case, you will see a small blurry blob on the screen.
DLP (Digital Light Processing)
DLP projectors work in a fundamentally different way. The heart of the DLP projector is a small chip, the DMD (Digital Micromirror Device, these chips are all made by Texas Instruments). It is a grid of tiny mirrors that determine whether the light is reflected or not. Some projectors use three such DMD devices, but these projectors are very expensive. For the home theater market, DLP projectors are so-called ‘single-chip’ devices. How do they get color? A color wheel for the light source sequentially creates red, green and blue light. The DMD switches in a flash for those three colors. Because this happens very quickly, we perceive the image normally, but in reality a red, a green and a blue image is projected very quickly one after the other.
DLP projectors can be very inexpensive, and because the light path is completely sealed, it doesn’t suffer from dust. The image is very sharp and the pixel grid is almost invisible. Since he only uses one DMD, convergence problems are impossible. DLPs have slightly better motion sharpness and generally the best image uniformity.
But DLP projectors unfortunately also have drawbacks. Some people are sensitive to the ‘rainbow effect’. Due to the sequential projection of the colors, they sometimes see rainbow edges around objects. The easiest way to test this is to look at the image from your peripheral view. DLP projectors also provide only moderate contrasts. But they are also not the best choice in terms of color. After all, the color wheel always filters out part of the light. As a result, colors on a DLP projector often appear much darker than on a comparable LCD projector. DLP projectors with a light output of 2,000 lumens, once calibrated, sometimes reach 700 lumens. When the manufacturer uses a color wheel with segments other than red and green (eg white, to improve clarity), that gets worse. Finally, DLP projectors are very noisy at times. A compact housing often requires faster (and therefore louder) fans, and the color wheel is also noisy.
LCOS (Liquid Crystal on Silicon)
LCOS is best known by the name of the brands that use it: Sony SXRD (Silicon X-tal Reflective Display) and JVC D-ILA (Direct-Drive Image Light Amplification). LCOS works as a combination of LCD and DLP. The light from the lamp is split by Dischroic mirrors and directed to three LCOS panels. But instead of allowing light to pass through (like an LCD), the LCOS panel reflects the light from the pixels it wants to turn on (like a DLP).
COS is the absolute winner when it comes to contrast, a very important part for projection. The black value of these projectors is astonishingly good. They have an almost invisible pixel grid and excellent color reproduction.
Like LCD, they are susceptible to potential convergence problems. LCOS projectors are also generally quite expensive. You can enter this category for no less than 2,000 euros, while with DLP and LCD you also score a decent projector for 1,000 euros. The LCOS models are generally very well equipped.
Resolution of a projector
For real Home Theater projectors, Full HD (1,920 x 1,080) is still the best choice. In this category you will find a very extensive range. If you still want to aim higher, you can, but you still have to take into account a number of limitations. To begin with, the price of an Ultra HD projector is significantly higher. In addition, not all Ultra HD projectors have a native UHD resolution.
Some models use a native resolution lower than Ultra HD, but use pixel shift to display an Ultra HD image. JVC was the first to introduce this on the market, but other manufacturers are now also using it.
n concrete terms, a pixel-shift projector works as follows. For each image, the projector’s image processing calculates two images that it displays in close succession, but the second image is shifted half a pixel diagonally. This way he (theoretically) doubles the resolution both horizontally and vertically. LCOS and LCD projectors work with a Full HD panel. Note that the projector drives twice the entire panel (2x 2 million pixels), but an Ultra HD resolution requires four times as many controllable pixels (8 million) as a Full HD panel. The result is clearly visible.
Recent DLP projectors that work with pixel shift start from a new DMD device with 2,716 x 1,528 mirrors, a slightly higher resolution, but still no Ultra HD. When this pixel shift is used, and you control the DMD twice, you have the same resolution as a real Ultra HD panel. In practice, these DLP projectors are almost indistinguishable from a real Ultra HD projector. Cheaper DLP projectors also start from a 1920 x 1080 pixel grid and shift four times to create an Ultra HD image. In theory, that does deliver a real Ultra HD image, but in practice this variant also loses some detail.
Buying a projector or beamer: Light output and contrast
One of the most important specifications of a projector is light output and contrast. Two specifications that you can easily read on the manufacturer’s website. But unfortunately you cannot always rely on it. The light output is always given in the most ideal circumstances, with the lamp in the highest position, and with the image preset in a much too cool (blue-green) white. The result in a properly calibrated preset depends on the projector technology. DLP projectors will generally lose more light output when calibrated. LCD and LCOS suffer less from this. With contrast, things are much worse. Some manufacturers use a dynamic iris (which lets more light through in bright scenes and less in dark scenes to improve the black value). By comparing the darkest black (with the iris as small as possible) with the brightest white (with the iris fully open), you get an enormous contrast that is never achievable in practice. So try to find a static contrast digit whenever possible. And keep in mind that DLP usually yields the least good results, and LCOS the best. LCD varies depending on the price.
How many lumens do you need? That depends on the screen size, the amount of ambient light and the gain factor of your screen. We give you a number of guidelines based on a matte white screen (gain 1). Divide the manufacturer’s stated lumen output by two for a rough indication of what to expect in a calibrated image mode (the movie preset).
|Screen diagonal in inches||Required lumen output when darkening||Required lumen output in moderate ambient light|
Buying a projector or beamer: Lamp technology
On the subject of light, what about projector lamps and their lifespan? Most projectors work with UHP lamps. In a modern projector, they last between 3,000 to 5,000 hours, depending on whether you put the lamp in eco-mode or not. In eco mode you typically lose 30% light output, but the projector is much quieter. Keep in mind that the projector does not suddenly give up after x hours. The lamp loses a slow light output from day one, reaching about half of its original light output on its designated life. A lamp with 1,000 hours on the counter will be significantly less bright than a new one. How long your projector lamp will be in service therefore depends strongly on how much you watch. If you watch three films a week, a lamp will last up to 10 years. Do you watch TV four hours a day, then you are through it after two years. In that case, it is best to inquire in advance what the costs of a new lamp are. Replacing a lamp is usually a very simple task.
In addition to UHP lamps, we also see LED lamps and lasers appearing in projectors. They last much longer (10,000 to 30,000 hours). In both cases it concerns an alternative light source, but otherwise it remains a DLP, LCD or LCOS projector. LEDs are widely used in business applications and small pico projectors. Lasers are mainly found in high-end business or large venue projectors. Home Cinema projectors with lasers exist, but are very pricey.
Buying a projector or beamer: Conclusion
Home cinema projectors fall into three categories based on technology: LCD, DLP and LCOS, each with specific advantages and disadvantages. Most projectors still use Full HD, but Ultra HD is available. You have to pay attention whether you are viewing a real Ultra HD projector or one with pixel shift, although the latter also give excellent results. Light output and contrast are two very important properties for your projector, but how well you can or want to darken determines the final image quality. More about that in our next projector section where we look at placement.