Projector technology, LCD vs DLP

The Technology War: LCD vs. DLP

We will look at the advantages and limitations of both DLP and LCD in turn. The most important advantages of DLP technology include the following:

Sealed imaging chip. Most DLP projectors have sealed DLP chips that eliminate the possibility of a dust particle alighting on the imaging plane, which could create a dust spot on the projected image. LCD projectors do not have sealed panels, and the possibility of getting a dust spot exists. This is especially true when air filters are not cleaned periodically as per operator manual instructions.

Filter-free. DLP projectors that have sealed DLP chips can operate without air filters. Thus maintenance is reduced since there is no need to periodically clean or replace filters. Some vendors represent their DLP products as maintenance free, other than the occasional lamp change and dusting of the case and lens. Others don’t go quite that far, and recommend a periodic vacuuming of the air vents to limit the amount of dust getting into the unit. The vast majority of DLP projectors on the market do not have air filters, but some of the most expensive high performance 3-chip DLP models do, as do a few earlier generation DLP models that may still be in use.

Whether filter-free design is a true advantage to the user is a matter of competitive debate and controversy. In most DLP projectors, components other than the imaging chip itself are not sealed and can be adversely affected by a build-up of dust. In particular, dust on the color wheel may affect color and image quality. Dust particles can burn or melt when coming into contact with the lamp surface, thereby accelerating the degradation of lumen output over the life of the lamp. The degree to which a filter-free projector might be adversely affected depends on how much dust there is in the operating environment. Texas Instruments maintains that the amount of dust found in a normal room environment will not adversely affect the operation of a filter-free projector. Those who advocate the use of filters maintain that air filters will prevent an accelerated degradation of the lamp’s lumen output, even in normal room conditions.

Recognizing dust as a potential problem, Mitsubishi has taken extra steps to combat dust contamination in their latest filter-free DLP projectors, the XD3200 and WD3300. They have sealed the color wheel to prevent dust from reaching it. They have also made design improvements to the light pipe and airflow channels which reduce the amount of dust that can reach the lamp. These changes are intended to help maintain the lamp’s lumen output potential over its lifespan.

Those who advocate using air filters on projectors maintain that dust is never good inside a projector, and that the user is better off with a filtered design that prevents dust from entering the projector to begin with. All LCD projectors use air filters, as do some of the higher end 3-chip DLP models from vendors such as Runco and Digital Projection.

Those who support filter-free designs point out that many users of filtered projectors do not follow recommendations for cleaning or replacing air filters. If an air filter gets clogged over time, it can inhibit airflow, increase internal operating temperatures, and adversely affect the life of the LCD panels.

No convergence problems. All projectors using three imaging devices, whether they are LCD, DLP, or LCoS, must have all three devices aligned perfectly so that the red, green, and blue information for each pixel is in convergence. Over time, these three device systems can slip out of alignment. On occasion they can come out of the box, brand new, with slight convergence errors. Convergence errors can soften the projector’s image and create color artifacts where there shouldn’t be any.

The single-chip DLP design has a unique advantage over all three-chip or three-panel systems: since there is only one imaging chip, convergence problems don’t exist. There is simply nothing to go out of alignment.

Contrast advantages. Most business class DLP projectors (those intended for portable presentation or conference room use) have much higher Full On/Off contrast ratings than comparably priced LCD models. ANSI contrast figures are rarely published in the projector industry, but our measurements indicate DLP projectors usually have an edge over the LCD competition in ANSI contrast as well. However, with the introduction of inorganic LCD panels that are now used in most LCD 1080p home theater products, DLP’s traditional advantage in contrast within the home theater market niche has been neutralized to a large extent.

No image persistence. If one displays a static image for an extended period of time, an LCD projector with organic LCD panels may have a tendency to retain a subtle ghost of that image even after the subject matter is switched to another image. This does not occur on a DLP projector. Nor does it occur on LCD projectors that use inorganic panels.

Some of the advertising hyperbole has blown the seriousness of this issue out of proportion. Anti-LCD ads have claimed that LCD projectors are subject to “burn-in.” Strictly speaking, this is not really true. Burn-in, in traditional usage, refers to permanent damage that can be suffered by CRT or plasma phosphor-based displays. Once a static image has been etched into a phosphor display through long term exposure, it cannot be removed. This is a different phenomenon than we see on LCDs. On organic LCD displays, when image persistence occurs, it is temporary and can normally be erased by displaying a white screen for a period of time.

Nevertheless, the point is that image persistence does not occur on either DLP projectors or inorganic LCD projectors. So on these products there is never any need to take steps to erase a persisting image.

No degradation of image quality over time. There is usually no degradation of image quality on DLP projectors when used over long periods of time, other than that which might result from excessive internal dust build-up. But in any event, the DLP chips themselves will not degrade. Conversely, LCD panels and polarizers can degrade with time, causing color shifts, unevenness of illumination, and reduction of contrast. The degree to which LCD degradation is a problem on current products is somewhat of a mystery since those who know the most about it (the LCD manufacturers) don’t discuss it publicly. This issue will be discussed further below.

Somewhat less pixelation/screendoor effect on low resolution products. One of the historical advantages of DLP over LCD has been a reduced level of pixelation in the image. Pixels tend to have sharper definition on an LCD projector, and this can produce a more visible pixel structure in the image. This is often called the screendoor effect, since the picture on low resolution projectors can look like it is being viewed through a screendoor.

However, the differences between LCD and DLP in this regard are not as great as they used to be for two reasons. First, LCD makers have achieved smaller interpixel gaps, making the screendoor effect much less visible. Second, the average native resolution of projectors being sold today has increased dramatically over what it was several years ago. With increases in resolution come smaller pixels and a less noticeable pixelation across the board. Nevertheless, on low resolution products like SVGA and even standard XGA, DLP projectors still have an advantage in manifesting somewhat less visible pixel structure than LCD projectors. (Note: There is a disadvantage to having less distinct pixel structure, which is reduced image sharpness. We will discuss this further below.)

DLP leads in miniaturization. The single-chip light engine affords the opportunity for extreme miniaturization that LCD cannot quite match. At the moment there are 15 DLP projectors on the market that weigh less than 3 lbs and put out more than 1000 lumens. By comparison, the lightest 3LCD projector on the market weighs 3.5 lbs and most are 4 lbs or more.