Thursday, January 23rd, 2014

DLP Projectors

DLP, or digital light processing, refers to a technology developed by Texas Instruments as far back as 1987. DLPs have grown a lot since then challenging LCDs in the home and business markets, and opening a wide lead in digital cinema installations.

A DLP chip, known as a DMD (or digital micromirror device) is made up of thousands, and sometimes millions, of tiny mirrors aligned in an array.  Each mirror represents one pixel. Light from the lamp shines on the mirrors, and depending on whether each mirror is angled towards the lens or away from it, each pixel is turned on or off.

Color in one chip DLP projectors is usually introduced by means of a color wheel that spins in sync with the DLP chip. When a certain color, say blue, is in front of the lamp, only the corresponding mirrors on the DMD for that frame’s blues focus light through the lens. Thus, each color is processed separately.

Initially, only red, green, and blue were processed, but more recent DLP units have incorporated cyan, magenta, yellow, dark green and “white” (a clear brightness booster) into the color wheel. Because the wheel spins quickly, as much as 10 times as quickly as the frame rate, the brain is fooled into thinking that all the colors are present on the screen at the same time.

That’s the idea at least. The dreaded rainbow effect is the name given to the red, green, and blue artifacts or “shadows” that are occasionally visible, most often in high contrast situations involving fast movement, like the white letters of the credits scrolling on a black background.

The Rainbow Effect

The Rainbow Effect

Avoiding the rainbow effect in DLP projection is possible by processing color differently. A more expensive three chip DLP projector, for example, uses a prism to split the lamp’s light, and then processes each color with its own DLP chip, before recombing the light at the lens. This means that all the colors are projected to the screen simultaneously, avoiding the slight sync delays that cause the rainbow effect.

However, three chip DLP projectors are usually much more expensive, so an easier solution is to simply look for projectors with higher color wheel RPMs, which tends to reduce the problem. Also, if the projector will mainly be used to display static images, like the information used in most business meetings, the rainbow artifacts are unlikely to be a problem.

By contrast, LED DLP projectors eliminate not only the color wheel and the rainbow artifacts, but the lamp itself, by using high power light emitting diodes. This means that portable LED projectors do not require lamp replacement, which can typically cost as much as or more than the initial cost over the projector’s lifespan. They also require less power and run cooler.

Laser DLP projectors use three DMDs aligned with three color lasers to avoid use of the color wheel.

Many DLPs are air filter free, meaning there is one less part to maintain, leading some LED projector manufacturers to claim they are “maintenance free.” Some argue that this can lead to dust ruining parts other than the sealed DLP chip over time, such as the color wheel, and most important, the lamp, leading to a degradation in lumen output. Even LED projectors would be susceptible to dust buildup on their control circuitry, creating excess heat, which is never a good thing.

Advantages

  • Generally accepted to have slight advantage over LCDs in overall picture quality
  • Easier to replace light source than LCDs, with newer models (LED, Laser) requiring no replacement
  • Does not suffer from “color decay” (yellowing) common in LCDs with bulbs toward the end of their lifespan
  • DMDs are sealed chips, eliminating the possibility of dust landing on the chip
  • DLPs traditionally maintained contrast advantages over LCDs, but this gap is shrinking
  • No image persistence, which is the tendency of some LCDs to retain a ghost image if something is displayed statically for a long time. (Note: this is not the same as the permanent burn-in damage that CRTs and Plasma screen are known to suffer)
  • Less pixilation/screen door effect (especially at lower resolutions)

Disadvantages

  • Often more expensive than comparable LCD models
  • Rainbow effect bothers some viewers, especially those with sensitive eyesight, causing distraction, eye straight, and even headaches in extreme cases
  • Suffers from dithering artifacts (uneven blacks) due to the way grays are produced, which does not affect LCDs
  • Less compatibility with zoom and shift lenses, greatly limiting projector placement in large rooms