Is there any movie buff reading this who would like to set up a home theater? There are many familiar items found in any home theater, and the projector is probably the most familiar of them all. New technology has been continuously developed to improve the resolution and brightness and viewing experience of home theater video equipment. Nowadays all color display devices, including projectors, are based on how the human visual system perceives colors, and as it turns out, with just the three primary colors (red, green, and blue – RGB) it is possible to create all colors that the human eye can perceive, by mixing the appropriate amounts of the three primary colors.
When color projectors were first introduced in the 1950s the technology for projecting images went through several generations of improvements and developments. The first projector using color images were based on CRTs. Later in the 1980s and 1990s LCD and DLP as well as LCoS technologies emerged. For many years now the consumer market is mainly serviced by LCD and DLP projectors. In terms of the technology used for projecting images both LCD as well as DLP projectors are in a form of presentation equipment. They can be classified into transmissive and reflective projection technology. While transmissive technology, represented by LCD projectors, is most mature, the reflective technology, represented by DLP projectors, is continuously developing. Both technologies, for many years now, are continuously being improved and in terms of their characteristics and advantages are being different.
LCD Projection Technology
LCD technology was first reported in 1968. The development of LCD panels for LCD projectors is mainly based on the number of LCD panels, that is to say, single-panel LCD projectors and three-panel LCD projectors. All of the early LCD projectors were single-panel LCD projectors. A single-panel LCD projects images of red, green and blue color separately on one single LCD panel and then combines them into one image and projects it onto a screen. This type of single-panel LCD projector is of low cost, but images and color have very poor quality and are not saturated, and images are very washed out. However, due to improvements in recent years of light sources and LCD panels, single-panel LCD projectors can reach up to 800CVIA of brightness, bringing very good images even under daytime conditions. The basic working principle of single-panel LCD projector is to take away the backlight unit from the LCD panel, using a high-power light source for backlight, through a condenser lens to illuminate the images on the surface of the transparent LCD panel. The images then go outward through a focusing lens and a projection lens to project onto a screen. The working principle of a single-panel LCD projector is similar to that of ordinary overhead projector, which are used in classrooms.
For three-panel type LCD projectors, the light first goes through an infrared/ultraviolet cut filter to remove light such as invisible infrared and ultraviolet rays that can damage the LCD panels. Subsequently, the light goes through two multi-element lenses to split and uniform the light, and to correct the conical distribution of light from the UHP lamp into a rectangular shape similar to that of the projected image. In the two-mirror system, a prism located between the two mirrors pre-polarizes the light. This has the advantage of reducing light loss as compared with an asymmetric light tunnel without a prism. Then, the light is split by dichroic mirrors into three primary colors of red, green, and blue and is reflected onto the corresponding panels. The light that has traveled through the above-mentioned stages goes through a condenser lens and a polarizer prior to incidence on the panels. The condenser lens collimates the light, and the polarizer polarizes the light so that the vibration direction becomes uniform and can be controlled by the panels. Subsequently, the light goes through the corresponding panels under the control of the driver circuit. The pixels on each of the panels open and close in a synchronised manner to form images. Furthermore, by adjusting the amount of light for each of the primary colors, rich colors can be produced. The three beams of light are then combined and pass through a lens to form the projected image on a screen.
DLP Projection Technology
DLP stands for “Digital Light Processing”. It is a display technology, for projectors and also for a range of different Rear-Projection Television sets. The DLP technology was developed by Texas Instruments and this US company still is the main supplier of DLP chips. In special fields there also are digital light processors from the Fraunhofer Institute of Dresden in Germany. In Spatial Light Modulators (SLM) they modify spatial light, i.e. light that is spread out over space. Micronic Laser Systems from Sweden for example uses Spatial Light Modulators from the Fraunhofer Institute in its machines for Sigma plate-making as well as for silicon-template engraving. These deep-ultraviolet images are essential for producing very small structures.
In DLP projectors, the image is generated by a DMD (Digital Micromirror Device) chip. The DMD consists of an array of microscopic mirrors (precision, miniature reflective mirrors) arranged on a semiconductor chip, with each micromirror controlling one pixel in the projected image. The number of micromirrors corresponds to the resolution of the projected image; common DMD sizes include 800×600, 1024×768, 1280×720, and 1920×1080 (HDTV). These micromirrors can rapidly change their angles under digital drive signals. Upon receiving the corresponding signal, each micromirror tilts by ±10°, changing the direction of reflected incident light. A micromirror in the projection state is considered “ON” and tilts +10°; if it is in the non projection state, it is “OFF” and tilts −10°. Meanwhile, the incident light reflected in the “ON” state passes through the projection lens to form the image on the screen, while light reflected in the “OFF” state is absorbed by a light absorber.
In DLP projectors a single mirror has two positions or two angles, ON and OFF. The frequency between these two positions is variable. As a result, the light that is reflected by the DMD and which is projected onto the screen, can generate many grayscales between black (OFF) and white (ON). Two methods are used to create a color image with DLP projectors, one is used in single chip DLP projectors, and the other in three chip DLP projectors.
A single-chip DLP projector uses just one DMD chip in it. There are two typical types of light sources for a single-chip DLP projector. One type of light source is a white light, such as that provided by a metal halide lamp or high-pressure mercury lamp. The other type of light source is provided by a number of new solid-state sources, or what are referred to as new light sources, such as for example LEDs, which provide separate RGB light. In the case of a single-chip DLP projector which utilizes a white light, such as for example provided by a metal halide lamp or high-pressure mercury lamp, the projector produces the three RGB colors by means of a color wheel, which is located between the light source and the DMD. The color wheel has four segments, which are red, green, blue, and a transparent section. In order to increase the brightness of a color wheel, the transparent section is often provided. This section, however, reduces the color saturation, and, as a result, this section is often disabled in the projector, or it is simply not provided in some types of projectors. As the DMD chip is synchronized with the color wheel, i.e. the DMD chip shows a part of an image corresponding to a part of the color wheel, which is in front of the light source, when the blue section of the color wheel is in front of the light source, the DMD displays the blue portion of the image, and similarly for the red and the green portions of the image. In this way, the red, green, and blue portions of the image are sequentially projected as a very fast succession, so that the viewer perceives the entire, full-color image.
A three chip DLP projector contains three DMD chips inside. The light from the source is split by a prism into three paths, which are filtered into red, green, and blue beams and then directed to the corresponding DMD chips. Finally, the three modulated light beams are recombined by a prism into a single beam and projected through the lens onto the screen. Three chip DLP systems can display up to 35 trillion colors, whereas single chip DLP systems can display only 16.7 million colors. Most DLP projectors on the market today are single chip designs; three chip DLP systems can achieve very high image quality or very high brightness, but at a much higher cost.
Some Comparisons Between LCD and DLP
Color Capability
LCD projectors are characterized by good color saturation and rich color gradation, but text edges often show shadows and fuzziness. When displaying dynamic video, LCD projectors produce clearer and more vivid color images, while DLP projectors suffer a slight loss in color saturation and less vivid color reproduction.
Contrast
DLP projectors are known for their high contrast, excellent black & white pictures with lots of shadows, and also excellent dark scene performance. In terms of black & white text rendering, the blacks are deep and strong, and the text is very sharp. This is quite different to LCD which has certain limitations in regards to expressing strong contrast and dark scenes due to the technology.
Color Separation Ability
LCD projectors use three separate LCD panels to process the three primary colors (red, green, blue) and thus they can be controlled independently for brightness and contrast. This allows all three color light beams to reach the screen almost at the same time and to project the colors very accurately. In contrast, single-chip DLP projectors use a single color wheel as splitter for the colors and the image is modulated by the same micromirror array for all three colors. Because of the limited rotational speed of the color wheel and the switching time of the individual micromirrors, DLP projectors have a worse color reproduction than LCD projectors. In dynamic video, high-speed modulation of all colors is required for sufficient refresh rate of the image on screen. In this area, LCD projectors clearly have the upper hand over DLP projectors.
Pixel Capability
The transistor of every pixel on the LCD panel (the light switch) does not transmit any light. Furthermore, there are spaces between the pixels. On the other hand, the control transistors of the DLP projector are located at the back of the micromirrors. Thus, they do not interfere with the light. Moreover, the gaps between the micromirrors can be made very small.
Focusing DLP efforts on Engineering Applications may allow DLP to better capitalize on its strengths.
For Home Consumer use LCD is generally recommended. For the professional user or for the specific application of Engineering DLP has distinct advantages. Firstly, display quality, the micromirrors on DLP pixels are packed much closer together than their LCD counterparts, this results in a higher fill factor of the pixels, in turn this leads to a smoother image with greater native resolution and less of the “screen door effect” that is prevalent with LCDs. Secondly, the DLP design is sealed; this seal is further enhanced by the cooling system for the DMDs, which are situated behind the imaging area and therefore do not affect the optical path. This makes DLPs far less sensitive to dust, than their LCD counterparts and means that there is no need for filters to be cleaned or replaced on a regular basis. Thirdly, long-term image persistence, the light sensitive panels of LCDs degrade over time, leading to reduced brightness and poor color and brightness uniformity. The micromechanical nature of DLPs does not suffer from these problems and they are able to provide a uniform, color accurate image with consistent light level over long periods of time.
Conclusion
LCD and DLP have their own strengths and weaknesses. LCD projectors produce the highest color saturation and smoothest motion for home cinema use. However, LCD projectors are very sensitive to dust, have a screen-door effect and deteriorate over time. DLP projectors, on the other hand, have the highest contrast and the blackest blacks. Because the DLP projector’s optics are sealed, it is also more dust resistant than LCD. The single-chip DLP projector may have slightly less color saturation than its counterparts, but it is more reliable and has better contrast. As a result, the single-chip DLP is ideal for professional use in professional environments or for users who intend to use their projector heavily. Both technologies are constantly improving thanks to the introduction of new light sources.
8 Frequently Asked Questions
LCD = Vivid Colors and Smooth Motion | DLP = Deep Blacks, high contrast, and some minor rainbow.
Screen-door effect is visible grid on a screen of an LCD panel. This effect is not visible on a DLP projection system because individual mirrors are packed almost without any gaps.
With singlechip DLP, a color wheel is used to switch through the 3 colors in sequence. As a result, DLPs are limited to roughly 16.7 million colors and, occasionally, you may see a rainbow. Threepanel LCDs process the 3 RGB colors simultaneously. They, therefore, have much higher saturation than DLPs.
Three chip DLP projectors are capable of displaying 35 trillion colors and provide excellent brightness and projection quality. Three chip DLP technology is very expensive, and therefore it is mostly used in professional cinemas and not in the home market.
DLP projector has a sealed DMD chip, which is not affected by dust and maintains the image quality over years without getting yellow or dimmed for long. It saves a lot of maintenance.
Yes. Dust can create dark spots by entering into the optical path. The filter of an LCD needs to be cleaned or replaced regularly. DLP projectors are sealed up and don’t have this kind of maintenance.
Yes. 16.7 million colors as used by single-chip DLP projectors equals color depth of typical TVs and computer monitors. Humans only very occasionally will notice the difference in viewing conditions typical for human observers.
DLP. The technology in DLP projectors for full black outs is superior to that of LCD. As a result of the above, for movies there is much more contrast with DLP than with an LCD projector. This means that there will be better dark areas with less light leaking from the backlight of the projector.
