Rear projection is a type of large-screen television display technology. Most very large screen TVs (to 100 inches [254 cm] or more) use rear projection. A variation is a video projector, using similar technology, which projects onto a screen.
Rear projection television has been commercially available since the 1970s, but at that time could not match the image sharpness of the CRT. Current models are vastly improved, and offer a cost-effective HDTV large-screen display. While still thicker than LCD and plasma flat panels, modern rear projection TVs have a smaller footprint than their predecessors and are light enough to be wall-mounted.[1]
Three types of projection systems are used in projection TVs. CRT projectors were the earliest, and while they were the first televisions to exceed 40", they were also bulky and the picture was unclear at close range. Newer technologies include DLP (reflective micromirror chip) and LCD projectors. A type of LCD projection technology, LCoS, has been capable of 1080p resolution, and examples include Sony's SXRD (Silicon X-tal Reflective Display), JVC's D-ILA (Digital Direct Drive Image Light Amplifier), and MicroDisplay Corporation's Liquid Fidelity.
While popular in 2005 and 2006 as an alternative to more expensive LCD and plasma flat panels, the falling price and improvements to LCDs have led to Sony, Philips, Toshiba and Hitachi planning to drop rear projection TVs from their lineup.[2][3] Currently, Samsung, Mitsubishi, RCA, Panasonic and JVC remain in the market. The bulk of earlier rear-projection TVs meant that they cannot be wall-mounted, and while most consumers of flat-panels do not hang up their sets, the ability to do so is considered a key selling point.[4] In the 1Q of 2008 a comparison of worldwide TV sales breaks down to 22.1 million for CRT, 21.1 million for LCD, 2.8 million for Plasma, and 124 thousand for rear-projection. [1]
Comparison of different technologies
A projection television uses a projector to create a small image from a video signal and magnify this image onto a viewable screen. The projector uses a bright beam of light and a lens system to project the image to a much larger size. A front-projection television uses a projector that is separate from the screen, and the projector is placed in front of the screen. The setup of a rear-projection television is in some ways similar to that of a traditional television. The projector is contained inside the television box and projects the image from behind the screen.
The following are different types of projection televisions, which differ based on the type of projector and how the image (before projection) is created:
- CRT projector: Small CRT's create the image in the same manner that a traditional CRT television does, which is by firing a beam of electrons onto a phosphor-coated screen. The CRT's can be arranged in various ways. One arrangement is to use one tube and three phosphor (red, green, blue) coatings. Alternatively, one black-and-white tube can be used with a spinning color wheel. A third option is to use three CRT's, one for red, green, and blue.
- LCD projector: A lamp transmits light through a small LCD chip made up of individual pixels to create an image. The LCD projector uses mirrors to take the light and create three separate red, green, and blue beams, which are then passed through three separate LCD panels. The liquid crystals are manipulated using electric current to control the amount of light passing through. The lens system takes the three color beams and projects the image.
- Digital Light Processing (DLP) Projector: A DLP projector creates an image using a digital micromirror device (DMD chip), which on its surface contains a large matrix of microscopic mirrors, each corresponding to one pixel in an image. Each mirror can be rotated to reflect light such that the pixel appears bright, or the mirror can be rotated to direct light elsewhere and make the pixel appear dark. The mirror is made of aluminum and is rotated on an axle hinge. There are electrodes on both sides of the hinge controlling the rotation of the mirror using electrostatic attraction. The electrodes are connected to an SRAM cell located under each pixel, and charges from the SRAM cell drive the movement of the mirrors. Color is added to the image-creation process either through a spinning color wheel (used with a single-chip projector) or a three-chip (red, green, blue) projector. The color wheel is placed between the lamp light source and the DMD chip such that the light passing through is colored and then reflected off a mirror to determine the level of darkness. A color wheel consists of a red, green, and blue sector, as well as a fourth sector to either control brightness or include a fourth color. This spinning color wheel in the single-chip arrangement can be replaced by red, green, and blue light-emitting diodes (LED). The three-chip projector uses a prism to split up the light into three beams (red, green, blue), each directed towards its own DMD chip. The outputs of the three DMD chips are recombined and then projected.
CRT projector
Advantages
- Not restricted to fixed pixel resolutions, able to display varying resolutions
- Achieves excellent black level and contrast ratio
- Achieves excellent color reproduction
- CRTs have generally very long lifetimes
- Better viewing angles than those of LCD display
Disadvantages
- Heavy and large, especially depth-wise
- If one CRT fails the other two should be replaced as well to maintain color and brightness balance
- Susceptible to burn-in because CRT is phosphor-based
- Needs to be 'converged' about every year
- Has focus problems
LCD projector
Advantages
- Smaller than CRT projectors
- LCD chip can be easily repaired or replaced
- Is not susceptible to burn-in
Disadvantages
- The Screen-door effect: Individual pixels may be visible on the large screen, giving the appearance that the viewer is looking through a screen door.
- Possibility of defective pixels
- Poor black level: Some light passes through even when liquid crystals completely untwist, so the best black color that can be achieved is a very dark gray, resulting in worse contrast ratios and detail in the image. Some newer models use an adjustable iris to help offset this.
- Not as slim as DLP projection television
- Uses lamps for light, lamps may need to be replaced
- Fixed number of pixels, other resolutions need to be scaled to fit this
- Limited viewing angles
DLP projector
Advantages
- Slimmest of all types of projection televisions
- Achieves excellent black level and contrast ratio
- DMD chip can be easily repaired or replaced
- Is not susceptible to burn-in
- Better viewing angles than those of CRT projectors
- Image brightness only decreases due to the age of the lamp
- defective pixels are rare
- Does not experience the screen-door effect
Disadvantages
- Uses lamps for light, lamps need to be replaced on average once every year and a half to two years
- Fixed number of pixels, other resolutions need to be scaled to fit this
- The Rainbow Effect: This is an unwanted visual artifact that is described as flashes of colored light seen when the viewer looks across the display from one side to the other. This artifact is unique to single-chip DLP projectors.
Comparison
Advantages
- Significantly cheaper than flat-panel counterparts
- Front-projection picture quality approaches that of movie theater
- Front-projection takes up very little space because a projector screen is extremely slim, and even a suitably-prepared wall can be used
- Display size can be extremely large, up to hundreds of inches
- Projectors that are not phosphor-based (LCD/DLP) are not susceptible to burn-in
Disadvantages
- Front-projection more difficult to set up because projector is separate and must be placed in front of the screen, typically on the ceiling
- Rear-projection televisions are much bulkier than flat-panel televisions
- Lamp may need to be replaced after heavy usage
- Rear-projection has smaller viewing angles than those of flat-panel displays
- Rear-projection is susceptible to glare
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