Rear-projection television

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45 inch Zenith CRT rear-projection TV

Rear-projection television or RPTV is a type of large-screen television display technology. Until approximately 2005, most of the relatively affordable consumer large screen TVs up to 100 in (250 cm) used rear-projection technology. A variation is a video projector, using similar technology, which projects onto a screen.

Modern rear-projection television has been commercially available since the 1970s, but at that time could not match the image sharpness of a direct-view 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. The latest models are light enough to be wall-mounted.^[1]

However, the projection technique is much older than this. Projection systems were used in the very early 1950s when it was still impossible to manufacture CRTs with a screen size much over 12 inches. Utilising a 2 inch monochrome CRT driven at a very high accelerating voltage for the size (typically 25 kV), the tube produced an extremely bright picture which was projected via a schmitt lens and mirror assembly onto a semi translucent screen of typically 17 to 19 inches in size. The resultant picture was darker than with a direct view CRT and had to be watched in subdued lighting. The degree to which the tube was driven meant that the tube had a relatively short life. Details of a specific TV set with its optical system can be found here.

Three types of projection systems are used in projection TVs. CRT rear-projection TVs were the earliest, and while they were the first to exceed 40", they were also bulky and the picture was unclear at close range. Newer technologies include DLP (reflective micromirror chip), LCD projectors, and 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 the early 2000s 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, ProScan, 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] On June 6, 2007, Sony did unveil a 70" rear-projection SXRD model KDS-Z70XBR5 that was 40% slimmer than its predecessor and weighed 200 lbs, which was somewhat wall-mountable, however on December 27, 2007, Sony decided to exit the RPTV market.^[5][6][7] Mitsubishi began offering their LaserVue line of wall mountable rear-projection TVs in 2009.^[8]

Types of rear-projection 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 cathode ray tubes 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 and then the image is projected to a large screen. This is done to overcome the limit of size of cathode ray tube which is about 40 inches which is the maximum size a normal CRT television set (see image). Normally 3 CRTs are used, one red, one green and one blue, aligned so the colors mix correctly on the projected image.

• 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.

See also

• Silk screen effect
• Screen-door effect
• DLP
• Large screen television technology
• Laser video display (LVD)

References