|Saturday, 17 February 2007|
Projector devices generally display information by illuminating an image with high intensity light to present the image on a screen. There are several different types of technologies utilized by the light engines of today's projectors: cathode ray tube (CRT), liquid crystal display (LCD), digital light processing (DLP) and liquid crystal on silicon (LCOS). Each of these image output devices will display or print the image at a spatial resolution defined by the physical parameters and limitations of the equipment. Digital mirror device (DMD) projectors, also known as digital light processor (DLP) projectors, direct high intensity light at an image generator having a large number of miniature mirrors that selectively direct the light at a screen to form an image. The cathode ray tube (CRT) method has been employed for many years as an apparatus which projects and displays on an enlarged scale an image, such as television receivers and computer monitors. The CRT projector comprises a CRT assembly projecting an image beam, a mirror reflecting the projected beam, and a screen on which the reflected beam is displayed as a picture. The liquid crystal display (LCD) panel is known as one providing an excellent display of a quality comparable to that provided by a cathode ray tube. Liquid crystal panels are used extensively in compact, portable information processing devices such as laptop computers, as a display device. The liquid crystal projector, as compared with the CRT projector, has many advantages such that a range of color reproduction is wide, miniaturization and lightening of weight are easy, and adjustment of convergence is not needed. LCD display panel is particularly advantageous compared to the CRT in that it can be made thinner than the CRT, since it can provide a color display merely by the addition of a color filter for each image pixel and by using an optical means such as an illuminating light source. Liquid crystal projector has excellent color reproducibility, is small and lightweight, and has low power consumption. On a CRT display, light is produced by three primary phosphors, red, green and blue (RGB), which are excited separately by the electron beam in the CRT. The resolution of the CRT will vary with the higher cost display screens providing the greatest resolution. CRT displays typically apply chroma feedback to control electrical drive signals and thus manage the quality of images generated by the impact of electrons on the phosphor screen. Obtaining high resolution images from CRT displays is difficult and often involves the use of bulky devices with relatively high power consumption. Liquid crystal display panels have a number of advantages that make them useful in optical projectors, especially projectors for displaying computer generated images and portable projectors. LCD projectors have been of wide interest because of their compactness and ease of installation.
Liquid crystal panels that modulate light in accordance with supplied image information are widely utilized as direct view display apparatuses and as light valves (light modulators) of projection display apparatuses. A liquid crystal projector uses a liquid crystal panel which modulates light in response to a video signal in order to obtain a project image screen of a large size. A liquid crystal based display device may include an illumination source, a plurality of liquid crystal display panels for modulating red (R), green (G), and blue (B) lights respectively, and an optical system such as a projection lens for magnifying and projecting thus generated color image. The light source includes a lamp, optics and logic for manipulating the light in order to generate the pixels and color. A liquid crystal projector has a plurality of dichroic mirrors that selectively reflect/transmit the light emitted from a light source according to wavelength in the illumination path of such a projector for changing the direction of an illumination beam created by the illumination source, to make the display device more compact. The liquid crystal projector generally uses a backlit or reflective LCD. The reflective liquid crystal projector is mainly constituted of a light source, a color separating optical element, polarizing beam splitter prisms (PBS prisms), three liquid crystal elements, a recombination prism, and a projection lens. Most digital projectors include a video decoder and a light engine. The video decoder converts video data received by the projector, e.g., from the display connection of a personal computer (PC), into pixel and color data. An image projection apparatus isprovided with an adjuster for adjusting the degree of elevation of the apparatus in order to adjust the height of the projected image. The adjuster is usually operated manually. Image projectors are often provided with a cooling system that takes in cooling air from the exterior of the casing to cool the components such as light source, the supply circuit, and the lamp drive circuit which are heat sources that generate heat in operation. The cooling system is classified into an optical-system cooling system, a light-source cooling system, a power-supply and lamp-drive-circuit cooling system.
In a LCD projector, light from a light source is condensed by a focusing lens and projected on an LCD panel, a video image is produced by a video drive image being applied to the LCD panel, and the video signal is displayed on a screen through a projection lens. An LCD projector which magnifies and projects an image on a liquid crystal display (LCD) panel by using such a light source as a metal halide lamp has so far been in practical use. There are two types of liquid crystal projector: one uses reflective liquid crystal elements, and the other uses transmissive liquid crystal elements. The liquid crystal elements modulate and reflect the three color illumination beams respectively, to produce three color optical images. White light from the light source is separated into a blue illumination beam, a red illumination beam and a green illumination beam. The three color optical images are combined together through the recombination prism, and the combined full-color image is projected through the projection lens onto a screen. A liquid crystal projector generally includes a plurality of liquid crystal panels for modulating red (R), green (G), and blue (B) lights respectively, a dichroic prism for compositing thus modulated R, G, and B lights to generate a color image, and an optical system such as a projection lens for magnifying and projecting thus generated color image. The video decoder converts video data received by the projector, e.g., from the display connection of a personal computer (PC), into pixel and color data. The pixel and color data is then supplied to the light engine, which converts that data into the actual projected image.
Liquid crystal projectors are classified into a three-plate type and a single plate type according to the number of liquid crystal panels to be used. The single-panel type comprises a single liquid crystal display device having a color separating means for red, green and blue such as a dichroic mirror. A single panel type LCD projector generally comprises a projection lens, a field lens, a liquid crystal material, an incoming-side polarizer and an outgoing-side polarizer, the incoming-side polarize cutting off half of the light. The single panel projector has a simple structure and is cheap and small. A three-panel type liquid crystal projector uses three liquid crystal panels for red, green and blue images. Pictures corresponding to the three primary colors of red, green, and blue are displayed on the three black and white display liquid crystal panels, and the liquid crystal panels are illuminated with beams of light of the three primary colors corresponding to the pictures. A three panel type liquid crystal projector polarizes RGB light emitted from a light source such as a metal halide lamp or a halogen lamp into a predetermined polarized wavefront with a polarizing splitter block and then decomposes the RGB light into, R, G and with the appropriate B colors with optical elements. The three-plate liquid crystal projector is superior in display properties (resolution, screen illumination, color purity). The single liquid crystal panel is provided with three liquid crystal cells respectively corresponding to three RGB colors. Accordingly, an efficiency of using a light beam is small, the bright image is hardly displayed. However, since the liquid crystal panels and optical parts for three systems are required, the optical system becomes complicated and miniaturization is difficult, and the cost becomes very high. In recent years single-layer type liquid crystal projectors with excellent display quality has begun to be commercially available. In recent years, the reason is that the demand for an active matrix type liquid crystal display device (liquid crystal panel) has been increased. An active-matrix liquid crystal display using a thin film transistor as a switching element for driving each liquid crystal pixel can selectively drive each TFT to select a particular liquid crystal pixel. The active matrix type liquid crystal panel is structured such that a TFT is disposed for each of several tens to several millions pixel regions disposed in matrix.
Liquid crystal light valves have been used in both reflection-type and transmission-type projection display. Twisted nematic (TN) liquid crystal cells have been widely used in transmission-mode active matrix liquid crystal displays of high quality. Twisted nematic liquid crystal and ferroelectric liquid crystal are well known as liquid crystals used in display devices. A twisted nematic liquid crystal display elements comprises a pair of transparent substrates which have transparent electrode film coats and between which a liquid crystal is filled. TN liquid crysta achieves higher response speed and contrast in comparison with STN (super twisted nematic) liquid crystal used mainly in conventional panels, so that images of higher quality can be displayed. The TN type liquid crystal device has a liquid crystal layer between a pair of substrates.