The basic working principle of liquid crystal display is to use the electric and optical effects of liquid crystals to make a display device, that is, changing the external electric field can make the arrangement of liquid crystal molecules change, and produce the modulation of external light, to achieve the purpose of converting electrical signals into optical signals. Liquid crystal projection TV display is the use of this characteristic of the numerous liquid crystal sheet (point) as a combination of light transmission switch (valve), such switches are also known as liquid crystal light valve. Since the liquid crystal sheet can change its light transmission under the action of an applied electrical signal, a pattern corresponding to the driving signal (such as a video signal) appears on the liquid crystal sheet. When the light from the light source is projected onto the screen after passing through the LCD sheet (controlled by the video signal) and the lens, the desired picture will be displayed on the screen.
The optical system of the LCD light valve projector uses only one projection lens, so a variable focal length lens is available, and the screen size adjustment and projection adjustment are easy, without the hassle of a CRT type projector. There are many types of LCD light valve projectors, and they are divided into two categories: single-chip and three-chip according to the number of LCDs used: transmission and reflection according to whether the light source’s beam passes through the LCD light valve. Usually the brightness of the three-chip type is higher than that of the single-chip type, but it is also more expensive, and the brightness of the reflective type is also higher than that of the transmissive type.
According to the image signal writing method of the liquid crystal sheet, the liquid crystal light valve type projector can be further divided into two types: electric writing and optical writing. An electrically written transmissive LCD light-valve projector is what is usually called a liquid crystal display projector, which uses the electric writing method to produce R, G, and B TV images on three liquid crystal sheets or one liquid crystal sheet, respectively, as a light valve, and under the irradiation of the corresponding three-color strong light source, the liquid crystal sheet modulates the light intensity and produces a large-screen color image through optical amplification projection onto the screen. It has the advantages of small size, simple structure, light weight and easy adjustment compared with CRT type projector.
White light from the light source through the dichroic mirror decomposition into red (R), green (G), blue (B) three primary colors of light. M1, M2, M3 are reflectors. M1 will reflect all the white light from the light source, UV/IR filter for ultraviolet / infrared filter, can filter out the interference of invisible light. The green light reflected by DM1 is then reflected by M2 through the spotting mirror and liquid crystal plate (G), and the green light modulated by the liquid crystal plate (G) is projected onto the screen through DM3, DM4 and the projection lens. the blue light reflected by DM2 passes through the spotting mirror and the liquid crystal plate (B) to form the blue light modulated by the blue liquid crystal plate. The blue image is projected onto the screen by DM3 reflection through DM4 and the projection lens. The red light modulated by the liquid crystal plate (R) is reflected by M3 and DM4 and then the red image is projected onto the screen through the projection lens. The three primary colors are combined to form a full color image.
The optical system of another LCD projection TV set consists of . The system consists of 3 CRTs, 3 LCD light valves, dichroic mirror, polarizing light prism, projection lens set, external light source (metal halide lamp, UV/IR filter, etc.) and screen, etc. This is a light-writing type LCD light valve projector. The red (R), green (G), and blue (B) image signals fed from the TV circuit are added to three CRTs, each displaying a base color grating and emitting red, green, and blue light signals, which are then projected onto the screen after passing through their respective liquid crystal sheets, by polarizing beam splitting prisms and projection lens sets, which add and mix the three colors of light. Each LCD light valve is driven by a special CRT projection tube, and the optical coupling between the two is realized by optical fiber. External light source is now commonly used metal halide gas discharge lamp (referred to as metal halide lamp) to achieve, but its high voltage up to 10kV or more, should pay attention to heat dissipation when lit. It is through the rotation of the color wheel, the white light beam emitted from the lamp through the RGB filter on the color wheel, filter out the color light, then through the lens, and finally on the screen screen to form a color image. The single-chip projector is relatively simple in structure and low in cost, but the color effect is not as good as the three-chip projector.
