Projection TV refers toPhotoelectric systemA television system that projects images from the screen of a television picture tube onto a larger special screen.The image size of projection television can be enlarged greatly as required.[1]
Projection television appeared in the early 1970s, and its sales increased after the 1990s. Over the past 20 years, its definition, brightness and contrast have improved significantly.
Projection TV is a variety ofLarge screen TVIt is the most widely used one in.From the perspective of the imaging devices of projection television, there are mainly two types of projection television: projection tube type and liquid crystal type.Projection tube type projection television is to use an optical system similar to a magnifying glass to magnify the TV image on a small high brightness CRT (cathode ray picture tube) and project it onto a special screen.This small high brightness CRT is called projection tube.Projection tube type projection television has low brightness and limited image definition.To solve this problem, useFilm projectorThe principle ofelectron beam(beam orlaser beams）It is not directly hit on the fluorescent screen, but first hit on a certain medium, so that the medium layer plays the role of film film - "photosensitive". In addition, it can use a separate light source to pass through this medium, or be reflected by this medium, and be equipped with a projection lens to obtain bright projection television images on the screen.
Until the middle of 1988, the heart device of projection television was CRT, that is, projection tube type projection television dominated the world.Although the CRT projection tube has excellent electrical and optical properties, its volume and weight are not satisfactory. In addition, the registration of three color images during installation is also complex and difficult.Therefore, manufacturers in various countries have launched LCD projection televisions.LCD projection TV has the advantages of small size, light weight and easy installation, so it is widely used in high-end home audio systems.
LCD projection TV is mainly composed of LCD light valve, optical system, light source and screen.LCD projection television belongs to the light valve type projection television. It uses the principle similar to the slide projector, uses the LCD panel as the light valve to replace the slide film, and then uses an external strong light source to project the image modulated by the TV signal on the LCD panel onto the screen.[2]
classification
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(1) According to display technology, projection television is mainly divided into three tube projection television (CRT projection technology), color LCD projection television (LCD projection technology) and DLP projection television.
(2) Projection television is generally divided into film and television projection television, data projection television and graphic projection television according to the purpose.
(3) According to the projection mode, projection television can be divided into front projection television and rear projection television.
The front projection TV is also called front projection TV. The projection TV is separated from the projection screen. The projection TV is on the same side as the audience. The projection TV projects from the front or back of the screen.
The projection part of the rear projection TV is enclosed in the chassis, so the brightness has been greatly improved, and it can be viewed in the daytime.Its appearance is similar to that of an ordinary TV set, eliminating the interference of stray light.[3]
characteristic
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Compared with ordinary TV sets, projection TV sets have the following characteristics:
1. Projection televisions with different screen sizes all use projection tubes of the same size (7 inches or 9 inches), which is conducive to forming a series of projection televisions with different screens.
2. The projection screen surface will not form specular reflection and glare.When there are electric lights, windows or other bright objects around, the picture tube surface of ordinary color TV will form images or reflect glare, making people feel very dazzling.
3. Projection TV is projected on the diffuse layer of the screen for imaging. The light is soft and not dazzling.There is no x-ray radiation on the screen.It is not easy to get tired after watching for a long time.
4. The projection tube is small in size and has no female hood.It is less affected by the geomagnetic field.
5. There is a large space in the chassis, which can be fully used to improve the sound design.
6. The projection screen has a certain light gain, about 5-8dB, to improve the image brightness.
7. Have a certain viewing angle.As the viewing angle increases, the brightness decreases.The angle of view where the brightness drops to half, usually in the horizontal direction:+35 °~+40 °, and in the vertical direction: ± 10 °.[4]
Technical indicators
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1. Light output
Light output refers to the luminous flux output by the projection television, in lumens (lm).A physical quantity related to light output is illuminance, which refers to the ratio of light energy emitted by the screen surface exposed to light to the screen area. The commonly used median of illuminance is lux (lx).
2. Scanning frequency
The number of horizontal scanning times per second of electrons on the screen is called horizontal scanning frequency. The horizontal scanning frequency of video projection television is a fixed value, which is 15625Hz（PAL system）Or 15750Hz（NTSC system）。Horizontal scanning frequency is an important index to distinguish the grade of projection television.
The number of vertical scans of electrons per second on the screen is calledVertical scanning frequency, also calledrefresh frequency, which represents the number of times the image is refreshed per second.The vertical scanning frequency is generally not less than 50Hz, otherwise the image will flicker.
3. Video bandwidth
The total bandwidth of the video channel of the projection television is defined as the corresponding upper signal frequency when the amplitude of the video signal drops to 0.707 times.Video bandwidth is also an important indicator of projection television. To improve image resolution, it is necessary to increase video bandwidth.
4. Resolution
The resolution often given in the technical specifications of projection television includes addressable resolution, RGB resolution and video resolution.
For CRT projection television, addressable resolution is an important parameter of projection tube quality index.The addressable resolution refers to the highest pixel that can be resolved by the projection tube. It is mainly determined by the focusing performance of the projection tube. The addressable resolution should be higher than the RGB resolution.
RGB resolution refers to the highest pixel that can be reached when connecting RGB video signals to the projection television.RGB resolution and horizontal scanning frequencyVertical scanning frequencyAnd video bandwidth.
Video resolution refers to the highest resolution of the projection television when displaying composite video.
5. Contrast
The greater the contrast value, the greater the brightness difference between the full black and full white of the picture, and the wider the range of gray scale performance.
6. Projection size
Picture size is also an important indicator of projection television. It is usually expressed by the diagonal length of the largest and smallest pictures that can be projected on the screen by the projector, and the unit is inches or centimeters.The projection picture size can be selected according to the living environment and audio-visual space.The projection picture size of household three tube projection television is generally 40~120in, which can be adjusted in a large range as required.[3]
