[Principle of Projector]What is the principle of projector? It has been used for conference room presentations and watching movies on large screens by connecting devices such as DVD players in the home. In movie theaters, too, digital cinema projectors, which have begun to replace older film stock, are used as screens for hard-disk digital data.
What is the principle of a projector
When it comes to the imaging principle of a projector, basically all types of projectors are the same. The projector first shines light on the image display element to generate an image, and then projects it through the lens. The image display element of the projector includes a transmissive type that generates an image using transmitted light and a reflective type that generates an image using reflected light. No matter which type, the light of the projection lamp is divided into three colors of red, green and blue, and then images of various colors are produced. Because the element itself can only display monochrome, it is necessary to use 3 elements to generate 3 color components respectively. The 3-color images are then combined into one image through a prism, and finally projected onto the screen through a lens.
Figure 1: Schematic of the projector
Using image display components to generate red, green, and blue images separately, which are then combined for projection .
There are 3 categories of image display elements (see Figure 2). Among them, there are two types of liquid crystals, namely, transmissive liquid crystal elements using light-transmitting liquid crystals and elements using reflective liquid crystals that can reflect light. The latter element is a DMD (Digital Micromirror Element), which uses a micromirror per pixel to create an image by changing the direction of reflected light.
Figure 2: Three types of image display elements
They are transmissive liquid crystal elements and reflective liquid crystal elements using liquid crystals, and mirrors are used to generate pixels the DMD. Each of the three components has pros and cons.
Figure 3: Measures taken by reflective liquid crystal elements
The reflective liquid crystal elements used in projectors generally take the following three measures: (1) The orientation film of inorganic materials is used to easily control the liquid crystal; (2) by reducing the thickness of the liquid crystal layer, the response speed is improved; (3) by eliminating the obstacle in the liquid crystal, that is, the spacer, the light utilization efficiency is improved.
Transmissive element with the same structure as the liquid crystal panel
The principle of image generation by the transmissive liquid crystal element is the same as that of the conventional computer display screen. LCD display is the same. In Japan, Seiko Epson and Sony have already begun to provide such components. Liquid crystal elements for projectors are made of high temperature polycrystalline silicon liquid crystals. Because it is different from ordinary liquid crystal displays, it is projected by magnifying the image generated by small pixels to hundreds of times, so extremely small defects will be very obvious after magnification, and require quite high precision during manufacturing.
The working principle of transmissive liquid crystal element is exactly the same as that of liquid crystal display. The direction of the liquid crystal molecules will change after power-on, and the direction of the liquid crystal molecules will adjust whether to let the light pass through, so as to display white and black.
The disadvantage is that the utilization efficiency of light is poor. This is because the transmissive liquid crystal panel is composed of multiple layers, so only about 30% of the incident light can pass through.
The size of transmissive liquid crystal elements is getting smaller and smaller. Transmissive liquid crystal elements are generally between 0.7 and 0.8 inches, but in order to control costs, the elements used in mainstream projectors are all about 0.7 inches. However, the smaller the element, the smaller the area that transmits light, and the darker the image. Therefore, in order to ensure the brightness when using small components, the projection lamp must be larger, and in order to improve the efficiency of transmitted light, the optical system will also be larger. “When using a small LCD panel, in order to ensure brightness, more light must be irradiated, so the body will be larger. If the size is about 0.9 inches, it can not only ensure sufficient brightness, but also can be designed to be smaller .”
Transmissive liquid crystal elements will degrade with prolonged use. This is because the alignment films used to adjust the direction of the liquid crystal molecules and the polarizers used to control the direction of the light are made of organic materials. Due to the high power of the projection lamp, it not only generates heat, but also the light is very strong, so it will cause chemical changes in organic materials. The degree of material degradation varies greatly depending on the lamp usage pattern and user usage.
Reflective liquid crystal elements suitable for video playback
One of the liquid crystal elements that can achieve high image quality is a reflective liquid crystal. The biggest feature is that the responsiveness, which is crucial when displaying video, is very fast, and because of the high contrast, blacks are displayed very clearly. This liquid crystal is suitable for displaying video playback such as movies.
This component has been successfully developed by three Japanese companies. JVC, Hitachi and Sony have released such components in 1997, 2001 and 2003, respectively. JVC’s component name is “D-ILA” and Sony’s component name is “SXRD”.
The reflective type liquid crystal element has higher light utilization efficiency than the transmissive type, so it is possible to manufacture a high-brightness projector. There is a layer of light-reflecting film under the liquid crystal part, which can reflect 6-7% of the light. The high contrast ratio is due to the vertical alignment of the liquid crystals when the voltage is turned off. This way is called vertical orientation. Since it is displayed in black when no pressure is applied, it is possible to express black more clearly. The advantages of reflective liquid crystal elements are easier to understand when displaying dark images. When displaying black clothes and hair on a pitch-black screen, it can be displayed without being affected by the background.
The high response speed of reflective liquid crystal elements for projectors is due to certain measures taken in the liquid crystal part (see Figure 3). By reducing the liquid crystal layer to 2 μm or less, the response speed is improved. Generally speaking, in order to ensure a uniform thinness of a liquid crystal panel, an auxiliary material called a spacer is added to the liquid crystal. The thickness of this spacer is the thickness of the liquid crystal layer. However, JVC’s D-ILA and Sony’s SXRD achieved a thickness of 2 μm without the use of spacers by making efforts in manufacturing methods and packaging materials. “By eliminating the spacer, the problem of the spacer appearing in the pixel display part is solved. The thickness of the liquid crystal cell is ensured by using the encapsulation material.
How to use a lens for reflection
Each pixel has a micromirror that reflects light.
Some projectors also use micromirror elements. This is the DMD developed by Texas Instruments. Since the DMD patent belongs to the company , so only this company produces and supplies. Projectors using DMD are called DLP (Digital Light Processing) projectors.
Each pixel of DMD is a mirror, which is arranged on a semiconductor backplane As many micromirrors as there are pixels. Micromirrors are only 14μm long. The DMDs that use the most micromirrors are about 800,000 pixel models. One by one through about 800,000 micromirrors on a 0.7″ (diagonal length) base plate Action to display the image.
Each micromirror is tilted left and right with the diagonal direction as the axis. Use electrostatic attraction to move the micromirror. The micromirror itself applies 20V voltage, and 5V is applied under one end of the diagonal line. After the other one is applied with 0V, due to the large potential difference at the 0V end, the micromirror will shift to this side.
Figure 4: DMD structure (left) , and the principle of image generation with DMD (right)
Using the angle of the micromirror to change the direction of the reflected light. When displaying white, set the angle of the reflected light toward the lens. When displaying black, the light is absorbed by the absorbing plate.
By tilting the direction of the DMD to change the light reflection angle to achieve white and black (Figure 4 right). When the micromirror is tilted 10 degrees to a certain direction, the light will be reflected to the lens direction by adjusting the direction, When tilted 10 degrees in the opposite direction, the light will be reflected on the light-absorbing plate. This way, the light will appear white when reflected toward the lens and black when reflected toward the light-absorbing plate. Midtones are achieved by switching white and black repeatedly in a very short period of time To achieve.
Compared with liquid crystal elements, DMD pixels have higher image display performance. First, the contrast ratio is high. The contrast ratio can reach up to 3000:1. In addition, the response speed to the signal is fast. Response speed About 15 microseconds, which is almost 1000 times that of liquid crystal. The faster the response speed, the smoother the video image can be displayed. Moreover, the light utilization efficiency of DMD is better. Since the pixels are composed of micromirrors, the irradiated light is 90% will be reflected. However, while the performance is high, the average price per pixel is also high.
Figure 5: Projector types and uses
Includes Category 4. Single-panel DLP projectors and LCD projectors using 3 transmissive liquid crystal elements are popular products for presentations and home theaters. LCD projectors and 3-panel DLP projectors using reflective LCD are for Cinema digital projectors and halls and High-priced products for various mass events.
Figure 6: Structure of a single-panel DLP projector
A single-panel DLP projector using only one DMD filters colors through high-speed rotation The device irradiates red, green and blue light to the DMD in sequence. The DMD continuously displays images of various colors, which are then projected through the lens. Produced according to the public information of Texas Instruments in Japan.
Single-board projectors suitable for miniaturization
Single-board projectors suitable for miniaturization
Components used in projectors There are 3 categories, and the products that actually use these components are divided into the following 4 categories:
(1) Single-board DLP projectors that use only 1 DMD;
(2 ) LCD projector using 3 transmissive LCDs;
(3) 3 panel DLP projector using 3 DMDs and
(4) 3 reflective LCDs components of LCD projectors. From the perspective of the principle of projectors that display red, green, and blue images, they are basically 3-plate type. However, only one component with high image display performance like DMD can constitute a projector. The high price of a single DMD is also one of the reasons for the single-board design. Except for some large-scale products, DLP projectors using DMD are basically single-board type (see Figure 5).
Single-board DLP projectors do not pre-separate the light, but switch the three colors in sequence through a color filter consisting of red, green, and blue three colors of light (see Figure 6). ). The color filter rotates 60 to 180 times per second. The light passing through the color filter hits the DMD. The DMD continuously displays three-color images at high speed, displaying images with red components when irradiated with red light, and images with green components when irradiated with green light. The three-color image reflected by the DMD is projected through the lens.
Single-board DLP projectors are suitable for video display fields such as home theater due to their high contrast ratio and fast response speed. And the optical system does not need to be too large, so the design is small, light, and portable, so it is also suitable for carrying with a computer (Photo 1).
However, it has also been pointed out that the use of color filters in a single-panel DLP projector to display three-color images in succession has corresponding disadvantages. This is the “rainbow phenomenon” that causes color separation due to high-speed image display. Some people pointed out that the color separation can “feel the eyes dazzling”. Products for home theaters mitigate this color separation by increasing the rotation speed of the color filter, dividing the color filter into six, or adding white in addition to three colors.
Photo 1: Small DLP Projector
The DLP Projector “V-1100” from PLUS Vision Japan. It weighs about 1.0kg and measures 180mm wide, 45mm high, and 141mm long. It can be held in one hand.
Special lens for light splitting
Projectors that use the basic structure of the projector, that is, the 3-plate type, include the following three categories: (1) The popular type is the use of transparent (2) High-priced products using DMD and high-priced products using reflective liquid crystal elements.
The structure of this type of product is described below by taking a liquid crystal projector as an example (Figure 7). First, the harmful ultraviolet rays and infrared rays that affect the temperature are removed from the light emitted by the projection lamp. The light is then separated into three colors: red, green, and blue according to wavelength. A special lens called a “dichroic mirror” is used to separate the light. Dichroic mirrors have the property of allowing only certain wavelengths of light to pass and reflecting other light, or reflecting only certain light and allowing the remaining light to pass. Red is separated first, then green is separated, and finally blue light is left. Some products are separated in the order of blue, green and red. A 3-color image is generated by the image element, and these lights are combined using a prism. To create a natural color, the light is combined in a ratio of red 3, green 6, and blue 1.
Figure 7: Structure of a liquid crystal projector
The light emitted by the projection lamp is first divided into ultraviolet and infrared. It is then divided into three types of light, red, green and blue, using a special lens called a dichroic mirror. After the three-color image is generated, it is combined with a prism and projected.
Figure 8: Keystone Compensation Technique
Reduces the pixels on the part of the component that cause keystone distortion. Resolution will drop as a result.
Different requirements for projectors in various occasions
Different requirements for conference room use and home use
Projectors have different characteristics depending on the type of components and the number of components used. However, the actual product is adjusted according to the application in addition to the components.
The purpose of the projector is roughly divided into 3 aspects. The first is for presentation purposes in conference rooms and other places, the second is for home theater purposes for watching movies at home, and the third is for digital projectors for digital movies in cinemas and other occasions.
Small products of around 120,000 to 600,000 yen have been widely used in presentations and home theaters, which have the largest number of products. In terms of sales of such products, about 80% of them use transmissive liquid crystal elements, and about 20% of them use DMD. In conference rooms and various events, the cinema uses DLP projectors with 3 DMDs and projectors with reflective liquid crystal elements.
Whether to pay attention to brightness, whether to pay attention to contrast and color performance, it varies for different purposes. Presentation projectors are mainly envisaged for use in conference rooms. In the conference room, in order to take notes, the surrounding environment must reach a certain brightness. Therefore, there are brightness requirements for the projector. If the brightness reaches between 1500ANSI lumens and 2000ANSI lumens, even if the surrounding light is bright, normal projection can be performed. Moreover, because of the need to display small text such as tables, the display also has a higher resolution. Recently, most of them have reached the same XGA (1024 × 768 pixels) specifications as computer monitors.
On the other hand, in home theater, the brightness requirements are lower because the room brightness can be reduced. Instead, it is a matter of whether the contrast can express blacks more deeply and fully express the reds that are very important for expressing skin. In order to improve the effect of color expression, “by improving the manufacturing process of the beam splitter, the red and green effects can be fully expressed” (Sanyo Electric Consumer Enterprise Group AV Solutions Co., Ltd. Projector Business Division Product Planning Section Chief Sugimura) . In addition, high-priced projectors using reflective liquid crystal elements have begun to use Canon projector lamps with light wave distribution close to natural light and better color performance.
Fixing the displayed image by increasing the pixel spacing
For projectors, although image quality is as important as other display devices, there are also Problems specific to projectors due to the “projection” method. One is that the projection angle must be adjusted, and the other is that there must be an unobstructed space between the screen and the projector.
The projector will produce a keystone distortion of the image due to the incorrect projection angle (see Figure 8 above). When the room is small, it is sometimes necessary to stagger the projectors horizontally, or to project diagonally from below. If it is projected horizontally, it will produce a trapezoid that is widened on the left and right, and when projected from the oblique bottom, it will form a trapezoid that is wide at the top and narrow at the bottom.
This keystone correction technology is now mature and used in most products (see Figure 8 below). This technique is called “Keystone Correction”.
The aspect ratio of the screen projected by the projector is 4:3 as an example. The trapezoid, which is wide at the top and narrow at the bottom, corrects the picture into a 4:3 rectangle according to the bottom line. To achieve this, the image generated by the component is changed. That is, increase the pixel spacing of the part of the element that would widen when projected directly. That is, by adjusting the display on the element to be trapezoidal, the projected image is displayed as a rectangle. However, the resolution of the part where the pixel spacing is increased will be reduced.
The requirements for projectors vary from place to place
And calibration technology is constantly improving. Some products can not only correct up and down and left and right widening, but also can detect the body angle through the tilt sensor during projection, and automatically correct the image distortion.
A unique way is the correction technology developed by NEC ViewTechnology. Use the included remote control to designate the four corners of the display screen, and press the setting button to correct it into a square formed by the designated four points. The company implements this correction technology using a self-developed chip.
In addition, research and development has been carried out to shorten the distance between the screen and the projector. Because in order not to project the obstacles in the middle, the shorter the projection distance, the better. Recently, the demand for products with shorter focal lengths is increasing. Short-throw lenses are currently mainly used in high-resolution, high-priced models. However, because the lens was expensive, it was initially only used in high-performance, high-priced products. Recently, as the price of the lens has been gradually reduced, it has been gradually applied to small products two years ago.
In addition, we are also developing products that achieve short focal lengths without using lenses. This is the DLP projector “WT600” developed by NEC ViewTechnology, which uses the reflected light from the lens to adjust the angle. The light is reflected on 4 aspherical lenses in sequence for projection. Since the projector can be placed between the presenter and the screen, no human figures are projected. “The biggest difficulty in production is to increase the brightness. At present, 1200ANSI lumens have been achieved by improving the color filter.”
The DLP projector “WT600” launched by NEC ViewTechnology. The projection distance of the 60-inch (0.9×1.2m) screen is 26cm.
