The requirement on larger image size is the major focus for projector. To achieve large image size, a variety of projectors have been developed. According to optical engines, projectors can be classified into DLP, 3LCD, and LCoS systems.
1.2.1 DLP Type Projector
DLP (Digital Light Processing™) [1][2][3] technology was developed by TI (Texas Instruments) and is based on DMD (Digital Micromirror Device), as shown in Fig. 1-3. DMD is a switch component, which controls the intensity and placement of projected light via thousands of mirrors.
Fig. 1-3 DLP core technology---DMD by TI.
In DMD unit, micromirrors which are fixed on the DLP board can tilt either toward or away from light source, as shown in Fig. 1-4. When the micromirrors tilt toward light source (the switch is on state), light is reflected on the projection lens by the micromirrors. Then the bright pixel is yielded (as the micromirror “a”). When the micromirrors tilt the other way (the switch is off state), light is reflected on the light absorber component, and the dark pixel is yielded (as the micromirros “b”). A gray level pixel is yielded by controlling the duty of between micromirror “a” and micromirror “b” states.
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Projection lens
Lamp
DMD array Light absorb
b a
Light absorber
Fig. 1-4 DMD mechanism. The micromirror “a” is at bright state, and the micromirror “b” is at dark state.
DLP technology uses DMD to form image. In the DLP system, red, green, and blue colors are generated by a color wheel which spins between the lamp and the DLP board. The color wheel alternates the color of the light which is emitting into the micromirrors. Finally, the micromirrors reflect the color light on the projection lens to form an image, as shown in Fig. 1-5 [1].
DLP Board
Fig. 1-5 DLP projector optical system [1].
DLP technology has two features. First, the dynamic image does not suffer from the blur phenomenon. Due to the characteristics of the DMD and digital control technology, the micromirrors can be switched on and off quickly. Therefore, response time of micromirrors is only a few microseconds (ms) [1], which reduce image blue.
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For the reason, DLP system is able to render dynamic image with high quality. Second, the optical engine is compact. Because of microseconds response time of micromirrors, only one chip is allowed to use in the optical engine. Thus, the single chip design helps DLP projector be compact. A single-chip projector system uses color wheel to yield sequential color, and the sequential color form the image. When there is a relative motion between the image and the observer eyes, the separated color on image fringes can be perceived by human eye. The phenomenon is called color breakup, as shown in Fig. 1-6.
Color breakup issue
Fig. 1-6 Color breakup issue for single-chip DLP projector.
1.2.2 3LCD Type Projector
LCD (Liquid Crystal Display) is a popular display technology. LCD is widely used in TVs, monitors, and projectors. 3LCD projector [4] uses three monochromatic LCD panels to produce the color images, as shown in Fig. 1-7 [5].
5 LCD Image Panels Screen
Combining Prism
Mirrors
Dichroic Mirror Dichroic Mirror Mirror
Projection Lens
Lamp
Fig. 1-7 3LCD projector optical engine [5].
White light source from the illumination system is separated into three colors (red, green, and blue color) by dichroic mirrors. Then each of the three colors passes through the corresponding LCD panel which is used as a light valve. The LCD panel modifies the color, and then three colors are combined by a dichroic prism to produce an image.
Due to 3LCD projector does not use field sequential technology, 3LCD projector has no color breakup issue. However, a disadvantage of 3LCD projectors is the
“screen door” effect. 3LCD projector is a transmissive type projector [5], light passes through the monochromatic LCD panels. The panels of 3LCD projector have to be transparent, but the circuits on the panels around each pixel form an opaque grid.
Therefore, when the image is projected, the “screen door” effect is easily perceived, as shown in Fig. 1-8. Besides, lower light efficiency is another drawback due to low aperture ratio and the light absorption by LCD panel.
Fig. 1-8 The “screen door” effect for 3LCD projector.
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1.2.3 LCoS Type Projector
LCoS (Liquid Crystal on Silicon) [6] technology combines a semiconductor with LCD technology. In LCoS projector, reflective mirrors unit used in DLP projector is replaced by liquid crystals applied directly on the silicon.
The LCoS optical engine is shown in Fig. 1-9. White light from the illumination system is divided into three colors (red, green, and blue colors) by a color wheel. The desired colors pass through the corresponding PBS (Polarizing beam splitter) [8]
which only reflects s-polarized light on LCoS panel. Then the polarized light is modulated by the LCoS panel. When LC signal is at bright state, LCoS panel modifies s-polarized light to p-polarized. Then light is reflected again by LCoS panel and emitted on projection lens. However, when LC is at dark state, light is kept as s-polarized and is not able to pass through the PBS. No incident light on projection lens resulted in dark image.
Color wheel
PBS
LCoS panel
Projection lens
Fig. 1-9 Optical engine of LCoS projector.
LCoS projectors is reflective type, light is reflected by LCoS panel [7][8]. Also, because the transistors and driving circuits are located under the LCoS panel surface, the aperture ratio is higher than LCD panel. Therefore, compared with 3LCD projectors, higher brightness and higher light efficiency are achieved. Moreover, light does not pass through the circuits on the panel, the LCoS projector has no “screen
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door” issue. However, the LCoS projector system also uses color wheel to produce sequential color and form the image. Thus, the LCoS projector has color breakup issue, too. Then the comparison of the three kinds of projectors between the three projector types is summarizes in Table. 1.
Table. 1 The comparisons of three kinds of projectors.
Type Color breakup
The projectors previously mentioned can provide large image size. However, the bulky volume of projector limits the practicality. Therefore, the lightweight mini-projectors start to get interest and attention. The development of mini-projectors is described as follows.