Projector with liquid crystal light valve using light emitting diodes of the three primary colours

Abstract: A compact and energy efficient projection display can be made by starting with relatively pure red, green, and blue light sources. Each light source includes at least one light emitting diode 12 1 , 12 2 , 12 3 . The output beams of the colored light sources are received by at least one spatial light modulator (18). The modulated output beams are collimated and combined. A projection lens (22) receives the collimated and combined output beams and directs them towards a projection screen (24). All of the above may be contained in a housing to provide a compact and lightweight projection display.

Description: Field of the Invention The invention is directed towards the field of projection displays. In particular, the invention relates to the manufacture of rear projection flat panel displays. BACKGROUND OF THE INVENTION Real image displays can be separated into three categories: cathode ray tubes (CRT), flat panel displays, and projection displays. Currently, CRT displays range from 1-40 inches along the image diagonal. These displays have good image quality and can be manufactured economically. The shape of the display requires a depth that can exceed the length of the image diagonal for high resolution graphic displays. The displays are bulky, especially for sizes above 20 inches because the wall thickness of the glass must be increased to withstand the atmospheric pressure. CRTs are popular in the desktop monitor market where the common length of the image diagonal is 14-17 inches. Flat panel displays are used in many portable applications. They are thin and light in weight but have an image quality that is inferior to the CRT. At this time, the majority of displays are used in applications requiring 2-12 inches along the image diagonal. Flat panel displays are more costly to manufacture than the CRTs. As a result, display applications that require an image size above 40 inches are dominated by projection displays: either front or rear. Both technologies can create an image by one of two methods: using three small high brightness monochromic CRTs for red, green, and blue (RGB) or creating a color image via a spatial light modulator. These projection systems have low image brightness and are costly to manufacture. It is not economical to reduce the conventional rear projection display to 17-20 inches along the image diagonal. The cost of the light valve is dominated by the drive circuitry. To illustrate, the drive circuitry of an XGA display (768x1024x3) is approximately $120 regardless of the screen size. A 70W metal halide arc lamp capable or producing 5000 lumens with its corresponding power supply, ballast, and hot re-ignition capability are required. The total cost is prohibitive and unattractive compared to the OEM cost of a 17 inch CRT. US 5,467, 154 discloses a rear-projection monitor having a housing with a reduced depth. In a particular embodiment the use of a single 150 to 300 W quartz- or metal- halide lamp is contemplated. IBM Technical Disclosure Bulletin, Vol. 40, No. 04, April 1997 describes on pages 201 to 205 a Liquid Crystal Display three panel projector using three primary colors light Emitting Diode light sources. WO 97/16 679 discloses arrays of vertical cavity surface emitting lasers ("VCSEL"s) used for illumination in both infrared and visible wavelengths. A white light source can be constructed from six VCSEL clips generating light at equally spaced wavelengths between 475 nm and 625 nm. SUMMARY OF THE INVENTION A compact, light weight, and energy efficient projection display can be made by starting with relatively pure red, green, and blue light sources. The output beams of the colored light sources are received by at least one spatial light modulator. The modulated output beams are collimated and combined. A projection lens receives the collimated and combined output beams and directs them towards a projection screen. All of the above may be contained in a housing to provide a compact and lightweight projection display. A projection display, according to the invention, is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a preferred embodiment of the projection display. Figure 2 illustrates an alternate embodiment of the present invention. Figure 3 illustrates another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a light weight and compact projection display that is based on a spatial light modulator, typically a 1 cm 2 CMOS IC that is in direct contact with a ferro-electric liquid crystal material. Light transmitted through the liquid crystal material will rotate as a function of its plane of polarization with respect to the local surface voltage of the IC. Illuminating the LC/IC cell with consecutive flashes of red, green, or blue light from three separate LEDs creates a color-sequential image of the surface voltage of the IC. The address circuitry is buried in the 1 cm 2 IC. All display information is sent to the IC on one or several high speed I/O lines. The IC/LCD combination is equivalent to a reflective active matrix liquid crystal display (AMLCD) light valve at much lower cost and higher performance. For instance, the transmissive design of an AMLCD light valve blocks 60% of the light due to the shadowing of metal lines and transistors. In contrast, the reflective cell has estimated losses of only 20%, mainly reflective losses from the aluminum pads covering the IC surface. By using sequentially strobed RGB light sources instead of a white light source, the more than 75% light loss in the color filter is also eliminated. The present invention, a rear projection display using a spatial modulator and three LED power sources can be optically efficient. Conventional Design (Prior Art) LED Design Collimation 40% 60% Polarizer 50% 50% Rect/Circular Conversion 70% 70% Color Filters 25% 100% Valve Aperture 40% 80% Misc. Refl./Absorption 60% 60% Total 0.8% 10% Table 1 illustrates the optical efficiency of a rear projection system based on conventional design and LED design of the present invention. Flux exiting the front surface of the screen,  exit , is defined as follows:  exit = BA Ω A is the area of the screen and B is the brightness of the desired Lambertian light distribution. If the screen has a gain g of 1.5 by reducing the angular distribution below a Lambertian distribution, then the exiting flux is:  exit = BA Ω/ g For a high resolution desktop monitor, the brightness required is 100 cd/m 2 . A 17 inch monitor has an image area of 25x30 cm 2 or A= 0.075m 2 . Because a desktop monitor may have a smaller viewing angle than a comparable sized television, the screen gain may be selected to be g=2. For these assumptions, the exit flux is computed to  exit = 12 lm. For an optical system design having an overall efficiency of 10%, a source flux of  source = 120 lm is required. This flux may be distributed over all three sources as follows: Red 30 lm Green 75 lm Blue 15 lm Figure 1 illustrates a preferred embodiment of a projection display 10 that includes three light emitting diodes 12 1 , 12 2 , 12 3 , emitting a red, green, and blue light beam, respectively. Each light beam is received by a collimator 14 1 , 14 2 , 14 3 . Each collimated light beam passes through a polarizing beam splitter 16 1 , 16 2 , 16 3 . A spatial light modulator 18, having MxN pixels, modulates the split beams. The spatial light modulator 18 is controlled by a display driver 20. A projection lens 22 receives the modulated beams and directs them towards a projection screen 24, positioned at the front of the housing 26. A sequence controller 28, attached to the three light sources, controls the red, green, and blue light beams. The spatial modulator and the three light sources are connected to a power supply (not shown). The red, green, and blue light emitting diodes, in combination, have a luminous flux of 10-1000 lm. It is preferred that the approximate flux ratio is 65% green, 25% red and 10% blue light. The housing may optionally include at least two folding mirrors to receive the red, green, and blue light beams and to fold the red, green, and blue light beams. The folding mirrors allow the housing to be compact. In this embodiment, the color subframes are sequential. Figure 2 illustrates another embodiment of the present invention. There are three spatial modulators 18 1 , 18 2 , 18 3 , one for each light beam to allow for parallel color subframes. In this embodiment, the color subframes are parallel. Figure 3 illustrates another embodiment of the present invention. The multiple light emitting diodes shown in Figure 1 are replaced by a single light source 12. The single light includes light emitting diodes on a unitary substrate that emit red, green, and blue light. There is only one light path in the system as contrasted to the embodiments shown in Figures 1 and 2.

Claims: A rear-projection display (10) comprising: a housing (26); a light source (12) including at least a red, a green, and a blue light emitting diode, emitting a red, green, and blue light beam, respectively, wherein the red, green, and blue light emitting diodes have a combined luminous flux of 10-1000 Im with an approximate ratio of 65% green, 25% red, and 10% blue light; collimator means including at least one collimater (14), said collimater means being optically connected to the light source for producing a collimated light beam; at least one beam splitter (16) optically connected to said collimater means; spatial light modulator means including at least one spatial light modulator(18) having MxN pixels and being optically connected to said beam splitter (16); a projection lens (22), optically connected via said beam splitter (16) to said spatial light modulator (18); a projection screen (24) optically connected to said said projection lens (22); a drive circuit (28) connected to the spatial light modulator means for generating color subframes; a power supply connected to said spatial light modulator and said red, green, and blue light emitting diodes; wherein said housing (26) contains said light source (12), said collimator means, said beam splitter (16), said spatial light modulator means, said projection lens (22), said projection screen (24), said drive circuit (28) and said power supply. A rear-projection display as defined in claim 1, wherein the light source comprises a first chip including an array of N red light emitting diodes, a second chip including an array of N green ligth emitting diodes, and a third chip including an array of N blue light emitting diodes, where for each chip N ≤ 10 and the array has a diameter of ≤ 3mm. A rear-projection display, as defined in claim 2, further comprising for each of the arrays of red, green and blue light emitting diodes a reflector that surrounds the respective array of light emitting diode and an immersion lens, receiving light from the respective array of light emitting diodes, operative to pre-collimate the light into a solid angle of 3 steradians or less. A rear projection display, as defined in claim 3, wherein the array of light emitting diodes has an apparent source size less than 5 mm in diameter. A rear projection display, as defined in one of claims 1 to 4, wherein one of the red, green, and blue light emitting diodes is a laser. A rear projection display, as defined in one of claims 1 to 5, further comprising: at least two mirrors, that receive the red, green, and blue light beams, operative to fold the red, green, and blue light beams. A rear-projection display, as defined in one of claims 1 to 6, wherein the spatial light modulator means (18 1 ,18 2 ,18 3 ) and three beam splitters (16 1 , 16 2 , 16 3 ), three spatial light modulators includes each spatial light modulators receiving one of the red, green, and blue light beams, respectively; and wherein the drive circuit (20) is arranged to generate the color subframes in parallel. A rear projection display, as defined in claim 1, wherein the drive circuit (20) is arranged to generate the color subframes sequentially. A projection display as defined in Claim 1, wherein said light source comprises at least one red, one green, and one blue light emitting diodes (12 1 , 12 2 , 12 3 ) on a unitary substrate, and wherein the red, green, and blue light sources are independently switched.

Inventors: HAITZ ROLAND H, HAITZ, ROLAND H.
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 1998
Main classification: H04N9
IPC codes: G09G3, H04N9, G02F1, G02B27