Compact LED projector design with high uniformity and efficiency Wen-Shing Sun,1 Chuen-Lin Tien,2,* Chao-Hui Ma,1 and Jui-Wen Pan3 1

Department of Optics and Photonics, National Central University, Chung-Li 32001, Taiwan 2

Department of Electrical Engineering, Feng Chia University, Taichung 40724, Taiwan

3

Institute of Photonic Systems, National Chiao Tung University, Tainan City 71150, Taiwan *Corresponding author: [email protected] Received 28 April 2014; revised 11 July 2014; accepted 27 August 2014; posted 28 August 2014 (Doc. ID 210969); published 2 October 2014

The aim of this study is to design a compact LED projector that is divided into two systems: an illumination system and an imaging optical system. The illumination system consists of an RGB LED light source with a collimator lens group and a mirror with a color filter and a lens array integrator instead of an integrated rod so as to improve the uniformity of the light intensity. By using a new total internal reflection prism and projection lens, the whole optical engine is smaller in size and has a higher contrast ratio for image quality. The imaging optical system consists of a total of eight lenses (six spherical lenses and two aspherical lenses), for a total length of 52 mm; a tolerance analysis is conducted. Optical simulation software is used for the system analysis in order to determine the efficiency and uniformity of the light intensity. In this design, the uniformity of the screen reaches more than 82%, and the efficiency increases by more than 44%. © 2014 Optical Society of America OCIS codes: (220.2740) Geometric optical design; (220.4830) Systems design; (230.3670) Light-emitting diodes. http://dx.doi.org/10.1364/AO.53.00H227

1. Introduction

In recent years, digital projectors have been used not only for image resolution and color, but their size and weight have also improved. The current generation uses an LED as the light source. The advantages of LEDs include energy saving, environmental protection, small size, and long service life [1]. With the development of LEDs, many projectors have been designed using LEDs as the light source to project images on the screen [2–7]. In 1987, digital light processing projection technology was developed by Texas Instruments. A digital micromirror device (DMD) is used as a light valve [8]. Using the true digital pixels, the screen shows a rapid response, and the relevant technology is also used in projectors. Therefore, the

existing projection display technology has included the use of LED on DMD projectors [9–11]. Pan et al. [12,13] proposed that in an LED projector, the illumination system achieves high uniformity through a microlens array. A total internal reflection prism (TIR) can also be used to reduce the size of both the illumination system and the telecentric projection lens. Zhao et al. [14] developed a 210 mm DMD projector composed of three RGB LEDs, three collimators, an X cube, a hollow integrator rod, a relay system, and a TIR prism. The efficiency of this system was 29.98%. With this LED projector design, a total length of 131 mm and a DMD average luminous efficiency of 51.1%, good results were obtained. 2. Design Method

1559-128X/14/29H227-06$15.00/0 © 2014 Optical Society of America

The projector’s optical system design in this study was divided into two parts: an illumination system from the light source to the DMD and an imaging 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS

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have no patent issue. For the power mirror system, the optical tolerance for the alignment is very sensitive. We used a lens array to reduce the longitudinal size of the light system without patent issue for our illumination system design.

Fig. 1. System layout of a compact LED projector.

optical system for the light reflected by the DMD through the lens to the screen. The former is a nonimaging optical system that focuses on the illumination uniformity and efficiency, while the latter is to eliminate aberration and chromatic aberration of the lenses. The design of the optical modules includes a collimating lens, relay lens, and dichroic mirror. An illumination system includes a microlens array of integrators, condenser lens, mirror, and TIR prism. The DMD projection lenses and imaging system that compose the entire projector system are shown in Fig. 1. A.

Illumination System

For a comparison with current products, we selected the LED projectors of Coretronic Inc. [15] and BenQ. The optical structures for the LED projectors of Coretronic Inc. and BenQ are not telecentric. This optical structure feature indicates that the chief ray is not parallel to the optical axis and so can reduce the relative illumination to 0.55 at the field corner. However, the optical structure of our design is typically telecentric. The relative illumination can be maintained at 0.77. Also, the homogenizer for the LED projector of Coretronic Inc. and BenQ is a light pipe. The length of light pipe will increase the size of the optical system. The relay system for Coretronic Inc. is a typical two-lens system. The relay system for BenQ is a power mirror system. The two systems

1. LED Light Source Our design utilized a Phlatlight pt54 projection chip [2] as the light source. It has an emitting area of 5.4 mm2 and an operating current of 13.5 A. The luminous flux emitted at different voltages is 920 lm for a red LED (R), 1725 lm for a green LED (G), and 320 lm for a blue LED (B). By pulsing RGB LEDs at a height frequency of more than 60 Hz and synchronizing them with the corresponding color that displays on the DMD chip, a full color image is projected. The luminous intensity distribution curve is shown in Fig. 2(a). The luminous intensity distribution curves of the G LED and B LED are the same. The spectral distribution of the different wavelengths of RGB LEDs is indicated in Fig. 2(b). 2. Design of Collimator Lens The light intensity of Phlatlight RGB LEDs is not a Lambertion distribution, as shown in Fig. 2(a). Owing to the photonic crystal structure on the green and blue LEDs, the light has been collimated. For the red LED chip without a photonic crystal structure, the divergence angle is larger. For the RGB LED collimating degree, the design includes a collimator lens for each LED in order to increase the light efficiency and to control the light path. The collimator lens consists of a relay lens and a collimating lens, as illustrated in Fig. 3(a). The designed size was 30 mm × 30 mm × 22.2 mm, and the intensity distribution was confined in 10°, as shown in Fig. 3(b). By using this condenser, the light could be collected at a divergence angle of 60°. 3. Dichroic Filter Because LEDs with three different wavelengths must share the same route, they must be combined. By using two different dichroic filters with a multilayer coating, red or blue light reflection and green light transmittance can be directed to the microlens

Fig. 2. Phlatlight RGB LED light sources: (a) luminous intensity distribution curve of Phlatlight RGB LED and (b) spectral distribution of Phlatlight RGB LEDs. H228

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Fig. 5. Design of microlens array.

dimensions of the lens array integrator shown in Fig. 5. The material of lens array is polymethyl methacrylate (PMMA). Each lenslet has a plane surface and a spherical surface. The radius of the spherical surface is 8.95 mm. The dimensions of the lenslet are L  3 mm, D  2.74, and H  2.06.

Fig. 3. (a) Collimator lens layout and (b) luminous intensity distribution curve after collimator lens.

array, as shown in Fig. 4. Thus, the light source module becomes a larger size, and the volume increases to 60 mm × 78 mm × 40 mm. The size of the entire projection system is larger than that of the compact projector. 4. Microlens Array Uniformity in the illumination system is crucial and thus an increasingly important requirement, but while the use of a lens array integrator is designed to improve the uniformity of light [12], it can also reduce the space. The area of the lens array is 329.6 mm2 , a calculation based on the light etendue conservation. Assuming that the integrator lens array width is w and height h is 0.75 W, then we have w  20.96 mm and h  15.7 mm for the sectional

Fig. 4. Light source group including the dichroic filters.

5. TIR Prism In a DMD projector, the TIR prism is the key component connecting the nonimaging system and the imaging system. A TIR prism consists of both a triangular prism and an equilateral triangle prism combination in the air gap, as shown in Fig. 6(a). Light is incident on the DMD through a TIR prism angle set at 26° to increase the contrast. When the DMD mirror is flipped 12°, it is in the so-called on state. The reflected light from the DMD is incident on the air space between prism 1 and prism 2, which forms the TIR to impact the projection lens. Using principal ray tracing, the angle of incidence of light entering the TIR prism from the DMD mirror flip angle of 12° can be calculated, as shown in Fig. 6(b). In this study, all raw materials of the prisms were BAK4, with a refractive index of 1.56883. The size of the TIR prism was 31 mm × 28 mm × 25.25 mm. B. Etendue Calculation and Conservation

When a light beam passes through an optical system, without considering the light scattering, diffraction, and absorption, the etendue of the beam will remain conserved. The etendue for each component of the optical system shows the same value [16]; the formula is given by

Fig. 6. TIR prism: (a) TIR prism group size. (b) Micromirror is 12°. 10 October 2014 / Vol. 53, No. 29 / APPLIED OPTICS

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Fig. 7. Layout of the project lens.

E  πAn sin θ2 

πA ; 4F∕2

(1)

where A is the surface area of the plane. This plane can be optical elements, such as light valves or other optical aperture. Thus, the surface of the plane can define the f number (F∕). This optical system allows the preliminary specifications for each element to be calculated. 1. DMD Etendue Projected light on the DMD is considered the overfill. The general requirement for the amount of overfill is 20%. That is, the length and width of the light irradiated area are designed to be 1.1 times that of the original light valve panel size. For the size of a 0.55” 4:3 DMD (active area  11.17 mm × 8.38 mm, the DMD irradiated area A is 113.26 mm2. In this study, the DMD projection system had a flip angle of 12°, in other words, a half-angle of 12° for the cone of the light illuminating light valve panel. The etendue of the light valve panel at the DMD should be 15.38 mm2 -steradian. 2. Light Source Etendue Using the above formula, the etendue of the LED light source can be expressed as ELED  π × 5.4 mm2 × sin2 60°  12.72 mm2 − steradian:

(2)

of the DMD panel and the throw ratio, the first-order specifications of the projection lens are as listed in Table 1. The lens design results allowed for an analysis of the quality of the imaging and the performance of the tolerance analysis using Zemax. A. Design Results

By using Zemax, it was possible to optimize the design of the project lens. The layout and performance of the lens were as follows: the modulation transfer function (MTF) at 37 lp∕mm in all fields was over 60%; the optical distortion was 1.2%; the lateral color was less than 17 μm; and the relative illumination was 78%. The layout of the project lens as optimized through Zemax is illustrated in Fig. 7. Figure 8 shows the image quality of the project lens. Figure 8(a) indicates the MTF plot and spatial frequency; the abscissa is the spatial frequency and the vertical coordinates for MTF. We used a spatial frequency of 37 lp∕mm and an MTF of more than 58.7%. Figure 8(b) shows the lateral chromatic aberration, the abscissa value for the lateral chromatic aberration, and the vertical coordinate of the image height. The lateralchromatic aberrationhad amaximumvalue of20.7μm. Figure 8(c) shows the fieldcurvatureand distortion; the abscissa is the optical distortion percentage graph and the vertical coordinate is the image height. The maximum value was 1.41%. Figure 8(d) depicts the relative illumination; the abscissa is the image height and the vertical axis is the relative illumination with a maximum value of 0.77. B. Tolerance Analysis

3. Lens Array Etendue Conservation for the Area Based on the above etendue conservation, this value could be used to calculate the lens array sectional area of the integrator. Because the etendue of the integrator rod outlet is the same as that of the light valve panel, the etendue of the outlet end of the integrator column is equal to 15.38 mm2 -steradian. In addition, according to the law of reflection, we can know that this value is also equal to the etendue of the inlet end of the lens array integrator. When the incident angle is 7° and the etendues of both the inlet and outlet of the lens array are given, the area of the outlet end of the lens array can be calculated as 329.6 mm2. 3. Image Quality Analysis of Projection Lens

To design the imaging system, the first-order specifications must first be set. According to the standards H230

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The tolerance analysis of the projection lens was done using a Code V assessment. We used a spatial frequency of 37 lp∕mm for the MTF tolerance Table 1.

Item 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Specifications of Projection Lens

Parameter

Specification

DMD DMD pixel size Active area of DMD Wavelength range Effective image circle Effective focal length (EFL) f∕ Field of view (FOV) Pupil of position Elements

0.55″ 4∶3 13.6 μm 11.17  8.38 mm2 450–680 nm 18 mm 14.2 mm 2.4 65° 236 mm 8 (6G2P)

Fig. 8. Image quality of the project lens: (a) MTF and spatial frequency; (b) lateral chromatic aberration; (c) field curvature and distortion; and (d) relative illumination.

analysis. The tolerance range in size was evaluated, as shown in Table 2. There were inaccuracies in the production process, and the performance was not equal to expectations; therefore, it was necessary to estimate the tolerance before production. By using Code V, we analyzed the tolerance of this lens, and we chose MTF as the standard. After assessing the tolerance, the performance summary was as shown in Table 3. The design that took the tolerance specification into consideration had MTF values greater than 42%. 4. Simulation and Analysis of the Projection System

The projection system combined the illumination system and the imaging system. The uniformity and energy efficiency analysis on the screen are now discussed. Table 2.

Tolerance Centered Tolerance

Decentered Tolerance

Tolerance Range in Size

Specification

Test plate fit Center thickness Material index

2 fringes 0.01 mm Index: 0.0005 V: 0.5% 1 fringe 1–2 arc min

Element wedge Element displacement/ Roll

We used nine points to measure the uniformity degree. The illumination value at P1 to P9 was captured by photodetector. The uniformity can be defined as the Japanese Business Machine Makers Association (JBMA) uniformity and expressed as the following equation:   P1; P3; P7; P9average; lux JMBA%  × 100%: P5average; lux (3) The efficiency loss of the projection optical system is defined as from the light source to the screen, as Table 3.

Relative Field

Performance Summary

Freq AZIM Weight Design Design Compensator l/mm Deg TOL Range 

X, Y

Item

Irregularity Element tilt

A. Projector Uniformity and Efficiency Analysis

1–2 arc min 0.01 mm

0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00, 0.00,

DLTS23 0.00 0.29 0.57 0.86 0.93 1.00 0.29 0.57 0.36 0.93 1.00

37.00 37.00 37.00 37.00 37.00 37.00 37.00 37.00 37.00 37.00 37.00

TAN TAN TAN TAN TAN TAN RAD RAD RAD RAD RAD

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

0.8583 0.8121 0.7558 0.7582 0.7582 0.7548 0.8818 0.8330 0.6799 0.6706 0.6865

0.7908 0.7217 0.6427 0.5664 0.5065 0.4246 0.8292 0.7703 0.5685 0.5447 0.5512

0.062611 0.062611 0.062611 0.062611 0.062611 0.062611 0.062611 0.062611 0.062611 0.062611 0.062611

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Fig. 9. Light-energy efficiency loss for each component.

efficiency of the optical element. The design results of the compact projection system demonstrated a color image size of 20 in. The optical efficiency on the screen by RGB LEDs was 44%, 44.4%, and 43.65. The JBMA uniformity on the screen was 82.8%, 91.9%, and 87.3% for the different LED light sources. The compact LED projection system size was 131 mm × 93 mm × 91 mm. This study was sponsored by the Ministry of Science and Technology of Republic of China, Taiwan, under contract number NSC101-2221-E-008-107. References

Fig. 10. Light-energy distribution at the screen: (a) red light source; (b) green light source; and (c) blue light source.

Table 4.

Efficiency and Uniformity on the Screen

Lumenlus R G B LED Total Lumen on DMD Efficiency ANSI uniformity

Red

Green

Blue

920 497 54% 88.8%

1725 835 48.4% 98.4%

320 160 51% 98.3%

shown in Fig. 9. The light-energy distribution of the RGB LED light source at the screen was observed, as shown in Fig. 10. Table 4 shows the optical efficiency and (JBMA) uniformity analysis at the screen. The second column shows the optical throughput of RGB LEDs. The third column indicates the optical throughput at the screen. The fourth column presents the optical efficiency at the screen. The optical efficiency at the screen for RGB LEDs was 44%, 44.4%, and 43.6%. The fifth column presents the JBMA uniformity degree. The JBMA uniformity at the screen was 82.8%, 91.9%, and 87.3%. 5. Conclusion

This study used single R, G, and B LED light sources with a collimator lens and dichroic filters to replace the ultra-high-pressure mercury lamp. The reduction in the light source group was from the reflector, ultraviolet/infrared (UV/IR) filter and color wheel. In the illumination system, a microlens array was used to replace the rod to increase the aperture ratio and improve the light uniformity. Using TIR prism illumination systems shortened the total length of the imaging system to 52 mm. The imaging lens system and nonimaging illumination system were obtained from a previous design. Optical simulation software was used to combine the two systems. We took into account the actual

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Compact LED projector design with high uniformity and efficiency.

The aim of this study is to design a compact LED projector that is divided into two systems: an illumination system and an imaging optical system. The...
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