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Contrast and colorimetry measurements versus viewing angle for microdisplays Olivier Moreau, Jean Noël Curt, Thierry Leroux ELDIM S.A. 1185, rue d’Epron (Ancienne), 14200 HEROUVILLE ABSTRACT The response of a microdisplay device as a function of viewing angle is a major element in the quality of a microdisplaybased system. Such measurements are tricky and time intensive when realized with mechanical equipment. A new system, optimized for microdisplay and based on Fourier Optics is described. It realizes luminance and color coordinates measurement up to +/-30° with 0-360° azimuth angles in one shot (less than one minute) with a resolution better than 0.1 degree. This paper shows how characterization of the microdisplay itself, or combined with its illumination, can be performed for emissive, transmissive, reflective, on and off axis illuminated microdisplay devices. In particular, how its long working distance allows the equipment to carry out measurement even through beam splitter or complicated combination cubes is described. The new tool is shown to be adequate both for R&D and manufacturing teams. Keywords: Microdisplay, contrast ratio, colorimetry, uniformity, viewing angle.

1

INTRODUCTION

The solution we are introducing is based on our 10 years experience on measuring contrast and color coordinates versus viewing angle for liquid crystal panels [1][2] with EZContrast equipment. We propose various solutions for microdisplay according to its technology. The microdisplay image can be either projected on a screen or directly viewed by the eye through a magnification optic system. At present, microdisplays are classified in four categories according to their technologies [3]. See table 1 for more details about companies working on them. The first category is the emissive microdisplay. For theses components, the light is generated by the display itself. Theses components are mainly used for virtual vision [4 to 8]. Organic Light Emitted Diode (OLED), Active Matrix Organic Light Emitted Diode (AMOLED), Field Emission Display (FED), Active Matrix Electro-Luminescent (AMEL), Vacuum Fluorescent Of Silicon (VFOS) are representative of this category.

Transmissive

Technology VF on Si OEL FED AMEL AMLCD

Reflective

LCOS

Emissive

MEMS Scanned

Company Display Research Labs, Ise Electronics eMagin, FED Corp , IBM, Sanyo Micron, Motorola Planar Epson, Hitashi, Matsushita, Samsung, Sarif, Sharp, Sanyo, Sony Canon, Colorado Microdisplay, Digital Reflection Inc., Displaytech, Hana Microdisplay, Hitachi, IBM, InViso, JVC, Kopin, Microdisplay Corp., MicroPix, Micropixel, Nova, Philips, Pioneer, Samsung, Sony, SpatialLight, Three-Five Systems, Unipac, Varitronics, Victor Company of Japan Ltd Daewoo Electronics, Nippon Signal, Reflectivity, Silicon Light Machines, Texas Instruments MicroVision

Table 1: Companies working on Microdisplays

The three other categories of microdisplays find applications in projection systems as well as in virtual images systems. The second category is composed of the transmissive microdisplays. They are Active Matrix Liquid Crystal Displays (AMLCD) [9 to 13] and require a backlight like conventional LCD. The third category is composed of the reflective microdisplays. They use an external light source and modulate the reflected light. There are two main technologies: the liquid crystal on silicon (LCOS) [10][11][14 to 27] and the micro electrical mechanical systems (MEMS) [28 to 31]. LCOS is an AMLCD that operates in reflective mode. The active matrix circuit provides a voltage between an electrode at each pixel and a common transparent electrode, which is separated from the pixel electrodes by a thin layer of liquid crystal. The silicon acts as a reflecting mirror. The LC layer modulates the light. For MEMS, an array of tiny mirrors that flip from one position to another according to an applied voltage is fabricated directly on silicon. The viewing angle characteristics of MEMS are good because there are no liquid crystal effects. At last, the last category of microdisplay is composed of the scanning displays [32 to 34]. In that case, information is displayed as a dynamic luminous transparency overlaying the view of the real world. Microdisplays manufacturers use tests in production chain or in quality control [21]. A wide range of testing instruments for microdisplays enter in the market [35][36]. For example, they allow to test reflectance, contrast ratio, uniformity, blemishes and pixel defects, missing column/row. We are developing characterisation systems that analyse reflectance, luminance and color coordinates versus viewing angle of the final image given by the microdisplay system, but it is also possible to test the different components that compose it. The aim of microdisplay systems is to produce a large image from the small display device. The user, looking at the projected image or the virtual image, sees the image corners under large incidence angle. If the microdisplay optical characteristics, like luminance or color coordinates, strongly depend on the viewing angle, the uniformity of the image could be bad. So, by measuring the viewing angle characteristics of the microdisplay itself, it will be possible to deduce the uniformity of the final image if other components like lenses, polarizers, beam splitter, light source, are well modeled.

2

VIEWING ANGLE MEASUREMENTS

There are mainly two families of solutions for measuring the optical properties of display versus viewing angles. The first one is the mechanical scanning of the display under test. It is known as the goniometric system. A two axis scanning system is used and associated with a spot detector. The main drawbacks (slow and complex to use) are linked to the mechanical nature of the system that limits the measurement speed. The second family is our OFT equipment (Optical Fourier Transform) [37]. We will describe this system in the next chapter. 2.1 FOURIER TRANSFORM OPTICS BASIC PRINCIPLE Our system is based on the combination of Fourier optics and a cooled Charge Coupled Device (CCD) head. It provides instant and accurate measurements of the optical properties, e.g. luminance, contrast, color and polarization of any devices versus viewing angles. Instead of rotating the detector to obtain data, our spatial photometers EZContrast or EZLite systems measure at several angles simultaneously. That reduces the amount of time required to collect data. The basic principle is described in figure 1. A first objective transforms the angle distribution of light in a planar distribution. It is the principle of optical Fourier transform. A second objective scales the image of the Fourier transform on the CCD sensor. The system has been also designed in order to add color capabilities by using a color wheel approach. Five filters are designed in order to match the CIE tristimulus response. They use stacked colored glasses and they are specially designed for each equipment in order to take into account the optics transmission and CCD responses. The accuracy is 0.005 on x, y for color measurement. The sample set up is the same that the one for luminance or contrast measurements. Only two filters are used for luminance measurement whereas all filters are required for color measurements.

Device Under Test

Cooled CCD Sensor

Fourier Transform Plane

Fourier Lens

Optical Relay System (Sizing, Spotsize adjustment, …)

Figure 1: Construction of the image obtained with Fourier optics measurement system.

The evaluation of the photometric and colorimetric characteristics of displays sometimes requires simulating different ambient lighting situations. Illuminations can also be used for measuring any reflective samples. A light source on the side of the equipment provides the illumination. An optical relay system combined with a beam splitter cube is added into the photometer and enables to conjugate the light source plane with the Fourier plane (c.f. figure 2). The light source distribution function allows adjusting the angular distribution of illumination. Device Under Test

Cube

Cooled CCD Sensor

Measurement Path

Optical Relay Systems

Fourier Lens Fourier Transform Plane

Lamp

Figure 2: viewing angle measurement under illumination with EZContrast.

2.2 EQUIPMENT ADAPTATION FOR MICRODISPLAYS EZContrast system analyzes a surface with a spot size from 83 µm to 2 mm diameter, for a maximum incidence range of 80°. The sample under test must be placed at a working distance of about 1.5 mm. Firstly, due to this working distance, this equipment is well suited for measuring samples in which analyzed surface is close to the output surface. Secondly its large incidence range is well adapted to sample which can be seen at large viewing angle. It is not the case for microdisplays. Generally, microdisplays optical characteristics are measured within a cone of 30°. Moreover, for reflective microdisplays, the analyzed surface is quite far from the output surface due to the polarizing beam splitter presence. For theses reasons, we have adapted our equipment to match microdisplay specifications. EZLite Micro is a spatial photometer, which has the same

functions than an EZContrast system, but whose incidence angle measurement range is lower (+-30°), and working distance has been increased to 50 mm. Microdisplay system colors can be generated from two methods: time sequential colors or mixed colors. In the first case, the color is multiplexed in the time whereas in the second case, colors are continuously generate and mixed together. Our spatial photometers achieve measurement during a given exposure time. The light intensity is averaged during that time. So true colors measurements can be achieved with the equipment in both cases of microdisplay color generation as soon as the exposure time is significantly higher than the display characteristic time (10-20 ms). This is usually the case and can always be possible.

3

MEASUREMENTS SCHEMES

When using microdisplay for virtual image applications, the image is seen through an entrance pupil. As a result there is no interest for measuring the characteristics of the final image versus viewing angle for virtual systems because the eye position is well known. Viewing angle characteristics is playing on uniformity. So characterizing the microdisplay system by the viewing angle characteristics without eyepiece optics is a good way to model the uniformity of the image (see also chapter 4). When using microdisplay for projection system, the viewing angle characteristics of the final image not only depend on the microdisplay system itself but also on the screen reflectance. In that case, it should be also interesting to measure each component in a first step and then model the final viewing angle characteristics. We analyze microdisplays according to figure 3 where the microdisplay system is broken up into its components parts.

Light Source

Microdisplay

Beam splitter

Optics

Screen or retina

Figure 3: Microdisplay and its components

3.1 MICRODISPLAY DEVICE MEASUREMENTS 3.1.1 Emissive and transmissive microdisplays measurements For analyzing emissive or transmissive microdisplays, the needed equipment is EZLite or EZLite Micro. Due to microdisplays aperture, only incidence angles less than 30° are required. The microdisplay is analyzed without eyepiece optics, directly by the system. It is possible to measure the luminance, the contrast ratio, and the color coordinates of the display. Results can be the base of computation for designing others components of the system. Figure 4 shows a measurement of a transmissive microdisplay illuminated by a solid state Led backlight acquired with EZLite.

Figure 4: Contrast ratio of a transmissif microdisplay

3.1.2 Reflective display measurements Thanks to the long working distance of EZLite Micro, it is possible to measure optical characteristics of the set of the microdisplay and its illumination system. There are two kinds of use: on axis system and off axis system [38].

Wavelength separating “X Cube”

µLCD Polarizing Beam splitter Input White Light

µLCD

µLCD

Output to lens

Figure 5: On-axis basic principle With the first one, the light output is parallel to the light input (c.f. figure 5). They are used in virtual imagers as well as in projection systems. The on axis system is generally designed with a polarizing beam splitter cube to illuminate the microdisplay. The cube coatings limit the angular aperture of the light source because they are highly angle dependent. The contrast ratio is reduced by the polarization crosstalk between the incoming and reflected beam. Finally, the stress birefringence can lower the polarization efficiency, hence the contrast ratio. The set {microdisplay + beam splitter} is optically equivalent to an image located behind the beam splitter. EZLite Micro measures viewing angles characteristics of this image. If the optical characteristics of the microdisplay itself are already known, it is possible to test the effects of various beam splitters and to compare viewing angles characteristics of each cube thanks to EZLite Micro measurements. Our systems are flexible. We can provide a reference beam splitter according to the customer requirements.

Projection Lens Analyzer Screen

Lens Microdisplay

Polarizer

Figure 6: Off axis basic principle. The second type of use, the off axis system, uses linear polarizers instead of polarizing beam splitters. That reduces problems due to beam splitter and so, might give better contrast ratio. They are used only in projection systems. In that design, the source light is injected in one direction and the output light is reflected in another direction (c.f. figure 6). We add a light source and a mirror in front of EZLite Micro in order to illuminate the microdisplay. Then we analyze the optical viewing angle characteristics of this setting (Fig. 7). From the contrast ratio curves, it is very easy to find exactly the output angle and the output aperture of the reflected light from the microdisplay in order to optimize the output contrast ratio. The system currently has a resolution of 0.1° (it can be less on request). For example, figure 8 shows an EZLite Micro measurement of the contrast ratio of an off axis system. The cross on the picture indicates the maximum contrast ratio position. It occurs for an incident angle of 17° and for an azimuth angle of 88°. By extracting data from the plot by automation software, it is possible to analyze the contrast ratio versus lens output aperture.

Light source

Microdisplay Mirror

EZLite Micro objective

Figure 7: Off axis EZLite Micro configuration

Figure 8: Contrast measurement with Off axis EZLite Micro configuration 3.2 COMPONENTS MEASUREMENTS 3.2.1 Source light The final image uniformity depends directly on the light source uniformity [39][40]. Our spatial photometers can provide the angular distribution of the light emission, the lamp source luminance, and we can also deduce the efficiency of the lamp. Figure 9 shows a shot of the output of an optical fiber (O.N.=0.22) illuminated by an arc source lamp.

Figure 9: Intensity angular distribution of light of an arc lamp.

3.2.2 Polarizers characterization. To illustrate this polarizers characterization, we just show a shot of a linear polarizer located in front of EZLite objective and illuminated by a conventional display backlight (figure 10). The light is polarized according to the direction theta = 135°.

Figure 10: Luminance of a linear polarizer illuminated by a backlight. 3.2.3 Displaying screen characterization. EZLite systems are not only designed for measuring LCD or microdisplay devices but also for measuring surfaces behavior under a light illumination. Figure 11 shows color x and y coordinates versus viewing angle of a painted surface.

Figure 11: Color measurement versus viewing angle.

4

SUMMARY AND EXTENSIONS

Uniformity of an image provided by a microdisplay system strongly depends on the viewing angle characteristics. We have developed testing equipment not only for the microdisplay itself, but also for all the components of the system. On the other hand, we have developed another equipment family, MURATest family, which provides 2D uniformity analysis of luminance and color coordinates of a surface. MURATest equipments are classified into two categories: the VIRTUAL Vision equipment and the TELEView equipment. The VIRTUAL Vision equipment is designed to reproduce human vision ergonomics with a +-16° field of view and from 2 mm to 8 mm pupil diameters. It reproduces how an eye sees the image. The application fields of this equipment are the virtual imagers and the projectors. In the first case, the MURAtest objective is located at the entrance pupil of the virtual imagers. In the case of projected images, MURATest is focused on the image as a camera or the observer’s eye will do. Figure 12 shows how the equipment carries out a measurement in that case.

Figure 12: VIRTUAL Vision equipment. For technologies where luminance highly depends on viewing angle, the viewing cone luminance variations influence the uniformity measurement. Another instrument, TELEView Micro, has been developed to avoid such variations. It allows a perpendicular measuring direction for every 2D position on the sample area (c.f. fig.13). Only the uniformity of the microdisplay is then tested. A 2” TELEView equipment has been specially designed for microdisplays. MURATest equipment

TELEView option

0° 0° 0°

Figure 13: TELEView equipment. This equipment is suited for directly measuring the uniformity of emissive or transmissive microdisplay. In the case of reflective microdisplay, the uniformity is tested through the beam splitter. As a consequence, by comparison with a reference cube, it is possible to analyze the influence of the cube on the uniformity.

5 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

28.

T. Leroux, “Fast Constrast vs. viewing angle measurements for LCDs”, Proc. EURODISPLAY’93, 1993, p.447-449. T. Leroux, C. Rossignol, “Fast Analysis of LCD Contrast and Color Coordinates vs. Viewing angle”, SID Digest, (1995), p.739-742 Chris Chinnock, “Microdisplay Overview”, SID Seminar Lecture Notes, (2000), Vol I, M-5. K. Petlig, B. Aitchison, T. Larsson, T. Nguyen, D. Tuenge, S. Wald, L. Arbuthnot, C. King, “Improved Gray-Shade Performance on Miniature Active-Matrix EL Displays”, SID Digest, (1999), p. 80-83. Neil Greenham, “Organic Electroluminescent Displays”, SID Seminar Lecture Notes, (2000), Vol. I, seminar M-3. A.P. Ghosh, W.E. Howard, I. Sokolik, R. Zang, V.M. Shershukov, A.V. Tolmachev, N.I. Voronkina, V.A. Dudkin, “Color Changing Material for OLED Microdisplays”, SID Digest, (2000), p. 983-985. Lauwrence N. Dworsky, Babu R. Chalamala, “Field-Emission Displays”, SID Seminar Lecture Notes, (1999), Vol. II, F-1. P.F. Seidler, “Organic Light-Emitting Devices for Display Applications”, SID Digest, (1999), p. 371. Arlie Conner, “What’s Inside That Projector”, Information Display, (1999), Vol. 15, n° 1, p. 12-16. C.W. McLaughlin, “The Beat Goes On”, Information Display, (1999), Vol. 15, n° 9, p 20-22. M. Amazi, M. Osame, J. Koyama, H. Ohtani, S. Yamazaki, Y. Kubota, S. Naka, M. Hijikigawa, “A 2.6-in DTV TFTLCD with Area-Reduced Integrated 8-bit Digital Data Drivers Using 400-Mobility CGS Technology”, SID Digest, (1999), p. 69. K. Uchino, T. Kashima, F. Abe, M. Hirano, M. Satoh, T. Maekawa, “Ultra-High-Brightness 1.8-in. XGA Poly-Si TFTLCD”, SID Digest, (1998), p. 1067-1070. H. Nagai, S. Kamizawa, H. Koizumi, K. Nishino, F. Suzuki, H. Shimomura, “High-Resolution High-Brightness LCD Projectors”, SID Digest, (1998), p. 117-120. R.L. Melcher, M. Ohhata, K. Enami, “High-Information-Content Projection Display Based on Reflective LC on Silicon Light Valves”, SID Digest, (1998), p. 25-28. E.G. Colgan, F. Doani, M. Lu, A. Rosenbluth, K.H. Yang, M. Uda, M. Shinohara, T. Tomooka, T. Tsukamoto, “Optimization of Light-Valve Mirrors”, SID Digest, (1998), p. 1071-1074. P. Alveda, C. Mullin, J. Allison, “Embedding LCoS Microdisplays in portable consumer products”, SID Seminar Lecture Notes, (1999), Vol. 2, F-3. O. Akimoto, S. Hashimoto, “A 0.9-in UXGA/HDTV FLC Microdisplay”, SID Digest, (2000), p. 194-197. J. Gandhi, Y. Ji, M. Stefanov, “Performance Enhancement of Reflective CMOS TN Displays in Projection Applications Using Compensating Films”, SID Digest, (1999), p. 990-993. P. Watson, P.J. Bos, J. Gandhi, “ Optimization of bend Cells for Field-Sequential-Color Microdisplay Applications”, SID Digest, (1999), p. 994-997. C.E. Derrington P.A. Smith, H.C. Stauss, “High Volume Microdisplay Manufacturing”, SID Digest, (2000), p. 371373. N. Bergstrom, C-L Chuang, M. Curley, Al Hildebrand, Z-W Li, “Ergonomic Wearable Personal Display”, SID Digest, (2000), p. 1138-1141. M.T. Pfeiffer, “Liquid Crystals for LCoS”, SID Seminar Lecture Notes, (2000), Vol. 1, M-13. J.H. Morrissy, M. Pfeiffer, D. Schott, H. Vithana, “Reflective Microdisplays for Projection or Virtual-View Applications”, SID Digest, (1999), p. 808-811. J. Chen, P.W. Cheng, H.C. Huang, H.S. Kwok, S.K. Kwok, C.S. Li, S. Young, “Generalized Mixed-Mode Reflective LCDs for Three-Panel Color Projection Applications”, SID Digest, (1999), p. 754-757. R.D. Sterling, W.P. Bleha, “D-ILA Technology for Electronic Cinema”, SID Digest, (2000), p. 310-313. H. Kurogane, K. Doi, T. Nishihata, A. Honma, M. Furuya, S. Nakagaki, I. Takanashi, “Reflective AMLCD for Projection Displays: D-ILA”, SID Digest, (1998), p. 33-36. T. Nagata, I. Takemoto, T. Miyazawa, A. Asano, K. Yanagawa, S. Nakamura, H. Nakagawa, K. Saitou, N. Okabe, K. Matsumoto, A. Iguchi, “Silicon-Chip-Based Reflective PDLC Light Valve for Projection Display”, SID Digest, (1998), p. 37-40. S.G. Kim, K-H. Hwang, M.K. Koo, “Thin-Film Micromirror Array (TMA) for Cost-Competitive Information-Display Systems”, SID Digest, (1999), p. 982-985.

29. A. Kunzman, G. Pettitt, “White Enhancement for Color-Sequential DLP”, SID Digest, (1998), p. 121-124. 30. D. Doherty, G. Hewlett, “Phased Reset Timing for Improved Digital Micromirror Device (DMD) Brightness”, SID Digest, (1998), p. 125-128. 31. L.H. Hornbeck, D. Darrow, G. Pettitt, B. Walker, B. Werner, “DLP Cinema Projectors: Enabling Digital Cinema”, SID Digest, (2000), p. 314-317. 32. H. Urey, “Diffraction Limited Resolution and Maximum Contrast for Scanning Displays”, SID Digest, (2000), p. 866869. 33. T.M. Lippert, C.E. Rash, J.J. Hauser, “A Helmet-Mounted Laser Display”, Information Display, (2000), Vol. 1, p. 1216. 34. A. Gross, C; Lorenson, D. Golich, « Eye-Safety Analysis of Scanning-Beam Displays », SID Digest, (2000), p. 343345. 35. W.K. Pratt, R.A. Pendergrass, S.S Sawkar, “Defect Detection in Reflective Liquid-Crystal Microdisplays”, SID Digest, (1999), p. 468-471. 36. A. Poor, “Manufacturing, Materials, and Components”, Information Display, Vol. 16, n° 9, p. 28-32. 37. “Flat Panel Display Measurements Standard”, Video Electronics Standards Association (VESA). 38. K. Thompson, M. Bone, “Photonics Brings the Big Screen to Life”, Photonics Spectra, (1999), p.196-202. 39. Y. Honguh, “Illumination Optics for LC Projection Displays”, SID Digest, (1998), p. 113-116. 40. M. Foley, J. Munro, “Polymer Fly’s Eye Light Integrator Lens Arrays for Digital Projectors”, SID Digest, (2000), p.870-873.