Introduction LIz'hfS! Backlights are used for compact,portable, electronic devices with flat panel Liquid Crystal Displays (LCDs) that require illumination from behind. Applications include devices as small as hand-held palm pilots and as large as big-screen TVs. Goals for backlight design include low power consumption,large area with small thickness, high brightness, uniform luminance, and controlled viewing angle, either wide or narrow. To achieve these challenging design goals with a cost effective and timely solution, it is necessary to use computer-aided optical design tools to expedite the design. This paper describes fea-tures in ORA’s LightTools? illumi-nation design and analysis software that enable the development of state-of-the-art backlight designs.
(LXYx< Optical Design and Analysis Tools for Backlights JwWxM3(%t Illumination or lighting systems take light from one or more sources and transform it in some way to produce a desired light distribution over an area or solid angle. Illumination design software must be able to model the geometric and optical properties of different types of light sources and transforming elements, and it must also be able to evaluate the paths of light using optical ray tracing through the model to calcu- late the final light distribution.
6<5:m:KE The light distributions are calculated using Monte Carlo simulations to calculate illuminance, luminance, or luminous intensity over the desired areas and/or angles. Rays are started from random locations and direc- tions from the source(s), traced through the optical system, and col- lected on receivers. Illuminance can be calculated for rays collected on surface receivers and intensity for rays collected on far field receivers. By defining a luminance meter for surface receivers, the spatial or angular variation of luminance can be calculated from that surface.
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540Lw'A In some cases, it may be important to analyze the chromaticity of a dis- play. The spectral energy distribu- tion of the sources (such as LEDs) can be specified. The output of CIE coordinates, together with corre- lated color temperature (CCT), quantifies the colorimetric behavior of the display. An RGB photorealis- tic rendering of the display output can also be generated. All of these analyses are available in LightTools.
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A#xFPYY{ Aspects of backlight displays make particular demands on illumination analysis software. As will be dis- cussed, the means by which light is extracted from a backlight relies on either dense patterns of paint dots or patterned microstructures. Model- ing microstructure arrays in particu- lar can result in extremely large model sizes if created explicitly as a
CAD model. LightTools provides the capability to define arrays of 3D textures that ray trace and render accurately but are not explicitly constructed as part of the geometric model, thereby resulting in much smaller model sizes and much faster ray tracing.
1Q9Hs(s A second aspect of backlight analy- sis involves ray splitting and scatter- ing from the surfaces of the light guide. Because Monte Carlo simu- lations are used to analyze the illu- mination performance, a potentially large number of rays must be traced to get sufficient accuracy for com- parison of designs. It is most effec- tive to trace rays that carry most of the flux. This can be achieved by using probabilistic ray splitting to trace the paths with the most flux, and allowing use of aim areas or solid angles for scattering surfaces to direct scattered light in “important” directions (i.e., toward the display observer).
L=HnVgBs What is a Backlight? q*a~9.i @ A typical backlight consists of a light source, such as a Cold Cathode Fluorescent (CCFL) or Light Emit- ting Diodes (LEDs), and a rectangu- lar light guide, which is also referred to as a light pipe. Other elements than can be used include a diffuser, which enhances display uniformity, and a brightness enhancement film (BEF), which enhances display brightness.
}o[<1+W(. The light source is usually located at one edge of the light guide to mini- mize the thickness of the display. Edge lighting typically uses total internal reflection (TIR) to propa- gate light along the length of the display. Figure 1 shows a schematic of a typical backlight design.
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{%y|A{}c The backlight designer has several options for modeling light sources in LightTools. CCFL sources of differ- ent shapes (e.g., straight, L-shaped, U-shaped, or W-shaped, shown in Figure 2) can be rapidly defined using the Fluorescent Lamp Creation Utility. Reflectors for the lamp can be defined using a variety of Light- Tools geometric primitives, such as cylinders, elliptical troughs, and extruded polygons; reflectors defined in CAD systems may also be imported via standard data exchange formats (IGES, STEP, SAT and CATIA).
_T8S4s8q If LEDs are used, the designer can choose the desired
LED model from pre-stored catalogs of models from Agilent, LumiLeds, Nichia, or Osram. Once the light is directed into the side of the light guide, the problem becomes extracting the light out of the light guide perpendicular to the direction of propagation.
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}'>mT,ytgk As shown in Figure 3, the available power is highest at the source end of the light guide and falls off with increasing distance from the source. To obtain uniform output, the extrac- tion efficiency must increase with distance from the light source. Developing a light guide that exhib- its the necessary variation in extrac- tion efficiency is one of the primary tasks in designing a backlight. Two extraction techniques can be used. The printed light extraction technique uses patterns of paint dots on the bottom of the light guide to scatter light upward and out of the top of the light guide. The second technique, molded light extraction, relies on TIR from microstructures or textures patterned on the bottom surface to redirect light out of the top of the light guide.
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R4k+.hR LightTools supports the design of light guides via the Backlight Design Utility. This tool (Fig- ure 4) assists the user in creat- ing the different parts of a backlight. There are options for adding source/reflector components to the model, BEF modeling, and setting up a receiver for illumination analy- sis. The main focus of the Backlight Utility is multiple tabs for setting up and modifying extraction mechanisms of different types.
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oaI Lh For backlights using the printed light extraction method, the Backlight Utility provides options for linear variation in paint dot size and aspect ratio, as well as linear variation of dot spacing along the length of the light guide. This type of pattern variation will often give a good starting point for a uniform display, but is not sufficient to meet the final uniformity requirements. Additional control of output uniformity can be obtained by allowing non-linear variation of extraction parameters.
q.@% H} An approach that gives very flexible control with a minimum of parame- ters is to define the variation of a parameter in terms of a quadratic Bezier curve. The LightTools 2D Zone Utility is used to set up nonlin- ear patterns.
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O1DUBRli!q Figure 5 shows an example using painted light extraction in which three parameters (paint dot width, height, and vertical spacing) are var- ied to create variable extraction behavior.
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The output uniformity is shown in Figure 6. The slice on the right shows that the average output lumi- nance is constant.
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cg9}T[A The second extraction method, molded light extraction, uses the 3D texture capability in LightTools. The 3D texture feature is designed to ray trace repetitive structures very effi- ciently and store the information very compactly. Models created using explicit geometry can trace more than 30 times slower and have model files more than 100 times larger than equivalent models cre- ated using 3D textures. Three differ- ent basic shapes are available: spheres, prisms, and pyramids (Figure 7).
Z6Kp-z(l3 The Backlight Utility provides a means for setting up linearly varying patterns of the microstructure types.
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zh\"sxL The 3D Texture Utility can be used to vary the texture parameters non- linearly using a quadratic Bezier curve. An example where a groove microstructure (modeled using the prism 3D texture) is used as an extraction mechanism is shown in Figure 8.
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