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2009-11-12 15:07 |
Optical Design Tools for Backlight Displays
Introduction 54cgX)E[x
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. 2pN JWYW"
Optical Design and Analysis Tools for Backlights e.ym7L]$O
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. $<%
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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. zp-~'kIJ
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. 0Ilvr]1a4
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. !,!tNs1 K
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). LF.~rmPa
What is a Backlight? RW [<e
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. +Qb/:xQu
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. %vYlu%c<
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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). Y
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