Optical design of small and medium size LCD backli

Backlight for small, lightweight, Flat panel liquid crystal display requiring illumination from the back (LCD) and other electronic devices, including handheld devices as small as palm size and large screen televisions. The objectives of backlight design include low power consumption, ultra-thin, high brightness, uniform brightness, large area and different width angle control. In order to achieve these challenging design objectives, control cost and rapid realization, computer-aided optical design tools must be used for design 。  This paper introduces the characteristics of LightTools optical design and analysis software of ora company in the United States, which can be used to develop the most advanced backlight design applications.
Optical design and analysis tools for backlight
The backlight system needs to convert the light from one or more light sources in a certain area or at a fixed angle; Generate the required light distribution in the. The lighting design software must be capable of geometric modeling, setting optical characteristic parameters for different types of light sources and conversion units, and must be able to evaluate the path of light passing through the model and calculate the final light distribution by using the optical tracing method. The light distribution uses Monte Carlo simulation to calculate the illuminance, brightness, or luminous intensity of a specific area and / or angle. Light is emitted from the light source at random position and angle, tracked by the optical system and received on the receiving surface. The illuminance can be calculated from the surface receiver, and the intensity can be obtained from the far-field receiver. By defining a luminance meter on the surface of the receiver, the distribution of luminance with space and angle can be calculated. In some cases, it may be important to analyze the chromaticity of the display. Specify the spectral energy distribution of light source (such as LED), output CIE coordinate value and related color temperature (CCT), quantify the chromaticity of the display, and generate RGB real light rendered graphics on the display. These analyses can be done in LightTools software.
The characteristics of backlight display have special requirements for lighting analysis software. As will be explained, the light emitted by the backlight depends on the distribution density of printing points or the distribution pattern of microstructure. For the modeling of specific microstructure array, if CAD model is used directly, it may lead to very large model size. LightTools software provides the function of three-dimensional texture array definition, which can carry out accurate ray tracing and rendering. Because the geometric model built directly is not used, the volume of the model is smaller and ray tracing is faster. Another aspect of backlight analysis includes light splitting and scattering on the surface of light guide plate. Since the Monte Carlo method is used to simulate the lighting effect, it is possible that a large number of ray tracing must be used to obtain a design with sufficient accuracy. The most effective way is to trace the highest energy light. Trace the highest energy ray path by using the spectral probability, and use the target area or scattering angle of the scattering surface to orient the scattered light to the "important" direction (such as the observer towards the display).


What is backlight?
A typical backlight consists of a light source, Such as cold cathode fluorescent lamp (CCFL) or light emitting diode (LED), and a rectangular light guide plate. Other available components include diffusion plate to improve the uniformity of the display, and brightening film (BEF) to improve the brightness of the display. The light source is usually located at one side edge of the light guide plate to reduce the thickness of the display. Side light illumination usually uses total reflection (TIR) conducts light in the display.
Backlight designers have many ways to model light sources in LightTools software. The different shapes of fluorescent lamp light source (such as straight, L-shaped, U-shaped or W-shaped, as shown in Figure 2) can be quickly defined by the fluorescent lamp creation tool. The reflector of the lamp can be defined by various geometric primitives in LightTools software, such as cylinder, elliptical groove and extruded polygon. The reflector defined in CAD system can also be defined through standard data exchange format (IGES, step, sat and CATIA) are imported into LightTools software. If LEDs are used, designers can select the desired led model from the product models of Agilent, Lumileds, Riya, OSRAM and other companies pre stored in LightTools software. Once the light enters one side of the light guide plate, the problem becomes to extract the light perpendicular to the propagation direction from the light guide plate.
As shown in Fig. 3, the brightest side of the light guide plate is close to the light source. The farther the distance, the darker the brightness in the light guide plate. In order to obtain uniform light output, the light extraction efficiency must increase with the increase of distance. One of the main tasks of backlight design is to design a light guide plate that changes the light extraction efficiency as needed. There are two extraction techniques that can be used. Dot printing light extraction technology is to print a dot matrix structure at the bottom of the light guide plate to scatter light upward and emit light from the surface of the light guide plate. The second technology, molding light extraction technology, relies on the total reflection (TIR) of the microstructure of the bottom surface to emit light from the surface of the light guide plate.
LightTools software provides backlight design tools to realize the design of light guide plate. This tool (Figure 4) assists the user in creating various components of the backlight. Other options include adding a light source / reflector component to the model, modeling the BEF, and establishing a receiver to analyze the brightness. The interface of the backlight tool is multiple labels to set and modify various types of light line extraction mechanisms.
For the backlight using the dot printing light extraction method, the backlight tool can set the linear change of the size and aspect ratio of the printing point, and the linear change of the dot spacing along the length direction of the light guide plate. This linear structure is often a good starting point for display uniformity, but it is not enough to meet the final uniformity requirements. Further control of uniformity can use non-linear varying light extraction parameters. A method that uses the least parameters and has very flexible control is to define the parameter variables of quadratic Bessel curve. The two-dimensional area tool of LightTools software can be used to set up nonlinear structures. Figure 5 shows an example of using print extraction, in which three parameters (print point width, height and vertical spacing) change to obtain different extraction behavior. The output uniformity is shown in Figure 6. The right figure shows that the average output brightness is a constant.