Optical design and analysis tools for LED backlight display

Backlights are used in small, lightweight, flat-panel liquid crystal displays (LCDs) and other electronic devices that require backlighting, including handheld devices as small as palms and large-screen TVs. The goals of backlight design include low power consumption, ultra-thin, high brightness, uniform brightness, large area, and different width and narrow viewing angle control. In order to achieve these challenging design goals, and to control costs and achieve rapid implementation, computer-aided optical design tools must be used for design. ? This article introduces the characteristics of LightTools optical design and analysis software from ORA in the United States, which can be used to develop the most advanced backlight design applications today.

Optical design and analysis tools for backlighting

The backlighting system needs to convert the light from one or more light sources to produce the required light distribution in an area or at a fixed angle. The lighting design software must be able to model geometrically, set optical characteristic parameters for different types of light sources and conversion units, and must be able to use optical tracing methods to evaluate the path of light passing through the model and calculate the final light distribution. 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 positions and angles, traced through 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 the light source (such as light-emitting diodes), output CIE coordinate values and correlated color temperature (CCT), quantify the chromaticity of the display, and generate RGB real light rendering graphics on the display. These analyses can all be done in the LightTools software.

The characteristics of the backlit display have special requirements for the lighting analysis software. As will be explained, the light emitted by the backlight depends on the distribution density of the printed dots, or the distribution pattern of the microstructure. For the modeling of a specific microstructure array, if the CAD model is used directly, it may result in a very large model size. LightTools software provides the function of 3D texture array definition, which can carry out accurate ray tracing and rendering. Since no directly constructed geometric model is used, the volume of the model is smaller and the ray tracing is faster. Another aspect of backlight analysis includes the light splitting and scattering on the surface of the light guide plate. Since the Monte Carlo method is used to simulate lighting effects, it may be necessary to use a large number of ray tracings to obtain a design with sufficient accuracy. ? The most effective method is to trace the highest energy light. Tracing the highest energy ray path by using the splitting probability, and using the target area or scattering angle of the scattering surface to direct the scattered light to the "important" direction (such as toward the viewer of the display).

What is backlight? ?

A typical backlight consists of a light source, such as a cold cathode fluorescent lamp (CCFL) or light emitting diode (LED), and a rectangular light guide plate. Other available components include diffusers, used to improve the uniformity of the display, and Brightness Enhancement Film (BEF), used to increase the brightness of the display. The light source is usually located on one side edge of the light guide plate to reduce the thickness of the display. Sidelight lighting usually uses total reflection (TIR) to transmit light in the display. ?

Figure 1 shows a schematic diagram of a typical backlight design. ?

The backlight designer has many ways to model the light source in the LightTools software. Different shapes of fluorescent light sources (such as straight, L-shaped, U-shaped or W-shaped, as shown in Figure 2) can be quickly defined using the fluorescent light creation tool. The reflector of the lamp can be defined by various geometric primitives in the LightTools software, such as cylinders, elliptical grooves, and extruded polygons. The reflector defined in the CAD system can also be imported into the LightTools software through standard data exchange formats (IGES, ?STEP, ?SAT? and CATIA). If LEDs are used, designers can select the desired LED model from the product models of Agilent, Lumileds, Nichia, Osram and other companies pre-stored in the 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 Figure 3, the brightest of the light guide plates is on the side close to the light source. As the distance is farther, the brightness in the light guide plate becomes darker. In order to obtain a uniform light output, the light extraction efficiency must increase as the distance increases. 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. The dot printing light extraction technology is to print a dot matrix structure on the bottom of the light guide plate to scatter the light upward and emit it from the surface of the light guide plate. The second technology, 　compression molding light extraction technology, relies on the total reflection (TIR) of the microstructure on the bottom surface to make light emerge from the surface of the light guide plate.

?

LightTools software provides backlight design tools to realize the design of light guide plates. This tool (Figure 4) assists the user in creating various components of the backlight. Other options include adding light source/reflector components to the model, BEF modeling, and building a receiver to analyze brightness. The interface of the backlight tool is a number of tabs used to set and modify various types of light 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 printed dots, and the linear change of the dot pitch along the length of the light guide plate. This linearly changing structure is often a good starting point for display uniformity, but it is not enough to meet the final uniformity requirements. To further control the uniformity, non-linearly varying light extraction parameters can be used. A method that uses the fewest parameters and is very flexible is to define the parametric variables of the quadratic Bezier curve. ? The two-dimensional area tool of the LightTools software can be used to set the nonlinear structure. Figure 5 shows an example of using printing extraction, where 3 parameters (printing dot width, height and vertical spacing) change to obtain different extraction behaviors. The output uniformity is shown in Figure 6. The figure on the right shows that the average output brightness is a constant. ?

The second extraction method, compression molding extraction technology, uses the three-dimensional texture function of LightTools software, which makes the ray tracing of repetitive structures very effective, and the stored information is very compact. The ray tracing of the model created by the non-3D texture function is more than 30 times slower than the model created with the 3D texture, and the file is more than 100 times larger. There are three basic shapes for 3D textures to choose from: spherical, prismatic, and pyramidal (Figure 7). The backlight tool can define linearly variable microstructures. But the 3D texture tool can use the quadratic Bezier curve to change the texture parameters non-linearly. The example shown in Figure 8 is a trough-shaped microstructure (using prismatic 3D texture modeling) as the extraction mechanism. The resulting light guide plate and its simulation results are shown in Figure 9.

Backlight optical calculation

The two most important optical quantities of a backlit display are the display brightness and illuminance uniformity on the surface of the light guide plate. It is also important to calculate luminous intensity and various color metrics (CIE coordinates and correlated color temperature CCT). LightTools software has built-in these calculation functions and many other functions to help understand the data generated by Monte Carlo simulation.

?

Monte Carlo simulation is the basis for the calculation of illuminance in LightTools software. The random number generator is used to select the starting position, direction and wavelength of the light, and is used to sample the light distribution on the receiving surface. The choice of "random" numbers will greatly affect the convergence of the simulation. Using a low variance (Sobol) number sequence (it is not completely random), the error can be reduced to 1/N, where N is the number of rays at the receiving end. You can see the comparison result of using random number sequence (Figure 10) and Sobol number sequence (Figure 11) to calculate chromaticity. In this example, the simulation result using 128,000 random rays is equivalent to the accuracy of Sobol's 16,000 rays. The important thing is to compare the simulation convergence speed of different software. What we care about is the speed of achieving a certain simulation accuracy, not the speed of tracing a certain amount of light. In the LightTools software, the receiver is used to collect light data to calculate the illuminance.

The light data for analysis and display is collected from the data grid. The user can interactively control the size or number of the data grid. ? For a given number of rays on the receiver, the smaller the number of grids, the lower the spatial and angular resolution, but the higher the relative accuracy (low error rate). Conversely, the more grids, the higher the spatial and angular resolution, but the lower the accuracy (high error rate). The estimated error rate is displayed on each grid to help the user decide whether enough light is used for tracing simulation to meet the resolution and accuracy required by the design at the same time (Cassarly,?WJ,?Fest,?EC,? and?Jenkins,?DG,?2002). If more light is needed, the user can continue the simulation interactively until the goal is reached. ?

An important aspect of backlight analysis is the light splitting and scattering on the surface of the light guide plate. The function of the light guide plate is that light can be absorbed or emitted after multiple reflections on the inner surface. If the light is split into two parts of transmission and reflection on each contact surface, it will cause a very large number of split light rays, most of which do not carry much energy, thus slowing down the analysis speed. An example of this is shown in Figure 12, which shows a starting ray with many paths due to light splitting.

The following simulation uses 2,000 incident rays. Due to light splitting, the receiver collects 277,948 rays (Figure 13). Since most of the light reaching the receiver does not have much energy, the resulting error is 42%. On the contrary, if the Fresnel loss coefficient and surface scattering characteristics are used to determine the possibility of light transmission and reflection, to evaluate the possibility of optical path path, most of the time of ray tracing will be used to track the energy in the system, Thereby speeding up the analysis. A simulation result of 200,000 incident rays is shown in Figure 14. In this case, 118,969 rays reach the receiver, and the calculation error is 6%. The use of probability mode ray tracing reduces calculation errors by 7 times and reduces calculation time by 42%.

?

On the contrary, if the Fresnel loss coefficient and surface scattering characteristics are used to determine the possibility of light transmission and reflection, to evaluate the possibility of optical path path, most of the time of ray tracing will be used to track the energy in the system, thus Speed up analysis. A simulation result of 200,000 incident rays is shown in Figure 14. In this case, 118,969 rays reach the receiver, and the calculation error is 6%. The use of probability mode ray tracing reduces calculation errors by 7 times and reduces calculation time by 42%.

Finally, in order to improve the uniformity of the display, a diffuser is sometimes used on the top surface of the light guide plate. Since the diffuser diffuses the light to a wider angle, less light is scattered to the aperture of the brightness meter. According to the conventional display brightness test method, a very large amount of light is required for brightness calculation. The LightTools software maps the target area or angle to the scattering surface, allowing the user to specify which scattering should be considered. This is an important sampling form and another method to improve the convergence of Monte Carlo simulations.　Figure 15 shows a luminance meter, and a backlight with a diffuser, without specifying a target angle. After tracing 2000 rays, the brightness meter received 40 rays, and the grating of spatial brightness is shown in the figure.

?

Figure 16 shows the same example, but sampling by important value and specifying the target angle on the diffuser. The target angle matches the acceptance angle of the brightness meter aperture. When the light reaches the diffuser, the LightTools software will generate scattered light (the luminous flux that enters the target area calculated based on the angular distribution of the diffusion model) into the target angle, so that all the scattered light collected by the luminance meter will improve the convergence of the simulation. In this case, of 2000 incident rays, 1416 rays (71%) were received by the luminance meter.

Other considerations?

Backlight is widely used in liquid crystal displays (LCD), which is a polarization component. Modeling polarization components, such as linear polarization, quarter-wavelength plates, polarized light tracking evaluation, etc. are critical factors for successful analysis. LightTools software provides simple linear polarization and retardation models, as well as Jones-Mueller matrix specifications for polarization components. Users can use the polarization ray tracing function when needed to trace the polarization state of the light according to the Stocks? vector.

There are often various optical coatings with different transparency, reflection coefficient and polarization characteristics on the components. Coating is defined in LightTools software based on its performance, which is often the only information the user knows. The average or separate S or P values of reflection and transmission can be specified by any two of the following parameters: angle of occurrence, wavelength, X position, or Y position. The system provides tools for converting the coating stack into the coating format of the LightTools software.

Although most backlights use dot printing or compression molding light extraction technology, other methods are also possible. One is to use the particles in the light guide plate to scatter. If the size and density of the particles are properly controlled, Mie scattering from the particles can effectively extract light from the light guide plate (Tagaya, et al., 2001:6274). LightTools software can simulate the scattering of spherical particles in batches according to the Mie theory, or according to a user-defined angular distribution. ?

Exporting a complete optical design to a CAD system is often a necessary step in the manufacture of light guide plates. 　LightTools software supports standard format conversion such as STEP, SAT, or IGES to complete. Since the data conversion standard only supports external geometric data, in the case of compression molding design extraction, it is necessary to convert the shape defined by the three-dimensional texture into external geometric data for output. LightTools software supports standard formats and can selectively convert 3D textures into external geometric data, so that the entire backlight design is included in the converted file.

summary

Backlight design technology has been continuously progressing and developing, 　 to provide better performance and lower cost to meet the needs of the market. This kind of innovation requires lighting design software to continuously add new features, especially support for shortening the backlight design cycle. The main functions of LightTools software, such as model creation and file size, ray tracing and simulation time, and the function of calculating a large number of optical parameters related to backlight design, have all been recognized and verified by the industry.

Version 5.0 of the LightTools software released in 2004 includes illuminance optimization for noise redundancy, which is very practical in backlight design. This function can automatically define the light extraction template to maximize efficiency and uniformity. In addition, the backlight template optimization tool of the LightTools software provides an effective method for optimizing the output distribution of the backlight and light guide.

Keywords: LED backlight display, optical design, analysis tool