Small and Medium Size LCD Screen - Optical Design of Backlight Display
Backlights are used in small, lightweight, electronic devices such as flat-panel liquid crystal displays (LCDs) that require backlighting, ranging from hand-held devices as small as palm-sized to large-screen televisions. The goals of backlight design include low power consumption, ultra-thin, high brightness, uniform brightness, large area, and viewing angle control with different widths. To achieve these challenging design goals, with cost control and rapid implementation, computer-aided optical design tools must be used for design. This article describes the features of ORA's LightTools optical design and analysis software, which can be used to develop today's most advanced backlight design applications.
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 a fixed angle. Lighting design software must be able to model geometrically, set optical 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 through the model and calculate the final light distribution. Light distribution uses a 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. Illuminance can be calculated from surface receivers and intensity can be obtained from far-field receivers. By defining a luminance meter on the receiver surface, the distribution of luminance over 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 sources (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 be done in LightTools software.
The characteristics of backlit displays place special demands on lighting analysis software. As will be explained, the light emitted by the backlight depends on the distribution density of printed dots, or the distribution pattern of the microstructure. Modeling of specific microstructural arrays may result in very large model sizes if the CAD model is used directly. LightTools software provides the function of 3D texture array definition, which enables accurate ray tracing and rendering. Since no directly constructed geometric model is used, the model volume is smaller and the ray tracing is faster. Another aspect of backlight analysis includes the splitting and scattering of light on the surface of the light guide plate. Due to the use of Monte Carlo methods to simulate lighting effects, it is possible that extensive ray tracing must be used to obtain a sufficiently accurate design. The most efficient way is to trace the highest energy rays. By tracing the path of the highest energy rays using spectroscopic probability, and using the target area or scattering angle of the scattering surface to direct the scattered light in "important" directions (such as towards 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. Other available components include diffuser plates, which improve display uniformity, and Brightness Enhancement Films (BEFs), which improve display brightness. The light source is usually located on one side edge of the light guide plate to reduce the thickness of the display. Edge lighting typically uses total reflection (TIR) to direct light in the display.
Backlight designers have several ways to model light sources in 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 Lighting Creation Tool. The reflector of the lamp can be defined with various geometrical primitives in LightTools software, such as cylinder, elliptical slot, extruded polygon. Reflectors defined in CAD systems can also be imported into LightTools software via 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, etc. pre-stored in the LightTools software. Once the light enters one side of the light guide, the problem becomes to extract the light from the light guide perpendicular to the direction of propagation.
As shown in FIG. 3 , the brightest side of the light guide plate is the side close to the light source, and as the distance increases, the brightness in the light guide plate becomes darker. To obtain uniform light output, light extraction efficiency must increase with distance. One of the main tasks of backlight design is to design light guides that vary 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 at the bottom of the light guide plate to scatter the light upwards and emit from the surface of the light guide plate. The second technique, the molded light extraction technique, relies on total reflection (TIR) of the microstructure of the bottom surface to allow light to exit 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 the various components of the backlight. Other options include adding light source/reflector components to the model, BEF modeling, building a receiver to analyze brightness. The Backlight tool's interface is a collection of tabs for setting and modifying various types of light extraction mechanisms.
For backlights that use the dot-printing light extraction method, the backlight tool can set the size of the printed dots and the linear variation of the aspect ratio, as well as the linear variation of the dot pitch along the length of the LGP. This linearly varying structure is often a good starting point for showing uniformity, but is not sufficient for final uniformity requirements. Further control over uniformity can be achieved using nonlinearly varying ray extraction parameters. A method that uses minimal parameters and provides very flexible control is to define parametric variables for quadratic Bezier curves. The 2D Area tool of LightTools software can be used to set up nonlinear structures. Figure 5 shows an example using print extraction where 3 parameters (print dot width, height and vertical spacing) were varied to obtain different extraction behaviors. The output uniformity is shown in Figure 6. The graph on the right shows that the average output brightness is a constant.
