Liquid Crystal Displays (LCDs) are widely used in applications such as mobile phones, PC monitors, televisions (TVs), and multimedia devices. Since LCDs are non-emissive—they do not generate light on their own—external illumination is required to make images visible. For transmissive and transflective LCDs, a well-designed backlight module is therefore essential.
A typical LCD backlight module consists of:
- Backlight sources
- Light-guide plate (LGP)
- Reflector
- Light-diffusion plate
- Brightness Enhancement Film (BEF)
This application note introduces common LCD backlighting technologies and configurations, followed by an edge-lit LCD backlight design example.
LCD Backlighting Technologies
Cold Cathode Fluorescent Lamp (CCFL)
Cold Cathode Fluorescent Lamps (CCFLs) have historically been the most widely used LCD backlight source. Their popularity stems from:
- High brightness
- Broad, continuous spectral output
- Good uniformity over large areas
However, CCFLs require high operating voltage and are gradually being replaced by LED-based solutions.
Light Emitting Diodes (LEDs)
LED backlighting is now the dominant technology for small and medium-sized LCDs. Key advantages include:
- Low operating voltage
- Long lifetime
- Resistance to vibration and shock
- Precise brightness control
- Compact form factor
Electroluminescent Panels (ELP)
Electroluminescent (EL) backlights use phosphor materials that emit light when driven by an AC voltage. They are:
- Extremely thin and lightweight
- Uniform over large areas
- Suitable for low-brightness applications
EL panels typically do not require a light-guide plate.
LCD Backlight Configurations
Several illumination configurations are commonly used in LCD systems:
- Broad-Area Emitters: A uniform light source covering the entire display area, such as an EL panel.
- Point-Source Direct Emitter Arrays: An array of discrete point sources (typically LEDs) placed directly behind the LCD panel.
- Line-Source Direct Emitters: Multiple linear light sources (usually CCFLs) positioned behind the LCD cell and combined with reflective cavities.
- Edge-Lit Point Source with Light-Guide: Discrete LEDs positioned along one or more edges of the display, with a light-guide plate redistributing light across the panel.
- Edge-Lit Line Source with Light-Guide: A linear lamp (usually CCFL) coupled into a light-guide plate to produce uniform illumination.
Example: Edge-Lit LCD Backlight Design
In this example, an edge-lit LCD configuration is used to illustrate the optical design process. The design assumes constraints typical of a cellular phone display, where compactness and efficiency are critical.
Design Assumptions
- Display area: 75 mm × 75 mm
- Light-guide plate (LGP): Input face thickness: 4 mm, End face thickness: 1 mm
- Brightness Enhancement Film (BEF): Vikuiti™ T-BEF 90/24 (scaled model)
The light-guide thickness is selected to minimize overall package height while maintaining acceptable illumination uniformity.
The settings are shown in figure below:
The simulated intensity distribution shows effective light extraction from the LGP, with brightness concentrated toward the viewing direction due to the BEF.
The illumination uniformity across the display area is well controlled, indicating appropriate extraction feature density and effective light recycling.
The overall optical efficiency of the system is approximately 60%. Further improvements may be achieved by:
- Optimizing scattering features on the LGP
- Adding reflective or anti-reflection coatings
- Refining BEF and diffuser stacking sequences
Reference Source