Backlighting technologies: Light guide film (part 3 of 5)

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By Steve Baker | Oct 13, 2020
Keypad with light guide film backlighting

If you have read our previous posts in this backlighting series, you should already know what questions to ask before starting a backlighting project as well as when to use discrete LEDs. In this blog, we will be discussing the next backlighting technology – light guide film.

What is a light guide film?

Light guide film, much like it sounds, uses a thin film to guide the light from the LED(s) to the areas that need to be lit. The film has a reflective coating on the top and bottom layers of the film. The top layer is laser etched or abraded in the areas where we need light to escape and light the overlay. With varying depths and customized etching patterns, the distribution of light can be easily controlled, allowing specific areas to be brighter or dimmer.

Since the light feeds directly into the edge of the film, this technique uses side-fire or right-angled LED(s) to facilitate the optimal diffusion of light through the entire length of the film. The precise alignment and orientation of the film and LED(s) are extremely critical to the success of the design.

Advantages of a light guide film

The film’s low profile allows it to limit the impact on tactile feedback of metal snap domes or buttons. As a result, it is usually mounted directly below the graphic overlay and can be seamlessly integrated into thin and tight spaces. With minimal loss of light from the source to the other edge of the film, this technique promises uniform lighting across the entire plane with increased efficiency. It is a great solution for lighting large areas while still maintaining a mid-range price point. 

Some of the core benefits of a light guide film include –

  • Uniform lighting and brightness
  • Limited impact on the tactile feel of buttons or domes
  • Energy efficiency Ideal for lighting small, large, and curved surfaces
  • Suited for thin, compact, and flexible designs

Limitations of a light guide film

While light guide films are gaining momentum across several industries, few challenges need to be addressed while working with this technology. When the light travels from the LED through the film material, the edges are often illuminated very brightly, resulting in unwanted light leaks. This can be overcome by employing an opaque panel filler along the border of the film. Since LED(s) are butted up against one end of the film, it can create hotspots in areas around the light source. This can be eliminated by adjusting the printing process of the overlay to add a printed opaque layer. Due to the placement of the LEDs, light guide films generally struggle with lighting up the same area with multiple colors.

The main design concerns with light guide films include –

  • Light leaks from the edges
  • Potential hotspots around the LEDs
  • Limitations to backlighting the same area with multiple colors

Regardless of the drawbacks, light guide films are continuing to grow in popularity and the challenges will be mitigated as the technology evolves.

To see a few examples of light guide film projects and learn more about this backlighting technology, watch our short video below.