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

By Co Nguyen | May 13, 2015
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.  Now we’re going to go over the next backlighting technology, light guide film. 

Light guide film, much like it sounds, uses a thin film to guide the light from LEDs to the areas that need to be lit.  The film is has a reflective coating on a bottom and top layer of film.  The top layer is laser etched/abraded in the areas that you want light to escape and light your overlay. The etching pattern can be altered to control and allow certain areas to be brighter or dimmer, helping to eliminate hotspots.

Right angle LEDs are used when using light guide film. The right angle LEDs need to be butted up against the edge of the light guide film (whether the actual edge of the light guide or an edge inside a cutout in the light guide). Placement of the LEDs depends on several factors and will be determined by the engineer and vendor.  

Though it can be a more expensive backlighting option, light guide film may soon become the standard backlighting option because it has many advantages.  It can be designed very thin so it works well in small, light devices, and it has limited impact on the tactile feel of buttons. It is also a great solution for lighting large areas while still maintaining a mid-range price point. 

Some common challenges with light guide film can be light leaks around part edge and hot spots around the LED area. Light leaks can be overcome by using an opaque panel filler around the light guide. Hotspots around the LED can be avoided by making adjustments to the printing process of the overlay.

Light guide film generally struggles with lighting the same icon multiple different colors due to placement of the LEDs. Regardless of the drawbacks, we anticipate that light guide film with continue to grow in popularity and that the challenges will be minimized as the technology evolves.  

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