What is light guide film (LGF)?

Shanyo’s engineers can use any of the four different technologies (light-emitting diode (LED), electroluminescent (EL) lighting, fiber optic lighting, and light guide film) to integrate backlight into your membrane switch according to your requirements. This document focuses on the light guide film–what it is, when to use it, and precautions before designing the switch.

What is light guide film (LGF)?

LGF is a thin film that guides the light generated by side-emitting or right-angle LEDs to areas that require backlight. The film is placed directly under the graphics cover layer and above the circuit layer so that the light will not be blocked by any circuit traces or tactile devices. The film can be cut into any pattern or shape in the switch. Multiple LGF films can be used in one application to provide discrete backlighting for different graphic features. Different colors of LEDs can be used to achieve unique lighting effects, or white LEDs can be used to illuminate different printed graphics on the cover layer.

Since LGF is only a conduit for the light generated by the LED, the film must be adjacent to the right-angle LED. The placement of the LED in the switch depends on several factors and will be determined by the Shanyo engineering team.

When to use (and not to use) the light guide film as the backlight of the switch

Although LGF is usually a more expensive backlight option (compared to other backlight options), it has the following advantages：

LGF is very thin, so it can be integrated into membrane switches that cannot exceed a certain thickness (that is, small, lightweight equipment)

LGF has a limited impact on the touch of the buttons.

It can provide uniform backlight over large and small areas and is suitable for applications where the light remains on when the switch is powered on.
The disadvantage of LGF is that sometimes there will be light leakage and hot spot problems, and both of these problems can be overcome by proper design. Due to the use of single-source LED lights, LGF is not suitable for lighting the same area with multiple different colors.

Critical Pre-Design Considerations

The limitations of implementing LGF in switch design include：

• Leave enough space for the right-angle LED.
• There is not enough space between the light source and the backlight area.
• The distance from the LED to the backlight area, the number of LEDs/backlight area, and the distance between the backlight areas (please refer to the distance table on the back).