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Frontlights
Frontlights are an alternative to backlights for lighting liquid crystal displays that are used at night.
Frontlights are used only with reflective LCDs. As always, an LCD subpixel acts like a light valve, allowing
light to pass or not. On reflective color LCDs, ambient light enters from the front. When not disrupted, it
passes through the liquid crystal material and color filter to a reflector which reflects it back out the front.
A frontlight consists of light sources along one edge of the LCD, which emit light into the edge of a diffuser.
Fine optical structures in the diffuser cause the light to be uniformly reflected into the liquid crystal structure,
where it is handled in the same manner as ambient light.
Unlike the backlight diffuser, the frontlight diffuser must be transparent and it must be very highly one-sided.
It must be transparent to the controlled light being reflected out from the liquid crystal. It must be one-sided
so that all the light going into the diffuser is sent out towards the LCD. To the extent that it allows light to
escape towards the user, it will wash out the LCD image. Yet this one-sidedness cannot diminish its transparency
to LCD image light. See the Pen Computing article
Light Guides
for a better discussion of frontlight characteristics.
LED Frontlights
The light emitting diode (LED) makes an excellent light source because these devices are small and low power.
Small size is important because it must emit into the edge of a diffuser and the diffuser must be very thin
in order to be very transparent. Light is emitted from the surface inside the LED, so there is no need for a
reflector behind it. LEDs are powered by low voltage DC. No high-voltage inverter is required, although a much
smaller current limited power source is recommended.
CFL Frontlights
LCD vendors who lack transflective technology sometimes provide cold fluorescent light (CFL) based frontlights for their reflective LCDs.
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