Light Guides

The State-of-the-Art in Display Technology (September 2001 issue)

In all of three of the LCD Construction graphics, the light from the CCFL light source (the backlight or frontlight) makes what seems to be a right-angle turn in order to go through the LCD. Here's how that's accomplished.

In a backlight, the light guide - often called a "diffuser" - is a clear plastic slab with a highly reflective, white, ink-dot pattern printed on four sides and a CCFL butted up against the fifth side. The light from the CCFL is distributed throughout the slab via the printed dot pattern. To help even out the light distribution across the LCD, the plastic slab is usually wedge-shaped, and the density of the printed dots increases the further they are from the CCFL. All of the light from the CCFL thus has nowhere to go except out the sixth side and through the LCD. "Brightness enhancement films" are sometimes added, at significant additional cost, between the light guide and the LCD to help collimate the light. Because transmissive color LCDs have been used in portable computers since 1985, light guide technology for backlights is very highly refined.

In a frontlight, the light guide has a much harder job to do. Since you have to view the LCD through the light guide, it has to be as clear as possible and can't have a reflective coating on either of the two largest surfaces. It generally can't be wedge-shaped, since it has to sit right on top of the LCD and fit nicely within the computer's housing. Any light that escapes from the light guide without first bouncing off the LCD is wasted light - it decreases the observed contrast of the LCD. So what's the trick to making the light do a right-angle turn? The top surface of the light guide is covered with "microgrooves." The grooves are typically shaped like wedges 0.0004" deep and spaced 0.01" apart. The light from the fluorescent lamp bounces off the underside of each groove and heads towards the LCD. The grooves are too small to be seen clearly with the naked eye, so they don't interfere with viewing the LCD. And since the touchpanel is normally on top of the light guide, you can't feel them.

One of the big problems with frontlights is getting even distribution of light across the LCD. This is really difficult to do, since the only method of controlling and directing the light is via the microgrooves. With a frontlight size of 3"-4" diagonal (such as in the Compaq iPAQ H3600 and the Sony Clie PEG-N710c), light distribution is very even. With today's technology, the maximum practical size of a frontlight is about 6" diagonal. I know of only one product on the market with a frontlight larger than 6", and that's the Fujitsu PenCentra 200 CRF. This vertical handheld uses a 7.7" frontlight, and its light distribution is said to be somewhat uneven. Frontlights are still relatively young in development terms, since they've only been in production about 5 years. When frontlights become as highly refined as backlights (10 years from now), larger sizes will most likely be available.

Other problems inherent in using a frontlight include parallax error (due to the pen tip being further away from the surface of the LCD), and increased reflection of ambient light.

Based in Silicon Valley, Geoff Walker is a consultant with Walker Mobile. Geoff has worked on the engineering and marketing of pen computers since 1990 at GRiD Systems, Fujitsu Personal Systems (now Fujitsu PC) and Handspring. He can be contacted at

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