Marketing verbiage often muddles the terminology used to describe the location of LCD light sources. In order to make an LCD assembly as thin as possible, the CCFL light source in the color LCDs used in most portable computers is located at the side of the light guide (see the position of the "Fluorescent Lamp" in the three "LCD Construction" drawings). Thus it is always correct (but not very informative) to use the term "sidelight" (the term "edgelight" is also sometimes used). In LCDs with transmissive or transflective construction, the combination of the CCFL light source and the light guide are mounted behind the LCD. This is correctly called a "backlight" because the light comes through the back of the LCD. In LCDs with reflective construction, the combination of the CCFL light source and the light guide are mounted in front of the LCD. This is correctly called a "frontlight" because the light source is in front of the LCD.

How can you tell whether the computer in your hand uses a backlight or a frontlight? Turn on the light source, hold the computer so that the LCD is at a very oblique angle to your eyes (try about 15 degrees), and look at all four edges of the LCD. If you see a very bright white light on at least one of the edges, you've got a CCFL frontlight (such as in the Compaq iPAQ H3600 or the Sony Clie PEG-710c). If you see a very dim white light, you've got a white LED frontlight (such as in the Palm m505 or the Compaq Aero 2100). If you don't see any white light at all, you've got a backlight (such as in the Palm IIIc or the Handspring Visor Prism). If you see a very narrow line of bright white light, you're probably looking at a backlight with a small light leak, most likely due to assembly tolerances. Ideally all light from a backlight should go through the LCD, rather than leaking around the edges, but that's sometimes difficult to achieve in a mass-produced product.)

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 geoff.walker@att.net.