Watch marketers have recently brought out a bevy of lightpowered models meant to save both the environment and trips to the mall for new batteries. The watches are part of the trend toward eco-friendly, user-friendly timepieces that also produced the battery-less technology of Seiko's Kinetic and SMH's Autoquartz watches. How do light-powered watches work? How much light do they need to keep running? The following run-down answers these and other questions.

by Norma Buchanan

Yes, the names both refer to watches that draw their power from light. All such watches will run on either sunlight or indoor lighting.

These watches have their power replenished continuously by exposure to light, so they never need new batteries. This saves the wearer the trouble of bringing the watch into the repair shop and eliminates the risk of environmental pollution from discarded batteries.

Most have rechargeable batteries. Some have capacitors, which serve the same purpose as batteries: i.e., storing electricity.

Yes. Except for the source of energy used to make the quartz crystal vibrate, light-powered watches are just like traditional quartz watches.

The light enters the watch through the watch's crystal and dial. It then hits a solar cell underneath the dial. In some watches, the solar cell is a disk as big as the dial itself. In others, it's a smaller rectangle. The light knocks loose electrons in the solar cell, creating electrical current. The current is then stored in the battery or capacitor. From that point on, the watch operates just as traditional quartz watches do--i.e., the electricity sets the quartz crystal vibrating and the vibrations are divided into one-second impulses which are transmitted to the stepping motor, gear train, and finally the watch display.

The two biggest makers are Seiko, whose Pulsar brand includes the Pulsar Solar collection of light-powered watches, and Citizen, with its Eco-Drive models. Other makers include Junghans of Germany, with its Solar 1, SolarTEC and Phoenix Solar watches.

A light-powered watch will run indefinitely in normal lighting conditions, i.e. sunlight or indoor lighting. The energy it stores up during the day will keep it running through the night.

The light-powered watches now on the market have a low-power indicator that warns the wearer when the watch needs to be recharged. When power is running out, the watch's seconds hand jumps at 2-second intervals. Depending on the watch model, the low-power indicator starts operating when the watch has between one and five days' worth of power remaining.

It takes 3 to 3.5 hours of exposure to bright sunlight to fully recharge the watch. Indoor lighting takes much longer-175 hours for some models. For a day's worth of power, you need only a few minutes of sunlight (some watches require as little as one minute). Some watches feature a quick-start function that will get the watch running after just seconds of exposure to light. On Pulsar's models, for example, the seconds hand will start moving after just 2 or 3 seconds.

If the watch was fully charged when put away it will run from 40 days to 6 months, depending on the model.

There is no technical reason for this. Watch marketers believe the metal, which is recyclable, hypoallergenic and lightweight, fits the image they want to convey for their light-powered watches -- one of high-tech efficiency and eco-friendliness.

Manufacturers are starting to introduce multi-function models. Citizen, for example, has introduced a chronograph in its Eco-Drive line. Next year, Junghans will add solar models to its line of super-accurate, radio-controlled watches.

No. Although watch companies are now showing a renewed interest in them, some companies were selling rudimentary versions of them as long ago as the '70s.