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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

BlockR.gif (839 bytes)1. Do the terms "lightpowered" and "solarpowered" mean the same thing?

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

BlockR.gif (839 bytes)2. What are the advantages of light-powered watches?

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.

BlockR.gif (839 bytes)3. Do light-powered watches have batteries?

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

BlockR.gif (839 bytes)4. Do light-powered watches have quartz movements?

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

BlockR.gif (839 bytes)5. How do light-powered watches work?

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.

BlockR.gif (839 bytes)6. Who are the major manufacturers of lightpowered watches?

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.

BlockR.gif (839 bytes)7. How much light does a light-powered watch need to keep running?

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.

BlockR.gif (839 bytes)8. How can I tell if the watch's power is getting low?

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.

BlockR.gif (839 bytes)9. How long does it take to recharge a light-powered watch whose power is completely depleted?

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.

BlockR.gif (839 bytes)10. How long will a lightpowered watch continue to run if stored in darkness?

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

BlockR.gif (839 bytes)11. Many light-powered watches are made of titanium. Why?

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.

BlockR.gif (839 bytes)12. Can light-powered watches perform extra functions?

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.

BlockR.gif (839 bytes)13. Are light-powered watches new?

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.

Environment Time needed to charge watch from zero power to functioning power Additional charge time needed for full day power Time needed to charge watch from zero power to full 6-month power 
Normal office lighting  11 hours 50 minutes 175 hours
Under household lamp (30 watts/8 inches)  2 hours 10 minutes 40 hours
Outside, cloudy 30 minutes 3 minutes 10 hours
Outside, sunny  8 minutes 1 minute 3 hours
 Source: Seiko Corp. of America






Running time when fully charged 40 days 80 days 180 days
Sunlight exposure time need for:

full charge

One-day charge

 3.5 hours

2 minutes

3.0 hours

2 minutes

3.5 hours

1 minute

Low charge warning 1 day 5 days 3 days
Source: Citizen Watch Co. of America
BlockR.gif (839 bytes)HOW ECO-DRIVE WORKS


 Sunlight and any artificial light are absorbed through the crystal and dial ................................

A solar cell beneath the dial converts any form of light into electrical energy to power the watch. With regular exposure to light, Eco-Drive continuously recharges itself............................

Eco-Drive's lithium-ion rechargeable battery stores enough energy to power the watch for six months, (even in the dark)..........................

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