It is a well-known fact that the sun"s ultraviolet (UV) rays cause skin aging, sunburn, and skin cancer. Unfortunately, your eyes can not easily determine the intensity of UV radiation especially on cloudy and snowy days. Weighing less than an ounce and smaller than a pack of gum, the SunMate can be worn to the beach, the mountains, or anywhere outdoors. It will alert you when the UV intensity exceeds dangerous limits so you can apply the proper steps to protect your skin and eyes. Perfect for outdoor activities in all seasons and all weather conditions. Take it to a ski get-away, a play date in the park, or a sunny day on the beach. SunMate stands out from other UV detectors. Affordable high end UV detector covering full spectrum of UV rays (UV-A, UV-B, and UV-C). Fit for all weather conditions, sunny, cloudy, even snowy days. Superior accuracy, advanced photo-diode technology (specially for UV) ensures accurate reading of UV. Simplicity, with safe/caution/warning lights, you can use it without any technical background. Weighing only 0.7 oz and the size of a gum, easy to attach to a key chain or throw in the pocket/purse. Durable for years of usage instead of one time use. Indications (by UV Index): 0 - 2 no light, 2 - 4 green (safe), 4 - 7 yellow (caution), 7 plus red (warning). The SunMate is powered by a single 3 volt button battery (CR2025). Replace the battery after continuous use for more than 4 hours.