Patent UVWatch check low, moderate, high or very high/extreme level of Solar UV radiation; The center area of thePatent UVWatch change colors according toSolar UV radiation, higher theSolar UV radiation, deeper the color change, compare the color change to the 4 standard colors(low, moderate, high, very high/extreme) around and you can see low, moderate, high or very high/extreme level ofSolar UV radiation; Tell you when is the best time to use solar charger or similar solar equipment effeciently: Low: UVI 1-2,Solar energy equipment can not be used; Moderate: UVI 3-5,Solar energy equipment can be used but the efficiency is low; High: UVI 6-7,Solar energy equipment can be used but the efficiency is mid; Very High/Extreme: UVI 8-11+,Solar energy equipment can be used and the efficiency is high. Tell you UVIndex and SPF of sunscreen you should apply: Low:UVI:1-2,SPF <5,sun protection not necessary; Moderate:UVI:3-5,SPF <10,sun protection recommended; High:UVI:6-7,SPF< 15,sun protection necessary Very high/Extreme:UVI:8-11+,SPF<30,,avoid exposure to the sun; Tell you when to reapply sunscreen: When you apply sunscreen to your skin, also apply sunscreen on the transparent plastic film covering the center ofPatent UV Watch, when the center ofPatent UVWatchchanges color, it's the time for you to reapply sunscreen; China Patented and CE&ROHS Certified.