Measure the light near you. *** This is 15 days trial. DONT purchase the full version if this does not work at your end. Requires light sensor. ***The app gives you current measure of how intense the light focused on your phone is. It also gives maximum & minimum value of luminosity. (Intensity of light is measured in terms of lux/sq feet.)NOTE – THE APP REQUIRES YOUR PHONE TO BE EQUIPPED WITH LIGHT SENSOR.