Proximity and Ambient light Sensor (ALS) stick a light emitting device and a build-in digital conversion function of IC which unified the photodetector for both proximity and ambient light in one package. So this sensor has both function of the proximity and ambient light detection.
The light from emitting device returns from the measuring object. Proximity sensing is to detect far and near distance from sensor to measuring object by the light amount which returns from measuring object. Ambient light sensing is to detect the environmental lighting by the light amount which is detected with the photodetector for ambient light.