Proximity sensors such as those described here work by emitting infrared (IR) light and measuring the amount of light that is reflected by an object in close proximity to the sensor. However, when other sources of IR light are in view (such as the sun), the sensor cannot know whether the light it is receiving actually comes from the IR LED in the sensor and has been reflected by an object, or whether it is just ambient light. This can cause the sensor to falsely believe that an object is closeby, while in fact it just saw ambient light coming from the sun, an (incandescent) light bulb, or some other IR-light source (TL or fluoresent light bulbs usually don't emit enough IR light to pose this problem).