If it is truly smart, your Smartphone uses an ambient light sensor and a proximity sensor. These sensors are located behind the black glass near the speaker. The ambient light sensor optimizes the visibility of the LCD display based on light level. This allows adjustment to the power of the backlight; turning it down at night or in a movie theatre and cranking it up if you are outside in bright light. The proximity sensor is used to turn off the backlight of the display and disable the touch functionality when the phone is brought next to your ear, or any other object that isn’t your finger. This prevents your cheek from hanging up on that important phone call. Both ambient light and proximity sensor functions are designed to reduce power consumption and extend battery life.
So how do they work? The proximity sensor emits an infrared light which reflects off an object back to a photodiode. No object, no reflection, no signal on the photodiode. There is only one small problem, the infrared reflectivity of objects differ. And if that object is your head, well the color of your skin or hair will determine the amount of signal reflected. We used three optical cards to check the performance of a leading smartphone; white, grey and black. You can see the results in the video.
Spoiler Alert! The smartphone’s proximity sensor had no trouble sensing the white and grey card at a distance of about 2 centimeters. However, it could not “see” the black card. If you have black hair and talk on the phone for long periods, the proximity sensor of your phone may not be disabling the touch screen and your power consumption will not be reduced. You can test your smartphone by making a call and watching the display as you bring your phone to your ear.
To solve this reflectivity issue, Vishay recently introduced the VCNL4020 and VCNL3020. The VCNL4020 is a fully integrated ambient light and proximity sensor. Fully integrated means it contains the infrared emitter and photo detector for proximity, an ambient light sensor, and signal processing all in a single, surface mount package. The VCNL3020 is similar to the VCNL4020 but eliminates the ambient light sensing function. We tested this same phone using the VCNL4020 and it had no trouble “seeing” the black card.