Smartphones and other similar devices have a number of sensors on them for different purposes. In particular, there are two light-oriented ones that work differently and accomplish different things.
One is an ambient light sensor; it helps decide how bright or dim to make the screen, or perhaps whether your keyboard needs some backlight.
The other is the proximity detector. It works in conjunction with a small IR LED; the sensor measures the reflections of light from that LED to decide whether the sensor is near… something. This is typically used to shut off the touchscreen when the phone is next to your ear so that your face doesn’t inadvertently trigger various strange, unwanted events while you think you’re just on a call (something evidently referred to as “cheeking”). The sensor isn’t always on – apparently in some phones it’s disabled in landscape mode, or perhaps it’s only enabled when a call is in session.
A bit of Googling makes it clear that people very often confuse and conflate these two sensors.
Maxim noted that ambient light sensors can typically be thrown off by large amounts of ambient UV and IR light. So they’ve integrated in an optical filter to reject invisible light. This gives the ambient light sensor (a fusion of two actual photodiodes plus the filter) a range roughly like that of the human eye.
They’ve also integrated a third photodiode on the same chip, tuned to respond to the IR light from the pulsed LED – which is external to the chip. They’ve filtered this as well to reject – of all things – DC IR light. That enables the sensor to respond to the IR pulse from the LED without being thrown off by a constant level of ambient IR light.
So what you end up with is their MAX44000 sensor, just announced. You can find more info in their release…