When light hits the sensor, it allows more current to flow. This reduces the voltage across it. IC(ON) current is one way to determine what type of substance is between the LED and sensor; the more light a substance blocks, the higher the resulting IC(ON) current. Since the absolute IC(ON) current through the transistor can vary with a number of factors, a substance should not be identified by just its IC(ON) current. Instead, it should be identified by the ratio of  IC(ON) current in a known state to the IC(ON) current in another state. This ratio is called the “contrast ratio.”