What is an Active Light Sensor?
What is The Difference Between Passive and Active Light Sensors?
As you recall, a light sensor is a sensor that gives a reading based on how much light is being coming into its phototransistor (a device whose current changes based upon how much light it is subjected to).
Using a light sensor actively merely changes the function of the light sensor by changing its focus from the entire ambient lighting to a specific surface or object. When positioned close to an object or surface, the sensor only detects the reflected light from that object and can give a reading on how light or dark that object is. Lighter objects give off low readings (around 40) and darker objects give high readings (anywhere from 100-200, depending on how close the object is). Keep in mind that the closer the light sensor is to the object, the higher the value will be, as the sensor's shadow reduces the light reflected off the object. If the sensor is in direct contact with any object, even a lightly colored one, it will return very high values, as little to no light is coming into the phototransistor from the outside (since it is wedged between the object and its own body).
The Lego Light sensor is also equipped with a built in LED for use in dark conditions. If there is little to no ambient or direct lighting, the LED will provide enough light for the sensor to work properly. However, during normal lighting conditions, the light from the LED will over saturate the area which the light sensor is detecting, which may cause it to read low values even when it is "looking" at a dark object.