Light Sensor is a sensor that allows you to detect light and then convert it into signals to electrical signals are then used in a missing that use light as a triggernya.
There are several commonly used light sensors for example LDR (Light Dependent resistor), photo Transistor or a diode and a PhotoTransistor, Photodiode, photo/component this component is widely used in electronic circuits. The following is a more detailed explanation of each light sensor:
LDR
Photo resistor/photoresistor/ LDR is basically a resistor whose resistance value (in ohms) depending upon how much of the light that falls onto the surface of the sensors are.
The workings of this LDR is resistance will increase when it is not exposed to light (night) and will turn into a small resistance when exposed to light (daylight), LDR is typically combined with some transistor for membentul series automatic headlights. Fortunately there is no special code to read the value of the resistor is this light
Photodioda
Photo diodes or commonly referred to as Photodioda is a type of diode component which is capable to detect light. As well as ordinary diodes, this component has a P-N, it's just made sensitive to light.
This type of light can affect the diode of this photo which is infrared light, sunlight, ultra violet and x-rays. The use of the photo diode is very varied, as the countdown the number of vehicles on the highway automatically, as the light meter in digital cameras as well as the sensor on the equipment in the medical field.
Phototransistor
Next is a photo Transistor or Phototransistor. Phototransistor is 
simple is a bipolar transistor using contact (junction) base-collector who became a surface so that it can receive the light so it could be functioned as light sensor.
Phototransistor has better sensitivity when compared to diodafoto, this is because Electrons are generated by photons of light at the foot of the injeksikan junction in at the Base of the transistor to later reinforced at the foot of C/Kolektornya.
The disadvantage is, the response of the light Sensor Phototransistor will slower compared to a diode-photos.