While most people are content switching lights on and off manually, there are many practical applications that require the use of light sensors. Practically speaking, many people have lights on their house that come on when they sense movement. In the same way, lights can come on when they sense light. This is particularly effective for protecting your home. Some of the higher end outdoor flood lights have a combination light sensor and movement sensor so they could detect a flashlight or day light.

However, protecting your home is not the only application for a light sensor. You can also find a night light sensor. I have these down my hallway and in bathrooms. They're wonderful for night time because they don't use a lot of electricity and they turn off during the day when there is ambient sun light. These night light sensors are cheap and effective. They're also very useful in a dark garage or shed.

Other applications for a light sensor are screw in light bulb sensors that turn the light off when the sensor detects light. These come in a directional base and a 360 degree base that can detect light from all directions. LEGO® MINDSTORMS® NXT robots can be outfitted with a light sensor as well. This usage is very popular in robot contests that have a dark or night time component.