What type of light source can be harmful to a night vision device? 

Your night vision device is designed to assist your viewing in the dark. It may be harmful and will likely damage the device if you use it during the daytime or whenever there is sufficient light to see the object. Also, keep in mind that strong direct light, such as projectors, car headlamps, strong flashlights and so on, may be harmful to your night vision unit if you direct your device at the source of these intense lights.