Outdoor sensor lights are a great solution for changing regular on/off lighting to both PIR and daylight sensor based lights, this is a great solution for energy savings and security.
Photocell outdoor sensors are typically in a closed state (which allows the lights to be on), and when light is detected by the sensor, it deactivates the light. Typically, if the sensor fails, it the light should stay on 24/7 however this can vary depending on the brand.
Light sensors, especially outdoor sensors, operate by converting light energy into electrical energy. This is done when light hits a semi conductor material on the sensor, which then creates a flow of electrical energy which is detected and used to switch off a light during daytime hours.
Most motion sensors can be bypassed either by using the built in manual override (Only certain models have this) or by installing an additional light switch to control the outdoor light.
Outdoor sensor lights, specifically photocell sensors, can be tested in several ways - either cover the sensor with a light blocking material (electrical tape is commonly used) during the day time, or at nighttime use a flashlight and shine it directly into the sensor to see if the light dims or switches off.