What is a photocell primarily used for in lighting systems?

A photocell is primarily designed to control lighting based on the amount of ambient light detected in the environment. This device operates on the principle of light sensitivity; it detects the level of natural daylight and automatically turns lights on when it becomes dark and off when there is sufficient light. This automatic control not only enhances convenience but significantly contributes to energy savings by ensuring that artificial lighting is used only when necessary.

The other options, while related to different functions or utilities, do not accurately define the primary use of a photocell. For instance, measuring temperature is irrelevant to a photocell's function, as that relates to thermometers and thermistors. Similarly, improving energy efficiency in appliances relates to technologies like smart energy management or efficiency ratings, not the operation of lighting systems. Monitoring real-time energy consumption pertains to energy meters or monitoring systems, which serve a different purpose than that of a photocell. Hence, a photocell's core role in lighting systems is indeed to activate or deactivate lighting based on the present light conditions.