What type of environment can passive sensors measure?

Passive sensors are designed to detect natural energy that is emitted or reflected from surfaces, primarily in the form of sunlight. Therefore, they are reliant on the availability of light, which means their measurements are primarily taken during daylight hours when the sun is illuminating the Earth’s surface. These sensors capture this solar energy that has been reflected or emitted from the Earth, allowing them to gather data on various features such as vegetation, water bodies, and urban infrastructure.

While they can function under certain artificial lighting conditions, the key characteristic of passive sensors is that they do not have their own source of energy; instead, they depend on external light sources. Therefore, the correct choice emphasizes the reliance of passive sensors on the sun as an energy source, outlining that they perform best when this natural radiation is present.

In contrast, other options like measuring only in total darkness or solely during sunny conditions do not capture the broader operational range of passive sensors, which can still function in overcast conditions as long as some sunlight is available. This nuance highlights the limitations of those statements, reinforcing the importance of sunlight as the primary energy source for passive sensors.