Light is measured by either quantum sensors or pyranometers for either photosynthetic photon flux or irradiance, respectively. Because there is periodic shading in greenhouses from structural materials , quantum sensors are often placed outside on the greenhouse roof (arrow).

Pyronometers are better indicators of total radiant energy used to estimate heat load and are usually placed inside the greenhouse.

Image showing two photos. Top photo with arrow pointing out quantum sensors mounted on greenhouse roof. Second photo shows a pyronometer mounted inside a greenhouse.