Apogee offers two types of pyranometers; their original silicon-cell models and their new line of thermopile pyranometers, both of which are now ISO 9060:2018 Class C rated (previously known as second class). Silicon-cell solar radiation sensor models are excellent for applications that do not require the higher accuracy and cost of a thermopile pyranometer. They are less expensive and have a faster response time, but have higher errors under cloudy conditions.