For thermopile pyranometers, a thermopile is used within the instrument as the sensor, and the thermal gradients are measured across hot and cold areas (black and white). The radiation intensity is proportional to the temperature differences between the two sensing areas. Accuracy depends upon the sensitivity of the material used in the sensors, the response time, and the distortion characteristics of the material constituting the dome (if present) covering the sensors.
For silicon photocell pyranometers, electric current is generated by a photo-sensitive diode in proportion to solar intensity. Ordinarily, silicon photocell pyranometers are not fully sensitive to the full spectrum of visible light, and they cannot “see” a certain portion of the electromagnetic spectrum such as under cloudy conditions or vegetative canopies. Silicon photocell pyranometers will introduce errors under these conditions. In full sunlight conditions, however, they are calibrated to properly output solar radiation measurements.