Pyranometers are used to measure global solar radiation, most often on the thermoelectric principle. Their differential thermocouple, or thermobattery, indicates the temperature difference between a surface that absorbs almost completely the incident shortwave radiation and a surface that does not absorb the radiation or is shielded. A similar temperature difference is determined by the differential bimetal in a Robitzsch bimetallic pyranograph or by the temperature difference on the thermometers of an Arago-Davy pyranometer. Some types of pyranometers use photodiodes as sensors, which generate a photoelectric voltage proportional to the incident radiation. A distillation pyranometer or lucimeter measures global or circglobal radiation by using the radiation absorbed by the instrument's sensor to vaporize a suitable liquid, the volume of which, after recondensation, is a measure of the absorbed radiation. If direct solar radiation is removed by a shield, pyranometers measure the scattered solar radiation and operate as diffusometers. Pyranometers are usually fitted with two glass hemispheres to protect their sensors from the disturbing effects of wind, precipitation, internal air circulation within the sensor and from the deposition of dust and dirt. The hemispheres also prevent the passage of radiation of longer wavelengths than about 4 µm and cause the pyranometer to measure only short-wave radiation. If the pyranometer is exposed to hemispheres that allow the passage of long-wave radiation, i.e. it measures both short-wave and long-wave radiation, it is called a pyrradiometer.