Global Horizontal Irradiance (GHI) is the amount of terrestrial irradiance falling on a surface horizontal to the surface of the earth. GHI can be measured with a variety of instruments. The most common instrument used to measure GHI is called a pyranometer which has a hemispherical (180°) view angle. The hallmark of a pyranometer is a true cosine response to incident angle, i.e. the response of the pyranometer to a beam of light is proportional to the cosine of the incident angle of the beam. Most pyranometers utilize a thermopile sensor to sense incoming light; however, pyranometers such as the Licor LI-200 use a photovoltaic device with a diffuser. Thermopile-based pyranometers have a flat spectral response to incoming light, while PV-based pyranometers will have spectral sensitivity according to the PV materials used in the sensor. Pyranometers using a PV sensor respond nearly instantaneously to changes in irradiance, unlike their thermopile counterparts which take anywhere from 1 second to 15 seconds to achieve full response to a step change in irradiance. GHI may also be measured with a photovoltaic reference cell, which will have spectral sensitivity and generally will not exhibit true cosine response.
In some PV systems, GHI is measured and a model, such as the DISC or DIRINT models, is used to estimate the DNI or DHI.