The measurement of sun heat, or solar radiation, is typically done using an instrument called a pyranometer. A pyranometer is a device that measures the amount of solar radiation incident on a surface. It is typically mounted on a horizontal plane, facing upwards, and measures the total amount of solar radiation, both direct and diffuse, that strikes the surface.

Pyranometers are typically calibrated using a reference standard, such as a solar simulator or a blackbody cavity, and are then used to measure the solar radiation incident on a surface in watts per square meter (W/m^2). The output of a pyranometer can be integrated over time to calculate the total amount of solar energy incident on a surface over a given period of time.

Pyranometers are used in a variety of applications, including:

* Meteorology and climatology: Pyranometers are used to measure the amount of solar radiation reaching the Earth's surface, which is important for understanding the Earth's climate and weather patterns.

* Solar energy: Pyranometers are used to measure the amount of solar radiation available for solar power generation.

* Agriculture: Pyranometers are used to measure the amount of solar radiation available for plant growth.

* Building energy efficiency: Pyranometers are used to measure the amount of solar radiation incident on buildings and windows, which can be used to design energy-efficient buildings.

In addition to pyranometers, there are other instruments that can be used to measure solar radiation, including:

* Spectrometers: Spectrometers can be used to measure the spectral distribution of solar radiation.

* Radiometers: Radiometers can be used to measure the total amount of solar radiation, including both direct and diffuse radiation.

* Thermopiles: Thermopiles can be used to measure the temperature of a surface that is exposed to solar radiation.