To measure the solar radiation a special instrument is used known as pyrometer. Optical Pyrometer is a device which measure the temperature of distant objects. It is based on the principle that the intensity of light received by the observer depends upon distance of observer from source and temperature of distant source.
The output signal of the detector (Temperature T) is related to the thermal energy. Irradiance j* of the target object through the Stefan-Boltzmann law, so the constant of proportionality sigma, called the Stefan-Boltzmann constant and the emissivity of the object.
To measure the temperature of a human body we use clinical thermometer. If something is too hot handle we can use thermocouple (a kind of thermometer that works by generating electricity) instead. But what if we have to note the temperature of something like furnace or remote fire? In that case we use the handy device pyrometer.
Pyrometer is a remote sensing thermometer which measure the temperature of a surface from the spectrum of thermal radiation it emits. Modern pyrometer has a optical system and a detector. It based on Stefan law.