Kinetic temperature (Tkin) of an object is measured through direct contact of the object’s surface. Normally when we are referring temperature of an object, we are referring to kinetic temperature. It is called true temperature of the object. It can be measured using a thermometer.
Radiant temperature (T rad) of an object is measured at a distance from the object surface. It is apparent temperature of the object. It is also called as brightness temperature. Devices like radiometers are used for measuring the radiant temperature of the objects.
Radiant temperature of an object depends on various factors such as emissivity, kinetic temperature, thermal properties and rate of heating etc.
In thermal remote sensing, we measure radiant temperature(i.e., brightness temperature) of the objects. Using the relationship between radiant and kinetic temperatures it may be possible to identify what is the kinetic temperature of the object.
For real world objects Radiant temperature is normally lower than the Kinetic temperature.
Only for black body, radiant temperature is equal to kinetic temperature.
What is black body?
A black body is a hypothetical object which is capable of absorbing and re-emitting all energy incident on it. Emissivity of a black body is considered as 1.
Water has emissivity close to 1.
Relation between Kinetic and Radiant Temperatures:
Both kinetic and radiant temperature of an object are related by the following expression:
Trad = ε^(1/4)*Tkin
where, ε = emissivity of the object
Emissivity varies with different objects and it is dependent on nature of the surface, surface temperature, wavelength etc.
Because of this emissivity factor, different objects which are having same temperature may have different radiant temperatures.