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Such surfaces can be used to reduce heat transfer in both directions; an example of this is the multi-layer insulation used to insulate spacecraft. Since any electromagnetic radiation, including thermal radiation, conveys momentum as well as energy, thermal radiation also induces very small forces on the radiating or absorbing objects.
Naturally, a good reflector must be a poor absorber. This is why, for example, lightweight emergency thermal blankets are based on reflective metallic coatings: they lose little heat by radiation. Kirchhoff's great insight was to recognize the universality and uniqueness of the function that describes the black body emissive power.
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes.
For example, in a room in which air temperature is maintained at 22 °C at all times, but in which the inner surfaces of the house is estimated to be an average temperature of 10 °C in the winter or 25 °C in the summer, heat transfer from the surfaces to the individual will occur, resulting in a difference in the perceived temperature.
The amount of conduction, convection, or radiation of an object determines the amount of heat it transfers. Increasing the temperature gradient between the object and the environment, increasing the convection heat transfer coefficient, or increasing the surface area of the object increases the heat transfer.
The heat transfer processes (or kinetics) are governed by the rates at which various related physical phenomena occur, such as (for example) the rate of particle collisions in classical mechanics. These various states and kinetics determine the heat transfer, i.e., the net rate of energy storage or transport.
Two-way radiation between two systems is one of the two mechanisms of transfer of energy as heat. It may occur across a vacuum, with the two systems separated from the intervening vacuum by walls that are permeable only to radiation; such an arrangement fits the definition of a diathermal wall. The balance of radiative transfer is transfer of heat.
This mechanism is found very commonly in everyday life, including central heating and air conditioning and in many other machines. Forced convection is often encountered by engineers designing or analyzing heat exchangers, pipe flow, and flow over a plate at a different temperature than the stream (the case of a shuttle wing during re-entry, for example).
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