Interaction of Thermal Radiation and Matter

Let $Q$ be the total thermal energy incident on the surface of an object in some time interval, and let $Q_a$, $Q_r$, $Q_t$ be the respective amounts of that energy which are absorbed, reflected and transmitted. By conservation of energy,

Dividing through by $Q$ defines three dimensionless coefficients:

satisfying $a + r + t_r = 1$.

• Coefficient of absorption (absorptivity), $a$: the ratio of heat absorbed to total heat incident.
• Coefficient of reflection (reflectance), $r$: the ratio of radiant energy reflected to total energy incident.
• Coefficient of transmission (transmittance), $t_r$: the ratio of radiant energy transmitted to total energy incident.

Being ratios of energies, $a$, $r$ and $t_r$ are all dimensionless, pure numbers.

Special cases.

• If $r = 0$ and $a = 0$, then $t_r = 1$: all incident energy passes through, the object is a perfect transmitter (perfectly transparent to the radiation).
• If $t_r \ne 0$ but the object neither fully absorbs nor fully reflects, it is called diathermanous -- neither a good absorber nor a good reflector. Examples: glass, quartz, sodium chloride, hydrogen, oxygen, dry air.
• If $t_r = 0$ (so $a + r = 1$, no transmission at all), the object is opaque to the radiation. Substances that are largely opaque to thermal radiation are called athermanous. Examples: water, wood, iron, copper, moist air, benzene.
• If $t_r = 0$ and $a = 0$, then $r = 1$: all incident energy is reflected, the object is a perfect reflector. A good reflector is necessarily a poor absorber and a poor transmitter.
• If $r = 0$ and $t_r = 0$, then $a = 1$: all incident energy is absorbed -- this is a perfect blackbody, the subject of Section 3.11. …