Emission and Absorption

Emission and Absorption

The entire range of electromagnetic waves is referred to as the electromagnetic spectrum. One part of the electromagnetic spectrum, for example, is visible light. The electromagnetic spectrum is also broken down into different types of radiation depending on the wavelength range. Examples are gamma radiation, X-ray radiation, UV radiation and infrared radiation. Each part of the electromagnetic spectrum is known as a spectral range.

Infrared radiation predominantly occurs in the spectral range of 0.7 μm to 80 μm. The emission of electromagnetic waves in this spectral range is synonymous with the propagation of heat radiation. In contrast, the emission of radiation from other spectral ranges makes only a small contribution to heat radiation. The propagation of infrared radiation occurs, depending on the geometry of the radiation source, either in all directions (from the sun, for example) or in a single direction, such as with planar radiation sources. Radiation released from the radiation source as IR radiation disperses into space as waves.

IR radiation is not only emitted uniformly with a specific wavelength (e.g. 8.5 μm), but actually contains different wavelengths. The given wavelength range, e.g. of 3–10 μm, is of technical importance. The infrared spectral range is broken down into 3 bands for physical and technical reasons. These bands are, according to DIN 5031, IR-A (short-wave IR // 0.7 – 1.4 μ), IR-B (medium-wave IR // 1.4 – 3.0 μ) and IR-C (long-wave IR // 3.0 – 80.0 μ).

When exposed to IR radiation, objects absorb some of it. The absorption of this radiation is an influx of energy and increases the temperature of the object. 

There are different causes for IR radiation only being partially absorbed by the objects it is in contact with. One physical fact which is crucial to the absorption process is that thermal energy can only be transferred from an object of higher temperature to an object of lower temperature. This also applies to the transfer of radiated energy (e.g. contact heating). Another point for consideration is that radiation between the emitter and the absorber is interactive. In other words, the absorber emits IR radiation during the absorption process.

One reason why an object is only able to absorb some of the total IR radiation that it is exposed to is in the nature of its molecular structure. The nature of an object's molecular structure simultaneously defines its ability to absorb IR radiation in a definite wavelength range. This wavelength range is its absorption spectrum. There are bodies and materials with multiple maximum absorption values, within which ranges they absorb IR radiation parts equally well. Just as emitters can emanate IR radiation with different wavelengths, objects or material hit by the IR radiation can absorb it in the same way. Because materials have different molecular compositions, they can be shown to have an individual absorption spectrum at a certain temperature (e.g. room temperature). Radiation not corresponding to this absorption spectrum is able to pass through (transmission) or is reflected. This radiation does not contribute to the heating of the material.

The general basic rule that a good absorber must also simultaneously be a good emitter is thus derived from the physical principles above. There are, however, considerable differences at times with regard to the wavelength range in which a certain material absorbs or emits more at a given temperature. The following list of materials compares the total absorptivity α for solar radiation and the total emissivity ε at T=300 K:

Material

  α

  ε

Roofing felt

0,82

0,91

Snow

0,30

0,95

 

The example of snow shows that it can only absorb solar radiation at α 0.30. The reason for this is its reflective white surface for the given spectral range of solar radiation. Snow melts slowly in spite of solar radiation. If snow were to be exposed to long-wave IR radiation, for example, radiation from the warm wall of a house, its absorptivity would increase. In the process it could achieve a value of up to ε 0.91, which is also its emission maximum. The convection effect is disregarded in this example. Another example of material-specific differences with regard to emission and absorption is specially layered metal foils used in the manufacture of solar collectors. Their absorptivity and emissivity are α 0.95 and approximately ε 0.05 respectively.

The observation of these material-specific differences with regard to absorption and emission is not inconsistent with a good emitter simultaneously being a good absorber. The differences given in the examples are meant to show clearly that absorption and emission are dependent on the respective radiation intensity within a wavelength range.

A few laws of radiation that make the physical principles of IR radiation theoretically and experimentally comprehensible will now be mentioned.

Law

Meaning

Planck's law

Distribution of radiation intensity emitted from the perfect black body
depending on wavelength and absolute temperature

Kirchhoff's law

Emission and Absorption are equal

Stefan–Boltzmann law

The radiated power emitted from an object is proportional to the fourth
power of its absolute temperature

Wien's displacement law

Indicates at which wavelength a black body emits the most radiated power
depending on temperature

Inverse square law

Describes how much radiated power decreases depending on the distance to
the radiation source