Light Absorptance

Definition

Absorption is the process by which electromagnetic radiation is converted to another type of energy as it passes through a material. The absorptance of a particular object or instance of material measures its effective absorption, given as a ratio of the total amount of light or electromagnetic radiation that is absorbed compared to the total amount originally incident upon it. The absorptivity of a material is the amount of absorption that takes place per unit of distance light travels within it.

In opaque materials, absorptance is directly related to reflectance and therefore the surface color. Black materials have the highest absorptance values and white materials the lowest. Objects with a discernible colour will fall somewhere between these two extremes. In transparent and translucent materials, the relationship with reflectance is less direct, with colour and transmittance taking over.

Opaque materials obviously have the highest absorptance values as transmitted light is completely absorbed almost as soon at it enters. Translucent and transparent materials have lower absorptance values as some of the transmitted light is able to travel all the way through without being absorbed.

The Process of Absorption

When light passes through a material, it interacts with it at a sub-atomic level. All materials – be they solid, liquid or gas – are made up of molecules that contain atoms of one or more elements. These atoms contain varying numbers of protons, neutrons and electrons, with the electrons orbiting a nucleus made up of the protons and neutrons.

When a photon of light collides with an atom, it imparts its energy by changing the orbit of one or more electrons. Each atom can only support a certain range of electron orbit states. If the new state of the electron is supported then the electron will stay in its new orbit, increasing the kinetic energy of the atom which is then dispersed within the material as heat. When this happens, that photon is effectively absorbed.

If the new electron state is not supported, the electron quickly decays back to a supported state, releasing the excess energy as a photon of light. That photon either goes on to collide with another atom or, if it is close to the surface, escapes back out of the material as reflected light.

Different atoms support different ranges of electron states and, as light with different wavelengths contains different levels of energy, the confluence of the two means that some wavelengths will be absorbed more than others depending on the type and distribution of atoms within the material. This gives us the characteristic absorption spectrums of different materials and elements.

Transparent and Opaque Materials

One question that seems to come up reasonably often in this area can be summarised as follows: ‘if reflection and absorption are directly related to color for an opaque object, why do the Fresnel equations for transparent objects not seem to account for colour or absorption in any way?’. I am not sure I have the absolute answer for this one, but I do have an opinion on the subject.

When light is incident on the boundary between two transparent materials, Fresnel equations allow us to calculate the amount that is reflected and transmitted, but they do not include any consideration of the amount absorbed. Also, the equations themselves depend entirely on the refractive index of each material, suggesting that absorption plays no part in the reflected component. Reading further, absorption in transparent materials is only applied to the transmitted component as it travels through the receiving material. Thus, for transparent materials, the maxim seems to be whatever isn’t reflected must have been transmitted.

Fresnel equations do not apply to opaque materials and there is no meaningful transmission component. Thus, the behaviour of incident light is determined by the reflectance and absorptance of the material. In this case, the maxim becomes whatever isn’t reflected must have been absorbed.

The difference between the two is simply nomenclature as the fundamental mechanisms are the same in both instances. A photon of light strikes the material boundary and collides with an atom, imparting its energy by changing the orbit of an electron. Depending on the wavelength of the photon and the nature of the atom, the electron either retains its new state (thus absorbing the photon), or quickly decays to its original state, emitting the excess energy as another photon. One of three things can then happen to this new photon. The most likely is that it will collide with another atom and repeat the process. Alternatively, if it is near the boundary surface, it could escape back out and become part of the reflected light component. If, after many previous collisions, it is near the other side of the material, then it could escape out that way and become part of the transmitted component.

In opaque materials, there are simply more atoms that are able to retain their new electron states and therefore effectively absorb all of the photons that enter. In transparent materials, the photons either pass through without colliding with anything or the atomic structure of the material is such that very few electrons retain their new states and the emitted photons mostly travel in the same direction as the refracted light wave. In translucent materials, the atomic structure is such that few electrons retain their new states but the emitted photons travel in any direction. This mans that the distribution of both reflected and transmitted components are more diffuse.

In my view, confusion stems from the fact that there is so relatively little absorption within transparent materials that, when considering just the initial incidence of light on the surface, there is effectively only reflection and transmission. Thus, the fact that Fresnel equations do not consider absorption is absolutely right as they deal only with surface incidence. However, the reflectance of a transparent material also includes internal reflective effects from the rear boundary. As shown in the Reflectivity and Reflectance topic, these are not inconsiderable and this is where the absorption values of different glasses makes a real difference. When calculating the total reflectance, the Fresnel equations have to be applied multiple times as the light bounces back and forth between the front and rear faces. Each time, the incidence light value is degraded by absorption, thus affecting both overall reflectance and transmittance.

Units and Measures

Absorptance is measured as the ratio of the total amount of light or electromagnetic radiation that is absorbed compared to the total amount originally incident upon it. As such, it is a dimensionless fractional value between 0 and 1. Whilst a vacuum is the closest we can ever come to a material with an absorptance of 0, no real material other than perhaps a cosmic black hole has an absorptance of 1.

In some fields, the theoretical concept of a black body is used as a reference point for maximum absorbtance. This is defined as an idealised body that absorbs all radiation that falls on its surface, effectively having an absorptance of 1. Again, such a material does not exist, but it is possible to get reasonably close using a hole within a surface behind which is a cavity completely lined with black absorbing material.

Absorptivity

Absorptivity and absorbance are quite different. Absorptivity measures the attenuation of light radiant power with distance and is more commonly termed an attenuation coefficient. This is sometimes also termed an extinction coefficient, however it is important to note that extinction is a measure of both absorption and scattering. Absorptivity and attenuation coefficients are all given as fractional values per meter of distance travelled.

It is also very important that absorptance and is not confused in any way with absorbance which, like absorptivity, measures the attenuation of radiant power with distance but is a logarithmic ratio.

Example Values

The following are some calculated values of opaque surface aborptance for a range of common colours. Again, I used to have web references for these values but they have long since moved on or died, so you should treat these values as indicative.

Useful References

• https://en.wikipedia.org/wiki/Absorptance
• http://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29
• http://www.physicsclassroom.com/.../Light-Absorption,-Reflection,-and-Transmission
• http://chemwiki.ucdavis.edu/.../What_Causes_Molecules_to_Absorb_UV_and_Visible_Light
• http://violet.pha.jhu.edu/~wpb/spectroscopy/basics.html