Transmission occurs when light or electromagnetic energy passes into a material and some measurable amount is able to pass out the other side. Unless the material is a vacuum, the path of each photon of light typically involves innumerable collisions with atoms along the way, but the absorption is low enough that they still have energy when they reach the other side. The transmittance of an object or instance of material measures its effective transmission, given as a ratio of the total amount of light or electromagnetic radiation that is transmitted compared to the total amount incident upon it. The transmissivity of a material is a measure of its ability to transmit light or radiation.

Figure 1 - A set of spheres showing the visual effect of different levels of transmittance in a glass material.

The transmittance of opaque materials is typically zero as light energy is entirely absorbed before it has a chance to travel very far. The modifier ‘typically’ is used because it is possible to create very thin films from materials that we would normally consider opaque. When thin enough, in the order to 50 - 100nm, many such materials become transparent or at least translucent to light.

The transmittance of a vacuum is effectively equal to 1. A tangible example of this is the ability of light to travel millions of light years though space and still be detectable. Most inert gasses are also very good light transmitters, though many have very specific absorption frequencies that create characteristic spectral lines when light passes through them. Glasses and other crystals are the most commonly used building materials with relatively good light transmission.

The Process of Transmission

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. 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.

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 transmission spectrums of different transparent materials and elements.

Transparency, Translucency and Opacity

Transmission is not only affected by the density of atoms in the material, but also on its atomic structure. For example, the molecules in some materials such as water, glass and gemstones allow light to travel through without much scattering or absorption. This is because of the number of electrons in the atoms they contain and their energy states, they simply do not absorb photons with energy levels that equate to light in the visible spectrum. They do absorb photons with energy above and below this band, and some gemstones intrude into the visible spectrum at certain frequencies which give them a noticeable colour, but visible light is relatively unaffected and ends up traveling in virtually the same direction when it exits than when it entered the material. Such materials are termed transparent.

If there is a greater amount of scattering but still high transmission, light will still pass through but the direction each photon travels as it exits depends less upon the angle in which it entered. Such materials are termed translucent.

Finally, many materials have relatively high absorption such that all of the light energy is absorbed before it has a chance to pass all the way through. These materials are termed opaque.

It is important to note that many materials that we think of as opaque can actually be transparent if produced within a thin enough film. Similarly, if a transparent material is made much thicker than it normally occurs, it can turn translucent or even opaque.

Water and Transparency

Evolution suggests that the reason we see pure water as transparent is because our eyes originally evolved in water. Water is very interesting in that there is a small band of radiation for which it has very weak absorption. This band includes the area that we refer to as the visible spectrum (see Wikipedia). Thus it would make sense to be most sensitive to this band as the low absorption means that it penetrates the deepest into the oceans, more than 220m for some frequencies before being completely absorbed.

Units and Measures

Transmittance is measured as the ratio of the total amount of light or electromagnetic radiation that is transmitted through an object of material instance compared to the total amount originally incident upon it. As such, it is a dimensionless fractional value between 0 and 1.

Measuring Transparent Materials

To measure transmission, all you need is a highly directional light source and a radiometer, photometer or spectroradiometer as a light detector. To calculate transmittance, a measurement of the light source is first made by placing it in line with the detector without the sample material in place. The sample material is then placed between the light source and detector, and another measurement taken. The ratio of these two measurements gives the transmittance of that sample of material.

Measuring Translucent Materials

Figure 2 - An example of a LabSphere RT-060-SF integrating sphere.

Measuring the transmittance of translucent materials is a bit trickier as the transmitted light is more diffuse so it is difficult for a single detector to pick it all up. In these cases, it is possible to use an integrating sphere as the detector. The basic idea is that the inside of a sphere is coated with a highly diffuse reflective material with very low losses due to transmission or surface absorption, typically Barium Sulfate or PTFE.

With diffuse reflections internally and the fact that all the surface normals of a sphere point directly at its center, the light reaching any point on the surface of the sphere will be reflected to all other points so, regardless of the initial direction of light, its energy will be spread evenly over all parts of the sphere. Thus a detector with a known aperture size can be used to determine the overall total amount of energy within the sphere by simply multiplying the energy it receives by the reciprocal of the ratio between its area and the total area of the sphere.

Useful References

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