What is Differential Absorption?

Differential absorption is a phenomenon where a substance with mixed components will absorb and reflect radiation at different rates because of the differing makeup of the components. A classic example of this can be seen in x-ray imaging, where a medical practitioner can take a picture of the inside of a patient's body by irradiating the patient to expose film. The structures of the body are visible on the film because they absorb x-rays to varying degrees; bone, for example, absorbs a lot of x-rays and thus the film is left unexposed to show the patient's bones.

Medical imaging is not the only field where differential absorption is useful. It can also be helpful with activities like measuring concentrations of atmospheric gases, studying remote events in space, and analyzing soil and rock formations. In all these cases, people rely on the fact that materials absorb and reflect radiation at different rates to gather data.

The technician can adjust the clarity and quality of the image by using different kinds of radiation or altering the intensity. In medical imaging, people need to strike a balance between getting a good image and endangering the patient. In x-ray imaging, for example, it is possible to get a higher resolution, but this would expose the patient to unacceptably high levels of radiation. Scientific researchers taking advantage of differential absorption must also consider the effects of the radiation on the experiment.

In physics, scientists use differential absorption with techniques like light detection and ranging (LIDAR) to learn more about the composition of the atmosphere. This involves shooting pulses of light into an area of interest and reading the reflected light on its return. Some of the gases in the atmosphere will absorb the light while others will reflect it, and the amount of scattering that occurs can tell researchers which gases are present. They can compare data from other sites to study topics like the ozone layer, meteorology, and the effects of pollution on the atmosphere.

In soil sciences and geology, differential absorption can be a useful way to collect data about what is happening beneath the surface of the earth or the ocean. Archeologists also use this technique for studying buried sites. Most commonly, this involves the use of a device that emits sound waves. The device passes over the surface and the reflected sound waves tell technicians what lies beneath, as things like rock formations, shipwrecks, or buried buildings will reflect sound waves at different rates than soil. Different kinds of soil will also have different reflection patterns.