Transillumination is a diagnostic technique in which bright light is projected at or through an area of interest. It can be used in the diagnosis and evaluation of a number of medical conditions, and can sometimes be a very useful quick medical test when a doctor wants to make a rapid evaluation. There are no risks associated with transillumination, and the procedure is painless for the patient. This makes it especially appealing for examinations of infants and children.
Many people have engaged in a form of transillumination by shining light through their fingers. If the area is dark, the fingers light up with a red glow, because the blood in the fingers absorbs light waves in other areas of the spectrum. This trick also works on feet and toes, as some may have already noted. Basic transillumination of this kind done with a flashlight is not far off from the techniques used in the examining room.
By passing light through an area of tissue, a doctor can sometimes collect important information. Irregularities in the distribution of the light can indicate that there is a problem, such as a buildup of fluid where there shouldn't be, or a mass. Transillumination can sometimes be used to visualize clots in thin tissue, and it is also used in a procedure known as transilluminated phlebectomy, in which transillumination is used to guide a surgeon as she or he removes varicose veins.
Examinations of the ear, nose, and mouth often take advantage of transillumination. The bright light makes more structures visible, providing a complete picture, and variations in the light can be used to identify problems, as well. Transillumination is also used in the examination of breasts and testicles, or in the examination of infants, who are small enough that light can pass through their torsos as well as their extremities.
In some cases, transilluminating an area can rule out a potential diagnosis, allowing a doctor to move on to other diagnostic tests. In others, the transillumination may reveal a problem which additional testing can confirm, or the transillumination may be a diagnostic tool, with no additional testing needed to confirm a diagnosis. Over time, doctors become skilled at recognizing familiar patterns and shapes, learning to identify abnormalities of concern and to distinguish normal variations in the human body from pathologies which need to be addressed with additional testing which can be used to gather information about treatment options.