transillumination

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The passing of a light through the walls of a body part or organ to facilitate medical inspection.

from The Century Dictionary.

• noun A shining through; the process of causing light to pass through; specifically, in medicine, the throwing of a strong light through an organ or portion of the body as a means of diagnosis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine The examination of a body part, or a tissue sample, by shining light through it