Characteristic of mass Transillumination test shows high light transmission 2025-11-19T09:34:39Z Ganglion cysts may show high transillumination. Bright transillumination A transillumination test involves shining light through an area of the body or a mass. This term refers to a transillumination test that shows especially high (positive) transillumination, which may be observed when a mass or swelling contains clear fluid rather than solid tissue or blood.