When is a transillumination of the scrotum test used?

Check for hydrocele: Hydrocele is transilluminable as it contains clear fluid. Hematoceles, tumor, torsion, epididymitis should not transilluminate, however many testicular pathologies may have an additional hydrocele present. Thus not a very reliable test/examination. Scrotal ultrasound is much more accurate and should be used to make an accurate diagnosis.

Hydrocoel: Easy way to see fluid around testis.