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.