Transillumination through the chest is only possible on small newborns. Diagnosis of pneumothorax is always confirmed by chest X-ray . Areas filled with air or fluid that is not native to that location have increased light transmission and transilluminate when they should not. Benign tumors tend to appear red. Malignant tumors are brown to black.
Light passes readily through the fluid.
Transillumination is used to detect the presence of fluid within the skull.