A hydraulic unit for a braking system of a vehicle includes a body with actuator-side and brake-side ports. The light receiving element has a recessed portion 22 on the opposite surface of the surface confronting the detection surface. The golf ball preferably comprises a liquid, gel or fluid filled core andor core layer. Accurately quantifying 1,25-dihydroxy vitamin D3 is useful in differential diagnosis of vitamin D-related diseases and for monitoring vitamin D therapy in patients with chronic renal disease.