This exact reversal of the intensity of lighting lets the fish blend into its environment. In a top-lighted deep water environment, the light scattered to each side is equal. Looking up, one sees brightness, looking down there is dim blue-green. The same angle from the vertical will yield the same shade, regardless of direction. Stacks of reflecting crystals can be arranged to form an almost perfect mirror.
Many pelagic fish have reflector plates arranged vertically in the skin.
Viewed from the top it is dark like the depths. Viewed from the bottom, it is silvery white like the surface. The hardest to camouflage is a view from directly below from which viewpoint the fish would obstruct some of the intense light from above. Many clupeids like the threadfin shad, Dorosoma petenense are thin and come to a knife-blade edge at the belly. It has no belly to be seen from below.