A section of orange and a plastic cow lit from the front are approximately the same color.
That’s because we’re getting roughly the same volume and quality of light bouncing off the surface in what’s called diffuse reflection. This accounts for only part of the light that touches the surface of the forms. What happens to the rest of it?
If we turn things around so that the light is coming from behind, everything changes. The cow is darker because the dense plastic absorbs the light on the far side and there’s not much ambient light or reflected light filling the shadow on the near side.
The orange section is practically incandescent. Light enters the transparent skin on the far side and bounces around inside the fruit, eventually reemerging through the surface. Note that the glow is brighter where the wedge is thinner.
This effect has the fancy name of “subsurface scattering.” It shows up most strikingly when three conditions are met: translucent flesh, small forms, and backlighting. It’s also present on the lit side; it’s just not as obvious.
If you hold your hand up against the sun or against a bright flashlight at night, light traveling subcutaneously turns the spaces between your fingers bright red.
Subsurface scattering is what makes a person’s ears turn crimson when they stand contre-jour. The effect is similar to transmitted light, which describes light traveling through thin membranous surfaces like leaves.
Artists have known about this property for centuries. Peter Paul Rubens rendered skin not as an opaque surface, but as a transparent, glowing, luminous layer.
When academic students who have begun their training on plaster casts transition to the live model, they’re often amazed by the way skin glows, especially in the fingertips, nostrils, and ears.
Tomorrow we’ll see how subsurface scattering gives realism to sculptures, animatronics, and 3D animation.
Related GJ posts: Transmitted Light and Contre-Jour Lighting
The Rubens is thanks to the Fine Arts Museum in Belgium, link: