The angle of incidence equals the angle of reflectance is a law of light. It’s also photo-speak that (allegedly) explains how to evenly light a background, a copy stand or photograph a mirror. Unfortunately it means very little unless there’s an understanding of what the two types of light meters–incident and reflective–do to measure light. If you read the reflective and incident posts in my Exposure Tactics posts you have a working knowledge of the terms.

Here’s an example of the law in use. A client of mine is a Vietnam vet. His interest in military history, particularly the Civil War, has led him to collect pieces from the conflict. This makes perfect sense since he and I both live in Atlanta, the only American city ever destroyed by war.
He brought a full sized Bowie style presentation knife for me to photograph. The knife was made in 1861 by ironworks owner Mark Cooper for the then fire chief of atlanta, Charles Beerman. Presentation knives have extraordinary detail. The design is acid etched into the steel using beeswax to form the pattern. After that tiny embellishments are added with steel scribes much like scrimshaw in ivory. My challenge was to photograph the whole knife along with its wooden and brass hilt so that the detailed finishing work could be easily seen. This is an unretouched view of the knife with a ColorChecker. Notice the nut under the blade just before it starts to curve toward the tip. It levels the knife.

There are no specular highlights on the shiny brass finishings on the hilt. Nor are there any on the blade even at the upper left where it curves inwards toward the light source. A specular highlight is a mirror image of the source of light shown in a reflective surface of the subject. There aren’t any blown out highlights typically associated with specular highlights because the whole photograph is a specular highlight.
By making the source of light large enough that it covers the entire knife then angling its surface so it doesn’t show in the bright metal on its blade or hilt is a great example of the angle of incidence equaling the angle of reflectance. The setup is simple too. First I put down a 53 wide roll of Savage Super White paper over a table to serve as a background. Next I placed a Chimera 82 by 42 panel frame covered with diffusion fabric between two stands. Two Dynalite location electronic flash heads illuminate the diffusion material. Fill light is bounced back into the scene with a 48 by 48 piece of white foam core.

The graphic shows how the angle of incidence equals the angle of reflectance principle plays in the setup for the knife. The camera does not see direct light from the diffusion panel because the lights angle is illuminating the background then bouncing into the foam core reflector. The camera is not in the path of the lights reflected angle.
Since I shot this tethered to a MacPro I could view the results at 100% right away. The detail that this light reveals is better than any magnifying glass. Look at this close up view of the presentation mark made with a Sigma 150mm f/2.8 macro lens.

So whats that got to do with the types of light meters? Here goes…
Incidence is the light falling on a subject. Incident meters measure that light. The reflective meter measures light that has already illuminated the subject. As its name implies this meter takes into account the amount of light a subjects sends to the camera.
So there you have it. Demystified, this principle simply means the angle of the light falling on the subject is equal to the angle of the light bouncing off of it. It’s really useful in all kinds of lighting situations. More on that later.