Direct and Diffuse Reflection & the Family of Angles

3-types-of-interaction

In a previous article, I introduced light and some basic elements about how light travels and interacts with its environment. The next step is understanding different surfaces affect light.

An surface can interact with light in three different ways. It can transmit light through it, absorb the light, or reflect it. Most of what we see is reflected light.

Of the three types of interactions, reflection is the most visible. A large part of photography consists of managing reflections. Sometimes we want to see the reflections and other times we try to manage and reduce reflections.

Types of Reflection


Light can be reflected from a surface in several different ways: diffuse reflection, and direct reflection.

Most surfaces will exhibit a combination of the three and not just a single type of reflection. The combination of different types of reflections are what make different surface types appear differently.

Direct Reflection

Direct reflection (also called specular reflection) reflects a direct image of the original light source. Mirrors are a great example of a surface that directly reflects light.

Direct reflection follows the Law of Reflection, which states that the angle of incident equals the angle of reflection. In other words, the light is reflected a the same angle as it came from. The image below illustrates this principle.

law-of-reflection

The original light source is visible in a direct reflection, but only from a certain set of angles. This range of angles is referred to as the Family of Angles (more on this below). The family of angles is subjective, depending on the position of the light source and where the reflection is being viewed from.

The distance between the surface and a light source doesn’t change the brightness of a direct reflection. The size of the reflection, however, does change. The closer the light source is moved, the larger the reflection becomes.

Diffuse Reflection

Diffuse reflections scatter the light in all directions. In a diffuse reflection, the original light source can’t be seen. The angle at which the light source is placed doesn’t matter because the reflection is the same from all angles.

diffuse-reflection

Although, no subject creates a perfect diffuse reflection, white surfaces usually produce mostly diffuse reflections. A thick sheet of white paper creates almost an entirely diffuse reflection. When you look at the paper, it’s impossible to tell where the light source originates from.

For diffuse reflections, the distance between the surface and the light does matter. The closer the light is placed, the brighter the surface becomes.

The Family of Angles

The family of angles is important for photographers when dealing with direct reflections. It can be used to decide where a light source should be placed.

Lights that are placed inside of the family of angles will produce a visible direct reflection of the light source. If the light is placed outside the family of angles, it won’t be visible to the camera.

In the illustration below, only the green light will be visible to the camera. The orange light is not inside the family of angles and its reflection won’t be seen.

family-of-angles

If you want the entire surface of an object to show a direct reflection, the light source must be large enough to fill the family of angles. A small light source will only produce a bright reflection somewhere on the surface.

Applying this Information

Photographers are constantly managing reflections. If you know ahead of time what type of reflection a surface is going to produce, you can set up the lighting to get the best possible results.

Direct reflections can be a blessing or a curse. One situation where direct reflection improves a photograph is a portrait. The catchlights in a subject’s eye are actually small direct reflections of the light source. This tiny reflection can change how the entire portrait is viewed. Other times, we want to eliminate distracting reflections that divert the viewer’s attention.

Pay attention to reflection and learn how different surfaces reflect light, and you will gain more precise control over your photography.