Today I will be adding a bit more about diffuse reflection, but before I get started, I think it would be wise to backtrack a bit.
Reflection, Absorption, and Transmission
Reflection is just one phenomenon that light can experience when it encounters a boundary (surface) traveling from one medium into another. When light hits a boundary, it may also be transmitted through the new medium or absorbed by it. For the purposes of our discussion, these three phenomena (reflection, absorption, & transmission) account for all of what light can do when it leaves one medium and encounters another. What is not absorbed or transmitted is reflected. The degree to which light is absorbed, transmitted, or reflected will depend on both the substance and the nature of the surface boundary. For example, clear glass will transmit a large portion of the light that hits it, and absorb relatively little. It can be an efficient direct reflector, but a poor diffuse reflector. If the surface of that glass is etched to produce a frosted surface, it will reflect much more light in a diffuse manner and not nearly as much directly. Unlike glass, black soot will be poor light transmitter, but a very good absorber. As it absorbs much of the light, it will be a relatively poor reflector. I think you get the idea.
Diffuse reflection was described in the first post on reflection. If you didn't read that, you may want to jump back two posts. An ideal diffuse reflector reflects light in an omnidirectional way. As a result, the surface will appear the same to the viewer regardless of the angle at which the light approaches the surface and from which it is viewed. Surfaces that reflect in a primarily diffuse manner are usually dull in appearance, but this doesn't have to be the case. For instance, a surface covered in fine crystals or glass beads could be both brilliant and reflect largely in a diffuse way.
We are surrounded by diffuse reflection. Our perception of form, luminance, and color are largely a function of how we see diffuse reflection. Foliage, sidewalks, clothing, and painted structures reflect the vast majority of light in a diffuse manner. Light meters are calibrated under the assumption that the metered scene will consist largely of diffusely-reflecting objects, and that, on average, the scene will reflect about 18% of the light that hits it. This leads us to the next topic.
Reflectance is a measure of the percentage of light that is reflected relative to the amount that is incident on the surface. All other things equal, a lighter-toned surface will have a higher reflectance than a darker one. Darker surfaces generally absorb a greater portion of the incident light, leaving less light to bounce back. I'll explore reflectance in more detail, especially as it relates to balancing the specular and diffuse reflections on a face. The following posts will be more fun, with real-world examples showing applications of the boring stuff of the past three posts.