Friday, November 26, 2010

Reflections Part 3

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

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.

Wednesday, November 10, 2010

Reflections Part 2

In the previous post on reflections we covered the basic characteristics of direct (specular) and diffuse reflections. In this post I'd like to address three characteristics of direct reflections: direction, size, and intensity.


Direct reflections have a very definite sense of direction. In fact, on a plane reflective surface, the angle at which the light approaches the surface is exactly the same as the angle at which it leaves. This principle is most often stated as: the angle of incidence equals the angle of reflection. It can be extended to non-planar reflective surfaces using the tangent to the surface at the point of reflection. All of this is shown in the diagram below. This principle is very important and explains, among other things, why a light's placement can have a big impact on its apparent strength.


The size of the reflection will vary based on the curvature of the reflective surface. Compared to a reflection on a plane surface, convex surfaces will yield a smaller reflection and concave surfaces a larger one. For convex surfaces, such as spheres, a smaller radius of curvature will yield a smaller reflection. This is shown in the diagram below, where the grayed areas represent the reflection. The implications of this principle are numerous, and it explains why, for instance, a specular highlight on one's nose is much smaller than the one on the forehead.


The intensity of a reflection depends not only on the intensity of the source, but the size of the reflection. For instance, in the diagram above, the smaller reflection on the smaller sphere will be brighter than the reflection on the larger sphere. That is because both must reflect the same amount of energy, but must do so over differing surface areas. And, this explains why the that small specular highlight on the nose is brighter than corresponding hot spots on the cheeks or the forehead.

The next post will be a quick one on diffuse reflection, and then I'll try to bring it all together by applying these principles to real-world applications and examples.

Tuesday, November 2, 2010

Reflections Part 1

First, I apologize again to anyone who was looking for new content here. I made a promise I couldn't keep, and I'll not do that again. Nevertheless, I will try to add new content as time permits.

Today I'd like to start the first of several posts on light reflection. For me, it is the most important of all lighting concepts. I'll try to keep the technical stuff to a minimum, using visual aids to make the major points.

Reflections come in basically two forms: direct and diffuse. A direct reflection is a reflection of the light source, and is also called a specular reflection. Mirrored surfaces, for example, produce primarily direct reflections. A diffuse reflection, also known as an indirect reflection, is created when light striking a surface is scattered in a variety of directions. Matte paper, and talcum powder are two examples of diffuse reflectors. Keep in mind that no surface is a perfect direct or diffuse reflector, rather most surfaces produce a combination of both direct and indirect reflection. The differences between the two reflection types are shown diagrammatically below.

The diagram above shows the setup for a little experiment which uses a highly polished steel sheet as a direct reflector and a piece of white foam-core board as a diffuse reflector. The series of photos below shows the results for the two surfaces and two lighting positions. Note that the white board reflects back light similarly regardless of light position, while the polished plate produces strikingly different results. The direct reflector here acts in a kind of on-or-off fashion, either you see a reflection or you don't.

In the following posts, we'll dig a bit deeper into reflections and examine how they can affect our portraiture.