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4.2 Direct Reflection

In this lesson you will learn about direct reflections and see how they work in lighting and photography.

4.2 Direct Reflection

In this lesson, you will learn about direct reflections and see how they work in lighting and photography. According to the wiki, direct reflection, also known as specular reflection, is the mirror like reflection of a light from a surface in which the light from a single incoming direction is reflected into a single outgoing direction. The word specular is sometimes used to described the brightest part of a highlight. It's also used to describe highlights created from hard lights. Specular means having the properties of a mirror. So, sometimes these usages are correct and other times they're just nonsense. To make things clear, I am going to use the term direct reflection instead of specular reflection. This direct reflection behavior is described by the law of reflection which essentially states that the angle of incidence equals the angle of reflection. Let's check out an example. So in this demo, I have my green laser, a mirror, and a protractor that's printed out on a piece of paper. The light that's coming from the laser to the mirror is called the incident light and the light that's leaving the mirror is called the reflected light. If you imagine a line being drawn perpendicular to the surface of the mirror, right around this 90 degree mark. This is called the normal line. The normal line divides the angle between the incident light and the reflected light into two equal angles. The angle between the incident light and the normal is knows as the angle of incidence. The angle between the reflected light and the normal is known as the angle of reflection. And you can see, no matter where I put the laser, these two angles always equal each other. In this example, I'm shooting a hard drive platter here in a room with only one light source. And in the first two photos here, you can see that the hard drive platter looks black. That is until the camera is positioned right in line so I can see the reflection of the light source in the hard drive platter. Now, the interesting thing here is that the light source that's being reflected in the hard drive platter appears almost as bright as the light source itself. I say almost as bright because mirrors aren't perfect and they don't reflect all of the light. There's always going to be a little bit of light that's lost to absorption. But essentially, the light source in the reflection is as bright as the light source itself. And this would seem to break the inverse square law because if I pulled this mirror back 30 more feet. The reflection of the light source would still be as bright as the light source itself even though it's at a much greater distance. But, you see the thing that changes is the size of the reflection. Even though the light source appears to be as bright in the reflection because the actual reflection is larger, it's reflecting more light. So if I move this light to half the distance, it's going to reflect four times as much light. Exactly what the inverse square law says that it will. This leads us to an important concept when dealing with objects that have a lot of direct reflection. When you are trying to photograph an object that has a lot of direct reflection, you need to think more about positioning the lights in objects to reflect back to the camera. In other words, you don't think about how to light the object as much as you think about how to get the reflection of the light which is pretty much the only thing that you're going to see back to the camera. This leads us to the idea of the family of angles. I learned about the idea of the family of angles in a book called Light, Science, and Magic, an Introduction to Photographic Lighting. This is a fantastic book which I highly recommend. The concept is fairly simple, if you are shooting an object with a lot of direct reflection, there is a range of angles that produce this direct reflection. Anything outside this family of angles is not something that your camera can see. This is an important concept to understand because it will help you determine where to position your lights. Because of the law of reflection, you can fairly easily determine where the family of angles is located with respect to the camera. If you are shooting a mirror-like object and you want to see a direct reflection of your light source in the mirror, you need to use a light source large enough to fill the family of angles. If you don't wanna see a direct reflection of the light in the mirror, we need to position the camera and the light so that the light is not located in the family of angles. So now that you've learned about diffuse and direct reflection, it's time to look at polarized reflections, which is coming up next.

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