The Photon

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The Photon

Rays, as discussed in Chapter 3, are great in simulating the movement of light from one point to another, but they are not perfect. Remember you are trying to simulate electromagnetic waves in space as much as possible, and rays just can't do the job by themselves based on their simple definition in mathematics. Rays can describe most effects such as reflection and refraction (mostly the major makeup of an image) using ray tracing. Again a ray can be seen as a very thin beam of light. Rays are seen as single entities moving around the scene independently of one another. Rays are primarily used in ray tracing. If you recall from Chapter 1, energy flows in massive amounts and as a group . Rays simply lack the capability to correctly simulate the movement of energy as a whole. Therefore, you need another primitive that closely describes the electromagnetic wave based on its properties of wavelength and frequencyhence the photon!

The photon is the basic quantity of light according to physics. A photon carries energy on a given wavelength. When an electron changes state between high and low energy levels, an excited atom emits a particle called a photon. A photon can be seen as the quantum of electromagnetic energya particle of light released from an atom. The frequency of a photon is a related to its energy, where each photon has its own energy level. For example, a photon of blue light falls within the blue frequency portion of the spectrum by carrying that amount of energy. The different energy levels contribute to different colors of light as countless photons interact with matter.

Obviously, we don't have an electromagnetic spectrum in the computer, so the frequency parameter isn't needed. The wavelength parameter can be used to break up the components of white light into red, green, and blue compo-nentsfor example, consider light bending in a prism. The photon's energy level is of principal interest when developing the foundation for photon mapping. We will use its energy level to track the movement of energy as a whole as it interacts with matter. When you add the photon concept, you can then track the movement of energy from one point to another in time on a given wavelength. You can use rays to track the movements of light, and photons to track the intensities and movements of light given a time and place.

Many people classify the movement of rays as radiometry and the movement of photons as photometry . A light beam can be broken into a collection of small tiny photons moving together as a group in straight-line paths. You measure the amount of energy that is released from a light source as the wattage / power . See Figure 4.3 for an example of photons being emitted from a light source. Try to visualize the photons as small energy particles that leave the light source and scatter in the scene. The amount of energy that's released from a light source is carried in each photon. The power of the light source is measured by the number of released photons.

Figure 4.3. The photons emit ted from a light source moving towards a surface.

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