Flux

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Flux can be viewed as the radiant energy flowing through a surface per unit time or the time rate flow of pure energy in a vacuum . Flux is the movement of photons flowing across a given surface based on time. It is very important to understand the energy flowing around the scene because it's the fundamental basis for photon mapping. The energy that is arriving and leaving each surface in the scene must be labeled. We need to break down the incoming energy and outgoing energy into two terms. These two terms are the radiance and irradiance, described in the following sections.

NOTE

TIP

The radiance closely represents the final color of objects seen by the observer.

Radiance

This term is viewed as the flow of energy per unit of projected area. It can be viewed as the amount of electromagnetic radiation leaving a specific point on a surface. Radiance (or radiosity) is the total illuminative power of light leaving a specific point on a surface per unit area. The observer records the radiance. It represents the intensity of light at every point in the scene and mainly contributes to the coloring of objects in the scene. This term is very important to understand because it is extensively used in global illumination .

It also can represent the intensity of light for any point in space in a given direction. Consider radiance the amount of energy flowing from a surface. Radiance is a constant when flowing through space or in a vacuum. You learned (or reviewed) earlier that electromagnetic waves cannot increase or decrease in energy because of the energy conservation and electromagnetic induction laws. The speed can change only if the light goes through some special situations; for example, when passing through a prism or a glass window. Don't worry; you'll learn more about relevant points of this concept later. Radiance is a complex term that goes way beyond the scope of this book.

Irradiance

Irradiance is the total illuminative power of light striking a point on a surface per unit area. It can be seen as the flow of energy arriving at the surface from some specific direction. It is basically the amount of energy flowing to a surface. This is also important to know because it will be used in photon mapping.

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