Welcome for our computer graphics lecture.

So we are currently trying to develop

a physical description of light.

That's the basis for being able to do a simulation of light.

First, we have to understand how we describe it and so forth.

And of course, the first thing that we need to do

is to define or to derive quantities by which we

can describe light.

And so we started with a probably simplest possible way,

and that's using energy.

So light is just energy that is emitted

in the form of radiation.

And this is what we called E. So every photon

is a little, very small piece of energy.

And now typically, we are not really

interested in how much energy is distributed,

but how much energy per time.

And that's what we called flux.

So flux is just energy over time.

And this tells us how much light does a light source typically

emit per time.

And that's what you, if you buy a light bulb,

you typically look how many watts this light bulb has.

And that tells you how much light is emitted in the form,

or how much light is emitted per time.

And this defines the brightness.

OK, good.

But now this just tells us how many photons are emitted

per time, but it doesn't give us any hint about the direction

light is emitted and about emission of different points

on the surface.

So what we will want to have in the end

is something that tells us how bright is

a particular point on a surface when looking at it

from a particular direction.

So we want to have a spatial and a directional dependency.

And yeah, this has no dependency in that direction at all.

So the first thing that we did is

that we introduced a spatial dependency.

And we do that by simply deriving

the flux for the position.

So that means we take a little for a particular point.

We look at a little surface area around that.

We call that dAx.

And then we look at the emitted flux.

This one emits a flux d phi x.

And the smaller this area gets, the smaller

the emitted flux is, of course.

But if we take the quotient, then we

learn how bright that particular point is.