# reflectance function question

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I was just looking at Mario's reflectance function documentation and I have a bit

of a question.

if the flux density emitted by the light will is ΦE

and the density emitted by the source doesn't depend on the angle of the source to the surface

is the following really correct?

ΦE = P / A ( N

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is the following really correct?

ΦE = P / A ( N

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ΦE is indeed the symbol I'm assigning to the flux density emitted by the source. And no, it doesn't depend on some angle to a surface since it is an attribute of the light -- you can think of it as the number of photons emitted by the source per unit area of its surface, and as such, it is what it is regardless of how that area is positioned in space relative to some other surface.

Let me see if I get this

In the above explaination ΦE is being expressed based on the unit area of the surface

So the area is constant: ΦE=P/A

With this equation

ΦE = P / A ( N

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Hi Luca,

the equation above could be written:

ΦE f(θ)=P/A cos(θ)

15651[/snapback]

No, but I'm pretty sure from what you wrote that I know what the problem is now (and it's entirely my fault)...

The above is only true if you group it like this:

ΦE f(θ) = (P/A) cos(θ)

But I always intended it to be grouped like this:

ΦE = P / (A cos(θ))

It's just that in my haste to write it down I didn't put the brackets, and since I always read it that way, I didn't even notice that it was ambiguous. Also, since I can't write formulas directly in LaTeX, I got lazy and didn't make a picture to show it properly as:

```          P
ΦE =  -------
A cos(θ)```

which is the way it was always intended to be read... sorry about that

I'll fix that, and also I think I'll change it so everything is expressed in terms of A at normal incidence (and call it An) which should make everything a lot clearer.

Thanks for your comments. I'll make those changes when I get a chance, and please let me know if it makes more sense then.

Cheers!

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