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I can't get a spotlight in Radiance to work.

The doc states:

The spotlight primitive type is used for self-luminous surfaces that require a directed output. It is defined with red, green and blue radiance values as well as an orientation ( output direction ) vector and a full cone angle ( in degrees ). The orientation vector determines the distance of effective focus behind the source centre ( ie the focal length).

What exactly does "effective focus" mean? Isn't this the same as the opening angle?

I tried something like

void spotlight spot_light
0
0
3 128 128 128

or

void spotlight spot_light
0
0
3 1000 1000 1000 5 1.0 1.0 1.0
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1 Answer 1

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Found some explanation here:

The focal length produces only a subtle difference as it shifts the effective position of the source behind the actual emitting surface. A real spotlight has the effect of changing its falloff to being something less than r^-2 (where r is the distance to the emitter) in this way. The actual falloff is closer to (r+e)^-2, where e is the focal length of the spotlight's optics. It is a minor concern, I admit.

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