Radiance: How does the focus of a "spotlight" work? 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
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.