Area Lights and Soft Shadows

Hi,

I am working on implementing area lights and soft shadows.  What is the order of magnitude of the number of rays I need to send in order to get a good result (say what's in the book on page 240)?

I am also not convinced my algorithm is correct.  I changed the inShadow() function to send N number of rays to *random* points on the area light.  It counts how many rays were blocked and how many were not, and returns: unblocked / numRays (between 0 and 1).  (I've special cased 0 = no blocks and 1 = total blocks).

Then I pass this into the lighting function.

// as before, if in total shadow, just return the ambient
if shadowFactor == 1 {
  return ambient.Add(emissive)
}

Otherwise it does the normal calculations, and, at the very end, it scales the result by shadowFactor.  Which, with 200 shadow rays, produces something plausible (see attachment)...

result = (emissive.Add(ambient).Add(diffuse).Add(specular)).Scale(shadowFactor)

Is this the right general approach?  Is the number of shadow rays really that high?

Thank you
Dan