rendering meshes with interpolated vertex normals

Hi,

I'm having a problem rendering polygon mesh objects with interpolated
vertex normals (converted via obj2mesh). I'm running Radiance 3.7.2b
patch release 22 Aug 2005.

When I render a sphere mesh that's illuminated from above and to the
side, I get bad artifacts along the shadow line, with large dark
triangular wedges periodically protruding into the illuminated
area. (See link below for what this looks like.) The dark triangles
seem to correspond to those quadrilaterals in the mesh where only a
single vertex is illuminated. My guess is that it's treating the whole
quad as pointing away from the light source, even though one vertex
and a significant area of it are in the illuminated region of the
object.

Is there any way to avoid this?

Thanks for any help.

- Bob