I don't want a realistically bright light bulb to contribute to
radiosity, because its light already comes from a light_source.  So I
set no_radiosity on the bulb.  Problem solved, right?

But wait a minute!  What about the lampshade or hood?  Since they're so
close to the light source, they contribute significant radiosity to the
scene--except for the portion that is blocked by the light bulb.  But is
it blocked?  The documentation reads: "Specifying no_radiosity in an
object block makes that object invisible to radiosity rays..."  Yeah, as
if the light bulb isn't there.  Test renders reveal that, indeed, the
light bulb does not block radiosity from the surface behind it.

The issue is solved with a second, non-glowing bulb object with no_image
set.

The first image is a control with no physical light bulb.  The next has
the light bulb, and the scene is just as bright, indicating that the
bulb is not blocking any of the radiosity from the hood.  The last 3
images have a non-glowing, no_image masking object of various sizes.
The resulting illumination of the scene is slightly, but noticeably, dimmer.

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