[Radiance-general] Illuminance calculation on a virtual surface?

[Jim Estes]

Hi-

I have been using a script with rtrace to calculate the illuminance
values on a grid of points representing the work surface in my modeled
room. But now I would like to generate a false-color image of the
illuminance values on a plane .8m above the floor. I can easily do
the floor (or walls, etc.) with a parallel projection view, but I
don't see how to get the workplane illuminance values without creating
an opaque surface at the correct height which would change the light
distribution. Does anyone know of a way to specify a "virtual" surface
or other technique to do this?

Thanks,
Jim Estes