Rendered illuminance image on task plane (at a certain height)

Hello,

I am using Ecotect export to Radiance. In Ecotect, I placed camera to get
illuminance image, but it seems that the output image shows lux levels on
ground level. It doesn't give lux contours on a task plane (at any level).
Can anyone help me out in getting lux levels on a certain height.

Thanks!
Dharam