In software like AGI32, Visual and Dialux, the luminous shape is used for 2 parts of the calculation, the initial emission of light and the shadowing of light that comes back into contact with the luminaire. The luminous shape is also used for the rendering of the room.
For this paper, we looked at a 10' ring luminaire, similar to the photo above, but with only indirect light. The ring itself was narrow compared to the diameter, so there is a large open region in the center.
Without taking special care, many IES files for a fixture like this will have the luminous shape set to a cylinder or circle of 10' diameter with the light emitting out of the top. Since there is no way to describe the center of the ring as being open, light is emitted from the entire circle, which results in a large region of light on the ceiling which is actually brightest in the center (lower right image). In reality, light is emitted from the perimeter of the ring and thus creates a ring of light on the ceiling, which can be shown in a Photopia simulation of the actual ring geometry (upper right image).