[Radiance-general] Annual illuminance simulation anomaly

[David Smith]

Dear list,

I was beginning an attempt to look at annual illuminance using the
information in a weather file (Energy Plus). As an initial test, I
used gensky while providing explicitly the direct normal radiation and
diffuse horizontal radiation from the weather file.
There was no actual geometry in the scene, just the sun, sky, skyglow,
etc. I then tested the illuminance with rtrace -I for a sensor
directed in the +Z direction as well as looking directly at the sun
where ever it was at that time.

What I found was that at the end of the day, the sensor point directly
facing the sun spikes dramatically. I've tried it with two different
weather files with similar results. Also, other sensor points pointed
in the general direction of the sunset spike correspondingly.

Here are links to two images, each showing the hourly averages for the
month of June:

Abu Dhabi: http://i49.tinypic.com/cnuhw.png
New York: http://i47.tinypic.com/20gz991.png

Does anybody have any insight to what may be causing this?

Cheers,

--Dave