Lighting Analysis and MR: CIE Clear Sky

[Anonymous]

I'm in the middle of doing a series of day-lighting studies in Max 2010 Design and have come across a snag.

As background I've modeled the classroom wing of a new school in Revit and imported into Max via FBX. I've placed a new daylight system (using the CIE Clear Sky model) in the model and have been using light meters to get footcandle readings in a series of spaces. This is using MR Final Gather only, following the direction of the white paper posted on ADSK's website. I'm running calculations on three separate lighting conditions: clear sky at 9AM and 3PM, and an overcast sky condition.

The problem occurs in the MR Sky: CIE Parameters box. This gives the values for DIFFUSE HORIZ and DIRECT NORMAL ILLUMINANCE. I think I understand what these values represent, (the amount of energy landing on a horizontal surface from a diffuse sky and from a direct beam of sunlight.) What I don't understand is why I can change these values. It seems like they should be a function of the sky model (date, time and location.) Also as I switch between clear sky and overcast I have to zero out the DIRECT NORMAL ILLUMINANCE. When I switch back to clear sky I have to place a value in this box to get direct sunlight. When I do this max changes the value to a new number, but this number is dependent on the value I entered. So the question is how do I know I have the right value for my analysis?

Any help on this would be greatly appreciated, the white paper is vague on this point. Thanks.

Ian

[Anonymous]

Does anyone have an answer for this? This really affects the output of the lighting analysis and subsequently the accuracy of the tool. Any help would be greatly appreciated.