# Difference between irradiance calcs with LB cumulativeSkyMatrix and HB Run daylight Simulation

Hi all

for shading control we need irradiance values at specific points. Triggerpoint is 150 W/m2 at outer surface.

I first calculated them with the Ladybug cumulativeSkyMatrix as in example 05 Real Time Radiation Analysis.

When I realised, that with this component you can’t examine a whole period, because all values are summarized, I switched to the HoneyBee Run daylight Simulation method provided here. Thanks for sharing. (http://www.grasshopper3d.com/group/ladybug/forum/topics/hobeybee-ho…)

When I compare the results for particular HOY, then they differ slightly.

Example: at 04 18 17:00

LB cumulativeSkyMatrix: 786 W/m2

HB Run daylight Simulation: 816 W/m2

Can someone enlight me about the calc algorithm behind the two methods and which one is the one to be more trusted.

-rpict

Hi rpict, I can get back to you with a longer reply later today but I need more information about your case. Is the point inside a space or is it outside?

Hi Mostapha

the point is outside.

Well I noticed, that for HoneyBee radiance case, I also had the ground plane included in the simualtion. Without groundplane the result is 776 W/m2, which is almost the cumulative Sky value.

Ok, this leads me to another question:

The above calculations were done with a grid size of 3, resulting in a single test point on the exploring surface. When I decrease the grid size to 1, resulting in 16 test points and take an average of the values, the result is 464 W/m2 for the same HOY.

-rpict

Rpict,

You definitely want to have a small grid size in order to evaluate the radiation at a few different points (not just one) and average them together. The HB run daylight simulation component is pretty much always going to be more accurate than the LB Cumulative Sky MTX because HB computes bounces of light (like that happening off of the ground or your shade). The LB cumulative sky MTX component does not compute any light bounces (only direct radiation from the sky) and it is therefore only accurate in very open outdoor cases. In you case, it looks like it should not make that big of a difference but the light bounces off of the ground and your shade should change the result.

-Chris