LBT 1.5.0 diffuse radiation

Hi all. I’m working with the new LBT 1.5.0 version. On the way to split the direct and diffuse radiation, I found there are two ways in the figure below that

  1. No replace: using “LB Deconstruct Matrix (XMatrix)” component

  2. Replace: using “LB Deconstruct Data(XData)” component to replace all of the direct _normal_rad with 0

Although I guessed direct_normal_rad has nothing to do with diffuse, I found there are differences between the diffuses of “1. No replace” and “2. Replace” .

Which diffuse is correct? What is the difference between these diffuses?


Hey @chomi ,

The gendaymtx Radiance command that is used under the hood of the “LB Cumulative Sky Matrix” component uses the location to figure out where the sun should be in the sky and then uses the combination of direct and diffuse values to figure out how cloudy the sky is in order to distribute the solar energy over the sky patches. So setting direct radiation to zero and changing all of the diffuse radiation to be for a cloudy sky will change the distribution of the diffuse radiation over the sky patches. You can imagine that, for a sunny sky, there would be more diffuse radiation closer to the disc of the sun than there would be on the other side of the sky. For a cloudy sky, the radiation will be much more evenly distributed across the sky dome.

1 Like

Thanks so much, Professor Chris, for your reply. I understand the component.
Basically I can get better data with “1. No replace” than “2. Replace”.

Yes, I would not recommend zeroing out the direct radiation before you generate the cumulative sky since this means you’re creating a cloudy sky that does not exist in the climate you are studying.

If you’re trying to run a study that only looks at the diffuse component of the sky (acknowledging that the direct component is still there but you’re just not studying it), then you can create the sky matrix as you normally would but deconstruct it with the LB Deconstruct Matrix component, zero out the diffuse sky patches, then reconstruct the matrix with the LB Construct Matrix component. This is similar to the methods used to create a “Radiation Benefit” sky that you see in this sample file.

1 Like