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odd illuminance map pattern

asked 2016-10-10 17:14:57 -0500

Matt Koch's avatar

updated 2016-10-12 07:32:51 -0500


I decided to learn about illuminance maps in OpenStudio/Radiance. I think I implemented this properly for one of the spaces in my model. The relevant section of my model in SketchUp/OpenStudio with shadows turned on is shown in this image, where the lower and upper floor each have an illuminance map:


With this, the lower floor results as shown in this image:


... and the upper floor results as shown in this image:


In both images (July 21 at 6 PM local time), I can only assume that the left edge is the west facade and the lower edge is the south facade. Note then how the west facade has a continuous color pattern in both maps, while the south face is interrupted once for the lower floor (C-103) and multiple times for the upper floor (C-303). Both edges have identical windows over the entire length of the facade, so I have no clue where the interruptions might come from.

On a related issue, I am struck by how poorly the light seems to penetrate into the space with most of it being blue on both floors. These illuminance maps are the brightest observed throughout the day, and I would think with the sun relatively low at 6 PM, the space should be flooded with light more evenly or I might at least see the window outlined? I picked a clear double pane 3 mm with air gap glazing, so I cannot believe that should hold back this much light?

Finally, this is an image of the daylight controls for the bottom floor, placed at exactly the same height as the illuminance map, about 10 ft in from the western facade. Might that cause a problem?


Any advice would be much appreciated.


Matt Koch

Updated Images:



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answered 2016-10-10 21:29:26 -0500

My guesses are that you are seeing the effects of a stochastic algorithm, mixed with relaxed simulation parameters (low sampling density, few ambient bounces, and relaxed subsampling thresholds). Here's an old-school frown: =(

A couple of things to try here:

  1. Use the "fine" setting for the Radiance simulation parameters; this will help smooth things out and will probably manage to find more light deeper in your spaces.
  2. It appears that your illuminance map definitions extend to the edges of the building; offset the map boundaries one or two feet to the interior of the thermal zones. This will pay dividends in a couple of ways. When you calculate daylight metrics and include points right up against the wall, you are hamstringing yourself unnecessarily. Even the IES guidance for calculating spatial daylight autonomy allows you this "wall offset". Including those dark corners in your DA and other calcs is unfair. Secondly, by moving the points away from that infinitely thin wall polygon you reduce the chances for random exterior calculated values to influence your interior ones. This second issue is less likely to surface as long as you take care of item one and increase the simulation parameters.

It's hard to comment beyond that for now without better overall views of the building, but I'm guessing the maps will make a bit more sense when you make the changes mentioned here. Let us know!

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Thank you kindly, rpg777! I had already used the "fine" setting, so now I offset the map boundaries inwards, as you suggest, and it does seem to have improved the situation. I can't seem to post the images here in a comment, but the interruptions have disappeared. The light penetration also seems to be up just a little. Still, I have no way of knowing if any of this is close to reality.

Matt Koch's avatar Matt Koch  ( 2016-10-11 08:18:18 -0500 )edit

You could edit your original post with an "update with images" if you like. As far as reasonable daylight penetration depth, you should expect to see "useful daylight" extending to a plan depth of about 1x the window head height -- maybe 1.5x -- along the east, south and west facades most of the time. We're talking maybe 200-300 lux. Those spaces appear pretty deep but it's hard to tell without some dimensions. Some sense of orientation would also help us to say whether the results make sense or not.

rpg777's avatar rpg777  ( 2016-10-11 09:58:36 -0500 )edit

Thanks rpg777. I attached the new images as you suggested. The penetration does indeed seem to be on the order of a window height, but the lux are much higher than you suggest. I am not a lighting designer, and have not used lux much, so have no feel for what they should be. However, this is in San Antonio, TX, where the sun is very intense. Might that explain the higher lux values? In the meantime, I also inserted an OpenStudio partition on the top floor, just to see what the shading pattern would look like - for what it is worth, it does seem to make sense to me.

Matt Koch's avatar Matt Koch  ( 2016-10-12 07:39:30 -0500 )edit

By the way, the top floor measures 113 ft on the west facade and 86 ft on the south facade. The bottom floor also measures 113 ft on the west facade, but only 65 ft on the south facade. The bottom floor is 18 ft tall, the top floor is 15 ft tall.

Matt Koch's avatar Matt Koch  ( 2016-10-12 07:41:38 -0500 )edit

Yeah, thanks for the additional work, and for the images! These make sense to me. That space is indeed very deep, and you can't expect to get penetration back there. Values seem right on the money, too. When I said 200-300 lux, I was talking about the low end, the values you'd get at the back of that 1-1.5x head height dimension. That looks like what you're getting. The higher values by the window amake total sense to me and are in the range I'd expect.

rpg777's avatar rpg777  ( 2016-10-12 10:26:23 -0500 )edit

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Asked: 2016-10-10 17:14:57 -0500

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Last updated: Oct 12 '16