Comparison between reflected sun beam and potential glare from mirrored surface

Hallo dear @chris, sorry if I bother you,
I need to ask you, a somewhat curious question.

For a project, I need to see if there could be potential glare on the outside context, caused by a glass facade.
I built a script that generates the reflections coming from the position of the sun and everything that falls on the horizontal context is shown in a colored heatmap (yellow surfaces).

I wanted to go into detail and for individual positions (black marked circle), to see if there is a comparison between the potential glare derived from mirrored reflections and the potential glare seen by PITVIEW for the same time (Month/Day/Hours/Min)


One thing I noticed from this research is, that according to the size of the sun path aka “sun scale”, the point of reflection changes considerably, even in the exact same minute (see the left image).
For example the point of reflection with sun scale 1, is far lower than the mirrored sun in the fassade generated by radiance This also leads me to different results. Per example with sun scale 1, a certain sensor receives the reflected sun beam for 40 min in one day, while with sun scale 5, it receives 10 min more.

I would like to ask you 2 things?

  • Which sun scale is most suitable / realistic for this type of analysis?
  • Having also noticed that the position of the reflected sun between the geometric reflection and the sun generated from the CIE Sky mirrored on the fassade is not exactly in the same position, is there any inconsistency between the component of the LB Sun Path and the CIE Sky?

Thanks and nest regards

If this is the case, then it sounds like you did not set up your Grasshopper definition in a manner that accurately models the sun. Sun vectors should always be parallel to one another (given how far away from the earth the sun is). So the sun scale or the sun position should not be affecting your results if you have set up your script correctly using only sun vectors and not the sun position, which only exists in theory and does not exist in reality.

Thanks for the advice chris, stupid not to think about it.

I hope you come to bring the bounce from surface component to the LBT version as well, because there is so much potential like how to find potential glare or filter a certain point that you want to analyze and see the overall period or the exact timing in which that particular point is blinded.
It is a type of analysis that is in great demand, per example, when any new buildings or photovoltaic systems can create glare when trains pass.

Thanks for all and best regards

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No worries. I made the same mistake with using sun positions back in the day.

I hear you and I opened an issue about it on out GitHub. I think we already have all of the core methods that are needed to port this component over the the LBT plugin and it should not take me that long to add it. I’ll see if maybe I can do it tonight or this weekend.

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Hey @LaFleur ,

I jus wanted to let you know that I added a component that’s equivalent to the Legacy “Bounce from Surface” component. You can get it now by running the “LB Versioner”.

I also added a sample file to show how I recommend using it to study the focusing of light by a parabolic building geometry.

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Hello, may I ask you how you built this script that can simulate the glare reflected by the building glass curtain wall on the surrounding environment?

Hello, can you update this “rebound from the surface” plugin can change the surface material of the reflector? Or could you please make a follow-up update?

Hi @Wu ,

Here’s an updated sample Grasshopper file that makes use of the latest Surface Ray Tracing component.

Hello Chris, thank you very much for your reply, I still have some questions to ask you about this plugin. I am studying the influence of different photovoltaic curtain wall component materials on the degree of outdoor reflection glare. And I have noticed that the plug-in you shared cannot change the material of the reflecting surface, but only reflects as a mirror. Then can you develop a plug-in in the outdoor environment, by changing the surface material of the building curtain wall, so as to affect the outdoor glare intensity! I really hope to get your guidance!

Hi @Wu ,

If you are studying glare, then you should be running a Radiance simulation where you set up a view that you want to evaluate glare for and assign materials to the surrounding geometry. Here is a sample for a point-in-time workflow that evaluates an image and here is a sample for for an annual glare simulation.

Hello chris,
I am very glad to get your reply. However, my research is on the effect of glass curtain walls on reflected glare in outdoor environments, not indoors. This workflow you shared is only for indoor annual glare. But what I really want to achieve is to change the material of the building curtain wall to have an impact on the outdoor reflection glare. It includes the outdoor glare area and the light intensity within the area. Still looking forward to your guidance. Thank you!

What makes you say this?

Daylight Glare Probability is just as relevant in outdoor conditions as it is for indoor conditions. It’s not like putting a pane of glass between you and the glare source suddenly makes DGP an invalid metric.

However, DGP indicators are usually for interior design, not for outdoor light pollution studies. It is designed to help design lighting systems for indoor environments to mitigate glare problems. For the study of outdoor Glare and light pollution, the more common indicators and methods include the GR value (Glare Rating) of the International Commission on Lighting (CIE), which is used to evaluate the glare in the outdoor environment.Could you build a plugin that is rated by GR?

Ok, fair enough. If you want to use CIE Glare Index (CGI), the first sample file that I linked to for point-in-tone Image based simulation will compute GCI. You see that the HB Glare Postprocess component in that files gives you CGI as an output in addition to other glare metrics like DGP.

So you can use that for your case.

Hello Chris, after taking your advice into consideration, I simulated the outdoor glare of the glass curtain wall under outdoor conditions. As you said, HB Glare Postprocess component can get DGI, DGP, UGR, CGI and other indicators. However, the glare comfort determined by the above indicators is only applicable to the glare simulation in indoor conditions. I wonder if it is possible to add glare indicators for SGP and GR, two outdoor conditions, to this component. In addition, I have noticed that the HB Check Scene component can choose to analyze the brightness of the light source. I wonder whether the value of this brightness can be directly derived? Looking forward to your reply.

You can use falsecolor to render the HDR image, and toggle print extrama on, it will find the largest brightness region. Here is an image of facade reflection glare we rendered recently.The red region showed the reflection glare.

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Hello, thank you for your reply. So how do you quantify your glare? What evaluation index is used to evaluate the impact of light reflection on human eyes of your simulated building?

Is only my 2 cents, but many glare analysis (like Retinal Irradiance for critical impact on traffic, or potential reflected glare for residential areas, through p.e PV, Glassfasade,etc) are one of the most interesting themes wich has not been totally covered from LBT and wich had a lot of potential because is highly requested.

  • I think that to evaluete the potential retinal burn, caused from reflected glare, is the simplest analysis to do, because is more a formula calculation, after perform a Irradiance Analysis with LB.

    Maybe a after process component where take in account also the treshold from C. Ho would be great.

  • I would ask, dear @chris, if you think is possibile to have a component that calculate the specular reflection for a point or sensor grid, caused from the sun position (Backward raytracing?), to also have the information in wich PIT and for how long, each sensor had a potential glare?
    P.e. I have standards where residential, or hospital windows (on Center Point) should not be subjected to glare from a reflected source, for at least 30 minutes a day or 30 hours in a year.

Thanks and best regards