Discrepancies between the illuminances values read from DS annual analysis and calculated with climate based sky on an analysis grid


Good evening everyone,
I’ve been redirected here from the McNeel Forum.
I’m quite new to grasshopper and HB&LB, I have a question. I premise I saw something similar in the forum, but i did not understand if it applies to my issue (I’m referring to: Illuminance discrepancy from Honeybee grid based, Honeybee[+] grid based, and Honeybee[+] annual), if so I apologize for the inconvenience.

Currently I’m performing some daylight analysis on a case study for my thesis degree and I encountered a problem: I found some discrepancies between the illuminances values (calculated using the “Honeybee_Grid Based analysis” component with the Climate Based sky) and those I can read using the “Honeybee_Read hourly results from Annual daylight study”. I’m using the same model and the same epw file and I’m confronting illuminances values for the same day, month and hour. If you confront the values for March the 20Th the differences are small, but if you repeat the comparison for December the 21St the values are quite different. Could you explain me if I’m doing something wrong? I do not know which values I can interpret as correct.

Thank you for your kind help.
Best regards,

PS I tried uploading my .gh file here, but its size is over the 1.5 Mb limit. So I’m providing the link to the McNeel forum where you can find the .gh file (https://discourse.mcneel.com/t/grasshopper-discrepancies-between-the-illuminances-values-read-from-ds-annual-analysis-and-calculated-with-climate-based-sky-on-an-analysis-grid/73904). I tried compressing it but it didn’t work, sorry for the trouble and again thanks for the help.
PPS if anyone knows a way to upload the file (despite the oversize issue) I will upload it immediately.


Hi @barbongiuseppe, this is my first time trying to answer questions on this forum so please let me know if there is any misunderstanding of your question. The content below is based on my understanding and many others on this forum may as well correct me or weigh in with better answers.

Since you are using HB, the annual daylight study runs via Daysim. Daysim uses 61-65 sun positions for direct calculation. Here is a post and Sarith mentioned the Daysim sun position and its inaccuracy on post 5. HB+, legacy and DAYSIM results

In HB annual run, the sun position is taken from Daysim, and the irradiation is taken from EPW file. But in HB point-in-time (grid-based analysis, climate based sky), the sun position is calculated via Radiance by your input (the hour), and the irradiation value is still taken from the same EPW file. So the sun position difference may lead to the difference of illuminance lux values for the same time from 2 methods.

Also a similar post on this issue can be found here: Illuminance Annual vs Grid data

Hope that helps!



Thanks @Xiufang for your answer! I’ll start reading those threads right now :slight_smile:


I read the articles you posted and you were totally right. If I understood correctly by installing Hb+ the issue should solve itself.
Again, I’m sorry for troubling you, but when i tried I couldn’t install Hb+ successfully: I downloaded the latest update (Honeybee[+] 0.0.04) from food4rhino, when I run the file it specifies that the .gh file should be in the same folder of the libraries to work correctly. My question is: where can I find the libraries folder, or where should i paste the .gh file to make it work correctly? Once again I apologize for the stupid question.
Best regards,


Hi @barbongiuseppe No worries! I learn from answering questions too! The food4Rhino version may not be the latest. After installation you can use the HoneybeePlus 05 Honeybee Plus Installer to update it into the latest Github version.

After downloading the food4Rhino file, unzip, keep everything in a local drive, then drag installer.gh file onto the Grasshopper canvas. I think the “same folder of the libraries” refer to the folders that contain UserObjects and python files.

But if you prefer the Github version directly they are here: https://github.com/ladybug-tools/honeybee-grasshopper.



It will be improved but that doesn’t mean that the result will exactly be the same.


Perfect! I succeeded installing it, now I’ll start doing the analysis again and see what happens. Thanks a lot, you have been really kind and helpful! Have a nice day!


Good morning,
yes, sorry, I was “summing it up”. First I’ll work on my file and then I’ll redo the analysis to see if the resulsts get closer. Anyway just to be sure: I can assume that the annual values read from Hb+ are more accurate than the annual ones from Hb (which are based on Daysim), but the most accurate af all are the point in time values. Did I get it correctly?


Between Honeybee (Daysim), Honeybee+(matrix-based method with pure Radiance) and climate-based point-in-time calculations, point-in-time makes the least amount of simplifying assumptions. So, it is the most precise within the three. While developing Honebyee+, we had tested the deviation between some of these methods and found that at least for illuminance, the deviations do not matter much (with regards to annual metrics). Part of that work is documented in my dissertation: See chapters 2,3 and 4 . The results there have since been independently arrived at by a few other users on this forum.

Just to be clear, when compared to reality, none of these methods are “accurate” per-se as we are using TMY radiation data to generate skies. If you were to use physically measured radiation values in the simulation and then the results would close to reality.


Ok, thanks for the clarification @sarith, I’ll start digging in your dissertation!
Have a nice day!


Good morning Sarith, when you can spare a moment could you please upload your dissertion on ResearchGate again? I don’t know if someone else has this problem, but when i try to download it my antivirus keep telling me that it is infected with PDF:UrlMal-Inf [Trj]. I tried using two different antivirus programs but i always get this. Thnks again, have a nice day!


Hi @barbongiuseppe, that is a false-positive generated by the Antivirus software. I think any documents with hyperlinks to the certain webpages have that issue. At least one other person had told me about this issue specific to my document. Anyway, I have uploaded it again without the hyperlinks. Can you try this link

My dissertation is also hosted on the official Penn State archive: https://etda.libraries.psu.edu/catalog/15229sxs1106. However that has the same pdf document as the one you tried downloading earlier.


Thanks lot, the link works fine:) Have a nice day!