# Daylight factor simulations - unreliable data with Accelerad

**URL:** <https://discourse.ladybug.tools/t/daylight-factor-simulations-unreliable-data-with-accelerad/21458>\
**Category:** ladybug-tools\
**Tags:** daylight, honeybee, accelerad\
**Created:** [April 14, 2023, 9:46am UTC](https://discourse.ladybug.tools/t/daylight-factor-simulations-unreliable-data-with-accelerad/21458 "2023-04-14T09:46:44Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![andrea.botti](https://discourse.ladybug.tools/letter_avatar_proxy/v4/letter/a/c89c15/32.png) [@andrea.botti](https://discourse.ladybug.tools/u/andrea.botti)\
**Post date:** [April 14, 2023, 9:46am UTC](https://discourse.ladybug.tools/t/daylight-factor-simulations-unreliable-data-with-accelerad/21458/1 "2023-04-14T09:46:44Z")

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Hi there,

I am writing with a certain degree of discomfort as my colleagues and I have discovered some rather worrying discrepancy between daylight results obtained when the **same** model is run on 2 separate PCs, both have the same Honeybee and Radiance versions installed.

PC1 has a powerful graphic card and accelerad installed.  
PC2 is a medium power PCs without accelerad installed, thus uses CPUs to run the simulation.  
( as a further check I ran the same model on PC3, a laptop with same configuration as PCs 1 and 2, and the results converge with those obtained with PC2)

Other than causing crashes and requiring Nvidia software update, I ran a sample analysis on ONE individual room with the 2 PCs to provide for a (glaring) comparison.

![image](https://discourse.ladybug.tools/uploads/default/original/3X/5/c/5c3f3170d8b718f345e071b7a4a41d15f54a2c4c.png)  
Results with PC1

![image](https://discourse.ladybug.tools/uploads/default/original/3X/8/8/88e509dbf49d69373797624c5371eeccdbebe4b3.png)  
Results with PC2

Scene includes a number of complex shading as per image below:  
 ![image](https://discourse.ladybug.tools/uploads/default/original/3X/3/8/385be6657f9fe38013bd1162998a6861e5f6d837.png)

Radiance Parameters( HB Detail Level **High** ) = -aa 0.1 -ab 6 -ad 4096 -ar 128 -as 4096 -dc 0.75 -dj 1.0 -dp 512 -dr 3 -ds 0.05 -dt 0.15 -lr 8 -lw 0.005 -ss 1.0 -st 0.15

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**Author:** ![charlie.brooker](https://discourse.ladybug.tools/user_avatar/discourse.ladybug.tools/charlie.brooker/32/27164_2.png) [@charlie.brooker](https://discourse.ladybug.tools/u/charlie.brooker)\
**Post date:** [April 14, 2023, 11:20am UTC](https://discourse.ladybug.tools/t/daylight-factor-simulations-unreliable-data-with-accelerad/21458/2 "2023-04-14T11:20:12Z")

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Hey @andrea.botti, might be worth sharing on the Radiance discourse and seeing if Nathaniel Jones can input.

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**Author:** ![Nathaniel](https://discourse.ladybug.tools/user_avatar/discourse.ladybug.tools/nathaniel/32/11215_2.png) [@Nathaniel](https://discourse.ladybug.tools/u/Nathaniel)\
**Post date:** [April 14, 2023, 1:24pm UTC](https://discourse.ladybug.tools/t/daylight-factor-simulations-unreliable-data-with-accelerad/21458/3 "2023-04-14T13:24:08Z")

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Hi @andrea.botti,

I don’t have background on what type of simulation you are running, but given the complexity of your shading device, your simulation settings are probably not accurate enough. I think you will need a higher `ad`, lower `lw`, and possibly a larger `ar` depending on the size of your model. In general, you should keep `lw` \< 1/`ad`.

In stochastic ray tracing, you can never expect to get exactly the same results from two different runs. You should be aware that Radiance and Accelerad perform irradiance caching differently. In Radiance, you need to run an overture pass to produce smooth ambient lighting, and if you do not, then your results will be biased toward whatever ambient values were calculated first. In Accelerad, the overture is implicit and doesn’t need to be carried out as a separate step, but there are [additional ambient parameters](https://nljones.github.io/Accelerad/documentation.html#commandline) to be considered. You might want to turn off irradiance caching with `aa`=0 to get a more accurate comparison.