Luminance values in Lighting analysis tool

[tobias_link]

Hi there, I compared the results of the lighting analysis tool to my lighting calculation program (Dialux evo) by creating a single room with same materials / reflection factors of the walls (50%), ceiling (70%) and floor (20%). Whereas the illumination values are quite exactly the same, the luminance values in Vray are higher than expected (in my example about 30%). What could be the reason for that, and is there a way to solve that? For the calculation, I created the three materials from a generic, with just diffuse colours on the respective values (0.5, 0.7 and 0.2), no reflection/refraction. Any ideas?
best regards
Oliver

[konstantin_chaos]

Hi tobias_link,

We will have to investigate this a little before answering your question.
In the meantime, can you please share some more details for the test you've done?
If you can send us the test scenes and some example images, showing the discrepancy, this would really help.

Thank you in advance,
Konstantin

[tobias_link]

Hi Konstantin, thanks for your reply. I'll send you some more infos within the next days.
best regards
Oliver

[tobias_link]

... so there's been a while, but the topic came up again, so I tried to investigate the differences in lighting calculations betwenn my "Standard" lighting tool Dialux EVO (which is pretty common and regarded "reliable" when it comes to lighting calculations) and VRAY. Here's what I found out so far:

1. I created the same scene in Dialux and SKP: outdoor, no sunlight, no environment. So basically the scene is totally black. I created floor and one wall in both programmes with a standard material and a reflection factor of 0,7.
2. From a manufacturer's website (here. BEGA) I imported the EULUMDAT file to Dialux and the IES-File to Vray, and created a row of 10 lights in a row with equal distances (to floor, to wall and betwen the lights).

3. First thing I found out is that the IES-File is interpreted differently when it comes to the intensity value in VRay. I had to manually overwrite the luminaire's luminous flux (stored in EULUM/IES-file) with the value stored:

4. After doing so, the calculations of only direct light (in both calculations, indirect lighting resp. Global Illumination in Vray was disabled:


You see that the lux values are pretty much the same. Fine

.... the maximum number of attachments is 5, so I'll continue with another reply....

[tobias_link]

... continued...


5. Now I turned on Global illumination in both programmes (Indirect lighting in Dialux and Global illumination in Vray (I tried different methods, I found out that there are differences but not very much, so I leave out the comparison between Bruteforce and Irradiance map and stuff..):


Here you can see that the Lux values differ up to 30% (the VRAY calculation shows lower lux values than Dialux.

6. Now I compared the luminance values which is a bit difficult because Dialux doesn't show the value grid for luminance, only a false colour image. That's why I picked some values from the software itself and put it in the picture to compare both calculations:

here, you see that the values are totally different....

So, here two questions come to my mind:

- In VRay, the luminance values show "cd" (Candela) as units - whereas the unit for the luminance is cd/m2 (Candela per Square metre). Candela usually is the unit for the light intensity (the intensity that a light emits into a direction). So, am I trying to compare the same thing in both programmes? What exactly does the "luminance" values in the lighting analysis tool show?

- Well, for lighting designers and planners, the only relevant values are intensity (lx), and the luminance (cd/m2), which is the intensity of light reflected by a surface. Which leads me to the next parameter relevant for both the differences in calculations with indirect lighting and luminance values: the material / reflection factors of the objects that reflect the light rays.
In Nr. 5 (see above), I could adjust the calculations by increasing the colour of the diffuse channel of the object material: the brighter the material, the more light will be reflected. But is that the right way to make the calculation comparable? And what would be the impact on the rendered images? There's more to investigate...


So, this is what I found out so far.
Here's the Sketchup file of the calculations: https://ds.tobiaslink.com/drive/d/s/...p-tLLgwPF2owg_
and here the dialux file: https://ds.tobiaslink.com/drive/d/s/...0--rJAchJ3owg_


And last but not least , there's one more thing that is connected to all the issues/questions above:

When we do renderings for outdoor lighting designs (street lightings for example), the presentation renderings require a Sky environment and background, BUT: for the lighting calculations, only the light produced by the lights in the scene itself is required - the light that the environment component in Vray produces adds to the lux values. So for false colour renderings I have to do additional renderings with Environment disabled. It would be great if you could add an option in the lighting analysis tool, something like "disable environment light in the calculation". This would help a lot in producing the renderings and false colour pictures.

Wow, the post became a bit longer than expected

I'd love to hear responses and help !
Best regards
Oliver

[tobias_link]

Hi there, any updates to my questions yet?