light intensity adjustments

[coromax]

I´ve a couple of questions regarding the meassurement and units of light intenstities.

1.  I´ve created a corona spherical light with 1 candela light intensity.
     Switching the unit to lumen results the light intensity to be 3.14 lm.
     But IMHO it should be 12.56lm (4x Pi). A spherical photometric light in Max has this implemented. http://tiny.cc/yu168x
     So, what happens in corona?

2.  When rendering a comparison, of a corona spherical light with a photometric 3ds Max spherical light,
     with the same light intensity, the corona light is more than 10 times brighter. What´s going on here?

3.  With 3ds Max photometric light one can adjust the brightness also in lux units, but it´s necessary to adjust also the distance
     of the light. That makes perfect sense to me.
     What is the basis of the lux calculation in the light brightness adjustment of a corona light?

Since I´m evaluating corona for a upcomming project where I need linking to realworld light measurements
any help would be really appreciated.

[coromax]

Nobody answers - that´s really disappointing!
Are these questions above to quirky to answer? I´d really like to use corona
for an upcomming project since it seems to be a capable renderer to producing highly photorealisic renderings.
But it´s vital that the adjustments of the lights are reliable relating to realworld situations.
Obviously this is a little suspect as I described above.
So please, can anybody help with an explanation of how the light intensity settings should be done,
if comparisons with realworld arrangements is aimed.

[maru]

Hi, sorry for that lack of answer. I must have somehow skipped your thread. I should soon be back with answers - please allow me some time to gather the data.

By the way, the best place to ask questions with 100% certainty that you will be answered is our support portal: https://coronarenderer.freshdesk.com/support/tickets/new

[coromax]

After all, somebody out there :-) Thanks for the link...

[maru]

Sorry to disappoint you, but I don't have much useful information...

Ad 1. The difference could be due to the fact that photometric light is a point, and Corona light is an area light with surface.

Ad 2. We have some clues what might be causing this, but we will need additional time to look into the issue.

Ad 3. The lx intensity in Corona light is most probably lx at 1[scene unit], but I will need to think of a way to confirm this.

I have forwarded the questions to our developers, once I have any definite answers, I will post them here.

This also might not be very satisfying for you, but generally, as a "more or less" method I would recommend the following:
Set up your scene with photographic exposure controls, and then tune the lights until they look plausible compared to a real-life reference.

Maybe it would help if you could tell us more about your project. What is the specific task?

[pokoy]

3.  With 3ds Max photometric light one can adjust the brightness also in lux units, but it´s necessary to adjust also the distance
     of the light. That makes perfect sense to me.
     What is the basis of the lux calculation in the light brightness adjustment of a corona light?

Keep in mind that the values you see in Max with photometric lights are only valid for perfectly spherical lights and don't take into account energy conservation when the light type is a spot light. I once relied on what the photometric lights displayed but wasn't aware of the difference and my 'measured' values were useless in the end.

[coromax]

@maru - thank you for your answers so far. I´m happy to hear that you´re getting your teeth into it ;-)
Yes, the corona light is a surface based light and for that kind of light the equivalent of 1 candela should be 12.56 lm. That´s the case with 3ds Max photometric spherical light.
Only the 3ds Max point light with the uniform diffuse distribution has the 3.14lm as equivalent for 1cd. So it seems the corona conversion is refering to a pointlight despite it´s actually a "surface light".

The task of the mentioned upcomming project is to simulate the reflections of technical equipment with single LEDs and control lamps, screens and monitors in the windowpanes of a control room at night.
These reflections interfere with the observation of the outside and should be eliminated as far as possible.
This room is dim lit by some downlights for which .ies files should be used. Also some LED striplights burried deep in some recesses have to be taken into account.
From my experience so far I can say that standard Mental Ray doesn´t do good with such a situation.
Since the planning should process by the results of the simulation, the intactiv renderer is vital to arrange items back and forth.
I have to estimate which renderer will suit the best, and corona seems to be at the first place by now. In the end I have to generate some renderings where the final brightness of the reflections is appreciable.
A pseudo color rendering of the luminance values would be ideal. But since there is no such thing in corona I´m planning to create a series of renderings with graduated exposure values.
Eventually I´ll create my own hdri which can be associated with a colorful gradient.
And therefore I need reliable adjustment of the lights brightness in comparisons to the realworld situation.

@pokoy - thank you for the hint. I will be more careful with the adjusments of the spotlights.

[bluebox]

Dunno if I got the details right, but IMHO you're aproaching a very technical topic with more or less "rule of thumb" archviz oriented renderer designed to produce first and foremost a plausible image.

You might want to check out DIALux (free AFAIK) or other specialized software that is designed to check light intensities. Renders coming out of it are awful (at least the used to be a few years ago, not sure how things are nowdays) on one side, but on the other you get totaly reliable results and if I understood correctly this is most important here. DIALux also produces light intensity gradients etc.

This is by no means some "under the hood" marketing. Just trying to help.

[coromax]

@bluebox - Thank you for your proposal. I know how Dialux works. It´s made for planning diffuse lighting and some glare ratings and it´s for free for the end user.
Not bad at all. But it´s generating the global illumination via radiosity approach and that is a bit outdated IMO.
For the mentioned project the main focus is on the reflection calculation. I probably need a renderer which can output a reflection pass at least.
Corona is capable of producing physical plausible renderings and therefore must be based on physical correct algorithms.
The only thing that is a bit unclear to me, is the interconnection to a realword scenario which can be validated  i.e by comparing reliable .ies files
or via  setting the light brightnesses to known values.