How to match real-world lumen lighting output in Corona with IES files

[shakedalon]

Hey everybody,
I have a question that I’ve been trying to figure out for a long time:

Let’s assume I received a 3D model from the lighting manufacturer along with the real IES file of a light fixture. In the technical description of the fixture, it says that the light output is 500 lumens.

Now, I import the 3D model into 3ds Max, apply a Corona Light, load the real IES file from the manufacturer, and enter “500” as the lumen value in the Corona Light settings.
For this example, let’s assume that this is the only light source in the scene.

My question is: What are the correct values in Corona Post Processing (such as exposure, contrast, ACES, etc.) that will ensure the lighting output is as accurate as possible compared to real-world lighting?
In other words, how can I know that what I see in the render actually represents what 500 lumens with this IES would look like in reality?

Hope that makes sense.
Thanks!
Shaked

[Aram Avetisyan]

Hi,

The short answer - there is none.
You can capture the same environment/room with different settings in real Camera, and the image will be totally different, while the lighting and illumination (yes, these are different things) are constant.

You can go crazy and try untonemapped raw output by Corona, with real illumination values, which then can be processed further (no idea how) but I would skip it.

From my experience, setting the light intensity to Lumens is the closest thing to accuracy you can do in regards to light intensity. It has been really solid for me to e.g. compare in real life how many of the light sources I need and where.

For the simplest of tone mapping operators - an exposure change of 1 stop means 2 times more or 2 times less light reaching the sensor. So if you have a light with, say, 800 lumens, which has illuminated a surface 1m away from light source (the calculations are more complex of course, I am presenting a simple case) with 800 lux, exposure value of 0.0 would capture it as is. With exposure value of 1.0, you would get/observe as if the surface was illuminated with 1600 lux.

Why the reference? You can compare this against general illumination values, say for a living room it is/should be from 100-300 lux (work surface, usually 0.8m above floor), so you can calculate (or eyeball at least :)) the rest.

This most probably also means non-default Corona settings (no MSI at first) to get the result of the light source in a specific environment as accurate as possible.

[burnin]

for such cases, it might be helpful to familiarize w/ 'gray card', 'neutral gray photography' and of course, use of light meter...

example articles:
"Using a Gray Card for Photography: What is It & Do I Need One? by Jen Golay"
and
"How to Use a Gray Card for Photography" (@greatbigphotographyworld.com)



(Please, please update the forum engine)

[TomG]

"(Please, please update the forum engine)"

Already in progress :) The forums will be moved over to an entirely new system probably by the end of the year.