Author Topic: Light intensity  (Read 7400 times)

2015-10-31, 07:11:51

3di

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Can anyone shed any light.....on the light intensity units and how they relate to real world ratings found on lightbulb packaging for example?

I have to use specific bulbs in a restaurant scene so the effect of the lighting is accurate, but when I choose 400 lumen for example (as shown as the lumens on the gu10 led bulb i'm trying to replicate) and choose the correct radius for my light (disc) the lighting seems very dull compared to what would be realistic.

In the renderer I'm trying to keep my exposure settings etc at the default of 0, white balance at 6500 and contrast/highlight compress at 1.....as presumably these are neutral settings to replicate the human eye's perception?

My world units are set up correctly in 3ds max.

I'm wondering if the sr m2 unit measurement is different from what you would normally find on your average light bulb packaging?

2015-10-31, 11:13:57
Reply #1

romullus

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Corona's EV (let's call it CEV) is arbitrary number. Zero was chosen as middle point between typical interior and typical afternoon exterior i believe. For a home interior you may want to set it to 2 - 4. Exteriors goes about -4 -5 CEV. That is if you use Corona's sky&sun and lights with physically acurate luminous output. With HDRI lighting it's try&error, because usually those are not calibrated.
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2015-10-31, 14:03:37
Reply #2

Ondra

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Corona's EV (let's call it CEV) is arbitrary number. Zero was chosen as middle point between typical interior and typical afternoon exterior i believe. For a home interior you may want to set it to 2 - 4. Exteriors goes about -4 -5 CEV. That is if you use Corona's sky&sun and lights with physically acurate luminous output. With HDRI lighting it's try&error, because usually those are not calibrated.

well it is not arbitrary. Having EV=0 means the image output values are in watts per meter squared per steradian
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2015-10-31, 20:53:13
Reply #3

Juraj

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image output values are watts per meter squared per steradian

I never tire of this description = )
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2015-11-02, 01:59:37
Reply #4

3di

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Ok...well i'm still confused then.  I have a night time scene where the camera will travel from outside the building through the doors and then travel around inside the building.

The problem is, i've set up all of my lights internally with 90 watt's (1500 lumens approx), which seem accurate with EV set to 0.  However when the camera is outside and I have a corona sun set up at a night time angle....the whole image is incredibly over exposed.....surely this cant be right?  Shouldn't the exposure of 0 be correct for both the light bulbs and sunlight if they're all physically accurate?


2015-11-02, 03:41:50
Reply #5

Juraj

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In short no :- )

Quote
.as presumably these are neutral settings to replicate the human eye's perception?

There is no such thing as this though.
(any) renderer is not replicating human eye but camera sensor, although they behave similarly in exposing for different light condition (exposure in Corona) and tone-mapping to accomodate broad dynamic range (highlight compression in Corona).

ExposureValue 0 in Corona is nothing important to consider. To translate Ondra's scientific hogwash it's "arbitrarily" (hah) set value at which value specified in material will look identical under unidirectional(domelight) light with intensity of 1. It's not useful for anything though. You should always adjust exposure to fit your lighting, just like in real-world photography, and whether you do that with actual photographic controls ( aperture/shutter/iso ) or by simple ExposureValue offset [ - ∞ , 0, ∞ ] doesn't matter.

Quote
I have a corona sun set up at a night time angle

Corona Sun/Sky system doesn't support night, it goes from Dawn to Dusk. If you want to represent night correctly, use night-time HDRi in Environment slot.

Quote
the whole image is incredibly over exposed.....surely this cant be right?

Yes it can, that is exactly what happens in reality. But specially in your case, there might be some issue with trying to get night-time from Sun system, which as I wrote above, isn't currently possible.

Quote
Shouldn't the exposure of 0 be correct for both the light bulbs and sunlight if they're all physically accurate

Exposure has nothing to do with type of lights. For example "Sunny 16"  [ f-stop 16; shutter 1/125; ISO 100 ] would look correctly under broad daylight outside but not inside a room. It depends on overall light levels, which depend on spatial relations, materials, position toward lights,etc...


You should educate yourself little bit more about what is exposure, and how photographic exposure works (although you don't need it, or need to use it in rendering, it's still helpful to get the concept).

And unless you need linear file for compositing purpose, you should use highlight compression other than 1 :- ) That's exactly what your eye or camera sensor is doing.
« Last Edit: 2015-11-02, 03:52:13 by Juraj_Talcik »
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2015-11-02, 03:47:15
Reply #6

Juraj

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On other hand, I feel the Exposure 0 is

Quote
image output values are watts per meter squared per steradian

literally confuses every single user coming from other renderers that use some default camera exposure ( like f/8 1/256 iso 100 in Vray's example ) that is more aligned with common scenario for photorealistic renderer.

As much as I think simply typing [- 2 ] after my default Sun/Sky scene looks overspexosed in framebuffer is absolutely natural and instant thought, it seems most users don't think like this and it needlessly confuses them. The tons of posts I've read about this by now seem to confirm this.
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2015-11-02, 12:49:14
Reply #7

maru

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This might not be a very technically correct workflow, but I think an easy way to make everything look "pretty accurate" is to set camera exposure to some realistic value (imagine you are using a camera in manual mode in your scene - so for example ISO400, f4, 1/30s for a dim interior - just an example, this can be very different) and tune the light intensity accordingly without caring too much about the realism of the intensity value. Once again, this may be incorrect from the technical/scientific/realism point of view, but should allow some realistic looking renders.
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2015-11-04, 17:09:19
Reply #8

3di

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Thanks for your responses.  I tried your suggestion Maru, and it's working well for the indoor scene cheers.