Question about Iray Lighting in Watts
StratDragon
Posts: 3,249
and for the record I am looking up stuff on the web, I'm just not finding info for some questions.
Can anyone explain the lighting system using W? and I'm assuming W is watts.
I create my scene: floor plane
a primitive object with a Iray texture
Set render settings to default just to give base setting
turn off dome, change to scene only,
change the ISO to 400
create 3 primitive planes for lighting, ( say I start with 1 meter; then change the scale from 100 to 500%)
apply the emissive shader to the three panes
change the lighting model to W
change the wattage to 50,000
create a 1 meter sphere as an object and position the lights fairly close, then render.
My lighting results to me seem FAR darker than what I would expect reality (the concept, not the software) - even if I pan out and render the lights so they are visible I need to bump up to like 25,000W before they are anything other than gray.
Am I missing something?
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Comments
There must be something wrong somewhere. 50,000 Watts is more light than a searchlight and at an ISO of 400 should be just a white render.
Duplicate post, see below
I don't know if posting images like this works. It appears much has changed on these forums since I was frequent here.
The 3rd screen shot is the giveaway. You've set Luminance, NOT watts. You need to input the value in the Luminous Efficacy channel.
I regularly use watts and anything from 50 - 200w gives decent light in a room situation, so you won't need to go to 50,000
Hi Maclean,
see below if you can answer the question ; either way thanks!
Are you saying that he needs to change the Luminance Efficacy (Lumens/Watt)?
wait - I think I got it!
img src="https://i.ibb.co/GFHCcRL/Screen-Shot-2019-02-22-at-3-51-37-PM.png
efficiency is the "wattage" of the mesh (when set to W)?
Could you explain what luminance is ?
<please delete>
Luminance is the Wattage when Units are set to W.
Luminance Efficacy is the Lumen per Watt of the light source being emulated either Incandescent, Candle or Fluorescent etc.
https://en.wikipedia.org/wiki/Luminous_efficacy
For example a 40 Watt could be 10; a fluorescent could be 80; an LED could be 140 and a Candle <1. The better the Efficacy the more light for the wattage.
If you use the universal tool and select the plane that is emitting light, it will show you what way it is really facing. There is a chance that you have it facing the wrong way.
And Fishtales is correct
It looks far darker than it'd be in real life to me as well. I used to think it was because in iRay the light rays in the ray tracing didn't bounce around enough but Haseltine corrected me & said it does (that's what the -1 in the iRay settings means) so that leaves three things as being wrong
1) The ambient light present from all other light sources
2) The surfaces the light is interacting with
3) The atmospheric properties like dust, water vapor, and such.
I think it's must the ambient light from all other light sources is wrong and I'd guess also our eyes are far better than 400 ISO film & whatever camera to adjusting to and seeing in different levels of light.
Also, take into account that W is an absolute value for your emissive surface. The bigger the surface, the lower the total luminosity. If you want to mantain the overall luminosity you should use other units as cd/m^2 or similar.
I've also noticed that modifying the ISO to 400 also changes the Exposure Value to 15; you might want to lower it again to 13.
The size of the emitter plane at 1 Metre has less light because it is over a greater surface. Drop the scale from 100% to 10% and move it closer to the sphere and you will see the difference.
@Rafmer
If you are going to use the Exposure Value then just drop it to 10 and leave the rest on default and it will do the same.
Unless you want to pull your hair out or make your forehead flat from banging it on your desk, do not try to adjust Exposure Value AND the ISO / F-Stop in the Render Settings.
Unlike a real camera, there's no tradeoff of having an ISO of 400 or 800 or 20. You aren't going to get motion blur or differences in the grain. Likewise an F-Stop of 8 or 22 or 56 or 3 isn't going to change your depth of field / focus. In the case of Render Settings, the values are totally arbitrary and changing any of them will adjust the Exposure Value.
Honestly, it's easier to just adjust the Exposure Value and turn a blind eye to what it does to the camera equivalent values.
True that, I only use exposure value when tone mapping. I just pointed that change in EV when modifying the ISO in case the OP was aiming for an established lightning setup that required it or something like that. I don't know why there are that many parameters when they all do the same.
The only reason I can think of to use the "camera controls" in Render Settings would be if you were using the Sun/Sky Dome and wanted to match the exposure settings that were used for a background image. But yeah, since I was a photographer before 3D apps even existed, I started trying to adjust those like I would on my camera and got frustrated how Studio would try to be "helpful" and modify the ISO when I changed F-Stop so it could maintain the same Exposure Value.
The OP is also trying to adapt Luxrender Tone Mapping and Lighting ideas to iRay and there is no direct correlation.
@maclean is absolutely wrong do not follow that way .. @rafmer is right.
The lumen and watt options are global values. That is, it is the global luminance the surface will emit. This way the larger the surface the dimmer the emission will be. Try to reduce the emissive surface and get it close to the target and you'll see a drastic emission increase.
The lm/m^2 and w/m^2 options are luminance densities. That is, the emission will be constant on the surface. For this reason this is also good for morphing surfaces where we want to keep the emission constant when the surface changes its shape during the animation.