Bloom and emissivity

Robinson

Can someone explain to me the interplay between the following:

    Bloom Filter Radius (I understand this one)
    Bloom Filter Threshold
    Bloom Filter Brightness Scale

and
    
    Luminance (lm)
    Emission Color
    Emission Temperature

and

    Tone Mapping Exposure Value

First up, I'm going to assume Emission Temperature is light wavelength and that Emission Color is therefore taking that light and running it through the color filter before it escapes the surface, given it's not a laser there will presumably be a spectrum of photons with various energies emitted (yes, I'm using physics talk and this is RT but for the conceptual model in my head...).  Luminance is the number of photons, hence increasing luminance is increasing brightness. 

The question is basically this:  how do I bloom an emissive object such that it "glows".  And moreover, if I do this with two different objects with different emission colour and energy.  Basically I have some emissive lights I want to be kind-of purple/pink neon glow, with some bloom and there's also some tubes in the scene I want to provide white light to bring up some other geometry.  I'm playing whack-a-mole with the bloom settings to make them all look nice and glowing as it's a post-process applied to the entire scene and not just bits of it.

 

Robinson

I think I'm starting to work out how this goes.  If I have two rooms, one with bright white light (normal lights, interior) and the other dark, like a nightclub foyer leading into the nightclub itself, the energy in the pixels of the light room will overpower the bloom in the darker room, so the setting would be different and you won't be able to bloom the background neon in the nightclub.  When you walk the camera into the nightclub, you change the bloom setting.  Now in something like a game engine this would be done automatically (at least Unreal Engine has a dynamic tone mapping filter).  In iRay we have to set this up manually.

fastbike1

I can give you the Luminance answers.

Luminance technical description is Luminous intensity per units emitting area. SI untis care candela per square meter (cd/m2). Basically how strong your lights is. In Studio Luminousity is applied to emissive sources, ie a bigger source will be brighter for a given luminance value. Iray spot and point light use lumens for a unit (in a not quite technically correct fashion) and just use a single value. In iray the Lumen values do not translate to real world numbers you will see on a light bulb package. You typically need values in the 100000 and up range.

Emission color is the color the the light source if you want a tint.

Emission temperature is a vlaue in degres Kelvin, typically 2000K to 7000K. This is a real world value. Lower numbers are "warmer", trending towards reddish tones. Higher numbers are "cooler" tending toward bluer tones. 3500K is considered warm light for real world. 5000K - 6000K is cool light for real world.

Tone mapping exposiure value is how light or dark your render will be if you select that option. The exposure value is similar to, but not identical, to real world photographic exposure values. This is a way to get darker Iray scenes without noise. Iray likes plenty of light for noise free shorter renders so using a tone mapping exposure value for a darker exposure will will get your darker scene.

Robinson

Interesting.  Thanks.