One of the features I would really like added to the Cycles Renderer is a photo-metric workflow.
That is the ability to set light sources intensity using real-world photo-metric units, load IES photo-metric data, have a physical daylight system, and set photographic camera exposure and white-balance for the output image.
While Cycles currently doesn’t have a fully functional photo-metric workflow,
It is equipped with some important basic ingredients needed for the development of such a workflow.
One Of these features is the Black-Body color conversion node that allows specifying color by Kelvin color temperature,
Another is the procedural Sky texture featuring Hosek / Wilkie and Preetham physical sky models, that can also be controlled according to global position, date and time with this addon.
Recently I’ve had a pleasant surprize finding out that Cycles actually has another important feature for a photometric workflow, and that is that Cycles Area Lights maintain a fixed general light output (‘Luminous Flux’) while area is changed and changes specular intensity correctly to so that the smaller the light source area, the greater its brightness (as it should physically be).
* This in difference to the way a mesh light with an emission shader behaves where the light output is per area and therefore increases or decreases when changing the shape and size of the surface.
This makes designing light sources with a fixed total output of light yet different shape, and therefore different specular reflection, shading, shadow softness possible,
And is in itself a valuable feature in realistic light source design.
* Especially coupled with setting the light color using Kelvin color temperature (Black-body node)
The only thing missing is the an ability to specify the total output of the light source in Lumens (lm) units.
I have encountered a mentioning of Cycles having a physical scale conversion ratio here:
Marco Pavanello, the developer of the Blender ‘Real Camera Addon‘ wrote:
“In Blender the Emission Node Strength is measured in W/m^2”
I haven’t had the time yet to seriously find out how that should be translated to intensity in lumens..
* It should be noted that both the Cycles Area light and mesh light use the Emission shader as there source for intensity / color settings, but differently,
You can see in the demonstrations below that for a light source of the same surface area a significantly larger strength value is needed to produce roughly the same light output as the light mesh and this is probably due to the output being internally divided by surface area which is in fact the subject of this post.
Here are some renders to illustrate the point, and the diffrent behavior of light mesh (mesh with an Emission shader)
I’ve added a rough glare effect that depends on float color intensity to illustrate the way the specular highlight intensity increases as the area of the ligt source gets smaller while overall light output is the same:
Cycles Mesh Light using an Emission Shader with different sizes and strength changes to compensate