Why can't light intensity be specified in physical units?

[pro.bhat]

From what I understand:

1. Lighting in Arnold is physically based (reference #1)
   
2. Light intensity in Arnold is specified as an unitless number (reference #2)
3. Given 1 and 2, one would assume that the conversion from Arnold's light unit to a physical light unit should be simple (e.g., a single scale factor).
4. Lights in 3DS Max are specified in physical units (reference #3) and here is what their documentation says about rendering with Arnold: "3ds Max scenes (particularly architectural scenes) setup for other rendering engines will not necessarily convert well to MAXtoA. Arnold does not currently support physical units for lights, and a conversion process is not straightforward. You may need to adjust Intensity and Exposure parameters for your lights to match."

As an Arnold user who needs to be able to use photometric lights in their scene, it would be really useful get an answer or pointers on how to achieve this requirement (i.e., specify light intensity in physical units like 20 Lux). 

Reference URLs:

1. https://docs.arnoldrenderer.com/display/A5AFMUG/Understanding+Physically+Based+Rendering+in+Arnold
2. https://docs.arnoldrenderer.com/display/A5AF3DSUG/Lights
3. https://help.autodesk.com/view/3DSMAX/2015/ENU/?guid=GUID-96E37398-613E-495A-A80B-9C5D398982F9

[CiroCardoso3v]

I agree with you. We should be able to input real world data. You can still use Photometric lights to get accurate values. At least for now. 

[pro.bhat]

@CiroCardoso3v
If by photometric lights you mean this light type in Arnold (reference #1), then I'm not sure this light type resolves the question of how to specify light intensity in physical units. Arnold's Photometric light has same unitless intensity parameter used in its other light types (e.g., area light, mesh light, etc). When I attach various IES light profiles to this light type the intensity parameter is not automatically changed based on the data in the IES light profile. 

Reference URLs:
#1: https://docs.arnoldrenderer.com/display/a5afmug/ai+photometric+light

[CiroCardoso3v]

I am making reference to these ones