First of all, let me say that english is not my first language, so please excuse any mistakes that you could meet from here on out.  I read several articles and explanations about luminance and illuminance, I have almost understood what is the difference between them but I'm still a little confused and that's the reason why i'm writing here.  Avoiding the technical definitions, I understood that Illuminance is the amount of light perceived on a surface, while Luminance, then, is how much reflected light is perceived looking at that surface.  The question is: can I consider the light reflected like a form of illuminance?  For example: consider a subject, a female/male portrait on a white background, there are not light sources in this subject but sources of light are behind my back and bring light to my subject. I take a photo of this subject with my camera. What my eyes as well as the sensor of my camera perceive is only light reflected from the subject....uhhmmm...light reflected? I could say that what I catch with my eyes and with my camera is Luminance...or Illuminance?  I mean, yes, light is reflected from the subject but then reflected rays of light become incident rays of light for my eyes and for the sensor of my camera...don't you agree?  Hoping my question is clear, if it's not clear just tell me, I will try to explain myself better.
The more I read about this, the more confused I become. Illuminance, as I understand it,  measured in lux, is the perceived brightness of light on a surface. It's what you'd measure with an incident light sensor. Luminance, then, is how much reflected light you'd perceive looking at that surface. This is what the light meter in a camera measures. Am I correct so far?  In both cases, the word "perceived" is important, because the scales are weighted using a  to map wavelengths to . However, in the case of illuminance, there's only actual perception if your eye happens to be the surface in question.  I can basically grasp this, but then I come across charts saying things like "family living room: 50 lux". Wait, hold on! Does that really mean that the typical lights in a house are that bright, or is it just confused and wrong, or am I confused and wrong?  If you're not taking pictures of light sources directly, why would an incident light meter reading ever be useful in photography? Reflected light recorded on film or by a sensor is usually what makes a photograph. So, if I have an incident light meter, how does that reading relate meaningfully to my camera settings?  Since incident light meters are sold and used, this implies that there must be some useful conversion. But this is where my brain explodes. Google tells me that 1 lumen equals 1 candela, so therefore 1 lumen/m² (i.e. 1 lux) must equal 1 candela/m2 (i.e. 1 nit). But clearly something is missing from this. There's something called a "steradian". Cones are involved. I'd never heard of this before; how does it fit in? I can see how it might be determined when calculating the usefulness of LED home lighting, but for a photo, I'm at a loss.  Some part of my (exploded, from the last paragraph) brain is trying to relate this to the difference between flash metering with TTL and with an incident meter. But how can the incident meter work without knowing the reflectance properties of the objects in the scene? Is this what the "C" in the  is all about? Is it just an average-this-probably-will-work value, or is there more to it than that? And if it is just an average, what knowledge is necessary for compensating for off-averages scenes? (As with the , where the photographer simply judges if the scene should be rendered brighter or darker than the average given by the meter.)  So yeah, so much confusion. In short:   What's the difference? Can one meaningfully convert between the two? When and how are illuminance / incident light measurements useful for photography?     Update: I appreciate Stan's answer, which covers the third point of when and how pretty well. And I think I've basically got the first point figured out, as described above. But I'd appreciate some answers covering the issue of conversion as well, both in the mathematical abstract and as the practical for photography. And I wouldn't mind more why and how, either.
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