# What is meant by Candela/m2?

Dear forum,

I’m trying to wrap my head around the concepts and metrics involved in analysing glare, and hope that someone can help me understand the outputs from HB.

I’ve noticed when we create those fisheye images, the unit of the colour gradient is Candela/m2.

I had to refresh my knowledge on what a Candela is, and from what I can tell it’s a unit of how much light is emitted from a point light source. If I understand correctly, in the above example the selected point on the wall of the tunnel is emitting close to 0 Candela.

What I don’t understand is, why is it “per m2”? I thought that Candela is the light emitting from a point. If the m2 are referring to the tunnel wall in the above case, then what do the Candela/m2 at the end of the tunnel refer to, where I see the sky? Where are those points ‘measured’?

Please excuse my ignorance on the topic but you have to start somewhere

Candela (lumens/steradian) is the unit of luminous intensity, which is used to quantify the amount of light emitted by any light source. It does not depend on where it is being measured .e.g a light bulb that emits 10 candela in a certain direction will continue to emit that light irrespective of whether you observe it from 10 meters away or from 1 meter away.

Candela/m2 is the unit of luminance. It is the amount of light measured in a certain direction at a specified distance from the light source. So essentially it quantifies the impact of light at a point while you (actually the luminance meter) is facing in the direction of that light. Luminance can also be described as the illuminance produced at the measurement point by a given source divided by its solid angle. So, the per m2 is relevant because the measured quantity is incident luminous flux per area (which is then divided by solid angle).
In very practical and simplistic terms, Luminance correlates to perceived brightness. So essentially the falsecolor plots are simply pointing out how bright a particular region of the field of view is.

With regards to the simulation, this point is specified using the -vp x y z values.

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Hi @sarith, thanks for that detailed description.

It is getting clearer for me now, though I’m still having trouble with the “/m2”. You mentioned the “impact of light at a point”, and the “illuminance produced at the measurement point” - where do the m2 come into play, if we’re talking about a measurement point receiving light from another point?

Let me ask it this way: is the following diagram accurate?

If so, then there is another “/m2” unit, namely lux=lumens/m2. I understand that the m2 is referring to the area of that floor surface. For the luminance then, does the m2 refer to the surface area of my eye lens? Or is it also the area of that floor surface?

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Dear @sarith, are you aware if there are any standards/code, with minimum or maximum requirements not to reach and/or exceed, according with the type of use, for example psychological or physiological glare?
Thanks and greetings

Essentially yes. If you were to multiply the luminance values (in cd/m2) by the area of your pupil (in m2), you would essentially end up with the luminous intensity of the various parts of the Radiance image inside your eye (in cd). I think this is why @sarith says that luminance is generally well correlated with perceived brightness.

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Sorry for continuing the conversation. I really want to know more details about this part, trying to learn more about it. Would that area be a really small number since it is the area of our pupil and by dividing it, we will get a large number in the unit of cd/m2, as mentioned before if we are using a viewing angle perpendicular to the source so there will be no effect to the viewing angle? How small the number it is, what is the exact number used there in the component?

Best wishes,
Hao

Both luminance and illuminance are measured for infinitesimal areas (as per definition). So, the m2 comes from there. For empirical measurements, this area is the area of the transducer(sensing element) in the illuminance or luminance meter. I would suggest referring the 10th Edition of the IES Lighting Handbook for an exact definition and derivation. Actually, wikipedia does a decent job of explaining the definition too.

Conceptually, yes. They do convey that illuminance is a quantity measured on a surface and luminance is quantity measured in a specific direction based on the observer/measuring device. However, besides the Photopic efficiency curve/luminous efficiency function, the empirical measurement of luminance and illuminance have nothing to do with the eye. Both these quantities are calculated by weighting the electromagnetic radiation in the visible spectrum with the V(λ) function.

It’s been a while since I looked up any of the standards, but metrics like UGR, DGP etc. all account for luminance, contrast and source size. The dissertation work by Kevin Wymelenberg and the related papers published by him contain a thorough discussion on this topic.

Regards,
Sarith

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