1. ## Question Regarding Luminance.

[COLOR=rgba(0, 0, 0, 0.65098)]Recently I've been studying photometry lately and have been having trouble wrapping my head around the concept of luminance. Tried reaching out to TEEJ without much luck but I'm sure there are plenty others here who can "shine light" upon the topic.

Starting with luminance: I understand that it is the luminous intensity of a light times the area of the surface of that light source. (Differinciating it from luminous intensity) That surface being either the surface of an LED, a mirror, wall, pane of glass or a plane in space, etc. However, I am confused about the solid angle of light involved in luminance.
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[COLOR=rgba(0, 0, 0, 0.65098)]One I've tried to picture this is imagining a cone of light opening up from every single point on a plane (which represents the light source) where each point is the vertex of the cone.

This satisfies the math part where it's the luminous intensity (the cone) times the area of the source (the multiple cones sprouting up from each point on the plane)

However, this creates the phenomenon of the cones of light over lapping with each other. Do these areas contain more light or something? That shouldn't be the case.

Or I can see it as one big cone that uses the whole surface of the light source as sort of it's vertex instead of the usual "point" that were used to.

But if this is the case I don't see how this resulting "cone" of light is the result of a lights luminous intensity times the area of the light source.
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I am even further confused about this as I've read things like the solid angle is being subtended by the surface of the eye (which would mean the beam angle could be negative if the light source is bigger than the surface of the eye???) or that the solid angle is always parallel like a perfectly straight beam. (Although math wise might work, practically speaking we know that no light simply has a parallel beam.)
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[COLOR=rgba(0, 0, 0, 0.65098)]My apologies for just a lengthy, horribly composed, techincal question! Many thanks in advance to everyone![/COLOR]

2. ## Re: Question Regarding Luminance.

There are an infinite number of "cones", each with an infinitely small luminous intensity. You will either have to instinctively feel or just accept calculus for this approach to make sense. Most explanations focus on only one cone to make it more intelligible. Overlapping should be ignored - we are discussing the properties of a two-dimensional surface. There are not only an infinite number of cones but they are infinitely small - ie. they have a direction but no height. If you want to know what is happening at some distance from that emitting surface then you have to define a new surface and it will have its own set of infinitely many infinitely small cones representing the light passing through that surface. That light will be made up of light emitted from different areas of the original surface, that is different cones, and they can be related mathematically which you could call overlapping if you like.

What was the question again?

Perhaps best to start by fixing your definition. Luminance is luminous intensity per unit area. Hence it is luminous intensity divided by the area. The same intensity from a larger area means a smaller luminance. Does that answer any of your questions?

3. ## Re: Question Regarding Luminance.

Thank you so much for the detailed explanation! My question basically is what is luminance and how could one go about picturing this to fully understand it's meaning. For example the way I picture luminous intensity is that it's the density or how tightly the rays of light within a solid angle are. So would luminance be the number of luminous intensities in the area of a source? (#1 in picture) Aka the cone thing. Or is it just the luminous intensity of the light, but instead of considering the vertex of the lights solid angle as being the source you consider a cross section of the lights solid angle with the same area as the source as your source? (#2 in picture)

4. ## Re: Question Regarding Luminance.

Lithopsian, I think you need to go back to school. Per Wikipedia:

In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.

Taken to the limit, an infinitely small cone has an infinitely small luminous power, but not necessarily a small intensity. For a uniform distribution, as you look at smaller and smaller cones, the power per cone gets smaller but the intensity does not.

UrbanExplorer, let's cut through all the jargon and scientific crap and get to something that can be understood at a gut level. Luminous intensity is essentially 'how bright is your source in a particular direction'. In an ordinary (I'm tempted to say 'old-fashioned') incandescent light bulb, the intensity is nearly uniform in all directions. The exception is the direction coming out the base of the bulb. In the direction of the base of the bulb, much of the light is blocked by the base. Because the globe of the bulb is (usually) bigger than the base, some light still still goes this direction, but the intensity is substantially reduced.

Now consider an LED. In particuar, let's think about lighting class LEDs. The distribution of light is very different. Disregarding any mounting or other hardware, the actual LED die is mounted on a ceramic substrate. This essentially prevents any light from being emitted toward the back (luminous intensity is zero). So a large portion of the area that is illuminated by an incandescent bulb will not be by an LED. Then as you approach the front of the LED the intensity generally increases, with the highest intensity directly in front of the LED. This is not true of ALL leds, but it is true of most lighting class leds.

5. ## Re: Question Regarding Luminance.

DIW I think you need to go back to school. The post was wrt luminance not luminous intensity.

6. ## Re: Question Regarding Luminance.

Lol yeah I did look at Wikipedia and I have squared away luminous intensity with the kind help of a photometrist. I am sad to say I am no longer able to contact this person. Also just a random question but are the pics I'm posting showing up? It's not loading up on my screen so I cannot tell.

7. ## Re: Question Regarding Luminance.

I'm not seeing a picture, so I'm not sure what your #1 and #2 are. I think it would be good to stop thinking about point sources completely. That is just a simple case for drawing a picture showing angles in the classic "cone". Luminance is not defined for that case (or rather it is, but it is a brand of infinity), so really you have to move beyond it. You can obviously define a surface outside a point source and then define a luminance for it, but that isn't helping your visualisation much.

Luminous intensity is a "total" value for a light source, point or not. It is a sum (more generally an integral) of the luminance over the whole surface of a source. An individual "cone" from a point on the surface, if you want to consider such a thing, is effectively the luminance and the luminous intensity is the sum of all of them. Such cones will overlap and light from different points at different angles will "cross", but light in the same direction contributes to the luminous intensity wherever it originates on the surface of the source. Is that a better visualisation?

8. ## Re: Question Regarding Luminance.

I see. It seems like what I was thinking is the opposite of what you said. Mind if I PM you? Perhaps the pictures would go through that way.

9. ## Re: Question Regarding Luminance.

Originally Posted by ssanasisredna
DIW I think you need to go back to school. The post was wrt luminance not luminous intensity.
I stand my ground.

Originally Posted by Lithopsian
There are an infinite number of "cones", each with an infinitely small luminous intensity.
I believe this to be incorrect. For my reasoning, see my first post, or read below.

But now the definition of luminance is still a difficult one. Since luminous intensity is weighted power per unit solid angle, and luminance is luminous intensity per unit area, that would make it equal to weighted power per unit angle per unit area, which took me a few moments to wrap my head around.

Say you have an LED that emits one lumen, and is 1 cm square, for an area of 0.0001 m2. If you focused all of the light into a cone of one stearadian, you would have a luminous intensity of one candella. Block half the beam, now you have half a stearadian, half a lumen, and still one candella. No matter how small a piece of the beam you look at, the luminous intensity is still one candella.

The luminance of your light LED is one candella per 0.0001 m2, or 10,000 candella/m2.

Now add another identical LED shining in the same direction. Now you have 2 lumens per stearadian, or a luminous intensity of 2 candella. But since you now have 0.0002 m2 of emitter, the luminance is still 10,000 candella/m2.

10. ## Re: Question Regarding Luminance.

Originally Posted by DIWdiver
I stand my ground.

I believe this to be incorrect. For my reasoning, see my first post, or read below.

But now the definition of luminance is still a difficult one. Since luminous intensity is weighted power per unit solid angle, and luminance is luminous intensity per unit area, that would make it equal to weighted power per unit angle per unit area, which took me a few moments to wrap my head around.

Say you have an LED that emits one lumen, and is 1 cm square, for an area of 0.0001 m2. If you focused all of the light into a cone of one stearadian, you would have a luminous intensity of one candella. Block half the beam, now you have half a stearadian, half a lumen, and still one candella. No matter how small a piece of the beam you look at, the luminous intensity is still one candella.

The luminance of your light LED is one candella per 0.0001 m2, or 10,000 candella/m2.

Now add another identical LED shining in the same direction. Now you have 2 lumens per stearadian, or a luminous intensity of 2 candella. But since you now have 0.0002 m2 of emitter, the luminance is still 10,000 candella/m2.
A tad wordy but I get the gist of it. Kinda lost you at the 0.0001m^2 stuff tho. I understand luminous intensity. It's literally the quantity of light per solid angle. The equation says it. Lumens per steradian. If you had a 10'lumen light bulb and you covered half of it in black electrical tape it would emit less light and be 5 lumens. But the side that is still showing won't become any less dimmer because it has the same density of light. As for luminance is it the intensity of light coming out of each point of the source? For example. Let's say we have a surface that is 10x10 (100points make up the surface), and each point gives off 1 lumen per steradian. Would this mean that the luminance is the collection of all 100points and their lumen/per steradian values??

11. ## Re: Question Regarding Luminance.

Actually, I think the easiest way to think of it is the luminance is how bright the surface of the emitter is. This may not be scientifically accurate, but it's close.

The folks that build long-throw lights call this surface brightness. In case you don't know, 'throw' is the maximum distance at which you can light up an object. I won't go into what 'light up an object' means. It turns out that the theoretical maximum throw of a light built from an LED is not directly dependent on how many lumens the LED will put out. It is directly dependent on 'surface brightness', or more scientifically, luminance. This is why in the world of throwers, smaller LEDs like the XP-E and XP-G have not succumbed to their larger cousins like the XM-L and its successors. Lumens are greater in the bigger emitters, but luminance is higher in the smaller ones.

12. ## Re: Question Regarding Luminance.

This is an interesting thread, way above my current jargon. But I must ask, why? Intensity is easy to understand. It is independent of overall.

The overlapping cone, per pin point, mental image is grabbing. Reminds me of infinitely small point slope calculations, where logic says if you have infinite small point it could have no defined neighbors or slope... The cones would be additive, as lux is additive. In practice just take the die as a whole.

The phrase kept creeping up in my mind, perhaps as a question : how many angels do dance on the die of my LEDs?

I will need to reread when I have more time, to figure this out.

13. ## Re: Question Regarding Luminance.

So, a long time ago I emailed this light laboratory technician guy about questions involving luminous intensity, flux etc and although a tad off topic from luminance he did briefly mention the concept of luminance that we might find useful. Perhaps some of you more experienced folks can get more out of it than I did.

Hi Patrick,
Best of luck,
Andrew

Hi!

I am a student who is interested in photometry and have been doing some research on the topic but I've run into a little problem with conflicting definitions that I was wondering if you can assist me with.

From my research the following terms seem to be most commonly defined as followed (in simple terms):

Luminous flux: Total quantity of light energy emitted in all directions per second.
[AB: Light is not energy; it is the flow of energy, also called the time rate of energy, i.e. energy per time. It is measured in units of lumens which are spectrally weighted watts. A watt is a joule/s.]

Luminous intensity: Luminous flux per steradian in a certain direction.
[AB: The direction is defined by the particular solid angle. Consider if the solid angle is 4pi, then the "direction" is all directions. As a unit the flux and solid angle are infinitesimals, and so intensity can be found for any direction. For practical solid angles the intensity is really the average intensity over the particular solid angle.]

Lumen: Si unit for luminous flux/10^15 photons (depending on wavelength).
[AB: The lumen is the SI unit for luminous flux, period. It is dimensionally equivalent to the watt unit (a unit of power). You can convert a quantity of light with a particular spectral power distribution into photons/s, but because of the special spectral weighting contained in the definition of the lumen the units photon/s and lumen are not equivalent in general.]

Candela: Si unit for luminous intensity/density of light energy equivalent in brightness to about a candle.
[AB: Again, light is not energy; it is energy per time. Candela (cd) is defined as light per solid angle. [cd = lumen/steradian]. A candle flame does have an intensity of roughly 1 candela. Intensity is not brightness. Brightness is dimensionally equivalent to luminance (L) in units of cd/m^2 which is light per solid angle per area of the source. [cd/m^2 = lumen/(steradian meter^2]. Luminance is the density of intensity on the source.]

However this definition of "luminous intensity" confused me when looking at the example where an isotropic point light source of one candela emits 1 lumen per steradian and 12.57 lumens in total.

How does an isotropic point light source have an intensity measurable in candela if candela is used to measure the luminous flux in a unit solid angle??
[AB: Intensity is light per solid angle. To get total light output you multiply intensity by solid angle: flux[lumens] = intensity[cd] * solid angle[sr]. This is analogous to calculating total flux from illuminance (light per area)[lumens/m^2]: flux[lumens] = illuminance[lux]*area[m^2].

That previous example made more sense when I considered luminous intensity as the light flux density instead. However, I could not find this in any definition of what luminous intensity is.
[AB: intensity is a flux density quantity. Where illuminance is a planar density (flux/area), intensity is a 3-dimensional directional density. It describes the concentration of light for different directions in space.]

Thank you for your time and assistance!
Patrick

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