Talk:Luminance

message: If luminance and luminosity are supposed to be synonyms, shouldn't those two articles be merged? --Abdull 12:00, 23 Mar 2005 (UTC)
replies: text: Yes, same thing is true: they are distinct quantities.  One based on radiant flux, the other on luminous flux.  One an energy-based measure, the other based on ability to simulate the human eye to perceive brightness.  Dicklyon 05:33, 19 April 2007 (UTC)
thread_title:  merge articles 

message: In the original version of the page, it was stated that The word luminance, a synonym for luminosity, means "the quality of being luminous; emitting or reflecting light and also Luminance is also a technical term in photometry.  OK, those are both sort of true.  But now that the article of about the latter term, the former statement is no longer operative.  I suggest we remove the false suggestion that photometric luminance is sometimes called luminosity. Dicklyon 06:52, 19 May 2006 (UTC)
thread_title: Luminosity is not a synonym for Luminance

message: Is this diagram suitable? --pfctdayelise (talk) 13:31, 29 July 2008 (UTC)
replies: text: Not really. Besides the fact that it's in French, it's not clear to me what it illustrates. At the least, we would need some text explaining it.--Srleffler (talk) 23:56, 29 July 2008 (UTC)
thread_title: {{tl|reqdiagram}}

message: What does the variable "d" represent ? --Redbobblehat (talk) 23:36, 1 February 2009 (UTC)
replies: text: It's not a variable – it's the differential operator. Dicklyon (talk) 02:20, 2 February 2009 (UTC)
thread_title:  Definition Formula 

message: "if you form a demagnified image with a lens, the luminous power is concentrated into a smaller area, meaning that the illuminance is higher at the image. The light at the image plane, however, fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens."
Should that be "The light at the image plane, however, fills a smaller solid angle"?
Oops - just saw previous section, so please disregard or delete this one. Though perhaps this is evidence that the comment in the article requires slightly more explanation, e.g. using the term "angular spread" to distinguish it from the area of the image. I confess I'm still a bit confused because if the demagnified image looks smaller at my eye than the source then I would have thought that the angular area subtended by the image at my eye is smaller than the angular area of the source. The explanation seems to involve the derivative ("spread") rather than area itself.
:It's not the solid angle subtended by the image at your eye that matters. You're right that it's about the angular spread of the rays. If you have optics that produce a demagnified image of the source, the cone of rays from the last optical element to that demagnified image will inevitably have a larger cone angle than that of the cone of rays from the source that are collected by the first element in the optical system. It turns out that the product of the cone angle and the height of the image is an invariant—it can't be changed by any ideal optical system. This is related to the Lagrange invariant. Image area and solid angle of the rays are proportional to the squares of the image height and cone angle. Their product is also an invariant, called the étendue.--Srleffler (talk) 22:16, 7 September 2013 (UTC)
replies: text: That is nonsense. The same amount of power is concentrated into a smaller area. By definition that is higher illuminance/irradiance. Your statement about "light points" is just wrong. There are no "light points" that behave as you describe. An extreme demagnifying lens produces very high illuminance, not zero. This is exactly the case of using a lens to start a fire: extreme demagnification creates very high illuminance.--Srleffler (talk) 01:40, 20 July 2015 (UTC)
replies: text: At the end I understood what it's happening. I'm not using the word "demagnified" as probably the optics experts do. For me and for some people, "demagnified image" suggests the contrary of using a normal biconvex magnifying glass, it suggests using a biconcave demagnifying lens, and we probably are wrong. That's the origin of my confussion. Perhaps the expression "demagnified image" could be explained a little in the article, or stress that it's a normal lens, just for the layman. Grausvictor (talk) 09:20, 27 July 2015 (UTC)
replies: text:  “Demagnified” means that the image is smaller than the object. It's not about the lens type, it's about the magnification. — Edgar.bonet (talk) 13:11, 28 July 2015 (UTC)

text: I see. I rephrased the sentence to avoid the word "demagnified".--Srleffler (talk) 05:34, 29 July 2015 (UTC)
thread_title:  Error? 

message: === luminous flux|luminous power ===
"The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view."  This is ambiguous, at best.  It can easily be read as total power reaching the eye from all directions.  How about Illuminance on the retina or luminous flux per solid angle?  — Preceding unsigned comment added by David R. Ingham (talk • contribs) 
=== The statement "Luminance is invariant in geometric optics." is not general enough. ===
The conservation of luminance is a case of Liouville's theorem (Hamiltonian), because Maxwell's equations, and most other fundamental physical theories, can be stated in Hamiltonian form.  So this sentence should read something like "Luminance is invariant in lossless coherent optics."  In a sense, it is invariant under any process that does not change the entropy of something else.  However the tricky case is diffuse reflection, which effectively decreases luminance by tangling the phase space.  The other complicating point is that the definition given here may not be identical to a general measure of phase space.  Perhaps someone will find a reference with a more general but equally accurate statement.  David R. Ingham (talk) 15:58, 24 May 2014 (UTC)
replies: text: Generally these concepts are taught in terms of ray optics, not waves and phase space.  Do you have sources for the alternative approach?  It sounds interesting.  Dicklyon (talk) 18:46, 24 May 2014 (UTC)
thread_title:  Explanation 

message: The conservation of luminance is a case of Liouville's theorem (Hamiltonian), because Maxwell's equations, and most other fundamental physical theories, can be stated in Hamiltonian form.  So this sentence should read something like "Luminance is invariant in lossless coherent optics."  In a sense, it is invariant under any process that does not change the entropy of something else.  However the tricky case is diffuse reflection, which effectively decreases luminance by tangling the phase space.  The other complicating point is that the definition given here may not be identical to a general measure of phase space.  Perhaps someone will find a reference with a more general but equally accurate statement.  David R. Ingham (talk) 15:58, 24 May 2014 (UTC)
replies: text: Generally these concepts are taught in terms of ray optics, not waves and phase space.  Do you have sources for the alternative approach?  It sounds interesting.  Dicklyon (talk) 18:46, 24 May 2014 (UTC)
thread_title:  The statement "Luminance is invariant in geometric optics." is not general enough. 

message: I edited the health effects section. More information on this would be good. I took out the sentence about heating being proportional to the product of luminance and time, because it isn't really true and doesn't tell the whole story. The weighting of wavelengths used in calculating luminance is not correct for determining heating of the retina. Red light will produce more heating than one would expect due to luminance. Near infrared light can quite effectively heat (and damage) the retina, but provides no luminance at all. 
Stating that local heating is proportional to the product of luminance and time is also a bit misleading. Damage is clearly not proportional to this product in general. Exposure to a low level of light for eight hours is clearly not the same as exposure to 288,000 times as much luminance for a tenth of a second. We can talk about heating, but if we do we need to go into more detail, so as not to leave the impression that the product of luminance or radiance and time is all that matters.--Srleffler (talk) 03:33, 25 May 2014 (UTC)
thread_title:  Health effects 

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thread_title:  External links modified 

message: Current text: "In this case, the solid angle of interest is the solid angle subtended by the eye's pupil."
The unit of luminant flux is the candela = cd = lumen/steradian
The unit of luminance is candela per square meter = cd/m^2 = lumen/steradian/m^2
On the one hand, the text states:
"The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. "
Yet the definition of luminance is candela per unit angle per square meter, and there is nothing in this definition which presupposes the angle of interest is that "subtended by the eye's pupil", which angle it should be noted will change depending on the distance of the object.
Luminance refers to light subtended by an angle and reflected from a source over an area (cd/m^2), but there is no decent explanation for how this is a reasonable measure of an objects' brightness. 
To give a specific example: the luminance of an "incadenscent light bulb" is given as 130 kcd/m^2, which is 130 kilo-lumens per steradian per square meter. How does one understand this geometrically when looking directly at a lightbulb? Where does the square meter come into play?— Preceding unsigned comment added by 2601:643:8104:a620:bdf1:61b3:1e91:c6eb (talk • contribs) 13 August 2017 (UTC)
:Luminance can be kind of hard to get your head around. One thing that helps is to understand that in an ideal optical system with no absorption, luminance is conserved. This is related to the conservation of etendue, and the article on that has some explanation you may find helpful. Because luminance is ideally conserved, you can calculate it at any point in the system, by considering the area through which the light passes and the angular spread (solid angle) of the light. The luminance of the object will, assuming perfect imaging, be the luminance of the image it forms in your eye. You can consider the area of the object and the solid angle subtended by your pupil, or the area of your pupil and the solid angle subtended by the object. You'll get the same result either way. Yes, the solid angle changes according to the distance to the object. Luminance tells you how bright an object appears. An object further away appears dimmer, because a smaller angle is subtended therefore less light passes through your pupil and contributes to forming the image.--Srleffler (talk) 04:44, 14 August 2017 (UTC)
replies: text: Regarding the light bulb: I don't see where you get that number, but supposing you had a lightbulb with such a luminance to answer your question I would first need to know whether it was a clear or a frosted bulb (it matters!). Assuming a frosted bulb, the square meter would be the area of any given part of the surface of the bulb. If you consider a small element of area on the surface (think of it as a pixel in an image of the bulb), you could use the luminance, the area of the surface element, and the solid angle subtended by the pupil of your eye to determine how bright that part of the bulb's surface would look to you. If the surface of the bulb has uniform luminance and if the frosted bulb is a perfect (Lambertian) diffuse emitter, then the cos θ in the formula cancels and all parts of the bulb's surface would look equally bright. For a clear bulb, you would be looking at the filament, and the relevant area would be on the surface of the filament. A clear 100 W bulb will have much higher luminance than a frosted one.--Srleffler (talk) 05:01, 14 August 2017 (UTC)
thread_title:  Confusing or Incorrect text? 

message: In the beginning there is sentence "Brightness is the term for the subjective impression of the objective luminance measurement standard (see Objectivity (science) § Objectivity in measurement for the importance of this contrast)". But later in section "Mathematical definition" formula contains luminous flux, which is subjective value. 
Dvgavrilenko (talk) 09:02, 7 December 2022 (UTC)
replies: text: Luminous flux is also an objective measurable quantity.--Srleffler (talk) 05:06, 9 December 2022 (UTC)
thread_title:  Definition