# Is there any relation between wavelength and brightness?

by tris_d
Tags: brightness, relation, wavelength
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 Quote by tris_d http://en.wikipedia.org/wiki/Intensity_%28physics%29 ...intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term.
Interesting that you chose to quote that link. I can't find any mention of a definition that involves photons in the whole of the web page. Can you? The only place the word turns up is in relation to the word "confusion". That rather proves my point.
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 Quote by sophiecentaur To avoid confusion, I have to point out that it is the other way round. The stars with the lowest visibility are given the highest magnitude value. Magnitude 1 corresponds to the apparent magnitude of Vega. The Sun, therefore, has a large Negative Magnitude. It makes sense as the stars with the highest magnitudes hadn't even been seen when the magnitude scale was first constructed.
Thanks for clarifying ... a high magnitude having a low number is poor phrasing.
Also, historically, the magnitude scale was introduced as a way to talk about brightness of stars in a sensible way.

Hopefully this didn't undermine the basic point that OP needs to pick meaning for the word "brightness" and stick to it. I see above that this message has not sunk in and OP continues to jump from one concept to another so much it is starting to look like trolling. At best he is observing that different writers use the word in different ways ... English is not the only language with this characteristic but it is especially famous for it. But what is wrong with that - as long as one is prepared to learn.

 That is why I keep saying brightness should not be defined as a property of light but as a property of an image. Then it will fit the definition from Wikipedia.
Language does not work like that - "brightness" is not a scientifically rigorous term with a standard useage across disciplines or even within disciplines. People use words for their own convenience, not yours. What "should" or "should not" is neither here nor there - you have to deal with what "is" and "is not" and learn to live with it.

We can tell you what a particular use of the word means in a particular context, but don't go expecting the same meaning to apply in different contexts.

If you want to measure brightness as 255-<greyscale index> in an image[*], then go back to your original question: brightness is not related to wavelength (except as an equal mixture of rgb levels) and blue appears less bright than orange because of the way computer monitors are designed. Not quite what you wanted was it?

In defense: we do need to have flexible use terms in order to smooth communication when we are not being rigorous - or we'd all start to sound like published papers.

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[*] iirc: that is the white index or "lightness" in an image.
Look in the filters of a decent photo-editor like GIMP or Photoshop and you'll see some defined as "brightness" and "luminoscity".
Fiddle with them and you'll see how those terms are defined in relation to an image - compare with your definition.
P: 162
 Quote by sophiecentaur Interesting that you chose to quote that link. I can't find any mention of a definition that involves photons in the whole of the web page. Can you? The only place the word turns up is in relation to the word "confusion". That rather proves my point.
No, I don't see them, and therefore I have to make them. That is my point. My other other point is if we take light source is emitting photons of the same energy, then we can convert all those definitions to use number of photons instead of energy, or whatever they are using now. Would you agree?
P: 162
 Quote by Simon Bridge Language does not work like that - "brightness" is not a scientifically rigorous term with a standard useage across disciplines or even within disciplines. People use words for their own convenience, not yours. What "should" or "should not" is neither here nor there - you have to deal with what "is" and "is not" and learn to live with it. We can tell you what a particular use of the word means in a particular context, but don't go expecting the same meaning to apply in different contexts.
I can tell you don't realize what I said. Look, if the source is emitting photons of the same energy, then the brightness of each pixel will be directly proportional to the number of photons that hits them and vary according to nothing else, yes?
 Mentor P: 11,467 Ok this is ridiculous. This thread has been going on 5 pages now, mostly because of arguing back and forth over whether to use photons or not, and what "brightness" means. I feel we've argued both of those beasts to death. In WHATEVER model we use, whether it's photons or not, the end result is the same. The energy/number of photons fall with the inverse square of the distance. Tris, since brightness apparently has absolutely no set meaning, if you want to use it to mean the value of the pixels in an image then go ahead. As long as however it is being used IS MADE CLEAR, I think we can all sleep at night.
P: 162
 Quote by Drakkith Ok this is ridiculous. This thread has been going on 5 pages now, mostly because of arguing back and forth over whether to use photons or not, and what "brightness" means. I feel we've argued both of those beasts to death. In WHATEVER model we use, whether it's photons or not, the end result is the same. The energy/number of photons fall with the inverse square of the distance. Tris, since brightness apparently has absolutely no set meaning, if you want to use it to mean the value of the pixels in an image then go ahead. As long as however it is being used IS MADE CLEAR, I think we can all sleep at night.
I never meant for this to be any argument here, just to put all those definitions in the context of photons, and I expected you would help me do that. Never mind, I'll derive new definitions myself, if you can please just confirm whether this statement is correct: - If the source is emitting photons of the same energy, then the brightness of each pixel will be directly proportional to the number of photons that hits them and vary according to nothing else. True, false?
 P: 162 If light source emits photons of the same energy, then: 1.) Radiant flux = energy per unit time => number of photons per unit time 2.) Radiant intensity = power per unit solid angle = energy per unit time per unit solid angle => number of photons per unit time per unit solid angle 3.) Radiance = power per unit solid angle per unit projected area = energy per unit time per unit solid angle per unit projected area => number of photons per unit time per unit solid angle per unit projected area 4.) Irradiance = power per unit incident area = energy per unit time per unit incident area => number of photons per unit time per unit incident area This is basically what I need to do, plus somehow substitute 'unit pixel' instead of 'unit incident area' and/or 'unit projected area'. C'mon, my friends physics wizards, this is nice little fun problem to solve, for you... for me it's not, so help me!!
 Mentor P: 11,467 That looks fine to me. But I'm no expert.
P: 162
 Quote by Drakkith That looks fine to me. But I'm no expert.
I didn't even know flux and intensity are two different things until you told me the other day. I don't think it's about knowledge, information can be googled out, but understanding can not. I think to solve this properly the most important thing is to have understanding what originally those definitions represent, what they relate to, and regarding that you are expert compared to me. -- Can you tell me what 'incident area" relates to in definition of "irradiance", is it about area on the light source, area on the lens, or area on the image, or some other area? That's kind of stuff I need help with, to understand what is what and how it works, how it relates.
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 Quote by tris_d I didn't even know flux and intensity are two different things until you told me the other day. I don't think it's about knowledge, information can be googled out, but understanding can not.
Edit: The issue isn't that you didn't know what they were, but that it seemed like you hadn't even given any effort to even look up anything on it.

 Can you tell me what 'incident area" relates to in definition of "irradiance", is it about area on the light source, area on the lens, or area on the image, or some other area? That's kind of stuff I need help with, to understand what is what and how it works, how it relates.
The first two sentences in the wiki article explain it.

Do you know what incident and radiated mean?
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 Quote by tris_d If the source is emitting photons of the same energy, then the brightness of each pixel will be directly proportional to the number of photons that hits them and vary according to nothing else.
You still have not defined "brightness". Anyway: the signal from the photo-receptor, to a monochromatic source, will be proportional to the number of incident photons. (The proportionality will depend on the photon energy in question.)

You'll have some software to convert the signal strength to some number - you could call that number "brightness" if you want. This will be a received, or perceived, brightness - which will vary with the distance to the source and the size of the pixel.

 Quote by tris_d If light source emits photons of the same energy, then: 1.) Radiant flux 2.) Radiant intensity 3.) Radiance 4.) Irradiance
1-3 are about the light that leaves a source - the unit areas here are on or about the source and light passes through it or originates on it. 4 is about the light that arrives - the area in question is the illuminated surface rather than the source. Different surfaces with the same irradience may have a range of brightnesses (according to their greyscale number when photographed) depending on surface characteristics like color.
 This is basically what I need to do, plus somehow substitute 'unit pixel' instead of 'unit incident area' and/or 'unit projected area'. C'mon, my friends physics wizards, this is nice little fun problem to solve, for you... for me it's not, so help me!!
You need to find the area of a pixel. The detector will have an aperture, and some mechanism to spread the light through the aperture to a CCD array. You need to know how many pixels are in the CCD array, and how much of the light through the aperture is intercepted by it, and the area of the aperture.

You need to be conscious of the different "unit area"'s in the definitions above - they are different places.

Even better would be to state the problem you are trying to solve by making these definitions. Different problems will involve different methods and different concepts. How would you expect to use the data from a "brightness detector"?
P: 162
 Quote by Drakkith The issue isn't that you didn't know what they were, but that The first two sentences in the wiki article explain it. Irradiance is the power of electromagnetic radiation per unit area (radiative flux) incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. Do you know what incident and radiated mean?
English is not my first language, so I'd hate to assume. I guess 'radiated' refers to area on a light source from which light is emitted, and 'incident' relates to either lens area or projected are on the image. But I wouldn't bet more than $10 bucks my guess is correct, and if I try to interpret it like that, then "radiation per unit area incident on a surface" doesn't really make sense. Mentor P: 11,467  Quote by tris_d English is not my first language, so I'd hate to assume. I guess 'radiated' refers to area on a light source from which light is emitted, and 'incident' relates to either lens area or projected are on the image. But I wouldn't bet more than$10 bucks my guess is correct, and if I try to interpret it like that, then "radiation per unit area incident on a surface" doesn't really make sense.
No, you are correct. A light source radiates light outwards from it. The light incident on a surface falls on the surface and is absorbed, reflected, whatever. It just means that a certain amount of radiation falls on each unit of area of the surface. It could be square meter, or square centimeter, or whatever unit you are using. IE 100 watts/m2.
P: 162
 Quote by Simon Bridge 1-3 are about the light that leaves a source - the unit areas here are on or about the source and light passes through it or originates on it. 4 is about the light that arrives - the area in question is the illuminated surface rather than the source. Different surfaces with the same irradience may have a range of brightnesses (according to their greyscale number when photographed) depending on surface characteristics like color.
Thank you! That's exactly kind of stuff I want to understand.

 You need to find the area of a pixel. The detector will have an aperture, and some mechanism to spread the light through the aperture to a CCD array. You need to know how many pixels are in the CCD array, and how much of the light through the aperture is intercepted by it, and the area of the aperture. You need to be conscious of the different "unit area"'s in the definitions above - they are different places.
Now we talking. Yes, I have to model all that is relevant, so yes, I see now I will need to define pixel size in order to relate it to "area" given in meters squared. One other question is how to model lenses, focal point and such, but I think that will become obvious when I understand more of how other things come into play and depend on each other. -- I used to be a game programmer by the way, so I'm pretty sure I could simulate and animate all that, as long as I understand how it works.

 Even better would be to state the problem you are trying to solve by making these definitions. Different problems will involve different methods and different concepts. How would you expect to use the data from a "brightness detector"?
I decided to make this in relation to my crackpot theory for Olbers' paradox, but since then I became really curious to understand how all of it works. And perhaps such program might be useful to astronomers and photographers, maybe to calculate what kind of equipment and settings would be the best for certain situations, or something. Basically, it's not about solving problems but about satisfying curiosity, and it is also about entertainment since I enjoy making software, especially if it challenges me and makes me learn new things.
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 Quote by Drakkith No, you are correct. A light source radiates light outwards from it. The light incident on a surface falls on the surface and is absorbed, reflected, whatever. It just means that a certain amount of radiation falls on each unit of area of the surface. It could be square meter, or square centimeter, or whatever unit you are using. IE 100 watts/m2.
Great, thank you. So, does that mean irradiance and 'incident area' relates to lens area, to aperture size? While radiance and 'projected area' refers to area on the image and is relative to magnification and focus?
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 Quote by tris_d Great, thank you. So, does that mean irradiance and 'incident area' relates to lens area, to aperture size?
What do you think?

 While radiance and 'projected area' refers to area on the image and is relative to magnification and focus?
I don't know.
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 Quote by tris_d One other question is how to model lenses
Depends on what sort of brightness detector you are talking about.

Lenses are normally simulated in computers using a transfer matrix or by ray tracing ... but you could get away with just stating that the lens arrangement spreads the light through the aperture evenly over the surface of the detector.