Is there any relation between wavelength and brightness?

[tris_d]

Take a few images and count the number of photons that have fallen onto the sensor over time. It's not quite that easy, as we have to deal with all sorts of noise, but that's basically it. I can measure the light that has fallen onto the sensor from a star.

Then flux CAN be defined in terms of 'number of photons'.

Brightness, as I defined it in my post you are quoting, cannot be measured if the image of the star is much less than the size of the airy disk. For example, even nearby stars have an angular diameter of hundreths or thousandths of an arcsecond. This is FAR below the 1 arcsecond resolution of my telescope. The airy disk would be about 1 arcsecond across, which would correspond to about 4-5 microns on the sensor. Do you know what an arcsecond is? Have you read up on what an airy disk is?

If we can define flux and intensity in terms of number of photons, per something per something, then we will be able to define image brightness in terms of photons per pixel. That's what I want, to define everything in terms of 'number of photons', to Soph's utter disgust.

 

[Drakkith]

Then flux CAN be defined in terms of 'number of photons'.

Technically no, as number of photons is not watts. However if we "hand wave" all the photons as being from the same frequency EM wave, then we can find the flux.

If we can define flux and intensity in terms of number of photons, per something per something, then we will be able to define image brightness in terms of photons per pixel. That's what I want, to define everything in terms of 'number of photons', to Soph's utter disgust.

I already explained it in my other post then. The "brightness" the reference you quoted uses is exactly how I used it.

 

[sophiecentaur]

Technically no, as number of photons is not watts. However if we "hand wave" all the photons as being from the same frequency EM wave, then we can find the flux.
I already explained it in my other post then. The "brightness" the reference you quoted uses is exactly how I used it.

But of course, only if you happen to know the frequency of the EM you happen to be dealing with. That is why it makes such good sense to describe Flux in Watts. (Funny, I have read and written that several times before.)

We often have contributors who want to do their own version of things. They either grow wiser or poorer, in time.

 

[Simon Bridge]

The photon and em wave models are both drilled into students - I'm wondering if the links need to be clearer earlier?

Light intensity is number of photons per unit area per unit time?

I think "light intensity" and "light flux" are terms that could mean just about anything.

The "photon flux" is the number of photons through a unit area per unit time.

- which seems to be what is intended here by "light flux". The word "light" has a range of uses in physics as well as regular language. I suspect that sophiecentaur is probably on to something by insisting on a precise language here.

The "intensity of the light wave" would be the square of the amplitude of it's electric field? (sophiecentaur?) ... which would be related to the photon flux and the photon energy. (Photons are understood primarily as energy quanta ... though wavelength, momentum etc can also be used to characterize a photon.)

The "luminous intensity" of a light source would be the power per unit solid angle being emitted by a light source. So already, two different ways to define "intensity of light".

"brightness" is a subjective measure that means different things in different circumstances. In common language, we would understand one object to be brighter than another is it appears to glow more when you look at it. This is what I've been trying to talk about on the first page of this thread.

Some examples:
(1)Some colors look brighter than other colors (part of the original question) because they look more like the colors in fire - for example - so the common concept is to do with more that just the light itself.
(2)stars with a high visual magnitude will look brighter than those with a low visual magnitude. Originally the visual magnitude was a subjective measure related to the way the human eye perceives light. However, astronomy has objective ways to assign magnitude to stars (look this up for more). Astronomers may refer to "bright stars" informally, in this context.
(3) photographers may refer to brightness in terms of the tendency of part of the picture to wash out other parts ... they use a light meter to help them work out exposure times. The meter usually measures power per unit area, averaged across the detection surface and this can be called the "brightness of the light" but more likely it will be called the "light level". I have seen "light flux" used in old SLR camera manuals.
(4)Drakkith seems to be using "brightness" to mean the number of photons emitted, per unit solid angle, from a light source (Drakkith?)
[edit] in order to be consistent with the linked article (earlier)

Because of the very wide usage, it is possible to come up with a reasonable sounding definition of "brightness" to contradict pretty much any argument. The term should be understood only in the context of a particular description. I don't think it is useful as a concrete general term.

I suspect that the persistence of OPs confusion in the face of repeated answers may indicate that we have yet to identify it's source. Perhaps sorting out more rigorous terms will help?

 

[Drakkith]

(4)Drakkith seems to be using "brightness" to mean the number of photons emitted, per unit solid angle, from a light source (Drakkith?)

I'm merely using it the way I think the article linked was using it, which I believe is photons per solid angle.

 

[sophiecentaur]

You just can't beat a bit of "rigour" if you want to get somewhere in Science. It's not a matter of preferrence and one has to walk before running.

 

[tris_d]

Brightness, as I defined it in my post you are quoting, cannot be measured if the image of the star is much less than the size of the airy disk.

...if the image of the star is much less than the size of the airy disk.

What you describe, is that point light source?

Brightness, as I defined it in my post you are quoting, cannot be measured...

http://en.wikipedia.org/wiki/Apparent_magnitude
- Note that brightness varies with distance; an extremely bright object may appear quite dim, if it is far away. Brightness varies inversely with the square of the distance.

It seems Wikipedia say further away star would simply appear as darker/dimmer 'airy disc' than closer away star with the same absolute magnitude. Would you agree?

But that does not apply when stars can be resolved to have some angular size, it applies only when the star is so far away that it becomes a point source. Ok?

 

[sophiecentaur]

Perhaps if we want to simplify or if the source emits photons of the same energy, ok? And then intensity would be directly proportional to the number of photons, wouldn't it?

http://www.cv.nrao.edu/course/astr534/Brightness.html
The number of photons falling on the film per unit area per unit time per unit solid angle does not depend on the distance between the source and the observer. The total number of photons falling on the film per unit area per unit time (or the total energy absorbed per unit area per unit time) does decrease with increasing distance. Thus we distinguish between the brightness of the Sun, which does not depend on distance, and the apparent flux, which does.


You either tell me that article is completely wrong and that I should forget it, or use your marvelous intelligence and rephrase your amazing knowledge in these same terms so we can talk the same language here. How about it? C'mon, you can do it! Or can you?




"GOOGLE IT!" -- You could put that in your signature, then you would not need to bother answering any more questions but simply reply with an empty post and it will automatically answer all the questions anyone might have.

Those links do not define any of that in terms of photons, and that article does, and I like it, so I want to understand that article and thus I need explanation according to those same terms they use. And I like photons. I hate energies, they are so vague. Ok? Photons, photons, photons! Photons rule, energy sucks. Hah!

That article is not "completely wrong" and, if you read it in total, you will see that they do not, anywhere, 'define' flux / brightness / intensity in terms of brightness. (Read what they actually say) They make the mistake, possibly, of introducing photons conversationally, to make the subject approachable . The fact that you picked up on that, to the exclusion to their formal definitions, shows that they chose an unfortunate way of putting things. You will not have been the only one to get an inaccurate message. But their main statement about brightness is in terms of Power - as it should be.

You really should not reject the idea of reading around a subject. Every stroppy post you are making is interfering with your self-education time. If you love Photons then you should learn what they really are. You will only find that out by reading and not asking the 'wrong' questions.

You are clearly in the early stages of learning about physics and I recommend you get the basics sorted out before coming to shaky conclusions. This stuff would never have been sorted out if it had been approached in a careless and uninformed way.

This is a discussion forum and not a free tuition service. Any help you may get is your good fortune and not a right.

 

[sophiecentaur]

(2)stars with a high visual magnitude will look brighter than those with a low visual magnitude. Originally the visual magnitude was a subjective measure related to the way the human eye perceives light. However, astronomy has objective ways to assign magnitude to stars (look this up for more). Astronomers may refer to "bright stars" informally, in this context.

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.

 

[Drakkith]

What you describe, is that point light source?

Yes, we can treat the star as a point source.

It seems Wikipedia say further away star would simply appear as darker/dimmer 'airy disc' than closer away star with the same absolute magnitude. Would you agree?

Yes.

But that does not apply when stars can be resolved to have some angular size, it applies only when the star is so far away that it becomes a point source. Ok?

The way we've been using brightness, yes. But be aware that brightness is a very bad term to describe light with. There are just too many different ways people use it. For example the way wikipedia uses it in your post is different than the way we've been using it.

 

[tris_d]

You are clearly in the early stages of learning about physics and I recommend you get the basics sorted out before coming to shaky conclusions.

I find your condescending remarks are funny. I recommend you stop talking about me, it's unnecessary. Just address what I say, directly, point out what you believe is wrong and tell us what you think is correct... or ignore it.

This stuff would never have been sorted out if it had been approached in a careless and uninformed way.

http://en.wikipedia.org/wiki/Intensity_(physics)
...intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term.

- "And so God scattered them upon the face of the Earth and confused their languages, so they would never ever, ever get back together." -- This stuff is indeed handled in a careless and uninformed way.

This is a discussion forum and not a free tuition service. Any help you may get is your good fortune and not a right.

Aha. Let me help you understand then. I am developing a simulator to visualize these relations between light source, emitted light, lens, image and its consequent brightness. In order to do that I must model light as photons. Ok? Now, there is simply no other way to go about it but to define intensity, flux and brightness in terms of 'number of photons', and I will do it with or without your help. I guarantee you that this can be done if simplify the scenario by having the light source emit photons of the same energy, and if you help me it will happen sooner rather than later. My friend, it's take it or leave it. Your comments about my person are inappropriate, it's all up to you whether you are going to help or not, so suit yourself and please stop whining about it already.

 

[tris_d]

The way we've been using brightness, yes. But be aware that brightness is a very bad term to describe light with. There are just too many different ways people use it. For example the way wikipedia uses it in your post is different than the way we've been using it.

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.

 

[sophiecentaur]

http://en.wikipedia.org/wiki/Intensity_(physics)
...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.

 

[Simon Bridge]

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.

Langauge 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.

-----------------------

[*] 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.

 

[tris_d]

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?

 

[tris_d]

Langauge 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?

 

[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.

 

[tris_d]

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?

 

[tris_d]

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 areaThis 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!

 

[Drakkith]

That looks fine to me. But I'm no expert.

 

[tris_d]

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.

 

[Drakkith]

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.

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?

 

[Simon Bridge]

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.

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"?

 

[tris_d]

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.

 

[Drakkith]

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.

 

[tris_d]

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.

 

[tris_d]

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?

 

[Drakkith]

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.

 

[Simon Bridge]

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.

Basically, it's not about solving problems but about satisfying curiosity,

But you still have to have a context for the information or it is meaningless

 

[tris_d]

What do you think?

I think aperture size must be in the equation somewhere, so by the logic of reduction I find that one fits description the best.