Is there any relation between wavelength and brightness?

[tris_d]

So READ UP about it and stop asking to be spoon fed with each little bit of info on demand. I actually doubt that you really are trying very hard, rather than just enjoying a fruitless conversation. There is no excuse for not reading everything that Wiki and others have to offer. You will always have personal models in your head until you get some self discipline and start learning the real stuff for yourself from tailor made resources. (You clearly have the time available).

I will be quite prepared to respond once I can see that you are, in fact , making an effort for yourself. Good luck with it.

No, you just get angry because you can not really explain what you are talking about. You make empty statements without any explanation behind them and you then blame me for not taking what you say for granted. You attacked Simon Bridge, you attacked Drakkith and you are attacking me just because we do not think like you do. Good luck to you too.

 

[sophiecentaur]

No, you just get angry because you can not really explain what you are talking about. You make empty statements without any explanation behind them and you then blame me for not taking what you say for granted. You attacked Simon Bridge, you attacked Drakkith and you are attacking me just because we do not think like you do. Good luck to you too.

Drakkith and I get on very well. He has actually acquired a lot of knowledge and he has my respect. I also justified my problem with the way Simon was describing something and I thik we cam to terms with that. I get the impression that you have a different attitude, though, which is unfortunate if you really want to get to grips with this.

Perhaps you could quote one of my "empty statements"? I will try to justify it, if I haven't already. Where does the idea of an "attack" come from? Why so insecure, I wonder?

 

[tris_d]

Drakkith and I get on very well. He has actually acquired a lot of knowledge and he has my respect. I also justified my problem with the way Simon was describing something and I thik we cam to terms with that. I get the impression that you have a different attitude, though, which is unfortunate if you really want to get to grips with this.

Perhaps you could quote one of my "empty statements"? I will try to justify it, if I haven't already. Where does the idea of an "attack" come from? Why so insecure, I wonder?

Oh, boy. I'm "insecure" because if I took your attitude I'd be banned straight away. I'm not claiming to know this stuff, I told you three times in a row that I DO NOT KNOW, and that's why I'm asking these questions. I'm not insecure about my knowledge nor do I feel ashamed about the things that I do not know. It's only you who associate some egoistical meaning to the amount of information stored in someone's head and like to gloat about it, which might very well mean you are insecure about some other things, in real life, and thus are compensating your frustration in your virtual life on this forum by criticizing other people with unnecessary personal comments. Just stop pulling my tail and concentrate on the topic please.

You can - but the density of photons would change as the wavelength changes.

If individual photos each can have their own wavelength, what does addition of other photons (density) have to do with the wavelength of each individual photon?

Whatever conclusion you came to about light of 450nm would be entirely different for light of 600nm.

Can you explain what and how would be different? -- It was not explained to me what "450nm" actually refers to. Drakkith was talking about focal point and airy discs, so I assumed "450nm" referred to angular size of projected image after it went through a lens and thus is relative to magnification. Please correct me if I assumed wrong.

The bottom line of this is to ask yourself why flux is not measured, conventionally, in terms of photons.

How is light flux measured?

BTW, you need to make the distinction between Flux (Watts) and Flux Density (Watts/msqu)

So flux is some flow rate per unit area? Flow of what, photons, no? How is that different to light intensity? Please explain it in terms I can understand, which means photons, like this:

Intensity is number of photons per unit area per unit time?

Light flux is number of photons per what per what per what?

 

[sophiecentaur]

If you will do me the courtesy of reading what Wikki and others have to say about Photons then you will Know what I am talking about and getting it from another source may help you believe it.
There are definitions of flux and flux density all over - relating to light, magnetism and other things. In my definition, I don't mention photons - I use the word Watts - which you know is Power. Don't you want to accept it from me? It's the truth.
How can intensity be "number of photons" when photons for different wavelengths all have different energies. Intensity is not defined differently for each wavelength, is it (look it up before you get cross) so it just can't involve a photon count.
If you really insist that the only information you get must be from me then you will be disappointed. Just try looking it up (with an open mind) and you will find all you want to know. Why don't you want to do it that way? Don't you see why I doubt your motives in this? Do you actually need 'links'?
Whilst you are at it, I suggest that you look up Wavelength of Visible light and you will see what Drakkith was talking about. Is your browser stuck on PF? This is a free service, you know and you can hardly demand an answer. When I was Paid to help students, I used to accept all sorts of lazy sods for tuition but this is done for 'love' (haha - of the subject).

 

[Drakkith]

How do we measure light flux?

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.

Why can not we measure brightness of point light sources, would not image itself (photo/eyes) be a measure of its brightness?

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?

Also, the way I have used brightness may not be correct. I was merely trying to use it as the same way your quoted source linked earlier used it. Terms like radiant flux, irradiance, and other terms are far better to use if we want to get anything meaningful out of a discussion.

I mean light flux, the one you say we can measure and drops off with the square of the distance. How many different light fluxes there are?

Light flux is nothing. It's not a unit of measurement. The correct term would be Radiant Flux or something else. Note that things like Luminance and Luminous Flux are based on the human eyes varying sensitivity to different wavelengths. See here: http://en.wikipedia.org/wiki/Intensity_(physics)
And here: http://en.wikipedia.org/wiki/Radiant_flux

I don't think units of flux can be be just Watts. Flux is generally flow rate per unit area, which seems to me is just about the same as what light intensity is.

Click the link above and it's right there.

 

[Drakkith]

Also, Sophie is correct in that there is a wealth of information out there that you need to read. A LOT of what we have explained cannot be understood if you don't know the bare basics, such as what "nm" means. (Nanometers)

It sucks, but you may need to hold off on asking questions like these until you read up on the very basics of light.

 

[tris_d]

Also, Sophie is correct in that there is a wealth of information out there that you need to read. A LOT of what we have explained cannot be understood if you don't know the bare basics, such as what "nm" means. (Nanometers)

It sucks, but you may need to hold off on asking questions like these until you read up on the very basics of light.

I know what nm means, I sad I do not know what size are you referring to. Would it not be easier for you to just tell me straight then misunderstanding me in order to criticize me? No one is forcing you to talk to me. If you think my questions are stupid just ignore me please.

 

[Drakkith]

I know what nm means, I sad I do not know what size are you referring to.

My mistake then. I misunderstood your reply to Sophie. What "size" are you asking about? The size of the airy disk?

Would it not be easier for you to just tell me straight then misunderstanding me in order to criticize me?

I have been telling you straight.

No one is forcing you to talk to me. If you think my questions are stupid just ignore me please.

Calm down please.

 

[tris_d]

How can intensity be "number of photons" when photons for different wavelengths all have different energies.

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?

If you really insist that the only information you get must be from me then you will be disappointed. Just try looking it up (with an open mind) and you will find all you want to know. Why don't you want to do it that way? Don't you see why I doubt your motives in this? Do you actually need 'links'?

"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!

Whilst you are at it, I suggest that you look up Wavelength of Visible light and you will see what Drakkith was talking about. Is your browser stuck on PF? This is a free service, you know and you can hardly demand an answer. When I was Paid to help students, I used to accept all sorts of lazy sods for tuition but this is done for 'love' (haha - of the subject).

I have no idea why are you even talking to me. It would take less words to actually answer my questions than to explain in detail how and why I should not be asking anything until I learned everything. But if I do what you say, it would take me much more time and then when I have learned everything I would not have any need to come back here. Is that what you want, to get rid of me?

 

[tris_d]

I have been telling you straight.

Yes, you have. Don't stop!

 

[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.

 

[Drakkith]

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

Yes, the aperture would be an "imaginary" surface if you like. You could find the total irradiance coming into your optical system using the area of the aperture.

 

[tris_d]

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.

Yeah, I think for the lens it would be enough to model some "scale" proportion, that is some magnification property that will define how small or large projected image needs to be, in relation to focal point and aperture size, and whatever else is there that comes into equation. I still haven't started to think about it properly since I have yet to learn how all those parameters relate to each other.

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

I'm not sure what do you mean, but here is what I got so far in relation to objects and their properties:

1.) Light sources:
- size, number of photons emitted, location/distance... anything else?

2.) Telescope:
- aperture size, exposure time, focus/magnification... anything else?

3.) Image:
- image size, pixel size, sensitivity, shades of gray... anything else?


So basically that's input defined by user, and output is an image of those light sources.

 

[sophiecentaur]

It's a shame that so much time has been wasted on a notion for which there is no justification except an elementary gut reaction - i.e. the specifications of Luminance / Luminosity / Light Flux etc etc in terms of photons. I have looked quite hard but have found nothing to support it. Everywhere I look, these quantities are specified in terms of Power. Why persist with a basically flawed treatment of the topic? Do you really not see how wrong it is?

What is the point of indulging in non-Physics when, I should have thought, the aim is to learn something and improve your knowledge? Persisting with the Photon Thing is not helping Tris_d to learn what he needs to know.

Physics Forums is amongst the most scrupulous of discussion sites and our rules require contributors to avoid groundless speculation and the use of reputable references etc. etc. That's why people get involved with PF. You can see the rules by clicking the button at the top of this page. The way this is going is definitely contrary to the guidelines. There are many other forums on which anything vaguely scientific is acceptable. I suggest you go to them if that's really what you want.

 

[tris_d]

It's a shame that so much time has been wasted on a notion for which there is no justification except an elementary gut reaction - i.e. the specifications of Luminance / Luminosity / Light Flux etc etc in terms of photons. I have looked quite hard but have found nothing to support it. Everywhere I look, these quantities are specified in terms of Power. Why persist with a basically flawed treatment of the topic? Do you really not see how wrong it is?

You are wrong. I explained in post #71 what and why. The rest of the people understand, they've already helped me. I think I now know enough, just need to develop equations and after that I can write the simulator in one afternoon.

What is the point of indulging in non-Physics when, I should have thought, the aim is to learn something and improve your knowledge? Persisting with the Photon Thing is not helping Tris_d to learn what he needs to know.

Do you want to see how the simulator works once is done?

Physics Forums is amongst the most scrupulous of discussion sites and our rules require contributors to avoid groundless speculation and the use of reputable references etc. etc. That's why people get involved with PF. You can see the rules by clicking the button at the top of this page. The way this is going is definitely contrary to the guidelines. There are many other forums on which anything vaguely scientific is acceptable. I suggest you go to them if that's really what you want.

Here's scrupulous discussion for you...


If light source emits photons of the same energy, then:

1.) Radiant flux
= energy per unit time
=> number of photons per unit time True/False?

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 True/False?

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 True/False?

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 True/False?

 

[sophiecentaur]

I can find no references to justify your Photon Idea. You have not quoted any. Until you can, it is nonsense in terms of Physics.
You can make a simulator do anything you like. It doesn't need to be valid Physics. These pages are littered with the confusions caused for people who have believed what they have seen in a simulation. Your simulation could be very entertaining and fun to play with but it has no scientific significance. Fair enough and good if you don't claim any more than that.

One day you may learn more about Photons and you will realize where you are going wrong with your "=>" assertions. They are False and misleading.

 

[Drakkith]

Sophie, remind me what assertions he is making? It seems more that he wants to "convert" energy into photons for the purpose of understanding the problem and developing a simulation.

 

[sophiecentaur]

You are wrong. I explained in post #71 what and why. The rest of the people understand, they've already helped me. I think I now know enough, just need to develop equations and after that I can write the simulator in one afternoon.

That reference has no mention of a formal definition in terms of numbers of photons. Have you actually read it all? Photon flux is actually mentioned in an entirely different context.

"^ Standards organizations recommend that radiometric quantities should be denoted with a suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities."

You should try to go for a bit more rigour if you really want to be taken seriously. Most of what you need to know is in that reference and it is not actually about Photons. (The word occurs just twice: just in a footnote)

 

[sophiecentaur]

Sophie, remind me what assertions he is making? It seems more that he wants to "convert" energy into photons for the purpose of understanding the problem and developing a simulation.

He seems to want to equate number of photons with the energy. Now, as photons of different wavelengths have different energies and very few light sources are monochromatic, the number of photons per second for a given Energy Flux density will not be the same for two sources with different spectra (e.g. red and blue stars). As far as I'm concerned, that knocks the Photon Thing totally on the head for serious use in comparing luminosities (or whatever related quantity you choose).

I realize that, for a simulation, it may be very convenient just to use a number but that's just not Physics. His simulation could work if he is only dealing with a notional light source which has the same spectrum throughout. But he seems to want the whole of Science to revolve around his wish to simplify. The daft thing is that he could just as easily use a variable called Power as a variable called Numberofphotons. But I think it has become too much of a matter of misplaced principle for him to do that simple thing.

That is the problem, I think. Understanding can only be claimed when what you think you've understood holds up against external criteria. Without that, it can easily be misunderstanding. Simulations 'prove' nothing. They can be smoke and mirrors.

 

[Drakkith]

I honestly don't see the big deal, but I really don't feel like explaining why. It's been a long, frustrating, confusing thread that I think I'm done with.

 

[tris_d]

I can find no references to justify your Photon Idea. You have not quoted any. Until you can, it is nonsense in terms of Physics.

http://en.wikipedia.org/wiki/Photon

You can make a simulator do anything you like. It doesn't need to be valid Physics. These pages are littered with the confusions caused for people who have believed what they have seen in a simulation. Your simulation could be very entertaining and fun to play with but it has no scientific significance. Fair enough and good if you don't claim any more than that.

Mine will use valid physics though, and will be exact as much as this equation is:

One day you may learn more about Photons and you will realize where you are going wrong with your "=>" assertions. They are False and misleading.

Don't blame me for your inability to understand. Here's something for you to practice:Power to photon rate:
http://www.calctool.org/CALC/chem/photochemistry/power_photons

Energy to no. photons:
http://www.calctool.org/CALC/chem/photochemistry/energy_photons