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Is there any relation between wavelength and brightness?

by tris_d
Tags: brightness, relation, wavelength
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Drakkith
#91
Nov18-12, 10:24 PM
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Quote Quote by tris_d View Post
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
#92
Nov18-12, 10:34 PM
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Quote Quote by Simon Bridge View Post
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
#93
Nov19-12, 01:28 PM
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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
#94
Nov19-12, 02:07 PM
P: 162
Quote Quote by sophiecentaur View Post
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
#95
Nov19-12, 03:32 PM
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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 realise where you are going wrong with your "=>" assertions. They are False and misleading.
Drakkith
#96
Nov19-12, 03:39 PM
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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
#97
Nov19-12, 03:43 PM
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Quote Quote by tris_d View Post
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
#98
Nov19-12, 03:57 PM
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Quote Quote by Drakkith View Post
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 realise 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
#99
Nov19-12, 04:05 PM
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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
#100
Nov19-12, 05:20 PM
P: 162
Quote Quote by sophiecentaur View Post
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 realise 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/ph.../power_photons

Energy to no. photons:
http://www.calctool.org/CALC/chem/ph...energy_photons
sophiecentaur
#101
Nov19-12, 05:33 PM
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So, now you know the relationship between power and wavelength (and frequency) and it has taken 100 posts for that to emerge. (This is no news to most people on this Forum and it's what I suggested you should find out about, way back in this thread.) Can you not see that the power in the light from an arbitrary source cannot just equate to a particular number of photons per second - because the photons all have different energies. You need to know the particular proportions of each wavelength (i.e. the spectrum) in order to work out the Power - photon rate relationship. Can you get your head round that? What wavelengths do you intend to use? Will your light source be monchromatic? That's not a very useful model to simulate. If, on the other hand, you use Power Flux, the problem (and my objection) disappears. If you're clever enough to put a bit of computer code together then this should be a piece of cake.

Btw, did you not read my bit about E = hf, about a hundred years ago on this thread?
tris_d
#102
Nov19-12, 06:42 PM
P: 162
Quote Quote by sophiecentaur View Post
Can you not see that the power in the light from an arbitrary source cannot just equate to a particular number of photons per second - because the photons all have different energies.
#59
- "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?"

#71
- "I guarantee you that this can be done if simplify the scenario by having the light source emit photons of the same energy..."

#75
- "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..."

#94
- "
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?
"

Can you get your head round that?
Are your blind, or something? See above.


What wavelengths do you intend to use?
Defined by user or imported from a database with actual measurements.


Will your light source be monchromatic?
For start each light source will emits only photons of the same energy.


That's not a very useful model to simulate.
You wouldn't know.


If, on the other hand, you use Power Flux, the problem (and my objection) disappears. If you're clever enough to put a bit of computer code together then this should be a piece of cake.
Your objection is only your problem. It is necessary to quantize the light into photons because the image is quantized into pixels.


Btw, did you not read my bit about E = hf, about a hundred years ago on this thread?
Yes, thank you. Time flies, eh?
sophiecentaur
#103
Nov20-12, 02:46 AM
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The pixel and the photon issue are totally separate issues. However, because you are implementing your simulation in terms of discrete quantities, you may feel pressured into the quantised way of thinking.

I can't think of many light sources of interest (certainly not cosmic ones) that are monochromatic (why not use the right word, eh?) Funnily enough, if you were to be addressing the problem of laser light, you would really be forced into using a wave approach, which would add complication.
tris_d
#104
Nov20-12, 07:28 AM
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Quote Quote by sophiecentaur View Post
The pixel and the photon issue are totally separate issues. However, because you are implementing your simulation in terms of discrete quantities, you may feel pressured into the quantised way of thinking.
How else would you calculate brightness per pixel?


I can't think of many light sources of interest (certainly not cosmic ones) that are monochromatic (why not use the right word, eh?)
It's about stars and galaxies, and in actuality I guess they do not emit monochromatic light, but if it can be approximated by taking an average, or if the actual measurements are already approximated in such way, then that should be good enough for me too.

Besides, I could make light sources output any range of photons with different wavelengths in whatever proportion, if necessary. It's just a matter of how actual measurements look like, which I'm in the process of figuring out right now by looking at star databases and the way they present such information.


Funnily enough, if you were to be addressing the problem of laser light, you would really be forced into using a wave approach, which would add complication.
I thought lasers are monochromatic and that intensity of a laser is therefore directly proportional to the number of photons emitted.
sophiecentaur
#105
Nov20-12, 12:49 PM
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Yes. Laser light is monochromatic but it is so coherent that it can produce speckles and patterns that 'ordinary' light sources do not. It would be difficult to model (except that your beloved photons per second per sqmetre would actually apply)

You
guess they do not emit monochromatic light
? Well, that was my whole point. There is a 2:1 ratio of energies of the shortest and longest wavelength photons. It is hardly worth my writing all this again - it's all in earlier posts - but you will surely appreciate the difference between what you get when you count photons and what you get when you measure the energy, for different coloured stars. The accepted way of doing this will take away this problem. Star Magnitude is based on energy flow, so why not just join the club?
There is no more to be said, really. If you want your simulation to be as real as possible then why not just specify things more conventionally? As I have also said before - the actual code would be hardly any different and you would have learned something at the same time.
tris_d
#106
Nov21-12, 01:15 AM
P: 162
Quote Quote by sophiecentaur View Post
There is a 2:1 ratio of energies of the shortest and longest wavelength photons.
It's all the same to me. If they can measure it, I can simulate it.


It is hardly worth my writing all this again - it's all in earlier posts - but you will surely appreciate the difference between what you get when you count photons and what you get when you measure the energy, for different coloured stars. The accepted way of doing this will take away this problem.
I only see problems and limits if I don't convert energy to individual photons.


Star Magnitude is based on energy flow, so why not just join the club?
Energy flow is based on number of photons flow.


There is no more to be said, really. If you want your simulation to be as real as possible then why not just specify things more conventionally?
To simulate radial spreading of light rays, to be able to model such property as is time interval between arrival of two successive photons, and so I can count photons per pixel.


As I have also said before - the actual code would be hardly any different and you would have learned something at the same time.
Photon is more specific way to model light, energy is vague and ambiguous.
tris_d
#107
Nov21-12, 01:20 AM
P: 162
Quote Quote by sophiecentaur View Post
Yes. Laser light is monochromatic but it is so coherent that it can produce speckles and patterns that 'ordinary' light sources do not.
Simulating double slit experiment is on my to do list. The goal will be to explore if there is any other, less magical, mechanics that can produce interference pattern.
Simon Bridge
#108
Nov21-12, 01:52 AM
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Quote Quote by tris_d View Post
Photon is more specific way to model light, energy is vague and ambiguous.
Not so: energy is a well-defined core concept in physics. Photons only risks being an incomplete model - but it is your model. Have fun.


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