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There is a disparity here... I think an optical depth is something like transmission=exp(-depth). But I'm not sure. William M. Connolley 19:54, 1 January 2006 (UTC).
The optical depth tau is related to the fraction of light scattered. The following equation expresses this relationship:
I/I_0= exp{-tau},
where I_0 is the incident light and I is the light that passes through the medium without being scattered. Equivalently, for a homogenous medium, tau is the ratio of the path length to the mean free path.
Corrected definition inserted in the article, some inexactitudes corrected, some concepts in atmospheric science added. I hope it is appreciated. Marenco 26 September 2006 (UTC)
I think there is a cognitive problem here. As I understand it, optical depth is a property of the material you are looking through, not of you the observer or the object you are observing. However, the article talks about it in analogy to taking an object and moving it backward, which makes it sound like the medium is not what is being described by optical depth.
It might be better to speak of optical depth increasing as the fog gets thicker, and the value of optical depth being the farthest thing you can see through that fog.
Raddick 20:10, 4 November 2007 (UTC)
I think the explanation can be reformulated to avoid misunderstandings, but is important to state that the optical depth change when there is more medium between the observer and the object (more light get scattered or absorved). So the optical depth is not just a property of the material, but it also depends on the distance between the object and the observer. The farther the object is, the less you see of it (because there is more medium on the way). Hsxavier 23:58, 5 November 2007 (UTC)
I'm not convinced that the first sentence is correct :
Optical depth is a measure of transparency, and is defined as the fraction of radiation (or light) that is scattered or absorbed on a path.
The equation is correct, but if tau is the fraction of the radiation that is removed then tau=0.5 should mean that half of the radiation has been absorbed or scattered. However e^(-0.5) ~ 0.6 and e^(-1) is certainly not equal to zero. -- Maddoug ( talk) 15:59, 22 January 2008 (UTC)
The current (Apr. 25, 2009) first sentence "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that is scattered or absorbed on a path." is not correct. Given I/I0=e^-tau, tau=-ln(I/I0). Thus the first sentence needs to be "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that IS NOT scattered or absorbed (i.e., IS transmitted) on a path." —Preceding
unsigned comment added by
Random25 (
talk •
contribs)
18:47, 25 April 2009 (UTC)
For what it's worth: optical thickness (depth, whatever) is an important concept in understanding the design of thermonuclear weapons.
Basesurge ( talk) 07:41, 22 March 2010 (UTC)
Is there no difference between the 2 concepts in English stellar physics ? I can't be sure this is a mistake, I am neither an English native-speaker nor a stellar physicist, so... Anyway, French physicists would see a big difference between optical depth (profondeur optique) and optical thickness (épaisseur optique), although the terms are often subject to confusion and related mistakes. In stellar physics :
Same in English ? Should it be clearly mentioned both
here (the article I have been partially translating) and in
Optical depth ?
Mianreg (
talk)
02:39, 25 August 2010 (UTC)
There is one thing I do not understand with this concept. According to the scattering properties of light, it can be influenced even in a far distance from a small scatterer (but this influence is very small). So if we say that light which is scattered (even a little bit) has been 'influenced' and is taken out from the original light beam, than optical depth would be infinite everywhere (in our universe). So what is the minimum scattering angle or minimum momentum transfer until which scattering has not to be taken into account for optical depth? Or is optical depth dependent on observer properties like a camera resolution (and this would define the minimum angle not to care about)? 134.76.234.36 ( talk) 10:08, 14 October 2010 (UTC)
The camera aperture is obviously of practical concern. We could always choose an aperture so small that the measurements become diffraction limited, or so large that most of the scattering is not observed. But that does not invalidate the definition. Think of optical depth as a way to compare two similar things. As long as the physical setups are similar the results can be compared. However, comparing camera one with camera two or three is of little value.
The equations in the article are significantly simplified verses those given by wolfram and other references. The definition specifically uses the word "normal" to indicate the radiation path and a camera (in fact, any sensor) can only approximate "normal". (Well, a telescope looking at the Sun is a pretty good approximation. A spectrophotometer in the lab, not so much.) Also, the actual definition of optical depth is actually an integral which, at least to me, appears to cover most of your concerns. However, I think that that is too much detail for a general audience.
You have presented some very interesting points, but I still don't understand why this bothers you. Q Science ( talk) 16:42, 20 October 2010 (UTC)
I don't think optical thickness should be equated to optical depth in general. In optics literature, I've learnt to know "optical thickness" of thin films as the physical thickness times the refractive index of the film, which is something very different. I think the entry on Optical Thickness at Encyclopedia of Laser Physics and Technology is clarifying. Danmichaelo ( talk) 16:23, 23 May 2011 (UTC)
I agree with Danmichaelo. I thought I might get a quick definition for a training presentation but find that Wikipedia has erroneously "redefined" the thin film meaning "nd" (real part of the refractive index times physical thickness). Perhaps in meteorology the term is used differently. "optical thickness" in thin films would be synonymous with "optical path length" which is used in geometric optics. Generally when the "path length" is of the order of the wavelength, the term "optical thickness" will be used. See for example the Wiki entry for "Thin-film interference." Also see "Practical Design and Production of Optical Thin Films" edited by Ronald R. Willey, 2002, ISBN 0-8247-0849-0. Or "Optical characterization of low optical thickness thin films from transmittance and back reflectance measurements" Y Laaziz, A Bennouna, N Chahboun, A Outzourhit… - Thin Solid Films, 2000 - Elsevier. Bubsir ( talk) 03:04, 19 September 2014 (UTC)
If optical depth uses the natural logarithm, e.g., rather than base 10 or 2, then the article should say so, yes? 67.248.149.19 ( talk) 02:07, 14 February 2012 (UTC)
Inexpertly, an electric dipole moment has the units of "charge × distance", and reflects a separation of positive & negative charges, which are then further separatable, by incident EM radiations. I.e. the dipole moment is like a gap between the nucleus & core electrons (net positive charge) and the absorbing electron (negative charge) which can be wedged wider, by an incident photon. A correct & clear physical explanation, for the terms in the optical depth formula, could help improve the article. 66.235.38.214 ( talk) 00:02, 13 November 2012 (UTC)
At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the Sun) matches the observed total energy emitted.
Does this mean:
The energy emitted by a given star can be computed by replacing the star by a black body with surface temperature and radius corresponding to an optical depth of 2/3 for the star ?
178.38.105.43 ( talk) 17:27, 9 April 2015 (UTC)
Why is there so much italicization? What style guide does that follow? — Ben Brockert (42) 12:51, 12 June 2018 (UTC)
This
level-5 vital article is rated Start-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | |||||||||||
|
There is a disparity here... I think an optical depth is something like transmission=exp(-depth). But I'm not sure. William M. Connolley 19:54, 1 January 2006 (UTC).
The optical depth tau is related to the fraction of light scattered. The following equation expresses this relationship:
I/I_0= exp{-tau},
where I_0 is the incident light and I is the light that passes through the medium without being scattered. Equivalently, for a homogenous medium, tau is the ratio of the path length to the mean free path.
Corrected definition inserted in the article, some inexactitudes corrected, some concepts in atmospheric science added. I hope it is appreciated. Marenco 26 September 2006 (UTC)
I think there is a cognitive problem here. As I understand it, optical depth is a property of the material you are looking through, not of you the observer or the object you are observing. However, the article talks about it in analogy to taking an object and moving it backward, which makes it sound like the medium is not what is being described by optical depth.
It might be better to speak of optical depth increasing as the fog gets thicker, and the value of optical depth being the farthest thing you can see through that fog.
Raddick 20:10, 4 November 2007 (UTC)
I think the explanation can be reformulated to avoid misunderstandings, but is important to state that the optical depth change when there is more medium between the observer and the object (more light get scattered or absorved). So the optical depth is not just a property of the material, but it also depends on the distance between the object and the observer. The farther the object is, the less you see of it (because there is more medium on the way). Hsxavier 23:58, 5 November 2007 (UTC)
I'm not convinced that the first sentence is correct :
Optical depth is a measure of transparency, and is defined as the fraction of radiation (or light) that is scattered or absorbed on a path.
The equation is correct, but if tau is the fraction of the radiation that is removed then tau=0.5 should mean that half of the radiation has been absorbed or scattered. However e^(-0.5) ~ 0.6 and e^(-1) is certainly not equal to zero. -- Maddoug ( talk) 15:59, 22 January 2008 (UTC)
The current (Apr. 25, 2009) first sentence "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that is scattered or absorbed on a path." is not correct. Given I/I0=e^-tau, tau=-ln(I/I0). Thus the first sentence needs to be "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that IS NOT scattered or absorbed (i.e., IS transmitted) on a path." —Preceding
unsigned comment added by
Random25 (
talk •
contribs)
18:47, 25 April 2009 (UTC)
For what it's worth: optical thickness (depth, whatever) is an important concept in understanding the design of thermonuclear weapons.
Basesurge ( talk) 07:41, 22 March 2010 (UTC)
Is there no difference between the 2 concepts in English stellar physics ? I can't be sure this is a mistake, I am neither an English native-speaker nor a stellar physicist, so... Anyway, French physicists would see a big difference between optical depth (profondeur optique) and optical thickness (épaisseur optique), although the terms are often subject to confusion and related mistakes. In stellar physics :
Same in English ? Should it be clearly mentioned both
here (the article I have been partially translating) and in
Optical depth ?
Mianreg (
talk)
02:39, 25 August 2010 (UTC)
There is one thing I do not understand with this concept. According to the scattering properties of light, it can be influenced even in a far distance from a small scatterer (but this influence is very small). So if we say that light which is scattered (even a little bit) has been 'influenced' and is taken out from the original light beam, than optical depth would be infinite everywhere (in our universe). So what is the minimum scattering angle or minimum momentum transfer until which scattering has not to be taken into account for optical depth? Or is optical depth dependent on observer properties like a camera resolution (and this would define the minimum angle not to care about)? 134.76.234.36 ( talk) 10:08, 14 October 2010 (UTC)
The camera aperture is obviously of practical concern. We could always choose an aperture so small that the measurements become diffraction limited, or so large that most of the scattering is not observed. But that does not invalidate the definition. Think of optical depth as a way to compare two similar things. As long as the physical setups are similar the results can be compared. However, comparing camera one with camera two or three is of little value.
The equations in the article are significantly simplified verses those given by wolfram and other references. The definition specifically uses the word "normal" to indicate the radiation path and a camera (in fact, any sensor) can only approximate "normal". (Well, a telescope looking at the Sun is a pretty good approximation. A spectrophotometer in the lab, not so much.) Also, the actual definition of optical depth is actually an integral which, at least to me, appears to cover most of your concerns. However, I think that that is too much detail for a general audience.
You have presented some very interesting points, but I still don't understand why this bothers you. Q Science ( talk) 16:42, 20 October 2010 (UTC)
I don't think optical thickness should be equated to optical depth in general. In optics literature, I've learnt to know "optical thickness" of thin films as the physical thickness times the refractive index of the film, which is something very different. I think the entry on Optical Thickness at Encyclopedia of Laser Physics and Technology is clarifying. Danmichaelo ( talk) 16:23, 23 May 2011 (UTC)
I agree with Danmichaelo. I thought I might get a quick definition for a training presentation but find that Wikipedia has erroneously "redefined" the thin film meaning "nd" (real part of the refractive index times physical thickness). Perhaps in meteorology the term is used differently. "optical thickness" in thin films would be synonymous with "optical path length" which is used in geometric optics. Generally when the "path length" is of the order of the wavelength, the term "optical thickness" will be used. See for example the Wiki entry for "Thin-film interference." Also see "Practical Design and Production of Optical Thin Films" edited by Ronald R. Willey, 2002, ISBN 0-8247-0849-0. Or "Optical characterization of low optical thickness thin films from transmittance and back reflectance measurements" Y Laaziz, A Bennouna, N Chahboun, A Outzourhit… - Thin Solid Films, 2000 - Elsevier. Bubsir ( talk) 03:04, 19 September 2014 (UTC)
If optical depth uses the natural logarithm, e.g., rather than base 10 or 2, then the article should say so, yes? 67.248.149.19 ( talk) 02:07, 14 February 2012 (UTC)
Inexpertly, an electric dipole moment has the units of "charge × distance", and reflects a separation of positive & negative charges, which are then further separatable, by incident EM radiations. I.e. the dipole moment is like a gap between the nucleus & core electrons (net positive charge) and the absorbing electron (negative charge) which can be wedged wider, by an incident photon. A correct & clear physical explanation, for the terms in the optical depth formula, could help improve the article. 66.235.38.214 ( talk) 00:02, 13 November 2012 (UTC)
At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the Sun) matches the observed total energy emitted.
Does this mean:
The energy emitted by a given star can be computed by replacing the star by a black body with surface temperature and radius corresponding to an optical depth of 2/3 for the star ?
178.38.105.43 ( talk) 17:27, 9 April 2015 (UTC)
Why is there so much italicization? What style guide does that follow? — Ben Brockert (42) 12:51, 12 June 2018 (UTC)