# The Naked Scientists Forum

### Author Topic: How does luminosity decrease with distance from a light source?  (Read 2478 times)

#### jeffreyH

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##### How does luminosity decrease with distance from a light source?
« on: 29/01/2014 16:38:27 »
I am looking for a formula to describe the decrease in luminosity by distance from a light source. This important for 2 reasons. 1 Describing interactions with the Higgs field and 2 A new method of describing De Broglie waves.

#### jeffreyH

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #1 on: 29/01/2014 16:48:41 »
Correction. I am looking for luminance equations.

#### jeffreyH

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #2 on: 29/01/2014 17:01:04 »

#### lightarrow

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #3 on: 29/01/2014 17:07:58 »
If the source is point-like and the emission isotropic, its intensity at distance R is proportional to 1/R2.
To have luminance you have to define the wavelenght (if monocromatic) or the entire spectrum (if non-monochromatic); luminance infact is a photometric quantity, it refers to the perceived light, not to the radiant energy conveyed by light.
http://en.wikipedia.org/wiki/Photometric_quantities#Photometric_quantities

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lightarrow

#### Soul Surfer

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #4 on: 30/01/2014 00:02:54 »
The classic inverse square law is in effect defined by the conservation of energy. For a radiating point source (zero dimensions) the energy density falls off in proportion to the increase in the area of the surface that surrounds the point.  This increases as the square of the radius that is one less than the three of the dimensions of the volume.

There is an interesting extension to this relating to the dimensionality of the source and the space that applies generally.

For normal three dimensional space the radiation fall off from a one dimensional (long line ) source falls of as the inverse of the distance and the fall off from a large plane source is zero. this can be demonstrated by visiting the empire state building.  The basic rumble of traffic noise(excluding of course very local noise sources  at street level) is the same at the top of the tower as in the street at ground level because you can hear noise originating from a much larger area of the city.

This concept also applies to world with higher numbers of spatial dimensions than three.

#### jeffreyH

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #5 on: 30/01/2014 00:14:36 »
The classic inverse square law is in effect defined by the conservation of energy. For a radiating point source (zero dimensions) the energy density falls off in proportion to the increase in the area of the surface that surrounds the point.  This increases as the square of the radius that is one less than the three of the dimensions of the volume.

There is an interesting extension to this relating to the dimensionality of the source and the space that applies generally.

For normal three dimensional space the radiation fall off from a one dimensional (long line ) source falls of as the inverse of the distance and the fall off from a large plane source is zero. this can be demonstrated by visiting the empire state building.  The basic rumble of traffic noise(excluding of course very local noise sources  at street level) is the same at the top of the tower as in the street at ground level because you can hear noise originating from a much larger area of the city.

This concept also applies to world with higher numbers of spatial dimensions than three.

You lost me at "There is an interesting extension to this relating to the dimensionality of the source and the space that applies generally." The dimensionality of the source I assume is the radiating surface area but what is "the space that applies generally"?

Also how can a one dimensional line show an inverse square relationship but a plane show zero? You need to be clearer here. If you are saying the luminance of a larger surface has zero decrease in luminance show me the data. That would be of great interest. However I doubt it.

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##### Re: How does luminosity decrease with distance from a light source?
« Reply #5 on: 30/01/2014 00:14:36 »