<p>In astronomy, <strong>luminosity</strong> is exactly as you've defined it. </p>

<p>In <em>radiometry</em>, the usual term for this is <strong>radiant flux</strong>. So, yes, they are the same thing.</p>

<p><strong>Luminous flux</strong>, however, is different. It is a term from <a href="https://en.wikipedia.org/wiki/Photometry_(optics)" rel="nofollow noreferrer">"photometry"</a>, which is the measurement of light *as perceived by the human eye" (I put it in scare quotes because <em>in astronomy</em>, the word "photometry" usually means measurement of light without any reference to human perceptions, and is sometimes casually used to refer to imaging or analysis of images). It is <em>analogous</em> to luminosity/radiant flux, but not the same thing, because it involves weighting the energy of the light by how well or poorly the human eye can sense it. (See the linked Wikipedia article for more details.)</p>

<p>Imagine a radio source that doesn't emit any visible light. An astronomer would say it has a luminosity of (say) <span class="math-container">$X$</span> watts. From a radiometric perspective, it has a radiant flux of exactly the same amount. But it would have a <em>luminous flux</em> of zero, since the human eye can't detect radio photons.</p>

<p><em>this website ... says lumens are a unit of luminosity, but how if lumens are candela times steradians?</em></p>

<p>Technically, that website isn't really correct, since it's casually equating luminosity (or radiant flux) with luminous flux.</p>

CC BY-SA 4.0
2019-04-21T08:40:46.007Z
2019-04-21T15:17:16.463Z
2019-04-21T15:17:16.463Z
Peter Erwin
<p>I've looked this up, and was wondering the differences between them. Luminosity is the total energy emitted by a source, in watts. However, I'm confused about luminous flux and radiant flux. Since radiant flux is the total energy emitted by an object, would that make luminosity the same as radiant flux? Then how does luminous flux relate to luminosity? Is luminous flux just radiant flux, but only showing the visible light? And this website, <a href="https://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html" rel="nofollow noreferrer">https://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html</a>, says lumens are a unit of luminosity, but how if lumens are candela times steradians?</p>

CC BY-SA 3.0
2017-02-13T16:18:42.350Z
Recommend closing because this is a physics question, and further shows a significant lack of research. These terms are all well-defined.
Carl Witthoft
CC BY-SA 3.0
2017-02-13T18:52:15.853Z
I think this is a perfectly valid question. While these concepts are technically physics related, they're most prominently used in astronomy. Furthermore, just because they're well-defined, does not mean OP finds it easy to understand them. General relativity is well defined but I'd still expect anyone new to the material to struggle in understand it.
zephyr
CC BY-SA 3.0
2017-02-14T08:36:30.930Z
General relativity would also be off topic here, I think.
Rory Alsop
CC BY-SA 4.0
2019-04-21T08:56:56.683Z
This is a valid question, because it involves confusion between terms and measurements in astronomy, radiometry, and [photometry](https://en.wikipedia.org/wiki/Photometry_(optics)) (as opposed to [photometry](https://en.wikipedia.org/wiki/Photometry_(astronomy))). These fields use some of the same words (e.g., "flux") in different ways, so the confusion is understandable.
Peter Erwin
<star><light><luminosity>
Difference between luminosity, luminous flux, and radiant flux?
user15819
# Difference between luminosity, luminous flux, and radiant flux?

(Asked by: username_0 on 2017-02-13)

I've looked this up, and was wondering the differences between them. Luminosity is the total energy emitted by a source, in watts. However, I'm confused about luminous flux and radiant flux. Since radiant flux is the total energy emitted by an object, would that make luminosity the same as radiant flux? Then how does luminous flux relate to luminosity? Is luminous flux just radiant flux, but only showing the visible light? And this website, <https://www.ifa.hawaii.edu/~barnes/ASTR110L_S03/inversesquare.html>, says lumens are a unit of luminosity, but how if lumens are candela times steradians?

**Tags:** star, light, luminosity
> **Comments:**
> **username_1**: Recommend closing because this is a physics question, and further shows a significant lack of research. These terms are all well-defined.  
> **username_2**: I think this is a perfectly valid question. While these concepts are technically physics related, they're most prominently used in astronomy. Furthermore, just because they're well-defined, does not mean OP finds it easy to understand them. General relativity is well defined but I'd still expect anyone new to the material to struggle in understand it.  
> **username_3**: General relativity would also be off topic here, I think.  
> **username_4**: This is a valid question, because it involves confusion between terms and measurements in astronomy, radiometry, and [photometry](https://en.wikipedia.org/wiki/Photometry_(optics)) (as opposed to [photometry](https://en.wikipedia.org/wiki/Photometry_(astronomy))). These fields use some of the same words (e.g., "flux") in different ways, so the confusion is understandable.  

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## 1 answers

### Answer by username_4 on 2019-04-21. Score: 0

In astronomy, **luminosity** is exactly as you've defined it.

In *radiometry*, the usual term for this is **radiant flux**. So, yes, they are the same thing.

**Luminous flux**, however, is different. It is a term from ["photometry"](https://en.wikipedia.org/wiki/Photometry_(optics)), which is the measurement of light \*as perceived by the human eye" (I put it in scare quotes because *in astronomy*, the word "photometry" usually means measurement of light without any reference to human perceptions, and is sometimes casually used to refer to imaging or analysis of images). It is *analogous* to luminosity/radiant flux, but not the same thing, because it involves weighting the energy of the light by how well or poorly the human eye can sense it. (See the linked Wikipedia article for more details.)

Imagine a radio source that doesn't emit any visible light. An astronomer would say it has a luminosity of (say) $X$ watts. From a radiometric perspective, it has a radiant flux of exactly the same amount. But it would have a *luminous flux* of zero, since the human eye can't detect radio photons.

*this website ... says lumens are a unit of luminosity, but how if lumens are candela times steradians?*

Technically, that website isn't really correct, since it's casually equating luminosity (or radiant flux) with luminous flux.


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