# Luminous flux

Discussion in 'Lighting' started by Richard, Apr 26, 2007.

1. ### RichardGuest

Luminous flux is pretty much like the total amount of light put out by a
light source.

When you buy a lamp it often will state light output in Lumens.

If you took a light meter and measured the luminous emittance of a surface
it would give a reading in lux. Luminous emittance is Luminous flux / m2.

That bothers me a bit, because in the first case Luminous flux is said to be
the total quantity of light emitted by a light source. Whereas in measuring
Luminous emittance, luminous flux does not neccessarly carry the concept
of total light emitted by a light source at all.

So luminous flux does not seem to have one single meaning - does it?

2. ### RichardGuest

I think I am right. But I am only right because of the difference in
calculating and measuring. When you calculate, you are using luminous flux
as a total amount of light. When you measure light luminous flux can be an
apparant total amount of light. So, there is a subtle difference.

Say there are no losses. A lamp of 2000 lumens is emitting all it's light
through two lossless diffusers with an area of 2 square metres. Here the
actual total amout of light is 2000 lumens, and the luminous emittance
*calculated* from the surfaces is 2000/2 = 1000 lux.

The same two diffusers letting light through from the sun. You *measure* the
luminous emittace at 1000 lux. in fromt of the sun. The apparant luminous
flux is 2000 lumens. Of course the light source being the sun is giving out
a total amount of light in lumens much more than that!

So, I guess you have to be careful what you mean by lumens.

Anyone see any flaws in this? TIA.

3. ### Douglas G. CumminsGuest

1) Your light meter does not measure luminous emittance. It measures
illuminance upon the meter in units of Lux or Footcandles.

2) To measure the luminous flux of a lamp source using a light meter,
you need to measure the light emitted from the lamp at *all* angles then
calculate the total - it's a bit more complicated than a simple sum
since you need to include sphere geometry, but you are essentially
summing the total amount of light striking the surface of a sphere.
Typically, the source is considered a point source.

3) Luminous emittance, or more simply luminance, is the amount of light
emitted from a surface, typically an area source. There is a big
difference between luminance and illuminance - the terms are not
interchangeable, but they can be related when using a diffuse surface.

4) Just because you're measuring the same illuminance from a diffuser
using two different sources does not mean that the sources have the same
flux. Perhaps the same flux from both sources is passing through the
diffusers, but that's not the same thing as the total flux emitted from
the source. To use your example, the sun is emitting light in far many
more solid angles than you are capturing through your relatively small
diffusers.

In either case, luminous flux does mean the same thing. It is a
dimension of measure. Just like length. The length of your finger
doesn't indicate your height - but they're both measured in inches (or
millimeters, or whatever is your pleasure). You're not measuring the
same thing, but the dimension you're using is the same.

Do you have to be careful in how you use the term? Of course - just
like any other measure. If the topic is the flux from a small,
omnidirectional source (Incandescent, CFL, etc.), then I can be fairly
certain that the flux is the measure of total light emitted in all
directions. If the topic is a directional source (LED, bulb & reflector
system like a automotive headlamp, etc.), then I can be fairly certain
that it is the flux within the beam - for example, in a headlamp I
tested I measured the flux of its bulb at 1000 Lumens but the beam flux
was only ~350 Lumens (which is fairly typical) because not all the flux
from the bulb is captured by the reflector and redirected into the beam.
There are also reflector and lens efficiency losses.

4. ### Victor RobertsGuest

Correct. This data should be provided for all lamps, but by
convention, the total flux is not given for many reflector
lamps.
Well, not really. Lux is a measure of the illuminance,
lumens/m^2 falling on a surface. The light meter is
calibrated display to the value of the flux it receives
divided by the active area of the sensor.

Luminance emittance, now called Luminous Exitance, is also
measured in lumens/m^2 but is a measure of the total amount
of light leaving a unit area of the source. It is not
measured in Lux.
Actually it does. If you measure Luminous Exitance over the
full surface of the source and then integrate the measured
values over the full surface, the area term disappears and
you have the total flux.
Yes it does. Remember that the first of your terms is total
flux while the second is not flux but is flux density
(lumens/m^2).

--
Vic Roberts
http://www.RobertsResearchInc.com
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

5. ### RichardGuest

I think the truth of the matter is that often luminous flux is the total
flux emitted by a light source. Be it one lamp or many that is providing
light.

This is true when there is an assumption, or in fact, the flux, in lumens,
appearing in a formula, is considered to be the total flux emanating from
the source or sources. Luminous flux then directly relates to the source /
lamps and their entire or total light output.

But, it is possible that the luminous flux quanity does not directly relate
to the total light output of a light source.

The key is to understand that a source does not necessarily equate with a
lamp or other primary source of light. A source can just mean a surface area
that is emitting light, and that being considered the actual light source
being responsible for the light.

Where a lamp of 2000 lumens is emitting all it's light through two lossless
diffusers with an area of 2 square metres, the actual total amout of light
is 2000 lumens, and the luminous emittance calculated from the surfaces is
2000/2 = 1000 lux.

Say the same two diffusers are letting light through from the sun. The
luminous emittace from those diffusers can be 1000 lux. The luminous flux is
calculates to 2000 lumens, which is the total amount of light being put out
by those diffusers or the surface areas taken as a light source. But, of
course, the primary light source, being the sun, is giving out a total
amount of light in lumens much more than that.

So lumens is total flux, but it is not always the same as a primary light
source, as in lamp, or the sun.

6. ### Victor RobertsGuest

I don't disagree with your example, but I also don't think I
ever said that every measurement of luminous flux will give
the total flux of the primary light source.

If you integrate the Luminous Existance over any surface you
will get the total flux emitted by that surface. However,
if the surface being measured does not completely enclose
the light source, as is the case for your example, then the
total flux emitted by the surface will be less than the
total flux emitted by the source.

--
Vic Roberts
http://www.RobertsResearchInc.com
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.

7. ### RichardGuest

Yes, that's it. I'm just a layperson trying to understand. When one at first
thinks about luminous flux one might, like me, think it is always refers to
the output from a lamp. That is what I was thinking at first, it just seemed
natural to think like that. But often, what is taken as the source is not a
lamp or what I called a primary source, but a surface. I think I have got it
now. Lumens is total flux, which can be less than total flux emitted by a
source. But it is a measure of total flux either way.

8. ### RichardGuest

I think you are saying is, that whether the source is a lamp, which actually
is a source, in the sense that it's converting electrical power into light
(also heat of course, but lets ignore that) or whether the source is simply
taken to be a surface, like for instance a diffuser that is part of say a
lightbox, both are capable of being termed sources of light with a total
flux, measured in lumens, as well as a luminanance. A radiating lamp has a
surface area, and can be taken as a surface, it's just that that surface is
part of a lamp, or a source that it is actually the source of light.

So, when I look at a light box, I know there is a primary light source, that
is the lamp behind the diffuser. That radiates a luminous flux, the value
found in the manufacturers lamp catalogue. The lamp has a surface that emits
light and has a luminance. But, I can alternatively look at the light
coming out of the diffuser and think of that surface as a source of light.
It too emits light, and radiates a luminous flux of a certain value and it
too has a luminance.

In either case, luminous flux is the total flux, in lumens, emitted by the
source in question. The point to grasp I think is that whatever source you
are fixing on, the lamp, or the diffuser in a light box, there is a surface.
And luminous flux is the same thing all the time, namely, the total flux
emitted by a source, or a surface.

As Vic Robert says, if the surface being measured does not completely
enclose the lamp or source, then the total flux emitted by the surface will
be less than the total flux emitted by the lamp source. Here we are simply
considering two sources, not one, the lamp with it's value of luminous
flux, and the surface of the diffuser which is considered to have it's own
value of luminous flux. Both being measures of total flux concerning
the respective source.

9. ### Victor RobertsGuest

Let me add one more clarification. The "surface" is rarely
the surface of the lamp. The surface can be a real surface,
such as the surface of an integrating sphere, or a virtual
surface, such as that described by a sensor on a goniometer.
The issue is whether or not the surface fully encloses the
source. If it does, then the total flux passing through
that surface is the light output of the source, measured in
lumens. If the surface does not fully enclose the source,
then the flux passing through the surface will be a fraction
of the total flux emitted by the source.

--
Vic Roberts
http://www.RobertsResearchInc.com