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Luminous flux

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

  1. Richard

    Richard Guest

    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. Richard

    Richard Guest

    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. 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. 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
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    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. Richard

    Richard Guest


    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. 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
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    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. Richard

    Richard Guest

    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. Richard

    Richard Guest

    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. 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
    To reply via e-mail:
    replace xxx with vdr in the Reply to: address
    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.
     
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