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The correct quantity

Discussion in 'Lighting' started by Richard, Dec 19, 2006.

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  1. Richard

    Richard Guest

    I'm looking at a technical data sheet concerning signage. There is a graph
    showing "Light output" in lux, versus "Position across sign".

    The lux value for the middle of the sign reads about 2K lux.

    Just checking: "Light ouput" is a measure of Luminance Emittance is it not?
  2. Light Output is a broad generic term. For example:

    Illuminance (Lux) is the measure of light over an area (light flux per
    unit area, also called flux density) - the light output on an object.

    Luminance (Cd/m2) is the measure of light in a viewing angle (flux
    density per unit solid viewing angle) - the light output from a large
    area light source.

    Luminous Intensity (Cd) is the measure of radiometric power per unit
    solid angle - the light output from a point source in a single direction.

    Luminous Flux (Lumen) is the measure of total radiometric power - the
    light output in all directions.
  3. Richard

    Richard Guest


    The technical article relates to acrylic sheet made to be edge-lit.

    The graph is made with the condition that the signage is edge-lit with two
    T5 HO flourescent tubes spaced 650mm apart. No mention is made of the
    wattage or the part number of these T5 HO tubes or their length.

    However, I think the graph is trying to give what constitutes useful
    information and I think in the circumstances that will be brightness of the
    signage across it. It seems to me then that the measure is very probably
    lumens per square meter. This corresponds to Luminous Emittance. That would
    be Illuminance if we were talking of light incident on a surface.
  4. In the US at least, more properly called "luminous exitance"
    these days. The IESNA says that the term luminous
    emittance was formerly used but is now is depreciated.
    But many people would still use the term illuminance.

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