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Thread: Lighting Metrics 101

  1. #1

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    Default Lighting Metrics 101

    Lighting Metrics

    So theres a lot of ways to describe the amount, type and brightness of light that we use to grow our beloved crops. Some of the terms are more familiar than others with some appearing quite complicated. Hopefully I can lay the more common terms out here and describe or explain them in the voice of grower/smoker.

    Lumens and Lux




    As we know from looking at rainbows, lenses and prisms etc, light is made up from many colours. Each of these colours have a corresponding wavelength/ frequency. Lumens are made from the range of light that is visible to the human eye.
    So a lumen of light is an amount of light. It’s actually the number of photons that are given off by a light source or could be the number of photons received. More lumens, more light. The brightest lights in terms of lumens are highly green/yellow in colour.
    Lux is a short hand to lumens per square meter. It’s the concentration of those lumens mentioned above and how many are concentrated into a unit square. Try not to confuse them. One is density of light, one if the amount of light. You can get high lux with low lumens – hold a single led close to your eye ( no don’t!) and you can get low lux with high lumens – think dusk or dawn. So much light but spread thin across the hemisphere.

    PAR – Photosynthetically Active Radiation

    So referring to the chart above, us growers are more interested in what a plant can see or use. We can see that the lumen figure for a light isn’t very informative for a horticultural light. Red light is the most active colour in terms of photosynthesis. A high lumen light may not include much of this red for example. So PAR is the range that includes the colours between 400nm (blue) and 700nm (red) roughly. Beyond this for example ultraviolet or infrared, the plant gets absolutely no photosynthetic benefit. The plant sees this light at dark black, as do we.

    PPF – Photsynthetic photon flux

    Sounds a bit of a mouthful. Flux is just the outflow of something – light and photons in this case. So it’s the amount of light given off by a source of light per second. It’s the number of photons (a particle of light) emitted by a light per second.
    It important to understand this amount of light given off, not necessarily received by the plant so it’s not always helpful to the grower.
    Measured in moles of light per second or more often micro (millionths) moles per second. A mole is a fixed number of photons, a very large number of them
    Unit - umol/s

    PPFD – Photosynthetic photon flux density


    (This is the lux to the lumen)
    This is the number of PAR photons arriving in a square meter of canopy each second. Its a spot measurement made with a quantum sensor placed at a point.
    Unit - umol/m2/s


    Some lights produce more consistent PPFD spreads than others. You can have lights that produce a hotspot in the middle and dim areas to the sides ( think euro barn reflector ) and you can have others that are more consistent but lower peaks ( think parabolic reflector )
    DO NOT trust any horticultural light or manufacturer that cannot produce the following specifications for their light
    • The area over which the measurements were made
    • The vertical and horizontal distances the measurements were taken at
    • The number of measurements
    • The minimum, maximum and average values

    Efficacy or Photon Efficiency

    For me this is the most revealing metric regarding a horti light. Simply quoting the number of watts is not particularly helpful now we have numerous types and styles of grow light. The efficacy is how much light a single watt of electrical power can produce

    The higher the number, the more light the plant will receive for a fixed amount of power
    Units: umol/joule
    Last edited by gardro; 20-02-19 at 04:21 PM. Reason: Greek alphabet unrecognised

  2. The Following 7 Users Say Thank You to gardro For This Useful Post:

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  3. #2

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    Boom, top work Gardro as always mukka

    Sent from Thames Iron works

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    Fair play Gardro! Spot on mate!


    Sent from my iPhone using Tapatalk

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