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Thread: Calculating Light Amounts

  1. #9

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    Wow you sure have a good memory Nep_ak!

    I think some types of cameras can be used as a light meter as well, but don't know the procedure.

  2. #10

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    I agree that a light meter would be by far the best way to find out, because this method does not take into account 100% correctly the reflection, light loss factor, lamp lumen maintenance factor, luminaire maintenance factor, room surface maintenance factor, utilisation factor, light output ratio of luminaire, flux distribution of luminaire, room proportions, and spacing to height ratio.

    Later on in my free time I will try to make little ammendments and an extension to the article of how to do it with all these factors. For now I would stick with my sort of inaccurate average way to do it or consult this article that requires things like calling the actual light company to find the depreciation rate of lumens for specific lights and etc.

  3. #11

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    Oh and if you have anything to add to the article that could give more insight please pm me explaining it as somone helpful has already done and we can discuss it or extend the article. I think this thread should be for people to input on eachothers ideas and improve on them until we have a reached a final decision on a way to calculate the light. THEN a new thread can be made with the exact way to do it and it can be stickied.

  4. #12
    cp_produtos's Avatar
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    Quote Originally Posted by [b
    Quote[/b] ]This doesn't factor in everything so it is just a way to get a very close approximation
    not a very close approximation , remember that the formula that you use is for a pountual source of electromagnetic radiation , the lamps are not , especialy fluorescent tubs.
    The calculation made like you presented will give a much higher value than the real one.

  5. #13
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    Elandolf - thanks for the thread, as I’m also interested in the amount of light my setup delivers. I have four 2000 lumen lamps about 10” from the top of my pots and based on your post I figured I’m around “Cloudy Bright”. I decided to check the numbers with my SLR camera.

    I got a white sheet of paper, drew a cross in the middle, set my camera to F22, set it to Aperture priority (so the camera would pick the shutter speed), 400 ASA film speed, auto white balance, and full zoom (14mm). I let the camera auto focus on the cross and read the shutter speed. Here’s what I got today at noon (the sky was pretty clear with some scattered clouds that weren’t blocking the sun when I took the outside readings):

    3 – room with a block window facing north
    30 – outside on the north side of the house in the deep shade
    50 – on the CP shelf about 18” from the light
    60 – on west side of the house in the shade of a pillar
    80 – on the CP shelf about 14” from the light
    120 – on the CP shelf about 12” from the light
    160 – on the CP shelf about 10” from the light
    320 – pointing the camera directly at a shelf bulb
    640 – outside in the full sun

    From these numbers it appears that about 10” from my bulbs the light is 25% of the full sun, which is also “Cloudy Bright”. In this trial it appears that your formula and using the light meter on my camera agree.

  6. #14
    Hello, I must be going... Not a Number's Avatar
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    It's easier than that if you have a hand held light meter. My Gossen Luna-Pro F is calibrated to measure Lux when set to ASA 50. I just take a reading and get the EV (exposure value) reading and look at the chart printed on the bottom of my meter. EVs are used in photography for the Zone system made famous by Ansel Adams.

    Sunny, partially cloudy, over the sun - EV 12.33 ~= 22500 Lux or cloudy bright.

    My tank lights (I'm going to redo them) at the distance where my Drosera is (3-4" from lights) gives an EV of 9 ~=2800 Lux or cloudy dull.

    In order to get an accurate reading you either have to take an incident reading (with an incident lens cap - use a styrofoam coffee cup if you don't have one) over the lens or a reflected reading from a standard 18% gray card.

    Below is the table as calibrated for my hand held meter for a setting of ASA 50. A word of caution: just because my light meter is calibrated to use this scale it doesn't necessarily mean that this scale is applicable to any other light meter (other than another Gossen Luna-Pro F). However if your light meter can report Exposure Values then by all means experiment with this chart.

    LW EVLux ca.fc appr
    -5
    0.016


    0.02


    0.025
    -4
    0.032


    0.04


    0.05
    -3
    0.065


    0.08


    0.1
    -21.40.13


    0.18


    0.21
    -12.80.26


    0.33


    0.4
    05.50.5


    0.63


    0.79
    1111


    1.26


    1.6
    2222


    2.5


    3.2
    3444


    5


    6.3
    4888


    10


    12.7
    517516


    20


    25
    635032


    40


    51
    770065


    82


    103
    81400130


    164


    206
    92800260


    328


    413
    105500500


    630


    794
    11110001000


    1260


    1580
    12220002000


    2500


    3180
    13440004000


    5040


    6350
    14880008000


    10800


    12700
    1517500016000


    20100


    25400
    1635000032000
    Last edited by Joseph Clemens; 01-27-2007 at 03:10 PM. Reason: Too much empty space
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  7. #15
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    Not a Number – yeah it would be easier with a light meter, but since I’ve owned a number of cameras and never got a light meter I figured I would use one of my cameras. Actually, with the camera it’s pretty easy – all you have to do is measure the light at your plants, and then go outside (on a clear/bight day) and measure again, then use the table posted by Elandolf to see where you fall.

    I do own a sound level meter, maybe I could use it?

  8. #16
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    I say again , not a very close approximation , remember that the formula that you use is for a pountual source of electromagnetic radiation , the lamps are not , especialy fluorescent tubs.
    The calculation made like you presented will give a much higher value than the real one.

    I have a digital lux meter.

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