• Estimation of required LED strip density and plant distance

    Jonas • 02/04/2020 at 10:37 • 0 comments

     A common difficulty when using DYI grow lights is to find out the right distance to the plants. Most sources I have found online are either too detailed, measuring the PAR as PPFD (photosynthetic photon flux density, in μmol/m2/s see here, here and here) and then adjusting the distance, or they are too much "back of the envelope", not taking into account the power of the used LED light and just guessing the distance.

    I want to use another approach and use professional LED grow lights with their power consumption as a data basis for my DIY lights. This is difficult, because a lot of cheap lights out there say they are for example 1000W whereas they only consume let's say 180W. So for a proper comparison, the actually consumed power is required (see here)

    Of course, this is still all guesswork, but correlatiing the consumed power of professional LED lights and their distances to the plants recommended by the manufacturers is a good enough start for me.


    Target DLI for seedlings

    Several sources on the web (here, ) suggest that seedling need around 120-400 μmol/m2/s to thrive. Another indicator, the DLI (daily light integral) then sums up all the light received by a plant throughout the day.

    This page recommends 6-10 DLI for early seedlings and 10-15 DLI for late seedlings. Another opinion is 6-8 DLI. Below image is taken from that page as well. Excellent source check it out!

    To reduce the hassle, I am targeting a constant DLI of 10 with the lights being on for 18h a day. So looking at the table above, this corresponds to a PPFD of 150-200, which is consistent with the other values found online.

    Estimate PPFD based on power consumption

    I took some figures I found online and measured values from an Amazon review. These calulcations are not entirely correct, as they do not take into account the exact geometry of the lights measured, for example the weakining of the light towards the corner of the illuminated area.

    After putting the values into a spreadsheet, I created a chart:

    All devices checked have a similar electrical density at the source (i.e. the light itself) of 0.19 to 0.28W per m². On my current setup, the maximum distance I can achieve is 25cm away from the seedling box.

    So assuming the PPFD scales linearly with the elecitrical density at the source (which it probably does not) and using the King Plus PPDF curve as reference (because it seems to be most consistent with the measurements from the Amazon review), curves for the LED strips could be created:

    The tight LED strip has a electrical power density of 0.04W/m², where as the loose one has 0.022W/m².

    Zooming into the LED strip curves:

    It can be seen that, with the loose strip setup, where each strip is 8cm away from the next strip is roughly 18 to 27cm.