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I would like to design my own grow lights to grow indoor plants like strawberries, herbs, etc. But I have a problem that I need some help to work out:

  • How many LEDs do I need to light up an area of 0.25m²?
  • At a hanging height of 0.15m?
  • To get a PPFD of 750μmol/s/m²?

I want to work out how many LEDs I would need using this data I found on a particular single horticulture LED: Cree LED XPGDPR-L1-0000-00G01 Link:https://au.mouser.com/ProductDetail/Cree-LED/XPGDPR-L1-0000-00G01?qs=UkDUCjYnTB0Zr%252B2VeuKxwg%3D%3D

From reading this forum and doing a bit of research, I found out that you can use this formula to find the PPF of an LED: PPF = gradient flux/energy of a photon

  • energy of a photon = E = hc/λ

And then I can use the PPF to find the PPFD by dividing it by the area.

Units of measurement:

  • PPF - μmol/s
  • PPFD - μmol/s/m²

Let me know if this is correct or any other formulas I can use to work this problem out. And I wouldn't mind seeing any example calculations since I think I'm getting the wrong values, and I don't know how to implement height into the formula for PPFD.

TLDR: I need help to design some grow lights but how many LEDs do I need to get a PPFD of 750 μmol/s/m², on an area of 0.25m² at a height of 0.15m?

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750umol *0.25m = 187.5 umol/s.

Datasheet gives 2.72 umol/s/LED at the test current.

187.5umol/s / 2.72 umol/LED = 69 LEDs. Scale as appropriate if you use a different drive current.

Since you're interested in an even illumination, consider how widely spaced they would be. If you evenly spaced them, then they're 6cm apart, or about double the height you want. That will give relatively even illumination, especially if the sides of your space are white or reflective.

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  • \$\begingroup\$ Thanks for working that out, I now understand how to use the PPF with my measurements. But I was also wondering if there's no PPF, could I still work out the PPFD? Since I noticed other brands don't have PPF in their specs, or even a value for the radiant flux. \$\endgroup\$
    – Omuse
    Commented Sep 20, 2021 at 9:32
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    \$\begingroup\$ Calculating the PPF requires you to know the number of photons per second, so if you don't know the flux there isn't much you can do short of measuring it. \$\endgroup\$
    – user1850479
    Commented Sep 20, 2021 at 14:28
  • \$\begingroup\$ Fair enough then. If I did know the radiant flux (for example 600mW) could you show me an example of calculating the PPF? Everytime I try to calculate it with the formula I wrote above, I get a giant value, which is obviously wrong. \$\endgroup\$
    – Omuse
    Commented Sep 20, 2021 at 14:39
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    \$\begingroup\$ For monochromatic light, you can calculate the number of photons per second by dividing the flux (in watts) by the photon's energy. The photon energy is Planks constant times the speed of light divided by the wavelength in meters. Finally, you can convert photons to moles by dividing by Avogadro's constant. For white light you can get the approximate value by using the center wavelength, but there will be some error in your estimate. \$\endgroup\$
    – user1850479
    Commented Sep 20, 2021 at 14:53
  • \$\begingroup\$ Thank you so much. I was missing the part of the calculation when I had to use avogadro's number to convert it into moles. And thank you again for the tip on calculating white light. \$\endgroup\$
    – Omuse
    Commented Sep 20, 2021 at 15:16

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