Grow Lamp manufacturers provide several pieces of information about their product:

  1. PAR (Photosynthetic Active Radiation measured in W/m2)
  2. Photosynthetic Photon flux density (measured in µmol/s )
  3. Lumens
  4. comparison to HPS lamp

I found that you need around 70000 lumens/m2 for best results. I also found lamps that have significantly higher PAR, even though they have 50% less lumens.

I would expect that PAR is the most important so I am confused why some literature states lumens as important factor. What should I aim for? Lamp that matches lumens recommendation or lamp that has the biggest PAR number?

  • 1
    Lumen is defined for human vision, with human vision sensitivities.On the other hand lamps need to be used by humans, so the light is "white" and the spectral distribution is similar to black body (yeah, white of a black body, physicists are ..particular people). [Lumen is used also for colored light, but I assume you are not speaking of, e.g., a red light]. Sep 4, 2017 at 9:17
  • 1
    Not sure about the spectrum of a black body, since it is defined it that absorbs all light falling on it :D but yeah, white light contains all visible wave lengths. I am still not sure what is best for my plants :) oh and I will be growing tomatoes and peppers.
    – sanjihan
    Sep 4, 2017 at 10:15
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    "Black body" is the generic name of emitting body: en.wikipedia.org/wiki/Black_body . My are just comment, and not an answer, because I cannot properly answer. It seems that 1 and 2 are index of photosynthetic radiation, so more precise than a human specific index (lumen). Sep 4, 2017 at 11:47
  • 1
    If it absorbs, it must also radiate back :) makes sense
    – sanjihan
    Sep 4, 2017 at 12:01
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    No. Black body is by definition (and theory) only an emitting body. This is due to temperature (and the radiation energy come from temperature). Sun (yeah, also a black body) radiate mostly because of the surface temperature, not because the small absorbing light from other stars. Sep 4, 2017 at 12:22

1 Answer 1


Yikes do these labels offer opportunities for confusion! I'll address the first 3; the HPS lamp I imagine is characterized by some unit but I don't know which.

Here's what the metrics have in common:

  1. All are a measure of the 'output' of light. Lumens can be translated to watts: https://clark.com/technology/lightbulbs-watt-to-lumen-conversion-chart/. Mols per second also can be translated into watts: https://www.researchgate.net/post/Can_I_convert_PAR_photo_active_radiation_value_of_micro_mole_M2_S_to_Solar_radiation_in_Watt_m22.

Here's how they differ:

  1. One refers to the area over which the light is spread, a square meter. That's a function of how far the light source is from the surface on which it is incident. So to say that a light bulb generically provides x watts per meter squared is making some assumptions about how the bulb will be used.

  2. Some are specific to a subset of the electromagnetic spectrum and some aren't. Lumens refer to the visible part of the spectrum. PAR covers the portion that plants can absorb and use for photosynthesis. Visible and PAR are not identical, although not coincidentally they overlap heavily. There's a nice picture on this webpage: http://sdhydroponics.com/2012/06/13/par-the-light-spectrum/. Watts are the least specific, covering the entire spectrum, or any electromagnetic energy. Light bulbs are made because we want visible energy, and mostly that's the portion of the spectrum in which new kinds emit most of their energy. Old fashioned incandescent bulbs emit an appalling portion of their energy as heat in the thermal part of the spectrum instead of as light. It's why they are so wasteful.

Just to make it more confusing, the wattage of a light bulb classically refers not to how much light it emits but to the amount of energy (electricity, generally) that it uses. An incandescent bulb that consumes 40 watts emits a predictable amount of light, which is how (and why) the translation to lumens is increasingly made. Watts nevertheless can be a measure of energy emitted, as they are in the PAR label.

In a perfect world one could translate any of these labels into any of the other units, though one would need some information about the emission spectrum distribution for each. Intensity (whether watts or mols) of PAR is what matters for the plants. So that's where I'd focus. I'd keep in mind that the visible-light wattage equivalent of an x lumens lamp is not far removed from the watts of PAR that it emits.

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