Can I compare LED and HID grow lights using photosynthetic photon flux density (PPFD) as a unit of measure?

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Q:

There are a lot of comparisons of LED and HID lights for growing plants. Most use watts for the comparisons of such things like taut efficiency and heat vs. light, but I would like to see a comparison using photosynthetic photon flux density (PPFD) as units of measure.

For example, if plants do well with 300 umol/m2/s from an HID light (knowing that only a portion of the light spectrum from an HID light is efficiently used by plants, even if it falls within photosynthetically active radiation—400 to 700 nanometers), what is a comparable number of umol/m2/s from LEDs that will produce similar growth results? This is assuming the LEDs are emitting wavelengths efficiently used by plants—roughly 400 to 500 nm and 600 to 700 nm on a 1:7 ratio of the two wavelength regions. I know there are many issues here, but please assume I have a reasonable understanding of them except this one. I haven’t found anything on this yet. Hoping for an answer.

A:

Thank you for your great question. PPFD (expressed in umol/m2/s) is currently the best measurement horticulturists have to determine how much usable light energy there is. Unfortunately, a PPFD reading gives us the total photon flux density between 400-700 nm without taking into consideration the nanometers to which chlorophyll a and b are most sensitive. That being said, the amount of umol/m2/s or PPFD required to produce optimal growth with LED lighting should be slightly lower than with other horticultural light sources (this is assuming the LEDs are emitting the optimal nanometers for photosynthesis).

In theory, horticultural LEDs should also be able to produce higher PPFD readings per watt of energy consumed than other light technologies because of their ability to tailor their light output to specific nanometers. I’m afraid until we have a way to accurately measure individual nanometers within the range of photosynthentically active radiation (PAR), comparing horticultural LEDs to other horticultural lighting technologies is a bit like comparing apples to oranges.

Thanks again for your question!

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Written by Erik Biksa
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Erik Biksa holds a diploma in agriculture with majors in fertilizer sciences and crop production. Erik has amassed over 18 years of indoor gardening experience and intensive research. Since first appearing in Maximum Yield in 1999, the “Ask Erik” column and numerous articles have reached growers throughout the world.  Full Bio