Consider the following technologies:

For growing food, the preferred measurement is Photosynthetically Active Radiation (PAR), given in μmol/s-m2 (PPFD). Finding equivalent PAR measurements across various lights can be difficult (due to various ways PAR can be measured)--plus manufacturers don't always list the value.


From what I've read, LEP is being touted as superior to LED for growing plants, for a few reasons:

  • Less power consumption
  • Less heat, which has numerous benefits
  • Emits UV, which can inhibit mildew growth


I am wondering:

  • How can a device that operates at 94 lm/W be superior (i.e., use less wattage) to a device that operates at 303 lm/W (for growing plants)?

  • Is it because of the devices have drastically different PPFD values?

  • What is the highest theoretical lm/W that a LEP device can generate?

I understand that converting from lm/W to PAR doesn't make sense (as PAR is more concerned with wavelengths and lm/W is about brightness).


See also:


closed as off-topic by Andy aka, PeterJ, Null, Daniel Grillo, Fizz Nov 14 '15 at 23:26

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  • 1
    \$\begingroup\$ The quoted 303 lm/W is far from commercially available. I think some Cree LEDs hit around 160 lm/W now. Lumens also have a wavelength dependent component based on the characteristics of the human eye. If the LEP source has a better match of the required wavelength characteristic I can see why it could be more efficient as it needs less watts to produce the same amount of the right light as compared to a white LED. \$\endgroup\$ – Arsenal Nov 14 '15 at 9:16
  • \$\begingroup\$ I've gone thru the various links you have provided and have the impression that this isn't an EE question - it's more about growing food indoors and optimizing the light physics for the plant. \$\endgroup\$ – Andy aka Nov 14 '15 at 11:39
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    \$\begingroup\$ I'm voting to close this question as off-topic because it's not EE \$\endgroup\$ – Andy aka Nov 14 '15 at 11:39
  • \$\begingroup\$ This question would be better on biology.stackexchange.com. \$\endgroup\$ – Fizz Nov 14 '15 at 23:27

Lumens are tailored around the characteristics of the human eye.

If you take a look at the luminosity function, which is used as a wavelength dependent weighting for the lumens, you can see that the area around 550 nm or 500 nm depending on photopic (when a lot light is present) or scotopic (under low light conditions) is the most valued one, so it contributes much to the total lumens.

The black curve shows the photopic weighting, which is more relevant:

luminosity function

When I have a look at the PAR it looks completely different, in fact almost reversed:


(Copyright: John Whitmarsh and Govindjee)

On the other hand we also have this weighting:

Weighting for photosythesis

(Copyright: Hankwang)

I'm not an expert on this topic, so I can't tell which of the both curves is more relevant, and why they look so different, but both of them deviate strongly from the luminosity function.

Taking a look at the spectrum of a very high lumens per watt LED you have a color temperature of around 5000K and a spectrum which looks like this:

Cree XH-G spectrum

The cool white is more relevant as the 168 lm/W is achieved with cool white, where a warm white results in 146 lm/W (at 50 mA drive current).

The LEP spectrum (you linked) looks like this: LEP spectrum

So there is much less power in the region between 500 nm and 550 nm for the LEP. Which might turn out to be a good thing.

I tried doing an analysis which might support their claim and my statement. So I extracted some data of the plots, multiplied the weighting with the spectra and integrating over them.

As it seems my approach has a mistake somewhere as the cold LED turned out to have a smaller integrated number for the lumens figure than the warm LED, although it should be the other way round.

I guess I share it anyways, maybe someone can point out my mistake. (Do I have to divide it by the integral of the relative radiant power or by the integral over the weighting function or something completely different?)

So my results look like this:

  • Blue curve = Cold LED
  • Green curve = Warm LED
  • Red curve = Plasma

Weighting the spectra with the standard luminosity function:

lumens weighted spectra

Integrating over them and dividing with the integral of the relative power spectra results in:

  • cold LED: 0.46712
  • warm LED: 0.47969
  • plasma: 0.39181

So if that would be correct the plasma has a lower lumens output than both of the LEDs, but the warm LED should have less than the cold LED, so I am missing something here.

If I do the same with the PAR spectrum:

PAR weighted spectra

  • cold LED: 0.55813
  • warm LED: 0.51099
  • plasma: 0.59380

The plasma turns out with the largest number, so it seems to produce the most useful radiation for that purpose. But these numbers are flawed in the same way as the ones before, so until someone suggests a fix, I won't rely on those. It's just a hint that something like this might be going on.


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