LED power density guideline

N

nanobud

Member
I'm planning out an LED grow light and having a hard time tracking down useful guidelines that would apply to growing at a specific wavelength (660nm). I've seen 9000 lm/ft^2 suggested for HPS, but as bad as lumens are for plants using a somewhat balanced spectrum, they're next to useless when considering a specific wavelength. So I did some investigating...

I made a spreadsheet of wavelengths vs photopic luminous efficacy (data from http://hyperphysics.phy-astr.gsu.edu/HBASE/vision/efficacy.html#c1), added estimated output per band for a HPS (http://www.ledgrowlightsonline.com/images/hps_spectrum.jpg) and multiplied across the band to get relative lumen output by wavelength of the HPS. Then (big approximate, reading from graph http://upload.wikimedia.org/wikipedia/commons/7/72/Par_action_spectrum.gif) did the same for photosynthesis rate by wavelength, and this multiplied across the spectrum with the HPS lumen outputs to give the "plant lumens" (I think this is what PAR is??? I'll just call it "Plm").

Spreadsheet columns are:
A} wavelength
B}HPS output (from chart - eyeballed)
C}efficacy (from hyperphysics chart)
D}HPS output (arbitrary units, but some factor of lumen) = B*C
E}plant efficacy (eyeballed from wikipedia photosynthesis rate chart)
F}plant usefullness (arbitrary units, some factor of plant lumens) = D*E

Net result (total F / total D) was "plant lumens" = 39% of HPS lumens. This would mean for a 9000lm HPS figure, it only works out to ~3500 Plm.

For a pure 660nm source, efficiencies are:
human: 6.1%
plant: ~95%
So if our target is 3500 Plm, this would be ~224lm @660nm (3500*.061/.95).
Using the photopic conversion (hyperphysics link) of 41.663 lm/W @660nm, that means 5.4W

So in conclusion, the recommendation of 9000lm/ft^2 HPS translates into 5.4W/ft^2 600nm LED (luminous output, not electrical rating). Using LedEngin (LZ1-00R205) 5W 660nm as an example, they output ~700mW, so it'd take 8 of these to meet the requirements (so ~40W/ft^2 in electrical rating). 450-460nm have roughly the same human and plant efficiencies as 660nm, so would count at roughly the same rate (for luminous power output).


I know it's pretty rough, since I had to eyeball numbers from the charts, and will be different with different HPS spectrums (9000lm/ft^2 recommendation is rough for the same reason), but it seems reasonable to me. This would mean a set of 7 reds, 1 blue of the LedEngin 5W LED would work for each square foot.

Can anyone find any problems or errors with this? Sound reasonable? Thanks for reading!
 
N

nanobud

Member
Here, maybe this will be easier to follow (missing values in charts were filled in with average of previous and next, since some data jumps by 10nm not 5nm):

Code: 
Wavelength      HPS      Luminous    Human        PAR      Plant 
              Radiant    Efficacy    Lumens   Efficiency  Lumens
                Flux   
  (nm)         (arb)      (none)     (arb)       (none)    (arb)

  450            5         0.04       0.19         1        0.19
  455           15         0.05       0.74         0.98     0.72
  460            5         0.06       0.3          0.95     0.29
  465            0         0.08       0            0.93     0
  470            4         0.09       0.36         0.9      0.33
  475           11         0.12       1.27         0.87     1.11
  480            0         0.14       0            0.84     0
  485            4         0.18       0.7          0.84     0.58
  490            0         0.21       0            0.83     0
  495           30         0.27       7.95         0.82     6.48
  500            0         0.32       0            0.8      0
  505            4         0.44       1.76         0.69     1.21
  510            0         0.50       0            0.58     0
  515           11         0.61       6.66         0.49     3.26
  520            0         0.71       0            0.4      0
  525            0         0.79       0            0.36     0
  530            0         0.86       0            0.32     0
  535            0         0.91       0            0.29     0
  540            0         0.95       0            0.25     0
  545            2         0.97       1.94         0.25     0.49
  550            4         0.99       3.96         0.25     0.99
  555           12         1.00      12.0          0.25     3
  560           15         1.00      14.93         0.25     3.73
  565           20         0.97      19.47         0.27     5.16
  570           48         0.95      45.7          0.28    12.79
  575           20         0.91      18.22         0.3      5.37
  580            3         0.87       2.61         0.31     0.81
  585            4         0.82       3.26         0.33     1.06
  590           55         0.76      41.8          0.34    14.21
  595           42         0.70      29.19         0.36    10.36
  600           40         0.63      25.2          0.37     9.32
  605           36         0.57      20.34         0.4      8.03
  610           30         0.50      15.0          0.42     6.3
  615           25         0.44      11.0          0.51     5.61
  620           20         0.38       7.6          0.6      4.56
  625           20         0.33       6.5          0.7      4.52
  630           20         0.27       5.4          0.79     4.27
  635           17         0.23       3.83         0.76     2.91
  640           15         0.18       2.7          0.73     1.97
  645           13         0.15       1.89         0.76     1.42
  650           11         0.11       1.21         0.78     0.94
  655           11         0.09       0.94         0.87     0.81
  660           13         0.06       0.79         0.95     0.75
  665           10         0.05       0.47         0.87     0.4
  670            9         0.03       0.29         0.78     0.22
  675           11         0.02       0.27         0.69     0.19
  680            9         0.02       0.15         0.6      0.09
  685            7         0.01       0.09         0.43     0.04
  690            4         0.01       0.03         0.25     0.01

Total:                              316.69                124.52
Conversion Rate: 39.3%
Source of numbers:
HSP Radiant Flux values:View attachment 1051276

Luminous Efficacy: Luminous Efficacy Tables

 PAR Efficacy (lower chart):View attachment 1051277

Human Lumens = HPS Radiant Flux * Luminous Efficacy

Plant Luments = Human Lumens * PAR Efficacy

Analysis:
At 9000 lumen/ft^2 HPS, that would translate to 9000 * 0.393 = 3537 plant lumens/ft^2 that this HPS figure is giving.

From the chart above, plants are (0.95 / 0.06) 15.83x more sensitive to 660nm light. So to get 3537 lumens/ft^2 to the plant, that requires (3537 / 15.83) 223 lumens/ft^2 of 660nm light. Using the Photopic Conversion figure at 660nm from the same place as the Luminous Efficacy numbers above, of 41.663 lm/W, that is equivalent to (223 / 41.663) 5.35 W/ft^2.

My Conclusion:
9000 lm/ft^2 of HPS is roughly equivalent to 223 lm/ft^2 or 5.35W/ft^2 of 660nm light.

Note, those values are output light, so if your LED efficiency is 20 lm/W, then you'll need about 11 per square foot (drawing 11W of power). And since not all companies quote output in the same units... if you're using LEDs that put out 700mW of light power, then you'll need about 8 of those for each square foot.

I'd be interested to find out what sort of output power densities current LED growers are using to see if this works out...
 
G

gobbly

Well-Known Member
I'm certainly not an expert on LED's, but I'm fairly good with spectrum and PAR, and what you have looks sound to me. I'm curious though, is your intent to create a proper PAR style spectrum by mixing LED's, or are you trying to isolate the 660nm line? If the latter, why? Photosynthetic spectrum is quite broad, more so than ways of measuring luminosity.

Assuming you are looking to create proper spectrum, then awesome! This is the sort of thing it takes to be successful with LED's. Something I might suggest if you have trouble finding more experienced people here. Check out some reef aquarium sites, like reefcentral.com or thereeftank.com. LED's have been used for a while now for coral propagation, and you will probably find a lot more people with more experience with them, many will know a lot about spectrum, optics, and even particularities of different brands.
 
N

nanobud

Member
Thanks gobbly, I'll check out some of those aquarium sites! Do they tend to be open to pot-growing applications there?

My intent wasn't to hit the 4 peaks with 4 LED wavelengths. I was considering either: to add in some blue at ~460nm, or go straight 660s but adding in 2700K fluoros to hit the blue requirements.

I was actually looking for information on pulsing LEDs (not worthwhile) and came across this article: LIGHT EMITTING DIODES AS A PLANT LIGHTING SOURCE. According to their results, the blue requirements are for a fixed flux, and not a ratio of red-to-blue. The chart Fig%20Bul%209.jpg seems to bottom out around 40 u-mol/m^2-s, which equates to roughly 1 W/ft^2 at 450-460nm.

My main concern after the requisite flux levels are met, is meeting the needs of flowering, which seems to have different needs than the PAR for vegetative growth.
 
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