Lux?

[Bob13567]

Anybody know about Lux how many Lux does a plant need it how important is Lux I have a Lux meter on my iPhone? Looked through a lot of fourms couldn't find anything about Lux. ?


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[Icemud]

Anybody know about Lux how many Lux does a plant need it how important is Lux I have a Lux meter on my iPhone? Looked through a lot of fourms couldn't find anything about Lux. ?


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Lux is a unit that isn't really used in horticulture as it is based on lumens, which don't really translate to any specific plant process, so many growers, horticuturists and researchers use units of PAR (photosynthetic active radiation) and this is measured in micromoles per meter squared, per second.

Plants are somewhat of "photon counters" so the amount of photons they absorb, in the PAR range (blue 400nm to red 700nm), effects the rate of photosynthesis. So using PAR and micromoles/m2/s-1 or PPFD, this is how you can directly know how a light intensity will drive your plants.

Since Lumens are used for human eyesight, there isn't a direct application in relating the unit "lumen" to a plant since it is a weighted measurement based on our eyes sensitivity with green being most prominent.


You can convert Lux to PAR (PPFD) but you have to know what type of light you are using as the conversion factor changes depending on the light source.


If you google "Lux to PAR" one of the top results should be a site from Apogee, which explains how to convert Lux to PAR.

Basically to take LUX to PAR, you divide the LUX reading by the conversion factor and you get the PAR (micromoles)

Sunlight 54
Cool White Fluorescent Lamps 74
Mogul Base High Pressure Sodium Lamps 82
Dual-Ended High Pressure Sodium (DEHPS): ePapillion 1000 W 77
Metal Halide 71
Ceramic Metal Halide (CMH942): standard 4200 K color temperature 65
Ceramic Metal Halide (CMH930-Agro): 3100 K color temperature, spectrum shifted to red wavelengths 59


So for instance, lets say you are using a 400w HPS which would have a lux around 50,000, then to get a rough idea of the PAR light emmitted, then you would divide 50,000/82 which would equal 609 umol/m2/s-1. Now light spreads the further you get away from the source, this effect is known as the inverse square law so depending on where you take your readings, the measurements will change.



For PAR, in veg plants don't need much light during a 18/6 schedule but for flowering in a 12/12 schedule they need more intensity as the daylength is shorter. This is known as DLI or daily light integral. The optimal DLI of cannabis is around 65 mol/day.

Therefore how this breaks down into PAR (PPFD) is this...

at a 24/0 schedule: 752 umol/m2/s-1
at a 18/6 schedule: 1128 umol/m2/s-1
at a 12/12 schedule: 1504 umol/m2/s-1

These are the PAR PPFD measurements for OPTIMAL cannabis lighting under different schedules. Ive read that other light loving plants have a minimum DLI of 21 mol/day...(non cannabis specific, for plants like tomatoes)

This would break down into PAR (PPFD) like this:

at a 24/0 schedule: 243 umol/m2/s-1
at a 18/6 schedule: 364 umol/m2/s-1
at a 12/12 schedule: 486 umol/m2/s-1

These would be the MINIMUM PAR PPFD measurements you would want to see for baseline results.

So generally, depending on what light schedule you are on, you would want your PAR (PPFD) readings to be somewhere between the figures above, for flowering most lighting enthusiast growers shoot for at least 800 micromoles/m2/s-1 for a 12/12 schedule which is a DLI of about 35 moles/day.

 

[Bob13567]

Thank you! You seem very knowledgeable.


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[Icemud]

Thank you! You seem very knowledgeable.


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You are welcome Bob! Hope it all made sense cheers!