Audrey 1 (my first grow log - comments/help welcome)
- Thread starter ownfive
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jayzero
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You're right watts don't mean anything to plants, but neither do lumens. People use them as a way of loosely guaging how much light they have, without a meter watts seems the easiest. Most bulbs (of similar type) have comparable outputs/per watt, so watts, although irrelevent to the plant do provide a way to guage. That being said lumens can be used equally, neither really gives any usable information...but we need something to use
Well, both of these are sort of half-truths. Watts and lumens can both matter and they are sort of related, but only loosely. A massively intense UV/blue lamp, for example, could have a huge amount of power (watts), a fair bit of which could be absorbed by a plant (PAR Watts - Photosynthetic Active Region), but very little of which a human could see (very few lumens). This is why MH lamps have a lower number of lumens compared to HPS but actually have a higher number of PAR watts (useful to plants) than an HPS - only because the human eye sees more of what HPS puts out compared to what MH puts out.
Lumens and watts/m^2 can add when you add bulbs as well, but only in the places where the light from both lamps overlap. The closer together you can put your lamps (closer to a point-source approximation) the more lumen stacking you can do. Still, it's an uphill battle of sorts and there's no simple maths to figure out what you've got.
The best measure is PAR watts, which is a power equivalent of what lumens are to human intensity perception, but specific to what a plant will absorb. PAR watts tells you how many watts in your spectrum are available for plant usage. A bright green light would have high lumens, but very low PAR watts, for example. Blue and red lights have much higher PAR watts but would provide far fewer lumens...if that makes sense.
For the mathematicians/physicists - essentially lumens are a sort of integration over the multiplied spectrums of the lamp output and the absorption spectrum of the eye. PAR wattts are the same sort of integrated value, but with the power spectrum of the lamp and the absorption spectrums of the photsynthetic molecules. For HPS and MH, since they're so widely used and since the spectrums don't change, people have just figured out what luminous intensity you need from those spectrums to get good plant growth. But to say you need 10,000 lumens implies 10,000 lumens from a specific type of lamp. If you had a hypothetical lamp that was specifically matched to photosynthetic absorption then you would need far fewer lumens to achieve the same amount of energy transfer to the plant. Not all lumens are created equal.
Not sure if that makes any sense, but at least when it comes to CFL, you basically need all the light you can get. No matter how many lamps you have, you can probably benefit from a few more - I've yet to see a CFL grow that really makes monster plants like HID lighting does. Anyone with examples otherwise, mind you, I'd love to see how it was done!
