PAR readings and your lettuce

carbApril 23, 2013

Hello everyone,

for those familiar with PAR and are using it to measure their lighting system, I have a question..

According to an article, lettuce needs around 14-17 umol-m-d-1 or a daily light integral of 14-17.

Additionally, a 70watt metal halide could produce 1550 ppt at 1 feet distance according to another article.

Going by those numbers, am I missing something If I concluded that lettuce needs only around 3 hours of lighting under 70watt metal halide to achieve a daily light integral of ~16?

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Hi, the 70 w will not do 1550 at 1 feet. My 775w will do that, your 70w will probably do 150, which is barely adequate to grow from. I would use an HPS light which will send out more PAR than a halide - usually the minimum size most indoor growers use is 400W, and HTG supply sells a gorgeous reflectorized HPS lamp that will save you having to buy a reflector. A conventional 400w HPS ballast is also quite cheap, but you can consider buying a digital ballast because the prices for those have come down, and they are quite reliable now - especially the PHANTOM brand, or similar. HTG carries those as well.

    Bookmark   April 24, 2013 at 10:41AM
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Well, I don't have a PAR meter but doing the math (and using 1500 ppf) I come up with the same answer as you did. However, that seems like an awfully bright light for just 70 watts. Maybe we're both missing something? :-) Out of curiosity, where did you get the information about that light?


    Bookmark   April 24, 2013 at 11:14AM
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I got it here.

anyone know the par of a 105w CFL at 3 inches?

    Bookmark   April 25, 2013 at 7:13AM
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Thanks for the info on that 1550 ppf light. That was a 'spot light' (obviously a narrow flood) that concentrated all of it's energy into one small area. That would probably fry your lettuce :-)


    Bookmark   April 25, 2013 at 11:40AM
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I got a 105w CFL that says it outputs 6200 lumens. I placed it under a shiny aluminum dome reflector about 1 feet in diameter. considering all other factors as optimum, will I be able to grow GOOD quality lettuce like in the store with that? (about 4 lettuce spaced 6 inches apart forming a square)

    Bookmark   April 25, 2013 at 11:30PM
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Hi carb,

Well, I thought someone (familiar with growing lettuce) might come along offering some suggestions by now. I don't grow vegetables at all (mostly just annual flowers) so I don't know how much help I can really be :-)

I'm sure other factors are involved but as far as the amount of light goes, I personally think your CFL light (based on 6200 lumens) would indeed supply enough light for the lettuce.

There's an older thread on GardenWeb about growing lettuce under lights (see link below). One of the posters said that 1250 to 2500 foot-candles is about right for growing lettuce. So if we use 1500 fc as an example and convert that to lux, you would need about 16,140 lux for your lettuce plants (1 foot-candle = 10.76 lux ).

If you had the light about 6 inches above the plants, the 6200 lumens would result in about 21,000 lux. Using a good reflector, you would probably get about 80% of that concentrated on the lettuce; that would be 16,800 lux.

If you would still get enough coverage, you could even bring the light closer than 6 inches to increase the lux more yet.

Also, a daily light integral of 17 will convert to somewhere between 14,000 and 15,000 lux.

So in my 'no lettuce growing' opinion, that is enough light :-)

Hope this helps until someone who knows more about growing lettuce comes along.


Here is a link that might be useful: Growing Lettuce

This post was edited by art33 on Sat, Apr 27, 13 at 14:20

    Bookmark   April 27, 2013 at 1:38PM
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thanks art33, I am gonna go ahead and experiment with that bulb :)

I've looked at some videos of indoor lettuce and saw a lot of them a bit leggy and not quite compact as I see in the stores where they have really big 3/4" - 1" diameter stems and really lush and compact growth. And I thought to myself, I want that kind of quality :p

    Bookmark   April 29, 2013 at 12:05AM
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I wish you the best carb, should be a fun experiment!


    Bookmark   April 29, 2013 at 1:40PM
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