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Lumens vs. Kelvin

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Lumens vs. Kelvin

Post  mollyhespra on 4/21/2013, 9:54 am

Hi, all.

I'm getting anxious that my little seedlings aren't getting enough "light" as they seem to lean towards one of them: I have two, one 7800K and one 4200K; they lean towards the 7800K light. I have four shelves, each having two lights as described above.

Once I realized the plants wanted to lean towards the higher K lights, I swapped out bulbs in a shelf I wasn't using yet so that they are both 7800K.

OK! Seemed to be working...except that it occurred to me to get my light meter and check out what was happening, and it registered as "dark".

SO...I've been reading about the difference between color temp (kelvin) and light output (lumens), and I'm confused. I *think* my set-up is OK, being as I'm interested in seed-starting, not promoting flowering or growing year-round (for which a lower-K color (& thus more visible-light output/lumens would be more efficient?), but I'm wondering if I've "diluted" my K's too much by mixing the tubes that I have. Should I get more 7800K tubes or maybe swap out the 4200K ones for something more like 5000K?

I did a search in this forum (for "lumens" and "kelvin" either or together) and couldn't find anything that addressed this particular issue, so if anyone can weigh in, I'd appreciate it, since I'm needing to start using those other shelves and I'm going to have to go out & get more lights if that's the way to go.

Thanks in advance!

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Re: Lumens vs. Kelvin

Post  llama momma on 4/21/2013, 10:00 am

Try the search box again I know there are discussions on lighting. Mine are double t8's per light hood and working out fine. Almost certain they are 6500k's. Lights are left on minimum of 16 hours/day. maybe those with more experience will chime in.

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Re: Lumens vs. Kelvin

Post  RoOsTeR on 4/21/2013, 10:27 am

I'm no lighting expert and I ended up having plenty of window space for my plants, so I took the easy route. Very Happy Take what I say as a grain of salt and to narrow it down... a lot, a typical adequate light set up for SEEDLINGS is about 6500k Full spectrum light. You can get into lumens, kelvin, red, blue etc. and you will find a difference of opinion on each for every person that is an "internet expert" on the matter. Different stages of plant life will perform better under different spectrums of light and so on. For what we are trying to accomplish, the 6500 full spectrum range should be adequate to start your plants and get them going enough till you can get them outdoors.
Do your plants look nice and healthy? If so, I wouldn't worry to much about it. Most of my plants tend to get a tad bit leggy till I can start getting them outside and hardening off. Then it disappears quickly.

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Re: Lumens vs. Kelvin

Post  jazzycat on 4/21/2013, 1:07 pm

For seedlings, you want more of the blue spectrum and less (or none) of the red spectrum (red helps with flowering). They need to be more than 5000k for seedlings. I bought T5HO bulbs and they worked beautifully (mine are 6400k and 2232 lumens). They are, however, quite expensive, especially if you buy the reflectors.

I am by no means an expert on any of this. Before I bought the lights I did a lot research and talked with the hydroponics guy a lot. Here is a link that explains it little, and there are a lot of other sites that explain the difference between temperature, wavelength, etc. You just have search.

http://gizmodo.com/5903134/better-know-a-grow-light

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Re: Lumens vs. Kelvin

Post  brainchasm on 4/21/2013, 2:02 pm

Lumens is the intensity, or amount of energy per unit measured (square foot, square centimeter, whatever).

Kelvin is the color of the light. But you can have a high kelvin light that has hardly any energy, or a high lumen light of any color(kelvin).

6400-6500K is the apparent sweet spot for seedlings and growing plants. From there, you want to get as high a lumen as possible, and then configure your reflectors and such so that as many of those lumens get to your plants as possible.

And now, if you want the really deep voodoo...

Lighting 101 - Selecting the proper setup

Spectral Curves of CFLs vs. PAR (plant requirements)


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Re: Lumens vs. Kelvin

Post  RoOsTeR on 4/21/2013, 2:09 pm

You can for sure go sensory overload on lighting. In the past (aka "before kids") I used to do saltwater reef tanks and fresh water planted tanks. Lights are crucial to both and I've been guilty of going overboard with hobbies! For most intents and purposes, your seedlings should do just fine with full spectrum or Daylight bulbs at Home Depot or Lowe's, 6500k. Provide em with sun if you can Wink

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Re: Lumens vs. Kelvin

Post  brainchasm on 4/21/2013, 2:10 pm

I got one of these after I managed to get my first batch of seedlings all leggy:



6400K (color), and 3850 lumens (power).

I put it in a deep can of a lampshade that I made out of a plastic garbage can, then I covered the inside with reflective material to increase the lumens going out, instead of getting absorbed by the plastic.

I figure it'll either make my seedlings grow, or give them superpowers. It's really freaking bright.



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Re: Lumens vs. Kelvin

Post  mollyhespra on 4/22/2013, 11:04 pm

@brainchasm wrote:
{snip}

I figure it'll either make my seedlings grow, or give them superpowers. It's really freaking bright.



Now THAT's what I'm talking about!

I need some more lumens, then. I've got the right color temp, but not enough "light".

I think I need to go buy some bulbs with higher lumens to complement the ones with 7800K I have.

Thanks for all the info!

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Re: Lumens vs. Kelvin

Post  bwaynef on 4/23/2013, 9:29 am

I found a light meter app for my iPhone and compared what I had at ground level outside around lunch time w/ what I had under my 4-bulb t8 shoplight. It showed that my shoplight was yielding about half the Lux of daylight. When I asked a retired lighting engineer who has access to technical data (which he shared in the form of graphs) he showed that seedling growth is almost perfectly matched by 6500k bulbs. He then explained to me that the measure @ ground level outside was measuring the Lux of full-spectrum light (only about half of which a seedling is able to use ...or needs), whereas the measurement under my 4-bulb light was measuring the Lux of almost 100% usable light. That translated to the fact that 16 on/8 off was producing more usable light/light-time than if I had my plants outside.

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Re: Lumens vs. Kelvin

Post  lbmcse on 8/6/2013, 11:04 am

I know the thread is a bit old, but here goes. ..

As said above, lumens is the power, or intensity of the light; while Kelvins is the color of the light. 

You know how a flashlight shone in your eyes temporarily blinds you?  The more lumens, the greater this effect.  A candle is weak in luminosity, a searchlight is strong.  For gardening, the more lumens the better, but this equates to more heat if you're using halide or incandescent bulbs.  Fluorescent bulbs, compact fluorescents (the spiral bulb above is a CF) and LED's, create much less heat.  In fact, LED's are getting so good that they're replacing metal halide lights above reef tanks.   

Color of light is measured in K, and 6500 is considered the color of sunlight at noon.  The higher this number, the bluer the light.  20K (20,000K) is similar to being underwater at 30 feet or so.  Water blocks lower Kelvin colors, so only the blue makes it down to depths. 

We want high luminosity and low K (6500 is great).  A very strong blue light is pretty on a reef aquarium at night, but useless for photosynthesis.  HTH.

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