# let there be light



## monkeyman (Feb 1, 2010)

hello everyone this is my 1st post on apc ,although i have been reading everyones helpfull information for months.
i have just up graded my lighting from 4 t5 ho 54w to 2 t5 ho and two 150w MH 4200k im just wondering if the k rating on my bulbs is to low ,and how long should i be running the MH for


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## Troy McClure (Aug 3, 2005)

Welcome.

What size is the tank (dimensions)? What plants are you looking to grow?


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## medicTHREE (Feb 5, 2010)

K rating relates to what the human eye sees. Despite common myth, it is not relevant to plants. Anyone who disagrees with me needs to cite sources, and "so and so said" will not work.

Personally, I think that is too yellow for my taste, but plants willg grow in it just find.


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## Newt (Apr 1, 2004)

Yah, as long as the light output has emissions in the red and blue regions of the visible spectrum you are fine. A 4200K will have more red than blue. The europeans tend towards bulbs in this kelvin range.


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## monkeyman (Feb 1, 2010)

the tank is a standard 4 foot i know its a lot of light but i got the fixture cheap . the light is a little green to the eye could you recommend a bulb / brand . im trying to grow ammannia , macranda, blyxia,walichii and UG, those are probaly the more difficult plants in my tank


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## Seattle_Aquarist (Mar 7, 2008)

Hi monkeyman,

The K spectrum of the light is important to the plants, as it the appearance of the aquarium to the eye. Here is some information on K value and the effect on plant growth.

I run 6700K bulbs on my tanks for the plants but prefer the look of higher K value bulbs in the 8800K or 9325K range.


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## Newt (Apr 1, 2004)

Here is a link to a nice MH bulb:
http://www.giesemann.de/76,2,,.html

Seattle, I have to disagree about the kelvin being important to plants. It is only important as to the ascetics of the tank - how it appears/looks - you either like a particular type of color to the light or you dont. The plants only care if it contains light that they can use for photosynthesis - red and blue (most efficiently). Lighting for a planted tank should not be chosen on color temp alone. It is true that 'full spectrum' bulbs are referred to as bulbs between 5000 Kelvin (K) and 6500 K and are considered to be best for planted tanks. Yet this does not indicate what wavelength in nanometers the bulb is actually emitting. If you want to optimize plant leaf development (blue light) and stem elongation and color (red light) you need light in both the blue and red spectra for photosynthesis.

I've looked at the spectral output of bulbs that have been given the same kelvin rating and they can have very different spectral outputs and can make the tank look very different.


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## medicTHREE (Feb 5, 2010)

Seattle_Aquarist said:


> Hi monkeyman,
> 
> The K spectrum of the light is important to the plants, as it the appearance of the aquarium to the eye. Here is some information on K value and the effect on plant growth.
> 
> I run 6700K bulbs on my tanks for the plants but prefer the look of higher K value bulbs in the 8800K or 9325K range.


I will again disagree. For starters, Kelvin rating for fluorescent bulbs is horribly inaccurate. http://www.cameraguild.com/technology/kelvin.htm


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## Newt (Apr 1, 2004)

medicTHREE said:


> ..........For starters, Kelvin rating for fluorescent bulbs is horribly inaccurate. http://www.cameraguild.com/technology/kelvin.htm


I totally agree.


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## freshyleif (Jan 9, 2008)

medicTHREE Very nice link good info.


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## Seattle_Aquarist (Mar 7, 2008)

Hi medicTHREE,

I am not trying for "color reproduction" in a color picture although I do photography and in the past have developed my own photos. I am trying to provide light in the spectrum that my plants can effectively utilize. 

There is scientific evidence that the spectrum of light that a plant receives does effect photosynthesis and therefore plant growth. I can certainly discern a difference in a 4000K flourescent bulb verses an 8800K bulb and therefore there must be a difference in spectrum even if they are not "properly rated". I will continue to try to provide light to my plants that they can efficiently utilize and others will do as they choose.


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## medicTHREE (Feb 5, 2010)

Seattle_Aquarist said:


> Hi medicTHREE,
> 
> I am not trying for "color reproduction" in a color picture although I do photography and in the past have developed my own photos. I am trying to provide light in the spectrum that my plants can effectively utilize.
> 
> There is scientific evidence that the spectrum of light that a plant receives does effect photosynthesis and therefore plant growth. I can certainly discern a difference in a 4000K flourescent bulb verses an 8800K bulb and therefore there must be a difference in spectrum even if they are not "properly rated". I will continue to try to provide light to my plants that they can efficiently utilize and others will do as they choose.


The problem is simple. Take a 5000k bulb from one manufacturer and compare its spectral analysis to that of another 5000k bulb from another manufacturer. They very likely will not match up. Thus the reason that the common aquarium store bulbs for plants are good for plants, but are still in the 3000k range and look like yellow pee water.

The K rating of fluorescent bulbs is what the human eye can see. It is NOT the spectral output(what plants need for photosynthesis). There is often a chart indicating spectral output as well.

Fortunately, MOST fluorescent bulbs have enough output in the spectrum(s) needed for photosynthesis in plants. Using the K rating to do anything other than choose what looks good to you is useless.


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## Newt (Apr 1, 2004)

The light spectrum is measured in nanometers. It is a wavelength. 









Kelvin is a temperature scale that equates to a color along a line on a chromaticity map.

















A light bulbs light output is made up of many colors in the visible spectrum.









The combination of all those spikes in the visible spectrum form a single color that manufactures rate in the kelvin scale. Sometimes the marketing department deploys a 'scheme' and is not very accurate with the kelvin rating all the time. Take GEs 9325K bulb. Do you really think its 9325. Its too red to be that high but it sounds neat.









I use Sylvania (used to work for them) GroLux Std and WS for dawn dusk lighting. The GroLux Standard (a plant grow light) is not given a kelvin rating. But it is very purple and would probably show up around 20,000 to 22,000K on the chromaticity map. It has a huge (true) red signature very high in the nanometer range and a strong blue spike. Both red and blue are used in photosynthesis.









Here are the wavelengths used by plants most efficiently:

















A kelvin temp is not a spectrum; althou the light color of the bulb can be equated to a wavelength in the spectrum of visible light.


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