Kelvin and watts

[Fishnut]

Is Kelvin simply a number for the colour of the light or is there more significance to this number?  Does it have something to do with the intensity or is that the watts?

[wrm130]

Here's a good read.  Even myself, simple as I am, managed to learn a bit:)  Not sure if it directly answers your question but its very close.

http://www.plantedtank.net/forums/showthread.php?t=184368

As far as I can tell Kelvin is the 'temp' of the bulb.  Different temperatures/colors are going to have different penetrating powers.

Someone jump in and put me to shame

[Stussi613]

Kelvin is commonly used to denote the color of light produced by a bulb, but has nothing to do with its intensity.  Outdoor noonday sunlight is 5600k, with a full spectrum of wavelengths from infrared to actinic (and beyond actually).  It's measured by heating a black body radiator and looking at the light being emitted and correlating it to the temperature of the radiator.

A candle emits light at 2700k, the same as an old incandescent bulb with a filament or warm white fluorescent, a cool white fluorescent at 4100k, a daylite fluorescent at 6000k and so on. In order to make bulbs cheaper, most fluorescent lights only emit light in the spectrum that produces the colour at the desired temperature, and don't emit light in other spectrums.  Full spectrum lights, on the other hand, contain gases that emit light in most of the spectrums the sun would, but the end result is a combination that produces a single temperature, like 5600k, which are the most common full spectrum bulbs.  With the addition of extra gases to produce the full spectrum of light comes cost.

When you start getting into lights for saltwater tanks there are very specific requirements that simulate the light at the equator and produce light in spectrums needed by corals (actinic bulbs with lots of output at 420nm are a good example).  They also reduce light in the red spectrum which is filtered out by seawater based on its colour temperature. The interesting thing about actinic lights are that the same gases in a bulb without the coating on the glass that helps it produce its range, and look white when the bulb is off, produce 470nm light which is what is inside a uv sterilizer.

The last thing to consider is the CRI, or colour rendering index, of fluorescent and led lights.  These lights are pretty easy to manipulate to produce light in a desired temperature and spectrum, but its another thing to get them to render colors the way they should look in outdoor noonday sunlight.  There are full spectrum bulbs at 5600k with high colour rendering indexes available for photography, operating rooms and other applications. Again higher CRI means better materials and higher cost.  This also plays into effect when you look at your fish in your tank. With cheap quality bulges the actual colours may be off because the light bulb stinks at rendering colour.

Wattage refers to the amount electricity needed to generate a given amount of light, based on the candle...believe it or not.  Using the wattage as a guide is a throwback to the days of the incandescent bulb, generally a 60w bulb would throw off around 850 lumens of light. When fluorescents came around they used the 60w as a belweather and rated the bulb wattage based on how much electricity it consumes to produce the same amount of light. In a CFL it takes 13w of electricity to generate the same 805 lumens as a 60w incandescent. In all honesty it would be better if we just switched to lumens but society is understandably used to the old way of doing things, that's why you usually see the comparison on most CFL packaging.

For those of you who don't know, I worked at Buchanans selling industrial and medical lighting for years before getting into high tech. If you have questions about lighting principles feel free to PM me...but I have to admit I'm not up to date on all the advancements in LEDs, but I do remember a thing or two.

[Stussi613]

Here's a good read.  Even myself, simple as I am, managed to learn a bit:)  Not sure if it directly answers your question but its very close.

http://www.plantedtank.net/forums/showthread.php?t=184368

As far as I can tell Kelvin is the 'temp' of the bulb.  Different temperatures/colors are going to have different penetrating powers.

Someone jump in and put me to shame

That's a great link and I agree, the PAR of the bulb is more important than the number of watts...but the old watts per gallon rule seems to want to live forever 

[exv152]

That's a great link and I agree, the PAR of the bulb is more important than the number of watts...but the old watts per gallon rule seems to want to live forever 

It is an awesome article, and I've had discussions with Hoppy himself. PAR is the best measurement out there for plants. If you have a Lux meter you can figure out the PAR fairly easily.