kelvin, light & color temp
For me a good place to start: what is light?
Light is some kind of electromagnetic field that has associated property known as wavelength. The shorter the wavelength the more energy it carries. Conversely the longer the wavelength the less energy is carried so the intensity of light in a given wavelength might be different than the intensity of light at another wavelength. How light intensity changes with wavelength is the lightís spectrum.
We perceive lights different wavelengths as colors. The longest wavelengths are red and as the wavelength decreases (gets shorter) we see all the colors of the rainbow to violet, the shortest light wavelength we see.
Another way to talk about lightís wavelength and the associated color is the Kelvin scale. Kelvin ratings (degrees) measure the color temperature (spectrum) of light. Color temperature is not the heat of the source of light it is the color given off by the source light.
Why is the hue (color) of the light measured as a temperature?
Back in the late 19 th century a physicist named Kelvin heated a block of carbon. The carbon glowed in the heat giving off different colors at different temperatures. The carbon at first produced a red light that changed to yellow as the temperature increased and eventually produced a white-blue light at the highest temperature.
So the longest light wavelength with the least energy (red) has the lower k rating and lower color temperature than shorter light wavelength (blue) with more energy has higher k rating and higher color temperature.
In "light" of this information it should be cool red and hot blue.
When light travels through water it is absorbed. The longer wavelength are lost first, so the shorter the wavelength the deeper they penetrate the water column. The descending order of penetration by color is red, orange, yellow, green, blue, indigo then violet.
Why use a full spectrum bulb in the aquarium if most of the light wavelength is lost in the first few inches of the water column: is it because of algae?
"Temperature" when used with Kelvin does not mean temperature as we commonly think of it, being the heat/cold in the air that we feel. While it sort of relates to hot/cold it is in the colour more than the physical surrounding, if that makes any sense.
Now to the more important question of light and aquarium plants. Aquatic plants require primarily red and blue light in order to photosynthesize. They also do use the yellow in between, but much less. This is why the so-called aquarium or plant fluorescent tubes are so purplish; they emit basically red and blue light.
Blue light penetrates further in water, which is why the ocean coral reefs need so much blue (actinic) light. This is what drives the corals, etc. But freshwater aquarium plants need red slightly more than blue, and the more red the leaves the more red light--though here, the yellow can help if it is bright enough.
One must also remember that many of our aquarium plants are not strictly speaking aquatic, but marsh or bog plants that are semi-aquatic. All the Echinodorus, Helanthium, Sagittaria, Anubias, Cryptocoryne and many other species fall into this category. These plants tend to be at their most active with respect to leaf growth and flowering during their period as emersed plants, with the leaves in the air. They manage fine fully submersed, which is why they make such good aquarium plants, though flowering is usually (but n to always) impossible if they do not have a period of emersed growth corresponding to their natural habitat. So obviously red light would be more plentiful in air than in water.
Studies carried out by biologists have proven that most of our aquarium plants when grown submersed will respond best to full spectrum/cool white light. While it is perfectly true that Kelvin is not truly related with spectrum, it does seem to serve as a useful guide. Every planted tank author I have read will recommend light with around 6000K to 7000K as the best for plant growth. Provided this light is of sufficient intensity, and the required nutrients are all available, the plants will photosynthesize fastest under this light. This is why so many of us recommend the "Daylight" type tubes having a K around 6500K. This is fairly close in colour to the sun.
Thanks for the input I sorta get how light Ė color are connected to temperature of light source. I am glad you brought up the point about red and blue light and aquarium plants because I have been looking unsuccessfully for the ratio of water column penetration by light in aquariums in terms of inches of depth.
I take it from your post that light meaning red light is capable of reaching a depth of approx. 20 inches for plants with red leaves. I have the idea that red light and the correlated wavelength is the first to be adsorbed by water before yellow.
I guess itís a thing of light intensity that would be a question of watts.
what does the green circle mean on your post
Red light would reach the substrate if it was of sufficient intensity, since it is absorbed by water as you noted earlier. This is why red-leaf plants need brighter light. They appear red because they are reflecting red light, so they need even more of it than green plants which reflect green. However, sources have said that just having brighter light that has red, blue and green/yellow in the mix will provide this. Another reason to use the 6500K light, which has high red, blue and then green/yellow. This is one reason the "plant" tubes with their red/blue are not so good. Another is their intensity is always much less than the full spectrum/daylight, so they provide even less good light.
the amount of attenuation of light of any wavelength in a few feet of clear water as in an aquarium is insignificant
Thank you and Bryon for helping me get the story right,,,,,, I am still a little concerned about light intensity and the stress it causes with aquarium fish.
I read somewhere that red wavelength of sunlight only penetrates the ocean five or six feet so I thought red light from a bulb would only penetrate the aquarium water column a few inches. as you pointed out water clarity and floating plants will effect lights penetration. pop
This is what I pass out to the Aquarium Reef boys, this should help.
LED Aquarium Lights,*Lighting; emitters, PAR, DIY, How they work | Aquarium Article Digest
Aquarium Lighting; Reef, Planted Light Information. PAR, Bulb, Watt, Kelvin, Nanometers, MH, LED.
Hello reefing Madness:
Thanks for the help the site is full of information. I found out I can not use a t8 light because of the depth is over 20 inches. Is it feasible to build a reflector and mount the light at plant level directly behind the plant or does light work better coming from the top. Maybe I could hang a sho cfl low in the back of aquarium and reflect the light to the plants?? Will white sand help reflecting pur up into the plants?
This site has raised many questions and some answers. pop
white sand will reflect more light up to a plant but naturally fish prefer a darker substrate. sand still makes a good option though as roots can easily grow in this medium.
wouldn't adding a light to the back of the aquarium shine it directly into your eyes while viewing the tank? none the less it will have some effect on the stimulus of the plant, as it wants to grow "up" towards the sun ( in this case the light )
Good point about light in the eyes. My aquarium is not a square shape but one quarter of a circle that has three sides. I thinking of putting sand and plants in the front right corner of the tank and have the light shine toward the other side across the tank or towards the back corner. I am not very creative but also consider submersible lights.
Thanks for the suggestions.
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