Aquarium Lighting

This is a fairly extensive and advanced guide related to aquarium lighting. If you are a beginner, this may be more information than is really necessary for your lighting choice. For more simple instructions go to How to Setup an Aquarium. Below we go over the terminology for our lights including: Kelvin rating, PAR, Lumens, Nanometers, and Watts.
1) Kelvin Rating (color temperature)
Kelvin rating in lights is somewhat different than Kelvin rating as it is more commonly understood in thermodynamics.
Kelvin is a unit of measurement used in the International System of Units (SI) respresented by the symbol K. The Kelvin scale is an absolute temperature scale using at its lowest point absolute zero, the temperature where all thermal motion ceases, up to the triple point of water (0.01°C).
For Lighting, it has a somewhat different meaning. Various types of light bulbs are given a Kelvin Rating, based on the color temperature of light. This color temperature is related to the temperature and radiation emitted by a black body radiator. A black body radiator is an idealized concept used to help relate a color spectrum to a particular light source based on temperature alone, not including other factors such as angle of incidence or the composition of the body. This relates to the thermodynamic principle of absolute zero, since supposedly at a color temperature of 0 there is no molecular motion or in the case of a black body radiator, radiation.
When using an incandescent bulb as an example, the color temperature at full voltage is close to 3000K. As the voltage is decreased the light dims, which in turn also dims the color temperature. Lights that are more red in appearance have a lower color temperature, whereas blue or violet colored lights have a significantly higher color temperature. A good example of this is the color of stars. Old dim stars appear with a darker red coloration since the color temperature is significantly lower, whereas extremely hot stars with a color temperature much higher appear blue.
This connects to our aquarium lighting primarily in regard to plants, since chloropyll is able to absorb light within a particular range. Natural sunlight appears at around 6500K, which is ideal for plants to absorb.
Higher color temperatures also penetrate water more deeply, which can impact your lighting selection.
Kelvin Temperature interacts with other elements of our light such as nanometers. Different combinations of nanometers can be used to achieve the same Kelvin Temperature within a light source.
2) Nanometers
A nanometer is a unit of measure in the metric system, equal to one billionth of a meter. The scientific notation is 1×10-9 m, and it is generally used to measure dimensions on an atomic scale.
In terms of aquarium lighting, we are measuring the wavelength of light. This is relevant for how light penetrates various obstacles. For instance, an x-ray which has a short wavelength is able to pass through walls and other solid objects. Whereas, visible light from your desk lamp has a long wavelength and is blocked by more common obstacles. As shown on the spectrum below, "blue" light will penetrate water deeper than "red" light.

Above you can see the various ranges of light categorized by the measure of nanometers.
It also relates to the PAR for our plants. Only specific ranges of nanometers are used by the chlorophyll in aquarium plants to enact photosythesis. Light relevant to our plants is primarily contained within the 400 to 700 nanometer range. With several spikes in PAR value contained in the "blue" UVA range (315-400nm) and the higher "red" infrared range (700nm).
3) PAR
PAR, or Photosynthetically Active Radiation, is the spectral range of light from 400 to 700 nanometers. This is the range that allows plants to absorb light energy for photosynthesis. We refer to this as "actinic light".
The light within this 400 to 700 nanometers range can be more specifically refered to as UVA to infrared.
UVA (Ultraviolet A) is the light range from 315-400 this is on of the most important regions for cholorphyll to initiate the photosynthesis process.
Infrared is normally classified as starting at 750 nm, but for the purposes of defining actinic light we refer to the upper range close to 700 nm as being infrared.
The range inbetween UVA and infrared is often refered to as "green light", which is not as essential for our actinic light needs. It is, however, the more visible aspect of the light for our naked eye.
For an optimal lighting in our aquarium setting, we want a light with both the UVA and Infrared range present with less emphasis on the inbetween "green light". These different ranges enact slightly different process from the chlorophyll and both are ideal to have the greatest efficiency in the photosynthesis process. It is important to double check these ranges, because many lights may boast an excellent PAR rating but not be providing the relevant ranges for our plants.
4) Lumens
The lumen represented by the symbol lm is the SI derived unit of luminous flux, a measure of the total "quantity" of visible light emitted by a source. In lamens terms, this is how bright a light appears to our visible eye.
When determining the intensity of light desired for your aquarium, this is a more appropriate measure than watts. Lumens per watt can range widely, and the higher the value the more efficient your light source is at providing useful light energy to your plants and aquarium inhabitants.
The effectiveness of your light source's lumen rating is affected by its PAR rating. Different types of sources may having very differing results in your aquarium. To give an example, a flourescent bulb with 1000 lumens at 6500K will provide significantly less than an LED light with 1000 lumens at 6500K. LED's tend to have a much higher PAR rating, which means their light is more focused into the range plants can benefit from.
5) Watts
Watt is an unit of measure defined as on joule per second, that measures the rate of energy transfer.
For aquarium lighting it is important to note that this does not represent the quantity of light being produced, only the amount of energy being used to produce the light. Watts have become a rule of thumb for the measure of light necessary to light a planted aquarium. This is really not a accurate measure of your light source and should be taken with a grain of salt.
Somewhat more accurate is the ratio of lumens to watt, but this only shows the amount of light being produced in relation to the amount of energy used. It is still important to know the other factors about the light that is being produced to achieve the desired result.
Choosing your Aquarium Light
In a nutshell, all of these things should be taken into consideration when choosing your light. For optimal plant growth you want a light that has a color temperature close to 6500K with the relevant ranges of PAR for our plant growth, and has enough lumens to penetrate the water depth in your tank.

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