1. KELVIN RATING (such as 10,000K daylight bulb):
Although the classic interpretation of what Kelvin is in not what is meant by light Kelvin ratings, I believe it should be considered.
Here is a brief description of Kelvin:
Kelvin is defined by two points: absolute zero, and the triple point of pure water. Absolute zero is defined as being precisely 0 K and -273.15 C. Absolute zero is where all kinetic energy (motion) in the particles comprising matter ceases, and they are at complete rest. At absolute zero there is NO heat energy. The triple point of water is defined as being precisely 273.16 K and 0.01 C. Here are a few Kelvin
numbers:
*Absolute zero = 0K (-273.15 C)
*Waters freezing point = 273.15 K (0 C)
*Waters boiling point = 373.1339K (100 C)
What the true definition of Kelvin is that is a unit of measure of temperature on the thermodynamic (absolute) temperature scale.
Light Kelvins
Kelvin is used in the lighting industry to define the Color temperature of a bulb. Higher color temperature lamps above 5500 K are "cool" (green-blue) colors, and lower color temperature lamps below 3000 K are "warm" (yellow-red) colors.
Kelvins as applied to color temperature of lights/lamps are derived from the actual temperature of a black body radiator, which is the concept of color temperature based on the relationship between the temperature and radiation emitted by a theoretical standardized material and termed a "black body radiator". This is where the "classic" definition of Kelvin and that used for lights come together, as hypothetically, at cessation of all molecular motion (the black body state of this hypothetical radiator), the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.
An incandescent filament is very dark, and approaches being a black body radiator, so the actual temperature of an incandescent filament is somewhat close to its color temperature in Kelvins.
Incandescent lamps tend to have a color temperature around 3200 K, but this is true only if they are operating with full voltage. When a lamp is dimmed below its full potential, its filament is not as hot, and it produces less light. The reduced temperature of the filament also reduces the color temperature downward. An incandescent light dimmed to 10% is considerably more red in color than one at 100%.
Another consideration as to the color temperature as applied to lights; color temperature does not take into consideration the spectral distribution of a visible light source. In cases where a light source, such as a fluorescent lamp, arc-discharge burner, laser, or gas lamp, does not have a spectral distribution similar to that of a black body radiator.
A few notes about Kelvin:
* Plant chlorophyll absorbs light at wavelengths of 300 to 700 nm (a Kelvin rating of about 6400 strikes a good balance here, which is why this is the best Kelvin temperature for freshwater plants and symbiotic zooxanthellae in corals).
* The lower the "K", the more yellow, then red the light appears, such as a 4500 K bulb.
* The higher the "K", the bluer the light appears, such as a 20, 000 K bulb.
*Higher Kelvin Color Temperature lights penetrate water more deeply, even more so in saltwater, however there is less of the infrared "PAR spike" as well. * The human eye sees mostly sees light around 5500K.
* Candle flame = 1850 - 1900 K
* Sunlight (1 hour after dawn) = 3500 K
* Typical summer light (sun + sky) = 6500 K
* Cool white fluorescent = 3400 K
What Kelvin rating for Plants & Corals;
Here are some observations made by me and others in the professional aquarium maintenance community, some of these are simple observations, while others were based on more controlled tests.
*The 6500 Kevin bulbs have produced the best freshwater plant growth (as this Kelvin lamp generally has more of the infrared nm spike need by "higher" plants, but still has some of the 425-500 nm blue).
This Kelvin Lamp can also work with SPS, LPS placed high in the tank water column (nearest the lights) based on the symbiotic zooxanthellae needs found in these corals. For more depth penetration (& to aid in the first blue spike in PAR, please see PAR section), blue actinic, 50,000 K/Actinic or adjustable/multiple LED can be added (such as AquaBeam Marine Blue) can balance out 6500K lamp if used in marine reef tanks.
Please note that saltwater absorbs slightly more light energy than freshwater due to the higher density of the water, so 6500 will not penetrate as deeply.
*The 10,000 Kelvin bulbs also achieve good growth rates, although slower than the 6500 K bulbs in shallow aquariums. 10000 K bulbs have produced excellent growth with soft corals and LPS, although slower paced SPS growth.
The 10,000K can be a good choice for achieving PAR for better depth penetration than a 6500K bulb (such as 20 inch or deeper aquarium)
*The 14,000K light/bulb (often popular with Metal Halide and LEDs), will penetrate even more than the 10,000K light while still providing useful PAR (this would be the highest "Daylight" Kelvin temperature I would recommend and still expect good growth in corals, see PAR section).
*The 20,000 K bulb is more blue yet and brings out all of the fluorescent pigments in many corals (making for a very nice appearance). However the best tests and observations show that when used alone the growth rate of SPS corals come to a complete standstill with 20,000 K lamps. Although maybe a good supplement for appearance, these bulbs are over rated for use as the only reef Kelvin temperature and should be avoided when used as the only Kelvin temperature lights
*The 50,000K is generally the Kelvin rating of an actinic blue light source which is beneficial for the first "spike" in PAR. This temperature light (as with 20,000K) is best used with other light Kelvin Temperatures and is a better choice than the 20,000K light for such combinations. The 50,000K is a good compliment to the 6500, 10,000, 14,000 Kelvin lights, especially for zooanthellic algae necessary for stony corals, clams, nudibranch, anemones, and other sessile species.
Kelvin summary
The Kelvin rating is another area of comparing apples to apples in lights, not just watts. Although the above is a simplified explanation of Kelvin as it applies to lights; as such you cannot compare a 6400 K T8 light to a 6400 MH light (the MH is going to have much more output as you will read later).
However (using a CFL as an example, this can apply to any light type), a 6400 CFL will have a higher energy output than a 3500 CFL, this is why an incandescent filament is very low in Kelvin (dark) as derived from the actual temperature as it is just above a black body radiator. Another example using an incandescent light, this time looking at it from the aspect of watts; a 100 watt incandescent has a Kelvin of 2870 while a 40 watt incandescent has a Kelvin of 2500. So as you can see, there is some correlation between energy output and Kelvin Color.
This does not mean that a certain Kelvin bulb is necessarily "better" as factors such as "lumens per watt", watts, focused lumens and more MUST be considered as well.
References:
Color Temperature In A Virtual Radiator - This is an interesting resource with a virtual radiator worth checking out.
http://www.uky.edu/~jholl2/technology_pdfs/KelvinColorTemperature.pdf
http://www.sizes.com/units/color_temperature.htm
2. THE NANOMETER RANGE (SPECTRUM)
A nanometer scale is used to measure the wave length of light energy from cosmic rays to radio waves. An actinic bulb will have a Nanometer spike at about 420N, a UVC bulb about 265N, and a daylight bulb about 700N. The difference in the wavelength determines how the wave affects its surroundings. It is this wavelength difference that allows short-wave x-ray to pass through walls, while longer-wave visible light cannot pass though the same material; short-wave ultraviolet and x-ray can destroy DNA in living microorganisms and breakdown organic material while visible light will not. All light energy is measured on a "nanometer" (nm) scale. Nanometer means one-billionth of a meter.
This applies to aquariums when we consider the light spectrum and how it applies to our aquariums individual needs: Red light is the first to be filtered out and can only penetrate a short distance. As light waves penetrate deeper into the water, orange and yellow are lost next. Of all the colors of the spectrum blue light penetrates the deepest. Corals need intense equatorial UVA (actinic). Most higher plants need a balanced PAR light range (see section about PAR).
The Nanometer scale and Kelvin temperatures come together when applied to aquarium lighting this way; Natural sunlight on a clear day registers at 5500- 6500 Kelvin degrees. Kelvin temperatures less than 5500K become more red and yellow and the higher the Kelvin temperature the more blue the light is.
Most photosynthetic invertebrates should be kept with lamps of a daylight Kelvin temperature from 6400-14,000 K (higher Kelvin with deeper specimen placement, not necessarily tank depth). 20,000K daylight lamps can also be used for deeper tanks, however these often have poor PAR and it is better to use a 14,000K Daylight or less supplemented with more blue.
Photosynthetic invertebrates (many corals, anemones, clams, nudibranch, etc.) also need more blue (400-550nm) than "higher" plants, with 465-485 the optimum blue. Not only is blue/actinic lighting beneficial to photosynthetic invertebrates, it is also aesthetically pleasing to the eye when used to supplement "daylight" lighting.
Freshwater aquarium plants benefit from lighting with a Kelvin temperature in the range of approximately 6500 degrees. Freshwater plants prefer light with more red in the spectrum (see PAR Section).
It is also noteworthy than Fluorescent and even more so incandescent lights produce a lot of yellow and green nanometer light, which research indicates is mostly wasted energy in terms of the needs or freshwater plants and SPS Corals. This is where an LED Aquarium Light, Metal Halide, or even (to a lesser degree) T2 Lights excels as there is much less wasted yellow/green light.
3. LUX:
A measure of the intensity of light (referred to the photometry of light), one lux is equal to one lumen per square meter. This is important for aquarium plants. Once again this is another area of comparing apples to apples in lights, not just watts.
This is also VERY important to most corals in marine reef aquariums. When the Lux (intensity) is not enough the zooxanthellae (algae that are inside of corals tissues) do not create plentiful oxygen. The minimum light intensity should be no less than 3,000-lux when it reaches the deepest part of the aquarium. You can over light your coral to a light saturation point (quite hard in my experience, but this should be noted), maximum Lux should be no more than 100,000 to 120,000.
By comparison Lux in tropical reefs has been measured to be between 110,000 and 120,000 Lux at the surface of the reef and 20,000-25,000 Lux one meter below the surface.
4. PAR:
PAR is probably one of the most important considerations along with the related Useful Light Energy, Lumens per Watt, Focused Lumens and Watts per Gallon when choosing a light for your aquarium, yet is often over looked by both marine and freshwater pant keeping aquarists.
PAR is the abbreviation for Photosynthetically Active Radiation which is the spectral range of solar light from 400 to 700 nanometers that is needed by plants & symbiotic zooanthellic algae (Zooxanthellae are single-celled plants that live in the tissues of animals such as corals, clams, anemones, & nudibranchs) for photosynthesis. This is found from actinic UVA to infrared. UVA is 400-550nm (of which 465-485 has the highest PAR of the actinic range) which is the absorption bandwidth of chlorophylls a, c2, and peridinin (the light-harvesting carotenoid, a pigment related to chlorophyll). Infrared is 620-700nm which is the red absorption bandwidth of chlorophylls a and c2.
Photons at shorter wavelengths (Ultraviolet -C or UVC) tend to be so energetic that they can be damaging to cells and tissues; fortunately they are mostly filtered out by the ozone layer in the stratosphere. Green light occupies the middle spectrum (550-620nm; what is mostly visible to us) and is partly why chlorophyll is green due to the reflective properties.
Bulbs that emit mostly actinic light will have a lower PAR (although actinic UVA still occupies an spike in PAR as seen from the graph and improve the PAR of your lighting), bulbs that occupy mostly the middle spectrum (yellow-green) such as "warm White (2700K) will produce little necessary PAR, while bulbs that produce mostly infrared (as seen from the graph) will produce more important PAR light energy, however it is the balance of infrared and UVA that will generally provide your best PAR output.
PUR (Photosynthetically Usable Radiation) should also be considered. PUR is that fraction of PAR that is absorbed by zooxanthellae photopigments thereby stimulating photosynthesis. As noted above, PUR are those wavelengths falling between 400-550nm and 620-700nm.
It is noteworthy that whereas higher plants (generally kept in planted freshwater aquariums) require more of the infrared aspect of the PAR Spectrum while the zooxanthellae found inside many sensitive corals require more of the Blue spike (465-485 in particular). For this reason either/both higher Kelvin Daylight and actinic/blue is required for many reef aquariums.
Important Definitions as it applies PAR in plants and zooanthellic algae: See the graph below as it corresponds to each of these definitions.
*A: Phototropic response; having a tendency to move in response to light. Basically this is the Chlorophyll containing plant or algae "moving" to respond to a positive light source to begin the process of photosynthesis (initial growth of plants, zooxanthellae, etc.).
*B: Photosynthetic response; the process which begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls.
*C: Chlorophyll synthesis is the chemical reactions and pathways by the plant hormone cytokinin soon after exposure to the correct Nanometers wave length (about 670 NM) of light resulting in the formation of chlorophyll, resulting in continued growth of a plant, algae, zooxanthellae and the ability to "feed" and propagate, and without this aspect PAR (670 NM light energy), zooxanthellae and plants cannot properly "feed" propagate. The results of the lack of this high PAR "spike" would be stunted freshwater plant growth, and eventually poor coral health in reef tanks
Further PAR Information; As the reader here can see, there are three main spikes in the PAR spectrum, with all three being important, however the most important spike occur at the red side and all three are generally incorporated more or less in a daylight bulb of approximately 6500K, not just a bulb of only infrared or as is mistakenly believed by many reef keepers, only in the UVA/actinic spectrum (although a daylight bulb of 10,000-14,000K can provide this too for deeper "more dense" tanks as these higher Kelvin bulbs generally penetrate deeper than a 6500K, albeit with slightly less from the 670 NM spike).
This can vary with bulb type though as not all daylight bulbs are the same, see the Useful Light Energy section.
Unfortunately, despite anecdotal claims passed around within the aquarium hobby; in better funded tests outside the aquarium industry show that many stony corals, clams, and other sessile species that depend on photosynthesis of zooanthellic algae not only thrive but also propagate with light that achieves the optimum PAR, which includes daylight from 6500k to 14000k (Higher Kelvin outputs are required for tanks deeper than 24-30 inches to achieve maximum PAR such as a 14,000K, although high intensity 6400 SHO Lamps can generally penetrate deep freshwater tanks well and perform well in marine reef tanks when balanced with actinic/50,000K lights necessary for the higher need of 465-485nm light energy by symbiotic zooanthellic algae).
*It is also noteworthy that most green algae need more of the actinic spike than "higher plants", hence the popularity of actinic lights for reef aquariums, however the optimum nanometer range is about 465-485nm, not the lower 420nm many actinic lights produce or the more broad range many "blue" aquarium lights produce of 400-520 nm (this is where the latest technology LED lights "shine", having a more precise 465-485nm blue). For this reason it is a good idea to have extra actinic for corals/clams that depend upon zooanthellic algae, while at the same time limiting blue/actinic in freshwater aquariums to avoid excessive green algae growth.
More recent studies show that excessive low UVA (under 420nm) and especially UVB radiation can actually bleach coral and in an aquarium environment lights with more UVA or UVB are not generally necessary as although a lamp with more "blue" such as a 20,000 K MH may penetrate more deeply (due to the fact that infrared light gets absorbed the deeper the light must penetrate), this should not be the sole Kelvin color light for most aquariums (better would be to compliment a 6400 through 14000K bulb with a 50,000K actinic bulb)
Measuring PAR
Although Kelvins (as well as LUX conversions using questionable LUX to PAR conversion factors) are ways of getting rough estimates of PAR, only a Specific PAR Meter (also called Quantum Light Meters) can give you're the best measurement of this very important aspect of determining your tanks lighting requirements (both at the surface and under the surface)
(Here is a link to a PAR Meter Apogee MQ-200 PAR Meter)
Currently accepted numbers measured as uMol*m2*sec (also referred to as micro mols or mmol) are 50 mmol for most plants or low light corals such as Nemezophyllia, while Acropra can require PAR outputs as high as 300 mmol (any higher is simply a waste of energy/light)
Further PAR Info
Some organisms, such as Cyanobacteria, purple bacteria and Heliobacteria, can utilize light in regions such as the low infrared. These bacteria make use of the unusable light discarded by the plant kingdom, in this case, light outside the PAR range required by plants, which is why Cyanobacteria thrive in lighting conditions that include the more yellow 4000 K and below and why actinic (50,000K) as well as balanced light in the 6400 to 14,000K range combined with passing water through an ultra violet sterilizer (to kill free floating Cyanobacteria) is important for control.
In the case of Red Slime Cyanobacteria, these Cyanobacteria do not use the PAR spikes at 435nm and 675nm and instead utilize more of the middle yellow and green light spectrum that is most common fluorescent (even so-called aquarium fluorescent lights) and incandescent lighting.
Please do not confuse the term PAR (Photosynthetically Active Radiation) used in plant growth as discussed here with another use of the same term in lighting which is Parabolic Aluminized Reflector. This type of light is used for stage lighting and should not be purchased for marine, freshwater, or Greenhouse use under the mistaken belief that these lights are "great" for plant/coral growth.
For further reading (references) about PAR:
*Light and Plants
*Photosynthetically Active Radiation
*http://www.aquariumpros.com/articles%20PDF/lamptypes.pdf
Fish Health:
Many recent studies have shown the importance of full spectrum lighting (which will generally encompass a high PAR value) as it relates to health in humans, animals and can be extrapolated to fish as well for a disease prevention which is why good lighting should not be restricted to Reef Marine or Planted Freshwater Aquariums, but to fish only salt or freshwater tanks as well.
In fact the medical community is now utilizing 6400K SHO bulbs (& similar full spectrum lights) due to increasing studies that show better immune function, mental health, and more. Animal studies support similar results as well. This need not be a SHO light, but any high PAR/full spectrum light (generally 6500K) T2, T5, LED, CFL, etc.
See these references:
*New Science Sheds Light on Immune Deficiencies
*Light as a Nutrient
5. Useful Light Energy:
This is a simplistic way to explain light energy that a light/lamp produces that is of little or no use to aquarium life; in particular freshwater plants or marine corals that have high light requirements.
The best way I can explain this is to think about how mixing all paint colors will produce black, while the mixing of all light energy produces white. We as humans may notice this to some degree, however we do not have the ability to pick out particular colors such as a honey bee can.
The ability of newer technology lights to pin point the exact nanometer spectrums in output results in much less wasted energy and allows for a light of considerably lower wattage to actually out produce another bulb of higher wattage that wastes much of its energy in light bands that are not useful for life processes (this is major reason the "watts per gallon rule" is so useless when applied to modern technology, yet it is still thrown around by many).
Even the best of fluorescent lights that are a Kelvin temperature of 6500 K use a percentage of their light energy in the yellow and green light spectrum which is mostly useless for aquarium plants or corals (LEDs & modern T2s/ T5s do not suffer as much from this problem from what I have observed using this lens).
This is where although a 6000-8000K light generally will provide good PAR often there is also more yellow/green light. When used in water applications (especially as depths increase), one aspect of a higher Kelvin (10,000- 14000K) daylight lamp is more efficient penetration (although Kelvin temperatures much over 14000K loose much of the important 700nm "spike" & should be avoided as the only light source)
The picture above demonstrates this with two 15 Watt CFL (30 watts total) vs. one 12 Watt Marine White LED (all white 14,000K emitters). This picture is taken with a camera that filters out certain wave lengths allowing for a better viewing of the difference which is otherwise not easy to discern, however the picture shows how the LED on the left has less of this wasted yellow and green than the CFL lights on the right.
Otherwise the light output appears the same, although this is still important when you consider that this is achieved with only 12 watts of LED vs. 30 watts of Compact Fluorescent lights.
With some LED Lights, new technology LED emitters can be selected for the exact wavelength of light, thus almost no useless yellow or green light is emitted, so although the LED may seem less bright than some HO lights with the naked eye (such as T5s or MH) the actual output of light energy in spectrums we cannot see is much higher. This is why gauging a light by what you see is highly inaccurate.
The Picture to the left shows the light spectrum as seen through a special 3D lens which breaks apart the light spectrum. This provides a dramatic example of how much of the light energy the 4000K CFL is in the useless yellow spectrum (the 4000K is not a terrible bulb either, as many still use this for planted aquariums). The TMC Reef White shows a much more complete light spectrum and much less wasted energy in the useless light spectrums.
The graph to the below shows 'A' the main PAR curve while 'B' is what we see with our own eyes. The reason for displaying this is that many lights commonly marketed appeal to CRI, which does not mean optimum PAR, as well most fluorescent aquarium/plant/reef bulbs still have much of their energy in this basically more useless band of light energy in the middle. This includes most fluorescent bulbs, although the newer T2 and some T5s have less waster energy in this yellow/green band width than others.
One final explanation to hopefully convey the importance of the concept of "Useful Light Energy" is with two light comparisons;
*I have seen a dozen "hardware store" cool white fluorescent lights provide adequate lighting for a planted aquarium and even basic reefs, however it would only take a few correct kelvin (for the application) T5 lights to accomplish this same task. The point is that even a T12 cool white provides some useful light energy, but it takes copious amounts of these lights to work correctly (in other word many more watts, which again speaks against the "watts per Gallon rule")
*In another comparison, one can use one of the many cheap LED panels that use 90-120 watts or more to light a reef or planted aquarium, or you can use a LED Fixture that utilizes the best technology emitters available such as only found in many TMC AquaRay LED Lights. This is similar to the above example as lower end emitters do not pin point the important light wave lengths the best patented emitters can (think about lighting your aquarium with 100s LED flashlights, would you do this?).
The point of both of these comparisons is you simply cannot compare apples to oranges when it comes a high wattage of T12 cool white lamps versus targeted T5 (or T2) as well as the "cheapie" LED panels with high wattage versus high end LED such as the AquaRay.
6. LUMENS:
The international unit of luminous flux or quantity of light used as a measure of the total amount of visible light emitted. The higher the lumens, the "brighter" or more "intense" the light looks to the human eye. You can figure lumens per watt by dividing the lumens your lamp lists by the wattage the fixture lists.
Knowing your lumens per watt is often as or more important than watts per gallon. For example a T12 light that is rated at 20 watts with a total lumen output of 800 lumens has a lumen per watt output of 40. While a 13 watt T2 bulb rated at 950 lumens has a lumen output of 73 lumens per watt. This is a clear example that the watts per gallon rule is severely flawed as the 13 watt T2 (or two of these) is clearly the better choice for a 15 gallon planted aquarium (or reef) and this does not even take into consideration the PAR rating which is also important for plants/corals or lumens per length of bulb (space). This lumen comparison also applies to SHO, VHO, and Metal Halide all of which far out produce most T12 lamps in lumens per watt.
Focused Lumens;
It is also noteworthy that even the lumen output can be deceiving when considering aquarium lights; LED are a good example of this as these newer technology lights have extremely focused light energy with little essential light energy lost (such as by Restrike), unlike almost every other type of aquarium light currently available. With this focused energy a LED often requires half the lumens (or often even less) to provide essential light energy (such as PAR) to plants, corals, etc. The newer generation LED lights have considerable less loss of lumens at 20 inches than a CFL light (as per tests that show 166% more lumens for the same wattage LED as compared to a common CFL of equal wattage). As another example, think lasers, although not nearly as focused as a laser, modern LED emitters (such as the Aqua Ray) are much more focused than other types of commonly used aquarium lights.
Caution as to using Lumens as a useful measurement of Light Output:
While lumens are a important useful measurement for standard household light bulb comparison, it is only a part of the equation for aquarium use, especially when this measurement is applied to new technology lights employed by aquarium keepers (such as LEDs).
As an example of just one aspect where the lumen measurement falls short is when Kelvin is considered; a bulb emitting 1000 lumens at a color temperature of 20,000K will not emit as much PAR as a bulb emitting 1000 lumens at 6500K.
7. WATTS:
Watts equal one joule of energy per second. For us, it's a measurement of how much energy our light fixture is using NOT of light output! This why the 2-3 watts per gallon for FW plants (3-5) for reef can be deceiving, and this rule is only a starting point similar to the 1 inch of fish per gallon "rule". This archaic rule was more accurate when all that was used were T12 lamps which is what this rules is based on.
Keeping this in mind the average T12 has a lumens per watt rating of 40, which means you would need half as many watts of a bulb that produces 80 lumens per watt (assuming PAR, Kelvin and other aspects are equal)
The term "watts per gallon" is getting more archaic as newer T-2, T-5, compact Fluorescents, the SHO, and especially the new reef compatible LED lights have more watts spread over less distance. Keeping this in mind; 'watts', when applied to a standard fluorescent tube are spread over longer bulbs as the wattage increases. For instance a standard 30 watt T 8 bulb is 36" while a standard 20 watt T-8 bulb is 24". For high light requirements such as plants or reefs, at least 1 inch per watt is required when comparing tube style fluorescents bulbs.
Many high output light such as the Metal Halide or the more economical SHO PC bulbs use a lot of watts in a small amount of space. The 110 watt SHO bulb uses 110 watts in 10" or even less if mounted in a pendant.
Another aspect of watts is the output of lumens per watts actually used. The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).
8. CRI:
To help indicate how colors will appear under different light sources, a system was devised some years ago that mathematically compares how a light source shifts the location of eight specified pastel colors on a version of the C.I.E. color space as compared to the same colors lighted by a reference source of the same Color Temperature. If there is no change in appearance, the source in question is given a CRI of 100 by definition. From 2000K to 5000K, the reference source is the Black Body Radiator and above 5000K, it is an agreed upon form of daylight.
A CRI of 100 has a heavy red spectrum. The color temperature is 2700 K for incandescent light and 3000 K for halogen light. An incandescent lamp, virtually by definition, has a Color Rendering Index (CRI) close to 100. This does not mean that an incandescent lamp is a perfect color rendering light source. It is not. It is very weak in blue, as anyone who has tried to sort out navy blues, royal blues and black under low levels of incandescent lighting. On the other hand, outdoor north sky daylight at 7500K is weak in red, so it isn't a "perfect" color rendering source either. Yet, it also has a CRI of 100 by definition.
CRI is useful in specifying color if it is used within its limitations. Originally, CRI was developed to compare continuous spectrum sources whose CRI's were above 90 because below 90 it is possible to have two sources with the same CRI, but which render color very differently. At the same time, the colors lighted by sources whose CRI's differ by 5 points or more may look the same. Colors viewed under sources with line spectra such as mercury, GE Multi-Vapor metal halide or Lucalox high pressure sodium lamps, may actually look better than their CRI would indicate. However, some exotic fluorescent lamp colors may have very high CRI's, while substantially distorting some particular object color.
Technically, CRI's can only be compared for sources that have the same Color Temperatures. However, as a general rule "The Higher The Better"; light sources with high (80-100) CRI's tend to make people and things look better than light sources with lower CRI's.
Why use CRI if it has so many drawbacks? It's the only internationally agreed upon color rendering system provides some guidance. It will be used until the scientific community can develop a better system to describe what we really see. It is an indicator of the relative color rendering ability of a source and should only be used as such (Source: Color Rendering)
To be blunt, CRI is not a parameter that is important in determining the best aquarium light, but it is included here since many mistakenly tend to consider it an important parameter.
To See Entire Article Go To - http://www.americanaquariumproducts.com/Aquarium_Lighting.html
Although the classic interpretation of what Kelvin is in not what is meant by light Kelvin ratings, I believe it should be considered.
Here is a brief description of Kelvin:
Kelvin is defined by two points: absolute zero, and the triple point of pure water. Absolute zero is defined as being precisely 0 K and -273.15 C. Absolute zero is where all kinetic energy (motion) in the particles comprising matter ceases, and they are at complete rest. At absolute zero there is NO heat energy. The triple point of water is defined as being precisely 273.16 K and 0.01 C. Here are a few Kelvin
numbers:
*Absolute zero = 0K (-273.15 C)
*Waters freezing point = 273.15 K (0 C)
*Waters boiling point = 373.1339K (100 C)
What the true definition of Kelvin is that is a unit of measure of temperature on the thermodynamic (absolute) temperature scale.
Light Kelvins
Kelvin is used in the lighting industry to define the Color temperature of a bulb. Higher color temperature lamps above 5500 K are "cool" (green-blue) colors, and lower color temperature lamps below 3000 K are "warm" (yellow-red) colors.
Kelvins as applied to color temperature of lights/lamps are derived from the actual temperature of a black body radiator, which is the concept of color temperature based on the relationship between the temperature and radiation emitted by a theoretical standardized material and termed a "black body radiator". This is where the "classic" definition of Kelvin and that used for lights come together, as hypothetically, at cessation of all molecular motion (the black body state of this hypothetical radiator), the temperature is described as being at absolute zero or 0 Kelvin, which is equal to -273 degrees Celsius.
An incandescent filament is very dark, and approaches being a black body radiator, so the actual temperature of an incandescent filament is somewhat close to its color temperature in Kelvins.
Incandescent lamps tend to have a color temperature around 3200 K, but this is true only if they are operating with full voltage. When a lamp is dimmed below its full potential, its filament is not as hot, and it produces less light. The reduced temperature of the filament also reduces the color temperature downward. An incandescent light dimmed to 10% is considerably more red in color than one at 100%.
Another consideration as to the color temperature as applied to lights; color temperature does not take into consideration the spectral distribution of a visible light source. In cases where a light source, such as a fluorescent lamp, arc-discharge burner, laser, or gas lamp, does not have a spectral distribution similar to that of a black body radiator.
A few notes about Kelvin:
* Plant chlorophyll absorbs light at wavelengths of 300 to 700 nm (a Kelvin rating of about 6400 strikes a good balance here, which is why this is the best Kelvin temperature for freshwater plants and symbiotic zooxanthellae in corals).
* The lower the "K", the more yellow, then red the light appears, such as a 4500 K bulb.
* The higher the "K", the bluer the light appears, such as a 20, 000 K bulb.
*Higher Kelvin Color Temperature lights penetrate water more deeply, even more so in saltwater, however there is less of the infrared "PAR spike" as well. * The human eye sees mostly sees light around 5500K.
* Candle flame = 1850 - 1900 K
* Sunlight (1 hour after dawn) = 3500 K
* Typical summer light (sun + sky) = 6500 K
* Cool white fluorescent = 3400 K
What Kelvin rating for Plants & Corals;
Here are some observations made by me and others in the professional aquarium maintenance community, some of these are simple observations, while others were based on more controlled tests.
*The 6500 Kevin bulbs have produced the best freshwater plant growth (as this Kelvin lamp generally has more of the infrared nm spike need by "higher" plants, but still has some of the 425-500 nm blue).
This Kelvin Lamp can also work with SPS, LPS placed high in the tank water column (nearest the lights) based on the symbiotic zooxanthellae needs found in these corals. For more depth penetration (& to aid in the first blue spike in PAR, please see PAR section), blue actinic, 50,000 K/Actinic or adjustable/multiple LED can be added (such as AquaBeam Marine Blue) can balance out 6500K lamp if used in marine reef tanks.
Please note that saltwater absorbs slightly more light energy than freshwater due to the higher density of the water, so 6500 will not penetrate as deeply.
*The 10,000 Kelvin bulbs also achieve good growth rates, although slower than the 6500 K bulbs in shallow aquariums. 10000 K bulbs have produced excellent growth with soft corals and LPS, although slower paced SPS growth.
The 10,000K can be a good choice for achieving PAR for better depth penetration than a 6500K bulb (such as 20 inch or deeper aquarium)
*The 14,000K light/bulb (often popular with Metal Halide and LEDs), will penetrate even more than the 10,000K light while still providing useful PAR (this would be the highest "Daylight" Kelvin temperature I would recommend and still expect good growth in corals, see PAR section).
*The 20,000 K bulb is more blue yet and brings out all of the fluorescent pigments in many corals (making for a very nice appearance). However the best tests and observations show that when used alone the growth rate of SPS corals come to a complete standstill with 20,000 K lamps. Although maybe a good supplement for appearance, these bulbs are over rated for use as the only reef Kelvin temperature and should be avoided when used as the only Kelvin temperature lights
*The 50,000K is generally the Kelvin rating of an actinic blue light source which is beneficial for the first "spike" in PAR. This temperature light (as with 20,000K) is best used with other light Kelvin Temperatures and is a better choice than the 20,000K light for such combinations. The 50,000K is a good compliment to the 6500, 10,000, 14,000 Kelvin lights, especially for zooanthellic algae necessary for stony corals, clams, nudibranch, anemones, and other sessile species.
Kelvin summary
The Kelvin rating is another area of comparing apples to apples in lights, not just watts. Although the above is a simplified explanation of Kelvin as it applies to lights; as such you cannot compare a 6400 K T8 light to a 6400 MH light (the MH is going to have much more output as you will read later).
However (using a CFL as an example, this can apply to any light type), a 6400 CFL will have a higher energy output than a 3500 CFL, this is why an incandescent filament is very low in Kelvin (dark) as derived from the actual temperature as it is just above a black body radiator. Another example using an incandescent light, this time looking at it from the aspect of watts; a 100 watt incandescent has a Kelvin of 2870 while a 40 watt incandescent has a Kelvin of 2500. So as you can see, there is some correlation between energy output and Kelvin Color.
This does not mean that a certain Kelvin bulb is necessarily "better" as factors such as "lumens per watt", watts, focused lumens and more MUST be considered as well.
References:
Color Temperature In A Virtual Radiator - This is an interesting resource with a virtual radiator worth checking out.
http://www.uky.edu/~jholl2/technology_pdfs/KelvinColorTemperature.pdf
http://www.sizes.com/units/color_temperature.htm
2. THE NANOMETER RANGE (SPECTRUM)
A nanometer scale is used to measure the wave length of light energy from cosmic rays to radio waves. An actinic bulb will have a Nanometer spike at about 420N, a UVC bulb about 265N, and a daylight bulb about 700N. The difference in the wavelength determines how the wave affects its surroundings. It is this wavelength difference that allows short-wave x-ray to pass through walls, while longer-wave visible light cannot pass though the same material; short-wave ultraviolet and x-ray can destroy DNA in living microorganisms and breakdown organic material while visible light will not. All light energy is measured on a "nanometer" (nm) scale. Nanometer means one-billionth of a meter.
This applies to aquariums when we consider the light spectrum and how it applies to our aquariums individual needs: Red light is the first to be filtered out and can only penetrate a short distance. As light waves penetrate deeper into the water, orange and yellow are lost next. Of all the colors of the spectrum blue light penetrates the deepest. Corals need intense equatorial UVA (actinic). Most higher plants need a balanced PAR light range (see section about PAR).
The Nanometer scale and Kelvin temperatures come together when applied to aquarium lighting this way; Natural sunlight on a clear day registers at 5500- 6500 Kelvin degrees. Kelvin temperatures less than 5500K become more red and yellow and the higher the Kelvin temperature the more blue the light is.
Most photosynthetic invertebrates should be kept with lamps of a daylight Kelvin temperature from 6400-14,000 K (higher Kelvin with deeper specimen placement, not necessarily tank depth). 20,000K daylight lamps can also be used for deeper tanks, however these often have poor PAR and it is better to use a 14,000K Daylight or less supplemented with more blue.
Photosynthetic invertebrates (many corals, anemones, clams, nudibranch, etc.) also need more blue (400-550nm) than "higher" plants, with 465-485 the optimum blue. Not only is blue/actinic lighting beneficial to photosynthetic invertebrates, it is also aesthetically pleasing to the eye when used to supplement "daylight" lighting.
Freshwater aquarium plants benefit from lighting with a Kelvin temperature in the range of approximately 6500 degrees. Freshwater plants prefer light with more red in the spectrum (see PAR Section).
It is also noteworthy than Fluorescent and even more so incandescent lights produce a lot of yellow and green nanometer light, which research indicates is mostly wasted energy in terms of the needs or freshwater plants and SPS Corals. This is where an LED Aquarium Light, Metal Halide, or even (to a lesser degree) T2 Lights excels as there is much less wasted yellow/green light.
3. LUX:
A measure of the intensity of light (referred to the photometry of light), one lux is equal to one lumen per square meter. This is important for aquarium plants. Once again this is another area of comparing apples to apples in lights, not just watts.
This is also VERY important to most corals in marine reef aquariums. When the Lux (intensity) is not enough the zooxanthellae (algae that are inside of corals tissues) do not create plentiful oxygen. The minimum light intensity should be no less than 3,000-lux when it reaches the deepest part of the aquarium. You can over light your coral to a light saturation point (quite hard in my experience, but this should be noted), maximum Lux should be no more than 100,000 to 120,000.
By comparison Lux in tropical reefs has been measured to be between 110,000 and 120,000 Lux at the surface of the reef and 20,000-25,000 Lux one meter below the surface.
4. PAR:
PAR is probably one of the most important considerations along with the related Useful Light Energy, Lumens per Watt, Focused Lumens and Watts per Gallon when choosing a light for your aquarium, yet is often over looked by both marine and freshwater pant keeping aquarists.
PAR is the abbreviation for Photosynthetically Active Radiation which is the spectral range of solar light from 400 to 700 nanometers that is needed by plants & symbiotic zooanthellic algae (Zooxanthellae are single-celled plants that live in the tissues of animals such as corals, clams, anemones, & nudibranchs) for photosynthesis. This is found from actinic UVA to infrared. UVA is 400-550nm (of which 465-485 has the highest PAR of the actinic range) which is the absorption bandwidth of chlorophylls a, c2, and peridinin (the light-harvesting carotenoid, a pigment related to chlorophyll). Infrared is 620-700nm which is the red absorption bandwidth of chlorophylls a and c2.
Photons at shorter wavelengths (Ultraviolet -C or UVC) tend to be so energetic that they can be damaging to cells and tissues; fortunately they are mostly filtered out by the ozone layer in the stratosphere. Green light occupies the middle spectrum (550-620nm; what is mostly visible to us) and is partly why chlorophyll is green due to the reflective properties.
Bulbs that emit mostly actinic light will have a lower PAR (although actinic UVA still occupies an spike in PAR as seen from the graph and improve the PAR of your lighting), bulbs that occupy mostly the middle spectrum (yellow-green) such as "warm White (2700K) will produce little necessary PAR, while bulbs that produce mostly infrared (as seen from the graph) will produce more important PAR light energy, however it is the balance of infrared and UVA that will generally provide your best PAR output.
PUR (Photosynthetically Usable Radiation) should also be considered. PUR is that fraction of PAR that is absorbed by zooxanthellae photopigments thereby stimulating photosynthesis. As noted above, PUR are those wavelengths falling between 400-550nm and 620-700nm.
It is noteworthy that whereas higher plants (generally kept in planted freshwater aquariums) require more of the infrared aspect of the PAR Spectrum while the zooxanthellae found inside many sensitive corals require more of the Blue spike (465-485 in particular). For this reason either/both higher Kelvin Daylight and actinic/blue is required for many reef aquariums.
Important Definitions as it applies PAR in plants and zooanthellic algae: See the graph below as it corresponds to each of these definitions.
*A: Phototropic response; having a tendency to move in response to light. Basically this is the Chlorophyll containing plant or algae "moving" to respond to a positive light source to begin the process of photosynthesis (initial growth of plants, zooxanthellae, etc.).
*B: Photosynthetic response; the process which begins when energy from light is absorbed by proteins called photosynthetic reaction centers that contain chlorophylls.
*C: Chlorophyll synthesis is the chemical reactions and pathways by the plant hormone cytokinin soon after exposure to the correct Nanometers wave length (about 670 NM) of light resulting in the formation of chlorophyll, resulting in continued growth of a plant, algae, zooxanthellae and the ability to "feed" and propagate, and without this aspect PAR (670 NM light energy), zooxanthellae and plants cannot properly "feed" propagate. The results of the lack of this high PAR "spike" would be stunted freshwater plant growth, and eventually poor coral health in reef tanks
Further PAR Information; As the reader here can see, there are three main spikes in the PAR spectrum, with all three being important, however the most important spike occur at the red side and all three are generally incorporated more or less in a daylight bulb of approximately 6500K, not just a bulb of only infrared or as is mistakenly believed by many reef keepers, only in the UVA/actinic spectrum (although a daylight bulb of 10,000-14,000K can provide this too for deeper "more dense" tanks as these higher Kelvin bulbs generally penetrate deeper than a 6500K, albeit with slightly less from the 670 NM spike).
This can vary with bulb type though as not all daylight bulbs are the same, see the Useful Light Energy section.
Unfortunately, despite anecdotal claims passed around within the aquarium hobby; in better funded tests outside the aquarium industry show that many stony corals, clams, and other sessile species that depend on photosynthesis of zooanthellic algae not only thrive but also propagate with light that achieves the optimum PAR, which includes daylight from 6500k to 14000k (Higher Kelvin outputs are required for tanks deeper than 24-30 inches to achieve maximum PAR such as a 14,000K, although high intensity 6400 SHO Lamps can generally penetrate deep freshwater tanks well and perform well in marine reef tanks when balanced with actinic/50,000K lights necessary for the higher need of 465-485nm light energy by symbiotic zooanthellic algae).
*It is also noteworthy that most green algae need more of the actinic spike than "higher plants", hence the popularity of actinic lights for reef aquariums, however the optimum nanometer range is about 465-485nm, not the lower 420nm many actinic lights produce or the more broad range many "blue" aquarium lights produce of 400-520 nm (this is where the latest technology LED lights "shine", having a more precise 465-485nm blue). For this reason it is a good idea to have extra actinic for corals/clams that depend upon zooanthellic algae, while at the same time limiting blue/actinic in freshwater aquariums to avoid excessive green algae growth.
More recent studies show that excessive low UVA (under 420nm) and especially UVB radiation can actually bleach coral and in an aquarium environment lights with more UVA or UVB are not generally necessary as although a lamp with more "blue" such as a 20,000 K MH may penetrate more deeply (due to the fact that infrared light gets absorbed the deeper the light must penetrate), this should not be the sole Kelvin color light for most aquariums (better would be to compliment a 6400 through 14000K bulb with a 50,000K actinic bulb)
Measuring PAR
Although Kelvins (as well as LUX conversions using questionable LUX to PAR conversion factors) are ways of getting rough estimates of PAR, only a Specific PAR Meter (also called Quantum Light Meters) can give you're the best measurement of this very important aspect of determining your tanks lighting requirements (both at the surface and under the surface)
(Here is a link to a PAR Meter Apogee MQ-200 PAR Meter)
Currently accepted numbers measured as uMol*m2*sec (also referred to as micro mols or mmol) are 50 mmol for most plants or low light corals such as Nemezophyllia, while Acropra can require PAR outputs as high as 300 mmol (any higher is simply a waste of energy/light)
Further PAR Info
Some organisms, such as Cyanobacteria, purple bacteria and Heliobacteria, can utilize light in regions such as the low infrared. These bacteria make use of the unusable light discarded by the plant kingdom, in this case, light outside the PAR range required by plants, which is why Cyanobacteria thrive in lighting conditions that include the more yellow 4000 K and below and why actinic (50,000K) as well as balanced light in the 6400 to 14,000K range combined with passing water through an ultra violet sterilizer (to kill free floating Cyanobacteria) is important for control.
In the case of Red Slime Cyanobacteria, these Cyanobacteria do not use the PAR spikes at 435nm and 675nm and instead utilize more of the middle yellow and green light spectrum that is most common fluorescent (even so-called aquarium fluorescent lights) and incandescent lighting.
Please do not confuse the term PAR (Photosynthetically Active Radiation) used in plant growth as discussed here with another use of the same term in lighting which is Parabolic Aluminized Reflector. This type of light is used for stage lighting and should not be purchased for marine, freshwater, or Greenhouse use under the mistaken belief that these lights are "great" for plant/coral growth.
For further reading (references) about PAR:
*Light and Plants
*Photosynthetically Active Radiation
*http://www.aquariumpros.com/articles%20PDF/lamptypes.pdf
Fish Health:
Many recent studies have shown the importance of full spectrum lighting (which will generally encompass a high PAR value) as it relates to health in humans, animals and can be extrapolated to fish as well for a disease prevention which is why good lighting should not be restricted to Reef Marine or Planted Freshwater Aquariums, but to fish only salt or freshwater tanks as well.
In fact the medical community is now utilizing 6400K SHO bulbs (& similar full spectrum lights) due to increasing studies that show better immune function, mental health, and more. Animal studies support similar results as well. This need not be a SHO light, but any high PAR/full spectrum light (generally 6500K) T2, T5, LED, CFL, etc.
See these references:
*New Science Sheds Light on Immune Deficiencies
*Light as a Nutrient
5. Useful Light Energy:
This is a simplistic way to explain light energy that a light/lamp produces that is of little or no use to aquarium life; in particular freshwater plants or marine corals that have high light requirements.
The best way I can explain this is to think about how mixing all paint colors will produce black, while the mixing of all light energy produces white. We as humans may notice this to some degree, however we do not have the ability to pick out particular colors such as a honey bee can.
The ability of newer technology lights to pin point the exact nanometer spectrums in output results in much less wasted energy and allows for a light of considerably lower wattage to actually out produce another bulb of higher wattage that wastes much of its energy in light bands that are not useful for life processes (this is major reason the "watts per gallon rule" is so useless when applied to modern technology, yet it is still thrown around by many).
Even the best of fluorescent lights that are a Kelvin temperature of 6500 K use a percentage of their light energy in the yellow and green light spectrum which is mostly useless for aquarium plants or corals (LEDs & modern T2s/ T5s do not suffer as much from this problem from what I have observed using this lens).
This is where although a 6000-8000K light generally will provide good PAR often there is also more yellow/green light. When used in water applications (especially as depths increase), one aspect of a higher Kelvin (10,000- 14000K) daylight lamp is more efficient penetration (although Kelvin temperatures much over 14000K loose much of the important 700nm "spike" & should be avoided as the only light source)
The picture above demonstrates this with two 15 Watt CFL (30 watts total) vs. one 12 Watt Marine White LED (all white 14,000K emitters). This picture is taken with a camera that filters out certain wave lengths allowing for a better viewing of the difference which is otherwise not easy to discern, however the picture shows how the LED on the left has less of this wasted yellow and green than the CFL lights on the right.
Otherwise the light output appears the same, although this is still important when you consider that this is achieved with only 12 watts of LED vs. 30 watts of Compact Fluorescent lights.
With some LED Lights, new technology LED emitters can be selected for the exact wavelength of light, thus almost no useless yellow or green light is emitted, so although the LED may seem less bright than some HO lights with the naked eye (such as T5s or MH) the actual output of light energy in spectrums we cannot see is much higher. This is why gauging a light by what you see is highly inaccurate.
The Picture to the left shows the light spectrum as seen through a special 3D lens which breaks apart the light spectrum. This provides a dramatic example of how much of the light energy the 4000K CFL is in the useless yellow spectrum (the 4000K is not a terrible bulb either, as many still use this for planted aquariums). The TMC Reef White shows a much more complete light spectrum and much less wasted energy in the useless light spectrums.
The graph to the below shows 'A' the main PAR curve while 'B' is what we see with our own eyes. The reason for displaying this is that many lights commonly marketed appeal to CRI, which does not mean optimum PAR, as well most fluorescent aquarium/plant/reef bulbs still have much of their energy in this basically more useless band of light energy in the middle. This includes most fluorescent bulbs, although the newer T2 and some T5s have less waster energy in this yellow/green band width than others.
One final explanation to hopefully convey the importance of the concept of "Useful Light Energy" is with two light comparisons;
*I have seen a dozen "hardware store" cool white fluorescent lights provide adequate lighting for a planted aquarium and even basic reefs, however it would only take a few correct kelvin (for the application) T5 lights to accomplish this same task. The point is that even a T12 cool white provides some useful light energy, but it takes copious amounts of these lights to work correctly (in other word many more watts, which again speaks against the "watts per Gallon rule")
*In another comparison, one can use one of the many cheap LED panels that use 90-120 watts or more to light a reef or planted aquarium, or you can use a LED Fixture that utilizes the best technology emitters available such as only found in many TMC AquaRay LED Lights. This is similar to the above example as lower end emitters do not pin point the important light wave lengths the best patented emitters can (think about lighting your aquarium with 100s LED flashlights, would you do this?).
The point of both of these comparisons is you simply cannot compare apples to oranges when it comes a high wattage of T12 cool white lamps versus targeted T5 (or T2) as well as the "cheapie" LED panels with high wattage versus high end LED such as the AquaRay.
6. LUMENS:
The international unit of luminous flux or quantity of light used as a measure of the total amount of visible light emitted. The higher the lumens, the "brighter" or more "intense" the light looks to the human eye. You can figure lumens per watt by dividing the lumens your lamp lists by the wattage the fixture lists.
Knowing your lumens per watt is often as or more important than watts per gallon. For example a T12 light that is rated at 20 watts with a total lumen output of 800 lumens has a lumen per watt output of 40. While a 13 watt T2 bulb rated at 950 lumens has a lumen output of 73 lumens per watt. This is a clear example that the watts per gallon rule is severely flawed as the 13 watt T2 (or two of these) is clearly the better choice for a 15 gallon planted aquarium (or reef) and this does not even take into consideration the PAR rating which is also important for plants/corals or lumens per length of bulb (space). This lumen comparison also applies to SHO, VHO, and Metal Halide all of which far out produce most T12 lamps in lumens per watt.
Focused Lumens;
It is also noteworthy that even the lumen output can be deceiving when considering aquarium lights; LED are a good example of this as these newer technology lights have extremely focused light energy with little essential light energy lost (such as by Restrike), unlike almost every other type of aquarium light currently available. With this focused energy a LED often requires half the lumens (or often even less) to provide essential light energy (such as PAR) to plants, corals, etc. The newer generation LED lights have considerable less loss of lumens at 20 inches than a CFL light (as per tests that show 166% more lumens for the same wattage LED as compared to a common CFL of equal wattage). As another example, think lasers, although not nearly as focused as a laser, modern LED emitters (such as the Aqua Ray) are much more focused than other types of commonly used aquarium lights.
Caution as to using Lumens as a useful measurement of Light Output:
While lumens are a important useful measurement for standard household light bulb comparison, it is only a part of the equation for aquarium use, especially when this measurement is applied to new technology lights employed by aquarium keepers (such as LEDs).
As an example of just one aspect where the lumen measurement falls short is when Kelvin is considered; a bulb emitting 1000 lumens at a color temperature of 20,000K will not emit as much PAR as a bulb emitting 1000 lumens at 6500K.
7. WATTS:
Watts equal one joule of energy per second. For us, it's a measurement of how much energy our light fixture is using NOT of light output! This why the 2-3 watts per gallon for FW plants (3-5) for reef can be deceiving, and this rule is only a starting point similar to the 1 inch of fish per gallon "rule". This archaic rule was more accurate when all that was used were T12 lamps which is what this rules is based on.
Keeping this in mind the average T12 has a lumens per watt rating of 40, which means you would need half as many watts of a bulb that produces 80 lumens per watt (assuming PAR, Kelvin and other aspects are equal)
The term "watts per gallon" is getting more archaic as newer T-2, T-5, compact Fluorescents, the SHO, and especially the new reef compatible LED lights have more watts spread over less distance. Keeping this in mind; 'watts', when applied to a standard fluorescent tube are spread over longer bulbs as the wattage increases. For instance a standard 30 watt T 8 bulb is 36" while a standard 20 watt T-8 bulb is 24". For high light requirements such as plants or reefs, at least 1 inch per watt is required when comparing tube style fluorescents bulbs.
Many high output light such as the Metal Halide or the more economical SHO PC bulbs use a lot of watts in a small amount of space. The 110 watt SHO bulb uses 110 watts in 10" or even less if mounted in a pendant.
Another aspect of watts is the output of lumens per watts actually used. The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).
8. CRI:
To help indicate how colors will appear under different light sources, a system was devised some years ago that mathematically compares how a light source shifts the location of eight specified pastel colors on a version of the C.I.E. color space as compared to the same colors lighted by a reference source of the same Color Temperature. If there is no change in appearance, the source in question is given a CRI of 100 by definition. From 2000K to 5000K, the reference source is the Black Body Radiator and above 5000K, it is an agreed upon form of daylight.
A CRI of 100 has a heavy red spectrum. The color temperature is 2700 K for incandescent light and 3000 K for halogen light. An incandescent lamp, virtually by definition, has a Color Rendering Index (CRI) close to 100. This does not mean that an incandescent lamp is a perfect color rendering light source. It is not. It is very weak in blue, as anyone who has tried to sort out navy blues, royal blues and black under low levels of incandescent lighting. On the other hand, outdoor north sky daylight at 7500K is weak in red, so it isn't a "perfect" color rendering source either. Yet, it also has a CRI of 100 by definition.
CRI is useful in specifying color if it is used within its limitations. Originally, CRI was developed to compare continuous spectrum sources whose CRI's were above 90 because below 90 it is possible to have two sources with the same CRI, but which render color very differently. At the same time, the colors lighted by sources whose CRI's differ by 5 points or more may look the same. Colors viewed under sources with line spectra such as mercury, GE Multi-Vapor metal halide or Lucalox high pressure sodium lamps, may actually look better than their CRI would indicate. However, some exotic fluorescent lamp colors may have very high CRI's, while substantially distorting some particular object color.
Technically, CRI's can only be compared for sources that have the same Color Temperatures. However, as a general rule "The Higher The Better"; light sources with high (80-100) CRI's tend to make people and things look better than light sources with lower CRI's.
Why use CRI if it has so many drawbacks? It's the only internationally agreed upon color rendering system provides some guidance. It will be used until the scientific community can develop a better system to describe what we really see. It is an indicator of the relative color rendering ability of a source and should only be used as such (Source: Color Rendering)
To be blunt, CRI is not a parameter that is important in determining the best aquarium light, but it is included here since many mistakenly tend to consider it an important parameter.
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