By Carl Strohmeyer
Updated 4/30/14
PUR stands for Photosynthetically Useable Radiation. It is also known as "useful light energy" and although this is not a scientific term, it is more easily applicable to aquarium light use.
PUR differs from PAR because the basic definition of PAR is any light in a specific frequency range of 400 to 700 nanometers that is needed by plants & symbiotic zooanthellic algae.
PUR is the usable portion of PAR, and different photosynthetic species will have a different PUR range to which they respond.
Further Reference:
Aquarium Lighting; PUR
Useful Light Energy/PUR has become the most important aspect of choosing an aquarium light since the advent of high end Aquarium LED Light Fixtures.
By high end I mean better emitter bins, advanced drivers/circuitry versus low many low end LED fixtures still available that essentially not much better that household LED emitters daisy chained together.
Since even the best of Fluorescent lights such as T5, T2, and SHOs still emit a considerable amount of light energy in light spectrums unusable by photosynthetic life, acquainting oneself with this aspect of light energy is very important if you are considering a new light fixture, especially a high end LED (such as the TMC AquaRay).
Recommended Product Resources:
*Aquarium LED Light Fixtures; AquaRay
*T2 Planted/Refugium Aquarium Lights, Lighting
*SHO Planted Aquarium and Refugium Lights, Lighting
PUR is also affected by water depths, with blue light around 450-480nm having the best useful depth penetration for our aquariums.This is an important note for many zooanthellic algae symbiotic dependent corals, clams, etc. in choosing the correct light as well as per specimen placement to achieve the best PUR light energy.
The picture to the above left shows Spectral Light Absorption in certain water depths.
What is noteworthy is that essential near red light energy becomes less viable not much under water surface, as per PAR requirements of Photosynthetic response and Chlorophyll synthesis of plants and green algae.
Further Reference: Aquarium Lighting, PAR, Photosynthetic response
Plants and zooanthellic algae have adapted to certain depths and is why a 6500K light generally works well (or best) for planted freshwater or refugium aquariums under 20-24 inches of water. While zooanthellic algae dependent corals require more 480nm blue and higher kelvin daylight energy such as 9000K, 10,000K, 14,000K, & 20,000K daylight lights as aquarium depths increase (such as 24+ inches of water depth).
With this information about actual sunlight penetration, we can safely make some accurate assumptions, even if relatively general.
As you can see the Photosynthetic response and Chlorophyll synthesis do NOT penetrate much past 25 and 50 meters of water respectively. So using Acropora Coral as an example, these have been found at varying depths mostly under 25 meters but generally thrive at 6 meters, producing an environment that is deficient in these Photosynthetic response and Chlorophyll synthesis light PAR spikes would result in less than optimal results.
Reference: http://www.nova.edu/ncri/11icrs/proceedings/files/m24-15.pdf
What the above means to the aquarium keeper is no artificial light; not LED or Metal Halide or any other is going to be equivalent to the energy of the sun (which in the tropics at noon is closer to 6500K at the surface of water).
So using a 20,000K fluorescent, Metal Halide 20,000K, or LED setup that is 2/3 blue is likely NOT going to produce optimum results in a tank of approximately 20 inches or less of depth.
With this lighting in a shallow tank you are more than likely producing environment that simulates many meters deeper than optimum, which still might be fine if this is the depth the specimens being kept are naturally adapted to as per specific wavelengths of light (nanometers)
Of course this begs the question of what the comparison would be and at this point I do not have a scientific equivalent (maybe there is one, but this would be difficult with the plethora of lighting choices and their subtle differences).The picture to the left clears up the confusion in my opinion (& experience too), since this displays an aquarium lit with 20,000K lighting and a natural Acropora reef.
It is obvious that the aquarium is much more blue than the natural reef.
However it is noteworthy since NO artificial light, the best LEDs or even Metal Halide (such as the top notch "Radion" 20K) can approach the sun's energy even in the color temperatures that make up CRI (what looks best to us). The result is that it often takes more "blue" appearing man made light energy to achieve the best possible PUR/PAR for your aquarium 'reef'. Often man-made lights, in particular many of the Johnny-come-lately LEDs will utilize lower kelvin white emitters and red emitters in an attempt to make a more human pleasing light, but as a result often waste more energy outside the essential PUR spikes needed by photosynthetic reef life.
Further Information: Aquarium Lighting; CRI
To the left is a picture of a 150 gallon reef aquarium that also has Acropora corals (for 6 months at the time of the picture).
PLEASE NOTE; this picture is much closer in color to the natural Acropora reef than the 20,000K light aquarium to the left in the picture above that is unfortunately more common with how many light their aquariums with Metal Halide or LED lights.
The picture to the left is courtesy of "Aquarium Lighting, Facts & Information"
The Reef tank above is running AquaRay/AquaBeam NP 1500 & 2000 tiles
What is also noteworthy is that PAR readings taken in 2011 on Bali Island at a coral farm, is that they kept corals for best growth about 1.5-2 meters under water surface. For best pigment intensification, even deeper; 3.5-4 meters.
Here is the point, the average PAR reading was 160 and never exceeded 200, not the higher reading often cited by many for their high intensity lights, especially Metal Halide.
WHY, quite obviously they is much more at play than just PAR and many of the best lights, including Metal Halides simply make up for lack of optimum PUR with higher PAR numbers than needed!!!
The previously noted comparison picture also makes the point that while at 6 meters, an Acropora Coral colony is not all that deep by ocean standards, but try getting your Metal Halide, T5, or LED light to penetrate 6 meters (over 18 feet)!!
This why we MUST pick a light that will closest meet these key photosynthetic response 'spikes', rather than pick a light that most looks like the sunlight underwater at a certain depth to our eyes.
This said, one LED emitter SPECIFICALLY designed for reef life, the Osram Olson NP Blue emitter is probably the closest to this same softer white/blue found naturally in reef containing Acropora coral.
What is also noteworthy and a popular trend is the use of "Near Ultraviolet" emitters (basically roughly between 300-400nm UVB- mostly UVA). This is intended to give a cool "pop" to many LPS and SPS corals, but what readers should note is that EVERY watt of energy used for these emitters is useless for your photosynthetic corals as it is outside the known PAR/PUR spectrums. This simply is a fad, that many LED manufacturers are following, including EcoTech and TMC AquaRay. At least with AquaRay they are coming out with this NUV emitter in a 600 Ultima fixture that is supplemental, NOT as part of a premier self contained reef light fixture as EcoTech has done. While this AquaRay NUV fixture is only currently available in the UK, it should NOT be included as part of your lighting wattage calculations, rather only purchased as a supplement.
Spectrograms:
Spectrograms are often used to determine PUR (Useful Light Energy), however I have found these are not fool proof either.
Although when it comes to many of the LED Aquarium Lights now flooding the market, Spectrograms are often very difficult to come by.
Part of the reason in my opinion and from knowing many in the industry is that the development of new LED emitters and drivers to run the emitters is at a fast pace, while the cost of producing a spectrograph for such a narrow band of users is simply too cost prohibitive (aquarium use of just the better LED emitter bins is but a needle in a haystack in the high end LED emitter industry). The other part of the reason is a bit more deceptive and that is many simply are VERY lacking in essential PUR!
However there are both good and bad LED fixtures with either new spectrographs available or outdated spectrographs. As well while spectrographs are useful, these too do NOT show the whole PUR story anymore than a PAR meter accurately depicts the type of light energy delivered (especially in the blues).
However taken together (PUR Spectrograph and PAR reading), a more complete picture can be extrapolated where the mmol (µMol•m²•sec) reading of a PAR Meter can be combined with the spectrograph of actual specific light energy wavelengths.If a spectrograph is equal, PAR comparisons can be more accurate too!!
Here are a couple of spectrographs we produced;
*The first to the left displays 12 different spectrograms for singular emitter types, (no mixed emitters).
The light energy spectrograph of the high output 4.5 volt DC LED flashlight is interesting, as it displays a reasonably good spectrograph. HOWEVER the output of actual energy (in wattage and PAR mmol) is obviously much lower than the 3 watt XR, XG, XT, ML, & XB Cree emitters as well as the cutting edge Osram Olson NP Blue emitter.
Similar can be said about the 3 watts total of TMC Flexi Red (spread over 18 inches) and similar LED Strips.
Which is why neither of these are nearly as much of a factor in your aquarium lighting scheme for higher light requiring planted or reef aquariums. This is not to say the Flexi-Red cannot add some "Useful Light Energy" (especially in the near-infrared) as a compliment to say a GroBeam or similar high output LED Fixture.
The picture above can be clicked on to enlarge
*The second is a mixed spectrograph from the TMC Reef White Ultra 1000 (7 14,000K and 3 465-485nm blue emitters)
Please pardon the quality of the ultra 1000 spectrograph
What I will state is that unfortunately many builders of lower PUR output LED fixtures will often only state PAR or at best only briefly mention PUR.As well many reef keepers still seem to be obsessed with PAR while ignoring the vastly more important PUR (useful light energy) aspect of aquarium lighting.
The facts are you can have a light with a higher PAR be a considerably lesser PUR and thus an INFERIOR light.
EXAMPLES:
*Exhibit ONE that measures a 200 PAR reading at 12 inches
*Exhibit TWO that measures a 120 PAR reading at 12 inches
However Exhibit ONE produces 50% of its energy in the useless green yellow spectrum and the other 50% in the photosynthetic active range while exhibit TWO produces 90% of its energy in the exact spikes of the photosynthetic active range.
So with the math, this brings down exhibit ONE to essentially 100 useful light energy 100 PAR while exhibit TWO is at 108 useful light energy PAR.
Obviously what appeared to be the inferior light is actually the superior light and likely is more energy efficient besides with less heat loss and no need for a cooling fan!!
As a more extreme analogy, which no Aquarium LED light employs, so I;m using it only to make a point:
*Exhibit ONE that measures a 120 PAR reading at 12 inches
*Exhibit TWO that measures a 200 PAR reading at 12 inches using ONLY green emitters
In this fictitious comparison, Exhibit TWO produces basically ZERO PUR, even though it has a higher PAR.
Hopefully this fictitious comparison drives home the point that one can have a high PAR, but lower (or even no) PUR.
For those who doubt how useless green light energy is, please reference this article:
http://www2.estrellamountain.edu/faculty/farabee/BIOBK/BioBookPS.html
From the above article:
"Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light EXCEPT green"
A good example is the EcoTech Radion (with off the shelf, non-patented, but still excellent Cree XT-E & XG-E emitters) and AI Sol, versus the AquaBeam Reef White NP 2000 Ultima, which does not have as high a PAR reading, but their PUR is higher per wattage used due to more specific patented emitter bins, PWM, and better drivers (constant current drivers versus RGB control features). It is also noteworthy that the AquaRay 2000 NP as well as the 1500 utilizes the unique Osram Olson NP Blue Emitter which actually is a FULL PUR/PAR Spectrum light, not just blue.
While the EcoTech Radio Pro produces 1100 µMol•m²•sec PAR (at 6" as per EcoTech's published information), the TMC NP 2000 produces 380 µMol•m²•sec (at 15" as per TMC's published information).
The input wattage is 170 watts for the EcoTech and 30 watts for the AquaRay, therefore is takes about (5) AquaRay 2000s to equal the wattage used of the EcoTech Pro, which then equals 1900 µMol•m²•sec PAR for the AquaRay. ABOUT DOUBLE (and likely more than double since the AquaRay published numbers are at more than twice the depth for PAR measurements)!!
The above example means that it takes DOUBLE the input energy for the EcoTech to equal the AquaRay, ASSUMING the spectrographs are equal, which in the case of the EcoTech Radion Pro it is quite similar but for wasted energy with the NUV emitters used, unlike the AquaRay 1500 & 2000. This can also be said of some of the lessor brands too, such as the Taotronic, Ocean Revive, Evergrow, etc.
Why the difference?
Simply less efficient emitters, a not as good a choice of emitter, and certainly a LOT of wasted energy as heat.
This is not to say the EcoTech Radion (or others) cannot keep Reef life, but these have more wasted energy as a percentage energy wattage input then the light energy output.
Please see the graph in this article below which clearly displays the difference in PUR between emitters:
Aquarium Lighting; PUR including Graph
Before I seem to pick on the EcoTech, let me perform a comparison of the EcoTech Radion Pro versus the Evergrow LED. assuming PAR and wattage used are the same, but use the VERY IMPORTANT PUR Spectrograph to determine light quality.
Using the Evergrow (AKA Ocean Revive) as a comparison, and since this company does not publish a spectrograph (for obvious reasons in my opinion), I will compare emitters used to the superior EcoTech.Since it is well established that the warm white emitters used are inferior as per the graph in the above cited reference, we can safely assume the Evergrow has a lessor spectrograph than the EcoTech.
So say both produced 500 µMol•m²•sec at 15", would both lights be equal?
The simple answer is NO!!
The Ecotech would be superior, as when if all parameters are equal, including PAR readings, the PUR is going to trump the PAR. This is why PAR reading should only be taken with a grain of salt and PUR should ALWAYS be considered in the mix since as it is the "photostynthetically USEFUL Radiation".
What a reef keeper who is considering one of the many LED lights now available should consider, especially the Chinese brands such as TaoTronics or Evergrow, as per the "scientific numbers", which again relates to PUR versus PAR is the emitters used. in terms of wattage draw and PUR output.
Keeping in mind that one can have a great PAR output, but a poor or even 0 PUR output, lets look at the Evergrow LED that utilizes 17.44 watts of energy output of the 120 watts for green and warm white emitters that produce nearly 0 PUR. As the green emitter might look nice, it in fact has 0 PUR and the warm white is very low in PUR.
Couple this with the fact that with the other less than optimal bridgelux emitters, use of current reduction technology; One requires a 120 watt LED fixture for the Evergrow, Taotronics, and many similar LEDs to even questionably have the same output as say an AquaRay Reef White 2000 Ultima at 30 watts of energy consumed so as to keep reef life.
One then has to ask why bother replacing your Metal Halides of 150 watts just to save 30 watts of energy, not to mention the high heat output that may still require chillers, and in the end the need for cooling fans for current reduction technology that has resulted in fan break downs and even fires????
Another aspect of PUR which is noteworthy is popular use of RGB features where by the user can control the color output of the lights.
The problem with this feature is when one alters the colors, you also alter the spectral output.
This may not affect PAR, but it MOST CERTAINLY affects PUR!!
These features should be avoided unless being used as a decorative light. I would not recommend these features for a serious reef or planted freshwater aquarium keeper.
Depth Penetration:
(Please click picture to enlarge)Depth Penetration is another consideration when choosing lighting. This is where the Metal Halide used to "control the market".
However modern LEDs have closed this gap considerably with lights such as the AI Sol Vega Blue, Kessil, and the TMC AquaRay Fiji Blue, Reef Blue, 2000 Reef White, and Ocean Blue NP Ultima, among others high end LEDs intended for reef aquariums.
The blue spectrums are more important to many light sensitive corals for the Phototropic response aspect of PAR, in part since these corals, clams, etc. live in environments where little higher spectrums of light reaches these corals.
In an aquarium this becomes more important in depths much over 20-24 inches, depending upon the light used including its raw energy.
Even here consider what is used.
In the picture above where a sheet of computer paper is used to block light energy, you can see how the Fiji Blue with its unlensed XT-E emitters has a more deeper blue (down to about 420nm) light which generally would penetrate better than a higher nanometer color light.
However the Reef Blue with its more focused ML-E royal blue emitters has more depth penetration as shown in this picture.
For this reason, while the Fiji Blue might seem like the better choice for tanks over 24 inches in depth, this picture very simply shows this to not be correct.
This is not to say the Fiji Blue or similar violet emitters used by a few LED fixture are a poor choice, only that using focused emitters can also make a difference.
In fact, lights such as the Violet/Fiji Blue LED are useful in providing specific wavelengths many corals might require, my point is simply do not purchase these for optimum depth penetration.
Also sometimes a mix of "blues" such as in the EcoTech Radion can make a difference in depth penetration.
The "Reef White 2000HD Ultima NP" or "Ocean Blue NP Ultima" also includes the Osram Olson new "NP blue emitters" in their mix.
This is also where the new Osram Olson NP (Nature Perfect) Blue emitter excels since these are the first emitters designed specifically for reef aquarium life.
Basically these are a FULL SPECTRUM "Blue" LED emitter (see the spectrograph below)
Product Resources:
*TMC Ocean Blue 1500 Reef Aquarium LED Light Fixture
*Reef White 2000HD Ultima NP (for deeper aquariums)
*AquaBeam 600 Ultima LEDs, including the Fiji Blue
The picture below further displays some of the differences in "blue" depth penetrating lighting:
Now using the AI Sol Vega Blue as an example, it uses uses (4) Cree XM-L Cool White LEDs, (8) Cree XP-E Royal Blue LEDs, (4) Cree XP-E Blue LEDs, and (4) OSRAM OSLON Standard Deep Blue LEDs.
This is a good depth penetration light if only by virtue of the many blue emitters used and proprietary 40 and 70 degree lenses, however it does fall short in that it uses older bin low depth penetrating cool white and the non-patented XP emitters are not intended as a depth penetrating emitter (unlike the XR-E and even newer ML-E Blue). As well this light uses RBG features and Current Reduction, rather than PWM which further lowers PUR and wastes energy as heat instead of light energy.
Reference: Aquarium Lighting; PWM
Further References/Resources
*Aquarium Lighting; Facts & Information
*PUR and Reef Aquarium Lighting: What is PUR?
*http://disc.sci.gsfc.nasa.gov/oceancolor/additional/science-focus/ocean-color/oceanblue.shtml
*http://www.reeftank123.com/lighting/lighting.html
*Physical Properties of Water
*Small Polyp Stony Coral and Light Wavelengths
By Carl Strohmeyer
Copyright 2013
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