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1: KELVIN RATING (such as 10,000K daylight bulb):
What is meant by Kelvin Temperature of Lights is not the classic interpretation of what Kelvin is, however I think is should be considered before I move on to our lighting definition of "Kelvin Temperature".

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 total absence of heat). Water freezes at 273.16 Kelvin, and water boils at 373.1339 or 100C.

The true definition of Kelvin is that it 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, 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 how it relates to light, come together. 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.
See the picture example below:

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 of 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, do 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 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 in shallow, perfect conditions).

• 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 red "PAR spikes" as well.

• The human eye 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 = 4200-5500 K

• Different nanometer wavelengths can be used to reach the same Kelvin Temperature; just as 4+5 & 1+8 both equal 9, so can different nanometer wavelengths be used for the same Kelvin Temperature. This is why often comparing one 6500K lamp to another can often be "apples to oranges" as for necessary useful light energy (PUR) needed by plants or corals
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. Please understand that these are still generalizations!
• The Warm White (3500K) & Cool White (4200-5500K) are typical lighting kelvin ratings used in home lighting and is common of lights sold in hardware stores, etc.
While these sources were used years back for both planted and even reef lighting out of necessity (as there were not many options available), these are poor choices due to little essential PUR energy and many more cool white and especially warm white lights are required to delivery this light energy for photosynthetic life.

Unfortunately many LED manufacturers such as Evergrow, Ocean Revive, Maxspect, & even EcoTech include these kelvin rated emitters in their lights to achieve Photosynthetic Life PUR via a shotgun approach and/or more human pleasing colors.
If you are looking for the highest output in useful light energy for watt of energy consumed, these kelvin color lights/emitters should be avoided!

• The 6500 Kevin lamps have produced the best terrestrial plant growth and generally the better freshwater plant growth (as this Kelvin lamp generally has more of the infrared nm spike needed 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, blue actinic can be added, such as AquaBeam Reef/Fiji Blue LEDs to 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 6500K lights will not penetrate as deeply and are not a good choice for depths over 12 inches.

• The 9000-10,000 Kelvin lamps also achieve good growth rates, although slower than the 6500 K bulbs in shallow aquariums. 9000 to 10000K 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 12-20 inch or even deeper aquarium).

• The 14,000K light/lamp, 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). An excellent daylight color for tanks between 15 and 30 inches in depth.

• The 20,000 K light/lamp is more blue yet, and brings out all of the fluorescent pigments in many corals making for a very nice appearance. However the many tests and observations show that when used alone, except in tanks over 24 inches, the growth rate of SPS corals can be slowed or even come to a standstill with 20,000 K lamps.

See Reference:
PUR vs PAR in Aquarium Lighting

Here is a quote as per their experience with Kelvin Temperature for a freshwater Planted aquarium:

"I had a 175, 6700K Metal Halide over my 22 gallon cube, and then switched it over to a 14K bulb I had when I used to do saltwater just to see the difference. I didn't like the blue appearance nor how it made the plants look odd. True, my neon tetras glowed more blue as did my beta, but the plants looked weird. I went back to the 6700K last night to make my final decision. Everything looked much better under that light. It is under 6700K that I got explosive growth when I added a good, CO2 reactor.
After I switched back to the 6700K, my plants were pearling away, and they had their true colors and natural look again! If I decide down the road to change to a saltwater fish only with live rock, I can still use the 6700K lights [or shallow reef aquarium]. People need to use the proper spectrum - they shouldn't use what pleases them as so many lights are now sold and presented in forums; they must use what pleases what you are trying to grow/raise be it fish, corals, or plants. You become the shepherd so tend your flock accordingly."
Gary Sanders

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 6500K T8 light to a 6500 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 6400K CFL will have a higher useful energy output than a 3500K CFL of the same wattage, 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.
This does not mean that a certain Kelvin bulb is necessarily "better" as factors such as "lumens per watt", watts of energy used, focused lumens, PAR, and especially PUR MUST be considered as well.

References:
http://micro.magnet.fsu.edu/primer/java/colortemperature/index.html Color Temperature in a Virtual Radiator- This is an interesting resource worth checking out.

Kelvin Color Temperature

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 420nm, a UVC bulb about 265nm, and a daylight bulb about multiple spikes from 400 to 700nm.
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) as well as other aspects of PAR. Most higher plants need a balanced PAR/PUR light range which includes the blue and two red spikes required for photosynthesis (see section about PAR & PUR).
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 marine 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 (over 22 inches) and/or supplementation with more blue lights (400nm- 490nm).

Photosynthetic invertebrates (many corals, anemones, clams, nudibranch, etc.) also need more blue (400-490nm) than "higher" plants especially as tanks increase in depth, with 465-485 recently being shown the optimum blue. Not only is blue/actinic lighting beneficial to photosynthetic invertebrates, it is also aesthetically pleasing to the eye and the 420 nm blue in particular brings out the colors of many corals/clams.
Osram Olson now has a "patent pending" LED emitter (the NP Blue) that is the first 'blue' emitter SPECIFICALLY designed for the full PAR spectrum required by marine photosynthetic invertebrates (see LED Section for more)

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 noteworthy that 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.

See the picture to the left that shows a T8 5500 daylight aquarium light that is commonly sold, this graph clearly shows the wasted green/yellow energy as well as the incorrect spike in orange rather than the correct PAR spike in red 630+ nm

It is also noteworthy that many "terrestrial plant lights" as well as many aquarium plant lights (often of lower in kelvin temperature) have more "red nanometer spikes" than higher kelvin 6500k, 10,000k & higher lamps.
The problem with these lights is that while all plants utilizing photosynthesis require the same essential ABCs of PAR (see the PAR section), the facts of light energy penetrating water requires higher kelvin (6500k +) be added to provide maximum PUR (see Useful light energy/PUR section). Aquatic Plants and corals have adapted/evolved to the natural light energy at certain depth of water and the misguided attempt to adapt these terrestrial plant lights is not going to be 100% effective as a light with more water penetrating blue & slightly lower red nm energy.

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 another area of comparing apples to apples in lights, not just watts.
It is noteworthy that a LUX Reading ONLY reports light intensity to which the HUMAN EYE is most sensitive (green light)

While this measurement only includes light visible by humans, this can still be a useful tool for freshwater plants & most corals in marine reef aquariums.
When the Lux is not enough, the zooxanthellae (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 one of the more important considerations along with the even MORE important related Useful Light Energy (aka PUR which literally stands for Photosynthetically Useful Radiation).
I have noticed that PAR is both over looked & OVER-RATED by both marine and freshwater plant keeping aquarists (meaning PUR is MORE important).

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 for photosynthesis (Zooxanthellae are single-celled algae that live in the tissues of animals such as corals, clams, & anemones).
In particular, this is found from actinic UVA to infrared.
UVA to 550 nm contains the absorption bandwidth of chlorophylls a, c², and peridinin (the light-harvesting carotenoid, a pigment related to chlorophyll).
Low (near) Infrared is 620-720nm which is the red absorption bandwidth of chlorophylls a and c².I should note for the technical "purists" that might read this article, true infrared is beyond 750 nm, but as per my many resources the energy spikes in the red spectrum are generally referred to as near infrared

Photons at shorter wavelengths (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.
Green wavelengths (500 to 525 nanometers) are actually useless for zooanthellic/plant growth. Objects reflect colors that they do not absorb as noted by Oregon State Univ..

Lights that produce light under 500nm will produce a lower PAR reading for a given energy input (wattage), not because less energy is emitted, rather because PAR meters are less accurate below 500nm.
Lights that occupy mostly the middle spectrum (500-600/ green-yellow) such as "warm White (2700- 3500K ) will produce LITTLE USEFUL PAR (in other words less PUR, which is discussed in more depth in the next section). However it is the balance of near-infrared and near-UVA that will generally provide your best overall PAR output.

Important Definitions as it applies PAR in plants and zooanthellic algae: See the graph to the left 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.). This is found in the 410-500nm "blue" spectrum.

*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" & 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. This lack of near-red light over 630nm is common to many so-called aquarium lights (such as the diagram at the bottom of the Nanometers section).

* When the amount of energy used (in watts) is considered, this can include 6400 SHO lamps which can generally penetrate most freshwater tanks well and also perform well in marine reef tanks UNDER 18 inches of depth when balanced with blue lights necessary for the higher need of 420-485nm light energy by symbiotic zooanthellic algae).

Further PAR Information;
As the reader here can see, there are three main spikes in the PAR spectrum, with all three being important, and all three are generally incorporated more or less in a daylight bulb of approximately 6500K.
As a light energy penetrates deeper through water, these "red spikes" need to move slightly "left" (lower) on the nanometer graph so as to allow for proper use of PAR (in other words Photosynthetically useful radiation; please see the PUR section).
This means higher kelvin overall lighting color temperatures of 9,000k to as high as 20,000k may need to be employed.
This can vary with light type though as not all so-called daylight bulbs are the same, see theUseful Light Energy section of this article.
• Besides the obvious depth penetration, it is also noteworthy that most zooanthellic algae need more of the blue spike than "higher plants", hence the popularity of actinic lights for reef aquariums. However the general optimum nanometer range is about 450-485nm.
An advantage of the latest technology LED lights is having a more precise 450-485nm blue as well as some 420-450 nm Fiji/violet lights/emitters for certain deeper reef specimens.
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.

As well, a popular trend in LED lighting is to utilize NUV light /emitters that emit light energy between 300-400 nm, HOWEVER as per current known science this light energy is NOT USEFUL for your photosynthetic life and is SIMPLY WASTED LIGHT ENERGY! Part of the reason for this trend is that this makes many corals pop with color since this light range is similar to a black light, but if used, this light energy CANNOT be counted as part of your necessary light energy.
As an example, if your Light is 140 watts but has 20 watts as NUV blue, you essentially have a 120 watt light!

• Planted Freshwater Aquariums: It is also important to note that freshwater algae also prefer more of the "blue" light so the excessive use of actinic blue lighting should be avoided in planted freshwater aquariums, as well the use of higher Kelvin Daylight (14,000K or especially 20,000K) should also be avoided in all but the deepest tanks since higher Kelvin Daylight lamps produce higher amounts of "blue" light. Otherwise many freshwater plants may not be able to "out compete" against some algae such as hair algae.

• More recent studies show that UVB radiation can bleach coral, so the use of blue lighting under 320nm should be avoided!
 

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Discussion Starter · #2 ·
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 the best measurement of this aspect of determining your tanks lighting requirements (both at the surface and under the surface)

Currently accepted numbers measured as µMol•m²•sec (also referred to as micro mols or mmol) are 50 mmol for most plants or corals such as Nemezophyllia, while Acropora can require higher PAR outputs.
Keep in mind though, that if one light is using a shotgun approach to achieving a high PAR reading with wasted energy in spectrums outside known needed PUR, you may need a higher PAR reading, while the reverse may be true of a very efficient PUR light that might achieve excellent results at 50 mmol compared to the "shotgun" light that might require 200 mmol for the same corals or plants to achieve good results.
It is noteworthy that a study of a Coral Farm in Bali showed no more than 200 PAR from noontime tropical sunlight at the depth the acropora were being grown!
Reference: PUR vs PAR in Aquarium Lighting

HOWEVER, keep in mind that a PAR Meter is NOT accurate in important light energy spikes WITHIN the 400 to 700 nanometer range, so while one light might measure a higher PAR mmol reading, another light might be still superior due to the more important PUR output.
This is where I have found the use of a PAR Meter to determine light efficiency VASTLY OVER RATED and why comparing lights based solely on PAR readings is folly!

As an example, just within the same brand of aquarium lights (AquaRay); A 12 Watt Fiji Blue LED fixture has a PAR of 38 at 400mm while the 12 watt Marine White LED has a PAR of 50 at 400mm.
BOTH use the same amount of energy, and in fact the Fiji Blue is actually deeper penetrating, YET the Marine White has a higher PAR reading. This is not to say the Marine White LED is bad, ONLY to show the folly of rating a light based solely on PAR readings!!!!

Some organisms, such as Cyanobacteria, purple bacteria and Heliobacteria, can make use of the unusable light discarded by the plant kingdom, in this case, light outside the PUR range required by plants, which is why Cyanobacteria thrive in lighting conditions that include more yellow light energy.

In the case of Red Slime Cyanobacteria, these Cyanobacteria do not use the PAR spikes at 465nm 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.

*For further reading (references) about PAR:
*Photosynthetically Active Radiation
*http://www.aquariumpros.com/articles PDF/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, 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 lights (full spectrum lights) due to increasing studies that show better immune function, mental health, and more. Animal studies support similar results as well.

See these references:
*New Science Sheds Light on Immune Deficiencies
*Light as a Nutrient

5: Useful Light Energy (PUR):

PUR (Photosynthetically Usable Radiation) also known as "Useful Light Energy" is what concerns us as aquarium keepers even considerably more than PAR in providing correct lighting.
Yet there is a lot of confusion, especially when considering LED Lights as many sellers will hype high PAR values while ignoring PUR due to less than desirable results.

PUR is that fraction of PAR that is absorbed by plants & zooxanthellae photopigments thereby stimulating photosynthesis. As noted above, PUR are those wavelengths falling between 400-550nm and 620-740nm.
A Spectrograph is an incomplete but still useful method to rate PUR.

The picture above demonstrates how three LED emitters of equal lumen and PAR output can be quite different in PUR and thus more wasted yellow/green energy.

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 400 -550 (465-485 in particular according to more recent research).
For this reason either/both higher Kelvin Daylight and actinic/blue is required for many reef aquariums while "near-infrared" is less essential and in fact does not penetrate water much anyway.

*As noted in the PAR section the exact nanometer spikes will need to be lower so as to provide the actual PUR, for this reason popular a cool white or "red" 5000-5500k plant light is NOT the best light for any water application except for very shallow water since the spikes are not adjusted for water penetration. Balancing these plant lights such as the "Aqua Flora" with 6500k to 10,000k lights is necessary for best results (such a combination may have the added benefit of certain plants growing more toward the surface).

*It is also noteworthy as per recent head to head studies with Acropora corals that photosynthetic corals found in waters increasing in depth have adapted to PUR of increasingly bluer light spectrums (although not all blue light either) and thus lights with too much red can slow certain coral growth!

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).

Even the best of fluorescent lights that are a Kelvin temperature of 6500K use a percentage of their light energy in the yellow and green light spectrum which is mostly useless for aquarium plants or corals.
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" which deeper water specimens have adapted to.

The picture to the left demonstrates this with two 15 Watt CFL (30 watts total) vs. one 3rd generation 12 Watt Marine White LED (daylight 14,000K).
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 much less 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 above/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 plants). The TMC Reef White in the picture shows a much more complete light spectrum and much less wasted energy in the useless light spectrums.

IMPORTANT Further/Newer Information as to Useful Light Energy:

As alluded to earlier in this section, EVEN a Spectrograph (which I admittedly use to make certain points) should not be the only determining factor for "Useful Light Energy" as these only rate the light spectrums of a light in air, NOT under water so as to determine PUR.
However when increasing depth of water are thrown into the equation, in particular for photosynthetic marine life, these spectrographs become less accurate as these corals and other photosynthetic marine life have adapted to the wave lengths of light energy that reaches their depths, which the spectrograph cannot show.
Obviously this is more blues and/or "bluer" daylights (such as 14,000k or 20,000k).
For this reason I prefer the less scientific, but in my opinion more accurate term of "Useful Light Energy" over PUR.

Further explanations to hopefully convey the importance of the concept of "Useful Light Energy";

*I have noted that a dozen "hardware store" cool white fluorescent lights can provide adequate lighting for a planted aquarium and even basic reefs, however it would only take a few correct kelvin 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 (& wattage) to work correctly (in other word many more watts of electricity)

*In another comparison; one can use one of the many LED panels commonly sold that use 90-120 watts (or more) that all use "Current Reduction" instead of the vastly superior PWM controllers to light a reef or planted aquarium, OR you can use a LED Fixture that utilizes the best technology emitters, PWM Controllers, & drivers available such as only found in a few "higher end" LED Lights which produce more PUR from less emitters & watts.
This is similar to the first example as lower end emitters, controllers, & drivers do NOT pin-point the important light wave lengths (spectrums) that the best patented emitters and Pulse Width Modulation Controllers can do (think about lighting your aquarium with 100's of LED flashlights; would you do this?).

*As a final comparison, it is important to note as per PUR, that for instance one 6500k light is not the same as another as there is more than one way to mix light "colors" (wave lengths) to achieve a certain kelvin temperature rating.
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.
As well photosynthetic aquatic life also has differing abilities to pick out the useful light energy it needs for life processes and even though the kelvin temperature or PAR readings may be equal, the light energy that provides this kelvin "color" is NOT.

The point 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 many "cheapie" LED panels with high wattage versus high end LED such as the AquaRay LEDs.

For further reading about PUR:

*PUR vs PAR in Aquarium Lighting; Why PUR is MORE Important

Below is a picture of a Reef Aquarium (88"x32"x24") that includes Acropora corals lighted with ONLY VERY high PUR but lower wattage AquaRay NP 1500 & 2000 LED lights (this tank has been running with these lights for 6 months at the time of the picture).

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 just one more small piece of the "aquarium lighting puzzle".
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 and this does not even take into consideration the PAR/PUR which is also important for plants/corals.

Focused Lumens;
It is also noteworthy that even the lumen output can be deceiving when considering aquarium lights.
The best LED Lights 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 "high end" LED often requires half the lumens (or often even less) to provide essential light energy 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, the best modern LED emitters are much more focused than other types of commonly used aquarium lights thus requiring less input energy for the same results.

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 cool white lamp emitting 1000 lumens at a color temperature of 5,500K will not emit as much PUR as a lamp emitting 1000 lumens at 6,500K.
More over, when compared to "Useful Light Energy" (PUR), Lumen output falls well short as useful comparison of light output.
This said, I am not saying Lumen output and focused lumens are useless, as these parameters are a piece of the light parameter "pie", but often it is overrated and these parameters should only be taken as a part and a small part at that when compared to PAR and especially PUR.
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 is why the old rule: "3-5 watts per gallon" can be deceiving, and this rule is only a starting point at best. This archaic rule was more accurate when all that was used were T12 lamps which is what this rule 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 PUR & other aspects are equal). Another example is the lumens per watt of the CFL when compared to standard incandescent lights.

The term "watts per gallon" is getting more archaic with the newer T-2, T-5, CFL, the SHO, and especially the new reef compatible LED lights.
Even within LED Lights, one 30 watt LED is not equal to another 30 watt LED.
An example, you cannot compare a 30 Watt TMC Ocean Blue to a 130 watt EcoTech Radion. However if you were to use an equal wattage of the TMC Ocean Blue or Reef White, you would have more actual useful light energy (PUR) with these per watt of energy used than the EcoTech (this is not to say the EcoTech Radion isn't reef capable LED).
Please read the FULL article to understand why I made this statement such as the fact the EcoTech uses COOL WHITE emitters & "current reduction" technology.

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".
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.

If PAR (& more importantly PUR) 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).
Wattage of lights versus PUR is where the actual watts used when comparing one light to another is simply not at all accurate, such as comparing a MH or LED of say 75 watts to T12 cool white Fluorescents also of 75 watts; these cool white T12s will simply not even be close to "useful light energy" output at 75 watts as the LEDs or MH (of coarse other factors apply to the MH and LED as not all are equal here as well).

The bottom line is 'watts per gallon' can be used when comparisons are "apples to apples" such as one Patented High output LED emitter of the same brand to another, but not when comparing a T8, to a T5 to a T2 , to a Metal Halide and especially to an LED.
An example of an "apples to apples" comparison would be the Patented emitters used by TMC require .8 watt per gallon (or less) under 24 inches of water for high light requiring reef life (.6 watt per gallon or less for planted freshwater aquariums).
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 that 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, in fact most lights sold with CRI ratings prominently displayed are intended for home or industrial use, NOT aquariums! However many low end aquarium lights such as the Fluval LED Lights still refer to CRI since their PAR and more importantly PUR IS POOR when compared to the true top notch lights now available.
 

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After working in the lighting industry as well as having both fresh water and reef tanks I think this is a very well written article. However for the average fresh water enthusiast it is packed with a lot of extra's.

The basic point is for planted tanks there are three important points to be considered.
1. Spectrum
2. Intensity
3. Duration

For spectrum the important things to consider are that plants need both blue and red light. Different plants prefer different ratios between the two. The plants have virtually no use for green light but without it the human eye would not see the reflective light in the green spectrum that we so love with our plants. The shallower the plants grow in nature the more important the red spectrum is and the deeper water plants become more dependent on the blue spectrum. Also the pigments within a plant have effect on what is absorbed and what is reflected.

Intensity. With plants brighter light is not necessarly meaning they have more energy for growth. Excessive intensity can be just as bad as a lack of light. Every plant has an optimum level where it will grow best. Increase or decrease the light from the ideal and will have less plant growth to even causing serious injury to the plant.

Duration Plants have a natural cycle that includes both photosynthesis as well as respiration. Dependent on the season location and other factors this natural cycle can vary. We can vary cycle somewhat in an artificial aquarium environment to make up for some variances in intensity but only to a very limited extent.

Finally in most aquariums we have an assortment of plants with various lighting needs in spectrum as well as in intensity and duration. Therefore unless we carefully select our plants for similar needs we can end up compromising resulting in an imbalance of growth and health in some species.

For the last 5 years I had experimented with LED's on in reef tanks. I'm presently doing the same with fresh water planted tanks. I am finding that with proper LED selection it is possible to push the efficiency limits well beyond what I would have guessed even 6 years ago, Right no I have several tanks where I thinning the plants on a bi weekly bases running only 1/4 of a watt per gallon on LED's. However these tanks also do not display the lush green colors we are used to seeing because of an insufficient amount of Green light.
 

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Nice write up.

I never obsessed about the kelvin temp of a bulb. Its rather meaningless when you have the proper data on a bulb. i.e. Normalized Spectral Output graph with a useful energy output in microeinstiens or other meaningful and measurable energy unit and PUR. Kelvin can be more of a manufacturers marketing gimmick like GEs 9325K. If you had a dozen bulbs with a kelvin of 6500 I think you would find they all had different spectral graphs, PUR and energy output. Manufacturers don't (necessarily) want to give you meaningful data as you would be able to easily determine whose bulb was the best.
 
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