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Lighting an Aquarium with PAR instead of Watts

436K views 453 replies 170 participants last post by  jeffkrol 
#1 · (Edited)
Before any discussion of aquarium lighting can proceed, first we have to debunk the myth about “watts per gallon” being a measurement of light intensity.

When the only practical source of light for a planted tank was T12 fluorescent tubes, someone decided that the way to pick out the best lighting was to figure out the “watts per gallon” that were needed to grow various types of plants. This would make sense if we could pour a teaspoon of watts of light into a tank, and get a light concentration of X watts per gallon of water, just as we pour a teaspoon of potassium nitrate in the tank to get a nitrate concentration of Y mg per liter of water. But light is nothing like a chemical - you can’t pour it anywhere, you can only shine it on something. That alone should debunk “watts per gallon” as a measure of light intensity.

But, there is more: Let us assume we have two 20 gallon tanks, with 40 watts of T12 Fluorescent light on each one - 2 watts per gallon. One tank is a 20L and one is a 20H. The 20L tank is 12 inches high, and the 20H tank is 16 inches high. If the fluorescent light sits right on top of each of the tanks, the light on the 20H tank is 4 inches farther from the substrate - 33% farther from the substrate. Because light intensity drops approximately proportional to one divided by the distance from the light squared, the intensity at the substrate in the 20H tank has to be about 56% of that at the substrate in the 20L tank. That alone should debunk “watts per gallon” as a measure of light intensity.

But, there is more: Let us assume we have two 20H tanks, one with a 40 watt T12 light sitting on top of the tank, and the other with the same light hanging 12 inches above the top of the tank. Again, because light intensity drops approximately proportional to one divided by the distance from the light squared, the intensity at the substrate for the tank with the light hanging 12 inches above the top of the tank must be about 32% of the light intensity of the tank with the light sitting on top of the tank. And, that alone should debunk “watts per gallon” as a measure of light intensity.

So, that is three strikes against “watts per gallon”.

But, there is more: Let us assume we have two 20H tanks, one with 40 watts of T5HO light, from a Tek light fixture, the other with 40 watts of T12 light. Anyone who has looked at both a T5HO bulb and a T12 bulb, when they are lit up, knows that the T12 bulb can be stared at without distress, but the T5HO bulb causes some temporary blindness if you look at it for more than a few seconds. The T5HO bulb is much, much brighter, and has to give much more light at a given distance than the T12 bulb.

“Watts per gallon” is dead!



PAR

Light intensity can be measured in lux, which is the intensity as perceived by human eyes. Or, it can be measured in PAR units, which is the intensity as perceived by plants. PAR is an acronym for “photosynthetically active radiation” - the radiation (light) that is used by plants for photosynthesis. The units of PAR are micromols of photons per square meter per second. So, a PAR of 1 is one millionth of a mole of photons striking a one square meter area every second.

Human eyes see the yellow green area of the spectrum of light very well - our eyes are very sensitive to yellows and greens, but we see reds and blues much less well. Plants are very sensitive to reds and blues, absorbing most of the light in those colors, but less sensitive to yellows and greens, reflecting a lot of the light in those colors. That is why most plants look green or yellow.



MEASURING PAR

The best way to find out how much light intensity we have in our planted tanks is to measure it. To do that we must use a PAR meter. A few years ago the only PAR meters available cost a few thousand dollars apiece. Now there are much cheaper PAR meters available.

You can buy a Quantum PAR meter, Model MQ, for $329 plus shipping. That is a near laboratory quality meter, with a guaranteed calibration, which can be re-calibrated at the factory when needed. It is the Cadillac of hobbyist PAR meters, usually bought only by clubs, where many members can use it.

A lower priced version of the Quantum PAR meter is just the sensor, Model SQ, for $139 plus shipping. To use this you need to either use a good millivoltmeter, which gives the best accuracy, or a cheap lux meter, like the Mastech LX1010BS, from Amazon.com, at about $20 plus shipping. Used with the lux meter you need to do your own calibration. You can use your digital multimeter, with a millivolt scale and the sensor, to determine the PAR from a light at a fixed distance, then connect the Mastech lux meter to the sensor to see what the meter reads at that PAR. This gives you a calibration constant for that combination of sensor and meter to convert lux to PAR.

Still cheaper is to buy one of the DIY PAR meters made by Mistergreen and/or O2surplus, for about $60. These are calibrated, and the meter reads in PAR units, but they may not be available when you want to obtain one.

Cheapest is to buy a Mastech LX1010BS, at $20 plus shipping, and modify it yourself per http://www.plantedtank.net/forums/showthread.php?t=179789 but you have to calibrate this yourself. However, the total cost should be $35 or less. If the Quantum PAR meter is the Cadillac of PAR meters, this is the refurbished Volkswagon bug of PAR meters.

SELECTING A LIGHT

Before we can even start to measure the light intensity, or PAR, that a given light will provide on our tank we first have to obtain the light. It may seem that we have to be working blind when we make this selection, given that knowing the “watts per gallon” won’t tell us anything about the intensity we will get. But, because there are now many PAR meters in hobbyist’s hands, we now have a lot of data on how much PAR we can get from several different lights, made by several different manufacturers. More data becomes available every month.

Today we can chose one of several different types of lights:
T5HO fluorescent lights with 1,2,3,4, etc. bulbs
T5NO fluorescent lights
T8 fluorescent lights with 1,2,3,4, etc. bulbs
PC power compact fluorescent lights with 1 or 2 bulbs
LED lights of many configurations - DIY or ready made
CFL screw-in fluorescent lights - DIY

For each of those types of lights a chart can be made showing the PAR produced by the light versus the distance from the light. These charts show the light intensity as measured without a tank of water being involved - just the intensity as measured in air. This is necessary to avoid the many variations in intensity caused by the tank dimensions and the cleanliness of the tank glass, both of which can have about a 10-20% effect on the intensity.





LOW LIGHT, MEDIUM Light, HIGH LIGHT

I don't believe there is any consensus about the definition of low, medium and high light. But, here is my definiition, subject to, and almost certain to change:
Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems

The following charts show the data that I now have for various lights. As I get more data I will keep updating the charts and adding new ones. If you want a light that isn’t included in the charts you can study the reflectors used in the light you want and compare them to the photos following the charts to see which charted light is closest to the one you want, to get a rough guess at what PAR that light will give you.

Fluorescent tube lights produce about the same light intensity for any length of tube, from about 24 inches to at least 60 inches. The longer bulbs are proportionally higher in wattage, so that the bulb wattage is mostly a measure of the bulb length, not the bulb brightness. For bulbs shorter than 24 inches, this may not be true.

CAUTION: Not all lights use a true, full power HO ballast. Some cheaper models use lower power ballasts, and will not produce as much PAR as those with good ballasts. Compare the chart for the FishNeedIt lights to the others for an example.






One layer of window screen over the bottom half of each bulb, right on the bulb, drops the PAR by about 30%






See http://www.plantedtank.net/forums/showthread.php?t=160396 for much more information on LED lights.

Photos of various reflectors used in T5 lights:




Hagen GLO 2 bulb light


ATI 4 bulb T5HO


Home Depot 2 bulb Diamond Plate Shop Light - note the reflections of each bulb.



One bulb T8 light with fairly good reflector


Aquaticlife 4 bulb Light
 
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#42 ·
Acturally....I do have a questions now....if you put 2 fixtures that each give say 60 PAR....does that mnean you have 120 PAR in the water.....or does that mean you just have 2 60 PAR fixtures directing light into the tank? Looking for the best way to get high light in my 90 Gal Planted Tank
 
#43 ·
PAR isn't something that you put into the tank. It is the intensity of the light at various points in the tank. The light can come from room light and/or one, two, three, etc. lights sitting on top of, or hanging above the tank. When you have two lights close together, their light overlaps a lot, so the intensity can double right under the bulbs. But, if the two lights are widely spaced, say 8 inches apart, the overlap is mostly of the lower intensity area away from directly under the bulb, so the intensity in the middle of the tank is more likely to be about the same as when only one light is over that spot.

A 90 gallon tank is about 18 inches front to back, so it is helpful to have two lights, widely separated, to get more uniform light intensity over the whole substrate. If you put two light close together you will get possibly double the intensity right under the lights, but near the front and back of the tank the intensity will likely be much lower.

If you are sure you want high light, you can use a Catalina Aquarium 4 bulb T5HO, with the bulbs in two groups of 2 bulbs, with the groups about 8-10 inches apart, in an extra wide housing. That will be a custom light, but they have made some like that for other people.
 
#237 ·
Hello,

I'm jumping in here a bit late, but reading the thread and the questions and difficulties that people are having are the product of a serious misunderstanding best encapsulated in the following statement:

PAR isn't something that you put into the tank. It is the intensity of the light at various points in the tank.
PAR is something you can put into your tank. It is measured in microEinsteins which is a measure of the number of photons (these are real things!). What is being measured with PAR-meters, and what Hoppy is talking about is Photosynthetic Photon Flux (PPF), also called Phososynthetic Photon Flux Density (PPFD). This is measured in microEinsteins per square meter per second. So, to summate,
PAR = uE
PPF = uE/m2/s

From the data accumulated at http://www.apsa.co.za/board/index.php?topic=4454.0 (in 2010) it is calculated that, on average, you get about 1 PAR/W from tube/fluorescent lamps (T8 & T5). (T5s are only marginally better for output and this is due, mostly, to geometry.) So a 20 W CFL will put out as much PAR as a 20 W T8 tube (because the physics of light emission process is the same). Of course the CFL is all twisted and a lot of light is lost among the twists and turns of the tube (especially as about 30-40% of the light emitting surface is facing another light emitting surface and gets lost by "squishing"). PAR output of LEDs varies from 1.55 to 1.03 PAR/W (depending on bias current) but the geometry of the lamps greatly increases efficiency as the light in concentrated over a narrow beam focus. We suspect that LEDs are, in the end, 3 times more efficient than tube lamps based on the number of LED watts needed to get plants to pearl compared to T5 tubes.

Using this data, and Tom Barr's PPF data for actual planted aquariums and scientific journals for attenuation coefficients* we worked out the following: http://www.apsa.co.za/board/index.php?topic=4454.msg109062#msg109062 (and have been fine tuning it since 2010 as we gathered more data). This is our best guess for how many watts you need over a tank to achieve 40 PPF at a certain depth.

The major factors (that will vary with each aquarium setup) are the turbidity of the water (the more light absorbing dissolved solids the more light is absorbed with depth), the amount of light lost to reflection due to surface disturbance or angle of lamp to water (90% of the light hitting the water at 60o is reflected and it gets worse the shallower the angle). You need the lamps as close to the water as possible to keep the angle of incidence steep. The quality of your reflector is also critical. With a bad reflector you are probably getting less than 50% of your light into the tank. With no reflector the value is closer to 30%. A model to determine reflector efficiency is available at http://www.apsa.co.za/board/index.php?topic=4379.msg41393#msg41393 .

Essentially, if you can engineer a reflector with high efficiency and place it so that as much light as possible hits the water's surface of the tank, then you can, with some certainty, use W/m2 to estimate PPF at specific water depths.

So, to summate by example: if you have two 30 W T8 tubes they are emitting approximately 60 PAR. If you have a good reflector that can focus all the light into the tank (which is impossible but lets just pretend) which is 36 x 12 inches in surface area you will get 222 PPF (PPF = PAR/(0.9 x 0.3 m)). Assuming 70% of the light actually enters the water, this will mean you will have about 99 PPF at 15 inches of water depth (not tank depth). More than enough for Glosso and HM and unless you have ample CO2 and nutrients you will get an algae soup. Without reflecors the value would be closer to 30-40 PPF which attests to the old adage that 2 full length tubes are enough to grow just about any plant.

* The issue here is that light moving through water is subject to the law of attenuation, not the inverse square law, and there is significant internal reflection of the light moving from the water to the glass. Light out of a fishtank follows the inverse square law just fine meaning the higher your lamps above the tank the less your photon density becomes with distance from the source.
 
#45 ·
It would be worth a phone call to Catalina to find out what it costs to ship there. They have great customer service. The Tek light would really be a lot of light for a planted tank. Using two bulbs only would probably make it work, but with 4 bulbs you would need to hang it high above the tank.
 
#49 ·
Thank you Hoppy.Outstanding analysis.

So, if I'm understanding the led chart correctly then; 2-36" TrueLumen Pro fixtures will produce app 40 par at 35" depth ?
Thanks again,
Kim
That is correct, providing you space them at 2.75 inches apart. If you have them too far apart you only get half of that PAR - what one strip would give you. But, I don't know how far apart that would be - probably several inches. But, then you would get more uniform PAR over more of the front to back depth of the tank.
 
#51 ·
having had a 220 with those dimentions and a odyessa 72" 4x80w fixture I can say I wish I had gone with 4 36" fixtures instead... Or at least 2 of their 72" fixtures. Light was excellent but only in a very limited swath under the fixture. Their 72" fixtures come with 3 switches, 2bulb 2bulb and led moons. Id just leave 2 bulbs off in each fixture for lower light. (or go for the afformentioned 4x36"ers)
 
#52 ·
I think tanks 18" or more in front to back depth will do better with two rows of widely spaced lights than one row of more intense light. So, I would use the 4 36 inch long lights, also because shipping 6 foot long lights and bulbs is more risky and expensive than shipping 36 inch long lights and bulbs.

However, if you hang the light a foot or more above the top of the tank, a single row of lights should work fine.
 
#53 ·
#54 ·
As I pointed out earlier: http://www.plantedtank.net/forums/showpost.php?p=1974604&postcount=43 it depends on whether you want high light or something lower. For high light, phone Catalina and ask for a quote on the custom light I described. If you want less than high light, do you want low light, so you don't need CO2, or do you want high enough light to grow anything if you also use CO2?
 
#61 ·
Cfl vs t5

Hello!

Am i reading the measured par charts in the first post of this tread right?

Is 23w cfl vertically mounted in a reflector giving more par than a Hagen Glo 2xT5?

I am setting up an ADA tank with dimensions 75x45x45cm, i do have a Hagen Glo 2x24 T5 that i can use, but when reading the par charts it seems to me that two reflectors (diameter 25cm) with 23W cfl will probably give better par than my Hagen Glo. Maybe also give better spread of the light beacuse the Hagen Glo fixture only have one reflector, both T5 tubes are placed under one reflector.

Do i reading the par chart right?

And when using a cfl with reflector, Does the reflector have to be shining aluminium color or is a white painted reflector ok?

Jnad
 
#62 ·
Yes, you are reading the charts right. A 23 watt screw-in CFL bulb in a 10.5 inch diameter dome reflector, polished aluminum suface, gives considerably more PAR than the Hagen Glo 2 bulb light. But, the Hagen Glo light covers more area with that light - it would take two of the CFL bulb/reflectors to cover the same area.

I don't know if anyone has measured PAR with those dome reflectors with white painted inside surfaces. I doubt that they would do nearly as good as the aluminum surface reflectors. Aluminum is inherently a very good reflector even if not well polished.

We have had some problems duplicating PAR measurements with CFL bulbs, especially when the bulbs extend slightly beyond the spherical surface of the reflector, so don't rely too much on the charts for them.
 
#67 ·
I just set up a 14watt 5k CFL with a aluminum dome reflector over my 10g for my plants. Its about 14" from the substrate. I'm trying to grow crypts, java fern, and micro swords. I plan to add dwarf hairgrass in the near future. Should I install another lamp over my tank? I want low-med lighting. Dosing Flourish once a week, and 1ml of API co2 daily.

Im confused because the PAR chart in this thread doesnt match one in another.
 
#68 ·
This thread supersedes the earlier one, as noted in the first post in that older thread (bold type).

The dome reflectors can't cover a 20 inch long tank with light, so it takes two of them to get good coverage.

It is still up in the air whether the charts for CFL bulbs in those dome reflector are correct. I suspect that they are good, and that we have misused our lux meters in getting later data. This will be worked out eventually.
 
#73 ·
Hoppy,

Which model of 24" ray 2 did you use for the par test? Was is the 7000k, 192 led model? So if I mount this on top of my 14" tall tank with a 2-3 inch substrate, I will be getting about 80-95 par?

Thank you for all your efforts into the research!


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