The Planted Tank Forum banner

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
 
See less See more
15
#75 ·
Just wanted to say that this thread is amazing and thanks for all the data. It's gone a long ways in helping me understand the lighting situation in my tanks and has greatly aided me in finding the correct lighting solutions. Well, correct being what I'm after, which is certainly not where the WPG rule led me when I set the tanks up a while back.

I'm currently putting together a brand new 100g planted setup for my beloved catfish. It should be a much more enjoyable experience with the right lighting :)
 
#76 ·
I have 2 questions that I wold like to address. One the them everyone could benefit from and the other question is more specific to my setup.

I have a coralife 36" T5HO fixutre with 2 39w bulbs
1. 6,700K BULB
2. Colormax BULB

I currently have the fixture lifted 15 inches from the substrate which is about 40PAR; however, I read somewhere that the colormax bulbs don't provide as much PAR value as the regular 6,700K would. Is this true? Has anyone tested the colormax PAR values?

I also have a recessed canister in my ceiling that give off light to this tank. It is a single LED 13w 800lumes bulb about 32" from the substrate at the front of my tank. My question is, how much will this effect my PAR?
 
#77 ·
Someone can contribute information if they have the two bulbs and a PAR or lux meter. Just take light readings at various distances from the light, and report the readings here. Without that data one can only guess whether the two bulbs produce about the same PAR.
 
#79 ·
I just finished reading through this thread and I want to thank you Hoppy and all those that have contributed. Now, if only I can wrap my mind around this and make sense of it I'd be well on my way ;)

My tank is 48" x 18" x 24". Right now I have two 48" T8 bulbs that say nothing on them other than they are 40 watts each. They are pink and came with the tank (bought off craigslist). I also have two 36" T5 bulbs on the front side of the tank. Both are 39 watts one being 6500K and the other 10000K. I have no clue the PAR rating of any of them. I am concidering just starting over with new lights all together choosing something from the charts you have provided so the PAR is already pointed out for me. I do not use Co2 and don't plan to start. So, I definetely need to stick with low light.

If anyone of you reading this have a tank similar to mine without Co2, and your plants are doing great, please share with me what you are using. Hoppy, if you have a recommendation, I'd greatly appreciate it!

Thank you for your time and effort you've put into this thread and helping those of us that are at a loss in the lighting side of the hobby.
 
#80 · (Edited)
Am in a similar situation as Amanda's Tank. Have four T8's on top of my 24"tall 80 gallon tank. It should be low light at ~40umol, I guess. I want to go up to medium at ~55umol since adding excel and ferts. Will be looking to get a double T5HO fixture with poor reflector and rest it right on top. No real question here. Thank to Hoppy, all my answers are out there.
 
#81 ·
Overwhelmed with info here. Looking to light a 38-gal with low-medium light. Tank is 20"H x 36"L x 12"W. I have a Coralife 2x96w PC fixture on hand, but the fan is noisy. There is a Current 3x39w T5HO available that is very affordable. Or, am I better off with something from Home Depot?
 
#83 ·
I see the Aquatic Life 36'' T5HO fixture, but I have 2 bulbs instead of 4..

2, 39w bulbs, one 6000k and the other is a 650nm roseate, im thinking of replacing with 2 6500k bulbs for better plant growth..

Anyway, can anyone tell me what my readings would be for that fixture currently? I have a 30 gallon tank and my fixture is about 16 inches from my soil?
 
#85 ·
Thank you hoppy. Would that be the same even with the stock bulbs that I listed? Or if i got 2 new 39w 6500K bulbs?

Reason being is my growth is significantly slower with the stock bulbs than before when i used 2 flat coral life fixtures with very poor reflectors.

Although it was still 4 bulbs. and they were 6500k each, not HO
 
#86 ·
Tank: 55 gallon, 19 inch tall
Lights: 2x Hagen Power-Glo T5HO 18,000k, 2150 lumens, 233 lux
Light Distance: 24 inch

Ok from what I've read so far, I should be under 40 par. I couldn't find anyone with a par reader for me to measure so I was hoping anyone can confirm this? I was planning to move it up by 12 inches from the top but I'm afraid of losing too much par.

What kind of bulbs was used to test out the Hagen lights were used? was it the 6500k or the 18000k or it doesn't matter, all 54w are the same? I was hoping to achieve around 50 par but I guess it won't be possible unless I want to dish out money for new lights.
 
#96 ·
Tank: 55 gallon, 19 inch tall
Lights: 2x Hagen Power-Glo T5HO 18,000k, 2150 lumens, 233 lux
Light Distance: 24 inch

Ok from what I've read so far, I should be under 40 par. I couldn't find anyone with a par reader for me to measure so I was hoping anyone can confirm this? I was planning to move it up by 12 inches from the top but I'm afraid of losing too much par.
To the degree you are confident of your current numbers, a bit of math should give you a workable number for your hypothetical setup.

As your space between the light and water surface doubles, the light intensity will be only 1/4 of what it was. Going the other way, you can quadruple your intensity by reducing the light's distance to the water by 50%.

You could expect a commensurate quartering (or quadrupling) of your value at the tank bottom.

(Effects are considerably more complex and severe when the water depth changes.)

-Matt
 
#87 ·
Those charts will give you approximate values for the PAR at the substrate, but if you want to really know what PAR you have with your tank and your bulbs and your light fixture, you have to measure it with a PAR meter.

There is still a lot to be learned, and probably un-learned about various lighting options. For example, what is the effect on PAR using various color temperature bulbs, or bulbs from various manufacturers, or year old bulbs vs new bulbs, etc. Also, do plants really respond the same to 50 micromols of PAR from a LED light made with cool white LEDs and one made with a mix of different colors? Any one with a PAR meter and enough interest can contribute a lot to our knowledge.
 
#88 ·
I have a GLA 36-L, same size as ADA Mini M, on the way. I am considering lighting options for it. I do plan to run CO2 with ferts. I am aiming for "high light". The fixtures I have pretty much narrowed it down to are the Archaea 27W Power Compact fixture and the 16'' Finnex Ray 2 DD LED fixture. Considering the size of the tank and what I want to grow (probably HC or glosso carpet and potentially some red stems), would both of these fixtures likely be sufficient?
 
#89 · (Edited)
Is a watt a watt regardless of the source?

Compare (2) 90 watt LED fixtures (180 watt total) to a hybrid fixture with (1) 150 watt Metal Halide & (1) 39 watt PC. (190 watts total). Will it actually cost less to run the LED lamps as opposed to the hybrid fixture with Metal Halide & PC lamps? Or is a watt a watt regardless of the source?
 
#90 ·
I have a 55 gallon tank and have 48" Nova Extreme 2xT5HO. When I bought it I thought I was getting the recommended light for medium light plants (2 wpg).

After continually fighting algae I started looking for answers and found that based on your par info that these are most likely considered high light for my tank.

I wanted to take one bulb out, but then realized it only works if I have 2 bulbs in.
If I take window screen and put a layer of it inside the light on the plastic slide out splash guard will that reduce the light enough to help keep the algae down but still be enough for all my plants on the bottom? The screen won't melt/burn will it?

I have dwarf sag that is finally starting to spread, but I'm sick of clipping bba and hair algae every day. I've already reduced my light period to 6 hours a day and its slowed down the algae, but has not stopped it.

I use diy co2, don't want pressurized co2 and is seachem fert dosing chart, but am soon hoping to switch to pps pro.
 
#91 ·
ok here we go i used to grow pot i dont know bout all these numbers and what not but here is my break down

hpsv and mh lights had 6 ft growable light spec
floresents only 3 to 6 inches
led i only used for clones and it streched the plants alot

you do need a full spectrum of lights to grow in fact dif specs will help plants grow in different ways such as in pot with veggie growth and flowering
i will expiement with my new fluval 12g tank for us i think water helps increase the usable light kinda like a tin can phone works with sound so for my 12 i will use stock lights plus about 60 watts soft white led and 30 of uv trust me uv does help maybe will kill some algae too ! this is a tall tank and thats part of why im going overkill and if it dont work i will get a high pressure sodium vapor and a metal holide and try that ! just hoping between the blues whites and uv i will have a good mix and maybe with those watts hit the bottom of the tank !!
 
#95 ·
A photography meter is probably reading in LUX. If so...

One answer is that "no" you can't convert from LUX to PAR with a standard conversion.

Another is that "yes" you can convert as long as you know the same bulb's PAR index. (I.e. someone else has taken both a PAR and LUX reading of the same bulb you are using and created a conversion factor from these measurements.)

There's a wide variety of "color profiles" in bulbs, and this can have a big impact on the actual PAR you calculate. Having said that, if you know for a fact that one of the index bulbs has a similar color profile to your bulb, then you can still get some idea of your PAR from the calculation.

If you search, I think Advanced Aquarist has some index values listed somewhere.

I'm using a LUX meter as well...I think it you're aware of how it works and the limits involved you can make due pretty well without worring about PAR. (OTOH, if you can borrow a PAR meter though...)

-Matt
 
#100 ·
As long as you are working only with fluorescent bulbs, with color temperatures between about 6500K and 10,000K, you can divide the lux reading by 76 and get reasonably close to the PAR value. Lights with more red or near UV in their spectrum can't be estimated with the same constant. The Lux meter is measuring a very small band of the spectrum compared to the PAR spectral range. (Sunlight also requires a different constant to convert from lux to PAR.)
 
#101 ·
Hi I was looking at your LED chart and it says "Finnex Ray II DX". I can't find this fixture. Is this supposed to be the DS with 7000K LEDS? If so I think this is going to be a great option for me - at 24" this fixture is putting out high par ratings and is right in my budget.
 
#114 ·
Light reflects off the water surface in increasing amounts as the angle between the vertical and the light increases - at 0 degrees, straight down into the water, virtually no light is reflected, but at 90 degrees, parallel to the water, or close to it, most of the light is reflected. When the surface is rippled, a lot of the surface is at lower angles to the incident light even way off to the side from the light. So, that might lead to a higher PAR. But, I haven't tested this idea.
 
#115 ·
The chart shows that the diamond plate, 2 bulb, T8, Home Depot shop light gives about 70-75 micromols of PAR at 15 inches. The other chart, of T5 lights, shows only the Hagen Glo, Coralife and FishNeedIt 2 bulb T5HO lights give less than that, and those may have sub-standard ballasts, and do have relatively poor reflectors. I agree that it is hard to believe that the Home Depot T8 light is that good, but someone tested it and that was where the data came from. I also measured the PAR at one distance for one, at a local aquatic plant club meeting, and the reading was consistent with that chart. I saw one on a tank once, and it certainly did look very bright.
 
#116 ·
Two questions:

According to the PAR vs Distance for Various T5 Light Fixtures chart on the first page, the AquaticLife T5HO 36" 4 bulb fixture has about 90 PAR at 25 inches.

Is it safe to assume that the same fixture with 2 bulbs has about 45 PAR at 25 inches?

I have glass covers on top of my tank and they build up quite a bit of condensation.

How much will the glass/condensation on the covers reduce the PAR of a light?
 
#117 ·
That is a safe assumption, that you would get half the PAR with half the number of bulbs.

The condensation on the underside of the cover glass should not appreciably reduce the PAR, but the hard water deposits it may leave behind will reduce the PAR. You need to keep the glass as clean as you can if you want to get the full PAR from the light.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top