Thoughts on effectively measuring tank lighting

[ataylor]

For a while now I've been dissatisfied with the 'Watts per gallon' expression for measuring aquarium lighting. This is due to two reasons:

1) Watts is a measure of the energy over time needed to illuminate a light source (for example the power required to light an incandescent bulb) but can not be used to measure the luminiscense or intensity of the bulb. Particularly in the case of compact fluorescents- where an input of 13 or so watts yields approximately the same light (in candelas) of a 100 watt incandescent bulb. For this reason, watts simply can't be used as an effective comparator of bulbs in general, and sometimes even when comparing bulbs within the same family (for example when comparing 2 fluorescent bulbs).

2) Aquarium illumination is spread over an area, not a volume. Therefore, unless you happen to be fond of putting your lights inside your tank, its only valid to compare the 'footprint' of the top of your tank in square units, such as square inches. That is the area you have available to you to illuminate. The depth of your tank water will determine how much of that lighting effectively makes it down to the substrate. In the general case of aquaria, even the taller tanks have only a foot or so more depth than a standard box, so in general the depth of your typical home aquarium tank can be disregarded, as the water depth in our cases will not significantly change the amount of light making it to the bottom.

So, we can establish that watts per gallon really doesn't fit for our uses. But what does?

Lux is a measure of lumens per unit area, so we could shoot for a given lux value per inch, by adding up the lumens of our bulbs and dividing them by our tank area...But lumens is derived from candelas, which is a measure of light at the wavelength most noticed by the human eye. As most of us know, plants utilize light across a wide band of wavelengths. While lux offers a more 'standard' way of measuring aquaria light, it still leaves something to be desired, at least for planted tank enthusiasts.

To add to the complication, lumens is a measure of candelas over an area, but in actual practice most of us are reflecting light down in the tank (the bulbs are circular after all), so we've changed the illumination in such a way that we can only estimate the true amount of light entering the water...and many of us have glass hoods between the lights and the water as well.

So I issue a challenge to the aquaria community - lets find a practical, 'user-friendly' way of accurately expressing tank lighting that allows each enthusiast to gauge his tank lighting and compare them against others 'in the field'.

 

[dapellegrini]

There is an interesting thread on Tom's forums about this (read through to at least page 3 or 4):

http://www.barrreport.com/general-plant-topics/2355-what-appropriate-lighting.html?highlight=lux

Tom seems to advocate PAR over LUX measures, but there are so many variables involved, I think it becomes impractical to come up with a solid rule measure. I have seen others build calculators that try and adapt the WPG rule to different types of lights, etc.

Interesting topic, wish I had more to add to it.

 

[Purrbox]

You'll find a very interesting discussion on this very topic linked in the stickies. Go to the lighting link, and towards the end you'll find a link to Wizzard of Ozz's thread. The end product resulted in the Oh Glorious Light which while not ideal, the calculator is fairly user friendly and more accurate than the WPG "rule".

It's still a work in progress, and I'm sure that the OP and several other people that helped work on would appreciate additional input.

 

[Zezmo]

Lighting and light levels are subject to many other conditions besides the light itself. Distance from tank, distribution (light zones), cover/no cover, water turbidity (clarity). Using a light meter and clasifying your tank by how much light strikes each tank zone (under water) may in the end be the only valid measurment..and even then your meter would have to be calibrated to the light spectrum utilization of the specific plant you want to plant in that zone....everything else is theoretical and therefore abstract from the actual situation.

Thing is, "exact" light amounts just are not that critical. That is why something as simple as WPG is a good rule of thumb to help folks generally classify their tanks and determine ferts needs. The reason we even care about total lighting usually comes down to CO2 or not CO2. Actualy growth of algae, and growth rates of plants tell you more about your light than any quantifyable theoretical measure.

 

[Plantbrain]

I and the rest of the botantist in the world prefer micromols(PAR) for units.
But we could be all wrong.......

I can measure any part of the tank, any depth, distance from the lighting source, it accounts for reflection etc.

No calculator will do this, no convertor will give good correlation between different lighting types either.

While some folks come along about every 6 months and poo poo the Watt gallon rule, I've yet to ever have had any issue with it. After doign tanks from 1 gal to 1600 gallon and using CO2 for about 15 years now, I think I'm a pretty well qualified honest person to say if a simple rule "works".

Many folks think it's their light, when often it's the CO2 or nutrients etc.
So you need to make a fair honest comparision and see if what you think, what your hypothesis is, can be falisified.

I can tell you, I've tried to push the lighting limits far more than most hobbyists I've read on line, their notions, their experiments and certainly their light types.

But you need to have a good handle on CO2 and nutrients before you can really test such notions, then you also need to use a number of plant species to make any generalization.
I've had hundreds of planted tanks over the years. Few aquarist really have that many unless they have been doing this for a long time or have too many tanks(You know who you are, haha)

I think the real question is why are you having an issue with the rule?
What specific observations do you think can be attributed specifically to lighting?
Several folks have offered alternatives, some mean a lot more work, some a little bit more, but most have the same types of issues, maybe a tiny bit better but on the practical level, not much improvement near as I can tell.

I report lighting in terms of micromols/m^2/sec.
I get around many of the issues this way.
I think a company called Apogee sells a nice PAR meter for about 250-300$.
A bit out of many folk's price range.

The other more basic way: experience.
I know the limits of lighting for specific tanks I've worked on and had a variety of lighting set ups on and changes I've made over the years.

I think in general, the W/gal rule ought to be lowered as PC, reflectors, T5's, HQI's, MH's etc have puished the amount of PAR up vs the old days when we have old HPS, MV and normal output FL's and generally no reflector.

1-1.5 w/gal of T5, HQI etc is enough in many cases.
Other tanks might need a bit more, but in most cases, it's much more due to light spread..........rather than intensity, this is true with smaller tanks, the angles are greatly reduce and less diffusion of the light in the water, reflection etc occurs, that affects plant growth and light strike.

I can add a 400 MH to a 125 gal tank, but have poor growth on each end.
If I added 400w of PC luighting, there'd be plenty and I'd have much better consistent growth along the tank's entire surface area.

A similar situation exsit with small, nano tanks but with the small PC bulbs this time.

I've had folks propose grand experiments, asking me(of course they are not willing to spend the $ and time to do the work) to do all these various experiments etc.

But I've still be unable to falsify the Watt/gal rule for most all standard sized tanks using FL's, PC's and MH's.
It's only when you get weird odd sizes or really large or very small tanks, do things get odd with the rule.

You are more than wlecomed to try, knock yourself out for 10 years if you want, use 200 species, dozens of tanks, but at the end of the day you'll agree with the same observations I've noted if you rule out CO2/nutrients etc



Regards,
Tom Barr

 

[Wizzard~Of~Ozz]

Tom, the major 2 issues that I have with the "WPG Rule" is 1, tank size.. but, this is something you seem aware of. The second is straight out efficiency. We here give advice to people, we tell them "2WPG will be fine without Co2" (Which I don't agree with, Co2 is beneficial at any light), but they go out and get 2WPG of CF Lighting, or worse, T8 lighting and can't figure out why their tank is pea soup.. The bulbs have become more efficient, CF is 35% more efficient then the T12 the "rule" was based on, and T8's are 141%.. so what advice should be given? the WPG can lead to disaster unless we add in efficiency and tank size..

What would be your approach to helping people understand the differences between high output bulbs, and NO? and how the size of their tank can affect the wattage needed?

 

[ataylor]

Using PAR is very useful if you are comparing your local value to a value you have confirmed works for your setup, parameters, and your plants. In other words, you have a database of measured values that from experience are known to work for you.

As Tom noted, if you have measurements for your hundreds of tanks, and you have a $250 meter to measure it, PAR measurements are superior to the WPG rule. And I agree with his thoughts on that.

But most of us need something a bit ... more practical.

I can only imagine taking a bulb back to the store and explaining to the clerk it didn't fit in your desired PAR range!!

Because frankly, PAR is useless for most of us because we don't have a database of PAR measurements for every combination of bulbs and tanks at various distances- that you could cross-reference to various plant species. And it won't ever exist. The vendors involved aren't going to do it, and most of us aren't going to rush out to buy PAR meters and start building one either.

Tom pointed out his experience lets him 'predict' what combinations give him adequate tank lighting. And he's right. We just need a better way of communicating that experience to others to prevent lots of frustration and unnecessary expense in the hobby.

As Wizzard noted, CF bulbs are quite popular now - and will only get more so due to their efficiency. Unfortunately they don't follow the established WPG rule whatsoever.

 

[Plantbrain]

Tom, the major 2 issues that I have with the "WPG Rule" is 1, tank size.. but, this is something you seem aware of. The second is straight out efficiency. We here give advice to people, we tell them "2WPG will be fine without Co2" (Which I don't agree with, Co2 is beneficial at any light),

Yep, I do agree here wholeheartedly.

but they go out and get 2WPG of CF Lighting, or worse, T8 lighting and can't figure out why their tank is pea soup.. The bulbs have become more efficient, CF is 35% more efficient then the T12 the "rule" was based on, and T8's are 141%.. so what advice should be given? the WPG can lead to disaster unless we add in efficiency and tank size..

What would be your approach to helping people understand the differences between high output bulbs, and NO? and how the size of their tank can affect the wattage needed?

I genenrally tell them more is not better.
But......they do not listen to me nor you.

Knowing that is also from expeirence 8O
I do not skirt human nature and habits, we have our own notions about things, many folks think more light = better.

I tend to focus on what the person's goal is, I try and see if they can defind that and what they ulmitately want out of the tank.
Some want fast growth, so I offer them that, then 3 months later they tire.

So I suggest changes as they grow and evolve with their tank.
If they want a lazy tank, I give that to them as well.

If someone wants a non CO2 50 gal breeder tank, I'll suggest 3 Triton 30watt lights, if they want a HC rug and lots of Tonia, they will end up with a 150 HQI, and/or a T5 or PC mix combo, maybe 200-220 Watts but not all the light cycle long.

I've done enough tanks to know what will work and how much is needed for specific plant species. I alos have light meters and can measure each tank and the measure is rapid and correlates quite well. so after measuring a lot of tanks over the years, I knwo what I can get away with in terms of real data as well as good old experience and Farmer Bob type of common sense approaches.

In general, folks have more than enough light and adding CO2 to a low light trank is a supper set upm, decent growth,e asy to keep up with and low algae issues.

But they still think more is better and that they can somehow have more success w/harder plants or red plants etc. That's simply not true.

It just means more work, dosing, pruning and algae potential and faster growth.

I think adjusting the W/gal rule down to 1- 1.5 w/gal for T5 and PC lighting is good. But you will never find a rule that meets everyone's unique goals and expectations, so I focus on things in terms of advice case by case instead.


Just see what the goal of the aquarist is, offer them some advice about what they may expect if they make certain choices and what they hope to gasin from planted tanks. That way everyone coming into the hobby has the highest success and retention rate. Takes more time, but quality is preferred over quanatity IMO.

Regards,
Tom Barr