Last night I had the privilege of seeing a PAR meter in use. Tom brought his PAR meter to our local plant club, and we tried it in the tank we were pruning. This tank was a 75 gallon size, with a pair of MH lights mounted about 4 inches above it. It was a well lighted tank.

I was shocked by the PAR readings. Right under one of the MH bulbs, at the water surface, the reading was 1500, which compares to full sunlight at around 2000. But, just a few inches down in the water, that reading dropped to about 150! And, at the bottom it was around 100, varying over the substrate. I didn't dream that a light, with a good reflector, which redirects almost all of the light into the tank, would suffer that much loss in the water.

Water does not absorb light nearly to that degree, so this had to be a geometric effect - an effect caused by the distance from the bulb. So, I sat down with my computer and my brain, which I managed to get up to speed, and figured it out. I made the "sketch" below to try to explain how I see this as happening. I hope it is clear.

This is why all the junk folks post about light equations and accounting for the modifications of the Watt/gallon rules, no matter how well written, how many graphs, rational etc, conversions .............cannot measure a tank in situ.

Inverse-square law - Wikipedia, the free encyclopedia

It's not just the source and type and wattage, it's a lot more.

And as plants grow through both space and time, the light changes dramatically influencing growth rates, color, CO2 uptake etc etc.

But some think that watt/gal or some manipulation will address this and make everything the same when doing comparisons.
Suggesting they do not even need to test for light

They assume it's all the same.
Anyone that knows a hill of beans about light and plant growth knows that it's far far from the same. I've not met a single person in this hobby or on line ever that uses a PAR meter to date.

That is well over 14 years.
I have met a few, and I mean very few, that do this in their reef tanks.
Still, reef folks are much much better about testing, measuring and chemistry than Planted folks.

Of course I'm the guy that some of my critics claim that does not test.
You have to test to verify your models.
And knowing what light intensity you have is a rather important parameter since it's where everything starts with respect to plant growth and CO2 demand.

Then there's the CO2 meter.

Again, items that hobbyists never bother to measure critically or closely, but instead, haggle over a ppm's of nutrients.

Seems if you want a global picture of plant growth, you'd be much better off focusing the time and resources on light and CO2, then go back and see how the nutrient/s respond.

Vaughn saw the tank and the results. He saw the tank with lots of KNO3 and ADA AS growing like mad. He saw the CO2 mist and we measured the light throughout the tank.

It's an eye opener to say the least.
Suddenly Vaughn realize how far off folks are about things and their assumptions.

Regards,
Tom Barr

Vaughn, you may recall the spherical LiCor meter probe I was talking about, take a look at thise link and you see why and how they decided this made more sense.

Inverse Square Law for Light
I heard some reef folks discussing this back in the early 1990's.
So I've had this in my head for many years.

However, we can take this a few steps farther:

Facts of Light – Part 2: Photons by Sanjay Joshi - Reefkeeping.com

We do not have tanks that are over more than 1 meter in general, there are cases, but very few and then not ever more than 2 meters.

So we can then discuss how the PAR is distributed with the spectral plots of PPFD.

He has a good set of useful readings:

Sanjay's Reef Lighting Info Pages

Still, this measurement does not tell you what the light actually is at the coral, or at the plant top, near the edge of the foreground plants on one side of your tank, within the plant groups, nor says anything about how the light changes through time as the plants grow.

Some plants of mine grow 4cm to 12cm a week.
They change their light status every day and pretty much each hour in many cases!!!

So that can change the CO2, and nutrient demand.
So maintaining lower light and pruning often will lead to more stability.

However, if you can handle high light systems, then you can handle the slower easier to deal with lower light systems as well.



Regards,
Tom Barr

One of the things I learned last night is that the only benefit to good reflectors, even parabolic reflectors, is that you get more light from the bulb into the tank, but that light still loses intensity per the inverse square law (approximately). If it were possible to have a point source light, of one discrete wave length, in a very deep parabolic reflector you could avoid the inverse square law. We can't do that. But, a very good laser would come very close to eliminating the inverse square law. To work as a tank light it would take an array of those lasers to cover the entire tank. I don't expect to ever see this.

Another thing I learned is that our plants are extremely adaptable to a wide range of light intensity. Within just one tank there is a 50 to 1 range of PAR and the plants grow in the full range.

One more obvious fact I learned is an explanation for why my Limnophilia aromatica grew so slowly - no pruning required - for a month, then suddenly it requires weekly heavy pruning. It grew taller, reaching out to a much higher light intensity, thus much faster growth. A corrolary to that is an explanation for the need for high light intensity for carpet plants - they are as far from the light as they can get, so they never, ever see that 1500 PAR intensity.

Another conclusion I draw from what I saw, and my thinking about it: a pendant light delivers a very wide range of light to the tank, depending on how high it is above the waterline. So, if the closest you can get the bulb to the waterline is 4 inches, you can cut the intensity in half by just raising it to the square root of 2 times 4 inches or a little more than 5 1/2 inches. For that situation, which is pretty typical, raising it 1.6 inches cuts the intensity in half. I had not realized how sensitive the intensity is to the hanging height.

However, another aspect of this is that a very powerful light, suspended 16 inches above the waterline, will lose only about 3/4 of its intensity by the time the light reaches the bottom of a 16 inch deep tank. Now, think about the problem lighting a 36 inch deep tank imposes! (Makes me wonder if WWII surplus carbon arc search lights would work best!)

This is very interesting to me as I have MH and a deep tank (24").

I feel that since even leaves and Riccia at the very bottom of the tank pearl, I am 'okay', but now wonder what the growth would be like if the light intensity was that of the surface!

Anyhow, based on this, how to determine an 'optimal' height above the tank for the fixtures?

Can it even be done with all of the variables that could go into the equation?

Sorry, never got past HS algebra lol

Great info. Thanks for sharing!

If I used a pendant light I would observe the plants. If they seemed to need more light I would look at lowering the light by an inch or so. If I was having intractable algae problems I would raise it an inch or so. Then repeat as necessary.

I can see that the best way for me to get more light on my tank right now is to get new reflectors. I don't even have the "simple" reflector...it's like a half of a simple reflector.

Carissa,

A reflector is huge and makes a marked difference in growth, visuals, etc.

I am pretty sure a Google search on 'DIY reflectors' or similar will result in some ideas if finances are an issue....

Not sure if metallic or white makes a better color... I have had both, but feel that the shiny metallic would be better. That is what my MH has..........

Vaughn,

Makes sense and is what I do now...........

I was hoping that there was a magic formula that could tell me that I am right in my estimating I love to be told that I am right about something and to have it confirmed scientifically would be great lol

Thanks.

Today someone at APC posted a link to Reef Tank, which shows PAR measurements using T5 bulbs. These measurements clearly do not show an inverse square drop in intensity. So, another APC poster did the math and figured out that my mistake was treating a tubular light as a spherical bulb instead of as a linear tube. When you treat it as what it really is, the drop in intensity is inversely proportional to the distance (not squared) from the bulb. A pair of pendant MH lights mounted side by side would also not have an inverse square drop in the area between the two bulbs, and a group of MH pendants mounted so the are all touching in a line, would give the same result for most of the length of the group. As you get near the end of the tubular light or the string of MH lights, the drop approaches an inverse square relationship again.

So, that is good news! Good, because it better explains how we manage to ever get enough light at the substrate for deep tanks.

Another couple of thoughts: First, we have long known that a deep substrate gives better growth than a thin substrate. Much of that could be because the deep substrate moves the plants closer to the light. And, when we first plant our tanks, the plants are short, no where near the top of the water. So, their growth rate has to be adversely affected by the lower light that results. That explains what I always see - relatively slow growth for a couple of weeks, followed by furious growth to the water surface. And, it suggests that our EI dosing could be reduced until the plant growth starts to accelerate.

Finally, all of us who couldn't afford or didn't want to mess with a MH pendant light can finally say we knew what we were doing - T5's have big advantages over them.