Light Intensity in an aquarium

VaughnH

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Edward's basic assumption is incorrect in that article. You can't equate 24 hours at 1 watt per gallon to one hour at 24 watts per gallon. His assumption is simple, looks very good, but isn't correct.

The article about reflectors and "spread charts" is very interesting - thank you for that. Today I will be starting to play with the PAR meter!
 

Tom Barr

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naman;25241 said:
VaughnH, this will help you to play with PAR-meter more rationally :)

Reflectors and Double Fixtures with UVB Fluorescent Tubes,
and Make Yourself a UVB Spread Chart.

As we know for light intensities, we need not PAR itself, but daily PPF (i hope you have read my posts before on this with some great links).

For those missed it at APC, this is nicely corresponds to this 24 Wpg per day rule by Edward.

naman

Naman, I've already discussed light with Vaughn in person and mentioned making such plots for his tank using the PAR meter. However, as time passes, the plants grow, reptile tanks do not.

Unlike the watt/gal/day thing, this is a far more rational approach and measures things through time and space for his specific tanks and plants. Something none of the links or rules can muster nor ever will, which is why botantist use PAR/PPFD etc. Things change as does uptake of both nutrients, CO2, flow rates through time and space.

PPFD also changes throughout the day and light cycles, plants do grow and change in space after all. The above method does not measure PPFD throughout a day nor at all for that matter. PAR is just the total PPFD integrated into one unit that plants use. They seem to adapt pretty well to most PFFD. Since we know the light intensity, we can easily measure that throughout the day. But taking measurements "where" and "when" specifically and for what plant species?

Do we assume they are all the same?
Is W/m^2 the same as PAR? I do not think it is, but it does illustrate the inverse square law through space and from the point source which is what Vaughn quickly saw when he used the meter and I showed him various points around the tank at Kyle's place.

However, w/gal, or w/gal/day are poor predictors of uptake. I discussed the issues with light measurement in depth in the Barr report article I wrote several years ago about such issues. So called watt/gal rules, whether they use watts per day per gallon or watts/gal based on a 10 hour day conversion charts are very rough estimators and offer little insight into the demands of the plants and how they influence uptake for CO2 and for nutrients. Which is why researchers and aquarist should measure things and not just claim them to be so and "better".

That's why Vaughn came over and borrowed the meter and I gave him some thought questions to ponder as he uses it for a few weeks. He will learn far more this way;)

The only thing the Watt/gal/day thing does is allow the Watt/gal rule to use more light for less time or a little less light for longer time vs the same intensity all day long for 10 hours. Total intensity is still the same over time from the bulbs.

However, bulbs are all different(Age, distance from the surface, clean or not, water spray etc), as are all tanks, as are all plants, and nutrient status, as are all ballast and bulb types(T5, T8, T12, HQI, MH's, LED)

All these errors are accounted for by measuring light in the tank at various points through space and a time in situ. Which if you sit down and think about it, it a lot more rational than w/gal or w/gal/day.

Which is also why I suggest this.
I also measure CO2 very critically now with a CO2 data logger, so that + light in situ tells you far more about nutrient uptake and behavoir.

Regards,
Tom Barr
 

naman

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Those parabolas…

Adjust-A-Wings® + Super-Spreader®

Adjust-A-Wings®

“adjustable nature and even spread of the light foot print”

Adjust-A-Wings® Instructions

“Removing the hotspot and spreading it strategically can deliver incredible benefits. Lights can be placed closer to plants than was ever dreamt of, area covered by each light is increased, light power and penetration is multiplied, and heat is greatly reduced.

…can operate two times closer to the plant layer than is otherwise possible giving the grower four times the light to spread around. ”

Adjust-A-Wing(R) Instructions.

See Adjust-A-Wings®, SUGGESTED LAYOUT for 150-250 Watt MH/HPS LAMPS
(Using Small Adjust-A-Wings Reflectors), Layout tips PDF 422Kb, page 2.

Super-Spreader®
“All horizontal reflectors create an over hot, over bright spot directly beneath the lamp.
The Super-Spreader® spreads the excess light and heat proportionally to all areas of the grow room (especially the corners and edges which are often underlit). This creates even light and more harmonious growing conditions throughout the entire grow room.
NOTE: By attaching the Super-Spreader® accessory, the Adjust-A-Wings® performance may be enhanced significantly.
Adjust-A-Wings® fitted with Super-Spreader® can be lowered to an unprecendented 10"-12" from growing plants (note by naman: it's about 1000W HID lamp!) to produce awesome growing power while maintaining maximum area coverage and minimum heat build-up. Yield increases of up to 50% are not uncommon with proper use of this truly outstanding product.” (Super-Spreader®)

We can try at least hot-spot reducers on any pendante with MH-HQI lamp Super-Spreader® for $30…
Similar product – The Equalizer™ - Hot Spot Diffuser.
I like this idea…


naman
 

naman

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Tom Barr;25245 said:
Naman, I've already discussed light with Vaughn in person...
I know and support what you say, but I can't get why you say "PPFD also changes throughout the day"?
PPFD is a cumulative value - all the light which plant get per day. May be you have been in a hurry and mean PPFD is not the same every day?
But we talk anout tanks, not lakes.

As for w/gal/day rule - this is forces aquarist to remember about PPF per day (it's a good sign :)
Without this folks often say "I use low light - 1wpg", while their plants get the same amount of energy for growth as his nabour's with 2wpg.
This is "why" I say it is a little bit better than wpg rule.

To be fair, W/gal/day is a good "quick rule of thumb" to estimate what you have without mesurements, but we must not forget the type of lamps and reflectors and other factors you mentioned. We can't measure everithing and everytime... and don't need that to grow lush plants after all.

You are just right, we can't take it as a precise rule.
Many folks know that, I hope, so I simply do not see any need in posting everytime all that factors you mentioned in last post so I omit 'em. ;)

Will you do the patterns like ADA's?
Will you try to do the same with Super-Spreader® ?
I think we can have some use from this device.


naman
 

naman

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VaughnH;25242 said:
Edward's basic assumption is incorrect in that article. You can't equate 24 hours at 1 watt per gallon to one hour at 24 watts per gallon. His assumption is simple, looks very good, but isn't correct.
Ah! You are just right, but he stopped this rule at 6 and 12 hours limits.
Less than 6h is detrimental for plant's health, more than 12h will stimulate algae too much.
Very clever and "a must" note however.

naman
 

Tom Barr

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naman;25251 said:
I know and support what you say, but I can't get why you say "PPFD also changes throughout the day"?
PPFD is a cumulative value - all the light which plant get per day. May be you have been in a hurry and mean PPFD is not the same every day?
But we talk anout tanks, not lakes.

Sure, it's just a density. It does not mean that it is the same all day long.
Nor everyday either. This is because plants grow every day(per day) and throughout the day(say per hour, per minute etc). If you want the total, you'd need to measure the leaf as it grows and moves through the aquatic weed bed.

Let me further explain by asking some questions to folks in general here:

Do plants grow throughout a day?
Some plants grow very fast, some several cm per day, some do not but get shaded by those that do.
The canopy changes very rapidly in aquatic systems.
Thus so does light/PAR/PPFD and any parameter you wish to consider through space and time.

PPFD changes as you move around in the aquarium.
You can measure this and reptile link supports this via the inverse square law.
Take a look at Sanjay's article and most Reef articles are about 20 years ahead of FW plant hobbyists in general, both in testing and measurements.

Sanjay's Reef Lighting Info Pages

Poke around on there and get a feel for the lighting calculator, the spectra plots.
I think if you are interested in coming up with a light calc to avoid measuring, you need this data but it still has some serious assumptions and plants grow much faster than cormals in general(but not macro algae!)
All in all, measuring light is neither simple nor easy.
youi need to be specific in the question you ask with respect to light.
One simple discrete point does not tell you much, several over time start to tell you a much better story.

Most folks use one discrete point in space and time.
My advice to Vaughn and anyone, is not to do that, measure over time/space.

Using light bulb maker's spectral data, or initial readings assumes a lot.
Distances from the plants change radically.
Growth rates and formations of the canopy also can go from zero to exponential back to near zero over a growing season or if you neglect trimming your tank.

You need to take many samples in an aquatic weed bed.
Since many folks do not trim daily, typically let's assume 2-3 week time frames, maybe less for a non CO2 method, how might the PPFD change over that time frame?

How might the Spectral Flux Density change as well?
Leaves absorb different wavelengths and wood and rocks reflect light etc, as does water, angles, distance that the user uses above their tanks to light source etc, different light sources also produce very different patterns.

HQI vs say LED vs T5's.
Are the SFD the same for these also?

Ideally you'd measure FSD for each point and this can be done.
But "PAR" just averages them together, which is okay and an easier number to haggle with for most folks.

Does PPFD change with respect to time as bulbs change over months and there is variation between bulbs, electrical sources driving the bulbs etc?

Lots of questions:eek:

As for w/gal/day rule - this is forces aquarist to remember about PPF per day (it's a good sign

Per second or per day, PAR per unit time still beats the snot out this method. It's not that much better than w/gal.
You still have the same total amount of energy, and this can change a lot of things, say compress it into 2 hours? Or 16 hours?
Then that rule falls apart.

Now if you simple suggest if you use higher light, try using it for 8-10 hours and if you have low light, 12 is fine.

However, you still have no reference standard to compare the light sources other than watts here between each other.

No "rule" can avoid that.
That's why measuring your tank in situ is critical.

Vaughn saw this rather quickly when he moved the meter around at Kyle's aquarium the other day. Sometimes you can learn a lot more in person than all day on the web.

You still are left with a large degree of error when you discuss light comparisons and aquatic plants. Much more so than the errors associated with EI.........

Without this folks often say "I use low light - 1wpg", while their plants get the same amount of energy for growth as his nabour's with 2wpg.
This is "why" I say it is a little bit better than wpg rule.

Yes, but the details of the information rather than a one shot rule is far more informational and useful. I do not rely too much on such rules.
I suppose it's a little better,m but it's not precise or that useful really anyway, which is why I use a PPFD meter and also use a spherical probe for lake studies.

Reflections etc likely play a large role in our tanks, but I'll stick with 2D dynamics for now. The sphere probe is extremely expensive, the Apogee is a relative cheap and many clubs/hobby groups can buy one and then share it to do data collection.

I often suggest that local clubs sell their plants on line to raise funds for high grade testing equipment and that they make a set of reference standards to share amongst their membership.

This helps folks learn much faster and better.

To be fair, W/gal/day is a good "quick rule of thumb" to estimate what you have without mesurements,

Yea, which is the same exact deal with EI!
So I find it ironic Edwards tries to hedge things with light while telling everyone how terrible and inaccurate EI is and how you are just guessing.

The main input of energy that drives CO2, all nutrient uptake is light yet he does not bother to even measure it in situ?

Humm........

The same applies to CO2, but it's not really something that cheap to measure accurate. I can measure reconstituted RO water using sodium carbonate and assume it to be all carbonate KH.
Then use pH and measure KH with a digital tritrator.
I've done this.
Then I added peat little by little over time and maintained the same CO2 flow rate.

But the CO2 dissolved meter method I suggested some time ago are made and there are LCO2 meters which get around all these RO/KH/alk variations and allows me to measure CO2 independent of such different in aquariums, tank to tank etc.

Then you can put it all together very well with high accuracy and precision.
If you are missing one large variable like light, you are really dead in the water and cannot compare independent observations.

Measuring nutrients is fairly easy compared to that.
But with organic fractions and influences and whether it's bioavailable or not?
Even that can get more complex.

Still, to answer such questions and to make sure our test are valid, you need to check. Then you go back and see how the model looks to use for predictions.

If all you consider for PPS or EI are the nutrients alone, then you will have the same poor correlations/poor success/plenty of algae.

Now plenty of folks do this and blame the method, however, lean EI vs rich EI, or non CO2, or PMDD, or ADA, or Dupla all work under some conditions.
Knowing what those are is key, but very few decided to measure and test it.

but we must not forget the type of lamps and reflectors and other factors you mentioned. We can't measure everithing and everytime... and don't need that to grow lush plants after all.

Yes, we have to account for bias in our assumptions in any model(rule).
You got it!

You are just right, we can't take it as a precise rule.
Many folks know that, I hope, so I simply do not see any need in posting everytime all that factors you mentioned in last post so I omit 'em. ;)

haha!
You crazy guy you:D

Will you do the patterns like ADA's?
Will you try to do the same with Super-Spreader® ?
I think we can have some use from this devise.


naman

Yes, I can, wait till Vaughn gives me the light meter back, I might drag the LiCOR out for some measurements at home or at another clients.

Since the tank is 1.3 meter's deep and we have 1000W MH's and can vary the light with sequential screens, that would be a nice tank to measure.

But I can show a much more relevant measure at home as a group of plants spreads out over say 2-3 weeks. Sorry to rehash over some of this stuff, I think out loud and have go through and ask myself each question.
This often helps me and other folks learn it. I'm fairly sure you have a decent understanding and if not, you will go get it.

Thanks,

Regards,
Tom Barr
 

Carissa

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I understand the desire for a rule to plug things into to determine light intensity, and why the wpg thing is so popular. Most people don't have access to a PAR meter and just want an easy way to get into the right ballpark with their lighting.

But, from what I can see, one of the major drawbacks of any such rule that is using gallons as one of the components of the formula, is that I don't think the volume of a tank really has all that much to do with the light intensity in that tank. Yes, it makes some amount of sense that a bigger tank would need more light. But wouldn't the actual dimensions of the tank be a much more accurate way of generalizing how much light a particular tank would need?

For instance a standard 10 gallon tank has a surface area of 200 square inches, a ratio of 20 square inches per gallon. A 50 gallon has 648, only about 13 square inches per gallon. So wouldn't it make sense that the 10 gallon would require substantially more light intensity per gallon to achieve the same effect as the 50 gallon? So just for purposes of this illustration, lets say we have 3 wpg over the 50, and we're trying to get the same over the 10g.... we would need 154% of that based on surface area, so we would actually need 4.62 watts per gallon.

However it's true that that doesn't take into account the depth of the tank. The 10 gallon is 12" deep and the 50, 19". There should be some type of calculation (which I'm not smart enough to figure out) that would account for the additional light needed to have the same intensity at 19" as you would otherwise at 12"..... or for this illustration since we're solving for the 10 gallon, the same intensity at 12" as we do at 19" in the 50g. Add or subtract this intensity from our previous calculation based on surface area, and maybe a more accurate watts per gallon would result.

Then the other issue is that you have a greater amount of surface area IN GENERAL in a smaller tank PER GALLON than you do in a larger tank. Light shining unimpeded in a tank through the water would seem to me to be much more powerful than light that bounces off glass, substrate, etc. and ends up getting either partially absorbed or lost. So maybe rather than just taking the surface area itself into account, the total of all the walls, substrate, and surface area should be calculated.

Anyway, all of this does not remove all the variables involved with the type of lighting and reflectors...but it seems to me that with some math, a formula or two should be able to be used to determine quite a bit more closely the lighting intensity in a particular tank, far more accurately than wpg could ever do, without the use of a PAR meter.
 

naman

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Tom Barr;25255 said:
However, you still have no reference standard to compare the light sources other than watts here between each other.
No "rule" can avoid that.
It's not that bad.
We use average lamps, with avarage PAR, in average pendants with average parabolic (I hope) reflectors haning over sdandard sized tanks.
When you do the science, that's right to mesure the way you say, but we are just "farmers" :)
For us, "farmers", mesuaring standard situations is enough.
If we get something similar data to that link posted by VoughH we will be happy with that.

naman
 

VaughnH

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My first use of the PAR meter which Tom was so generous to loan me was to try to learn more about my simple 10 gallon tank. That tank has a DIY modification to its Perfecto hood, shown here: http://www.aquaticplantcentral.com/...ects/45208-diy-mod-perfecto-10-gal-light.html

One thing I wanted to find out is whether a simple reflector like I used there is effective. The answer is YES!! Very effective. Here is a graph of PAR readings for that fixture, with and without the reflectors. (The "without" curve is only two data points, with the rest calculated.)
10GalReflector.jpg


You can easily see that those simple little reflectors come close to doubling the amount of light the fixture puts into the tank. This should help encourage people to make simple cheap modifications like this, when money is a problem.

The way I measured this data was to set the fixture up on a jig so I could do the measuring out in the open. To verify that this gives usable data I also got a single reading in the tank, in the water, to make sure the "air" data is close to the "water" data. Here is the setup for the "air" data:
IMG_1603.jpg
IMG_1604.jpg


I took readings at 11 places under the light, and at 8 distances below the light. I'm still working on the data to chart the variation in PAR throughout the tank, and I may post that later.
 

VaughnH

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Another issue that the measurements on the 10 gallon Perfecto modified hood could resolve to some extent is whether or not the light varies per the inverse square rule as you move further from the bulbs. To try to answer that I plotted the PAR values measured under the center of the fixture, and tried to fit an inverse square equation to the data. Of course it isn't possible to fit such an equation as you get too close to the bulb, because eventually the light source is just too far from an ideal source. At a quarter inch from the center of the outer surface of one bulb I measured 1000 on the meter (vs. 1700 outdoors in sunlight at 9:00 a.m.) I settled on fitting an inverse square equation to just the points near the bulb, but not that near. This is what I found:

10gallight.jpg


I have to conclude that for this fixture you can use an inverse square relationship to approximate the difference in light intensity between two places at different distances from the bulb, but you can't use it to judge the loss of intensity starting from right at the bulb itself. So, it is accurate to say that a tank that is twice as deep as another tank will have about one quarter the light intensity at the middle of the tank, for the same fixture on both tanks. And, much more interesting to me is that as a plant grows in this tank, the amount of light it receives grows in inverse square with its nearness to the bulb.

If I generalize this, it explains why my plants, in any tank, grow slowly for quite awhile, then accelerate in growth as they get taller.
 

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Now add some plants, some water, some canopy over time and space.
Inverse square law is pretty much what we'd predict to find.

However, in the real world, it deviates.
So measure that afterwards once you are done here.

Measure, mark and follow a single leaf through a week or two, or a set of leaves(say 11 at random points in the tank). Plants will also adapt readily to light SFD as well as PAR and also through time/space.

Measure the effects of pruning weekly vs every 3 weeks.
Measure the effects of a MH vs a FL tube, spacing in FL tubes(say a narrow band with two bulbs near each other vs 15" apart).

As you can quickly see, you have an infinite set of possible questions to ask and test for there, and with respect to each plant species available (300-400).
 

VaughnH

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Tom Barr;25271 said:
Now add some plants, some water, some canopy over time and space.
Inverse square law is pretty much what we'd predict to find.

However, in the real world, it deviates.
So measure that afterwards once you are done here.

Measure, mark and follow a single leaf through a week or two, or a set of leaves(say 11 at random points in the tank). Plants will also adapt readily to light SFD as well as PAR and also through time/space.

Measure the effects of pruning weekly vs every 3 weeks.
Measure the effects of a MH vs a FL tube, spacing in FL tubes(say a narrow band with two bulbs near each other vs 15" apart).

As you can quickly see, you have an infinite set of possible questions to ask and test for there, and with respect to each plant species available (300-400).
Sounds great! I only spent all day on the first little problem, and I figure I have about 10 more possibly good years left, so I should be able to complete all of that. Can I keep the PAR meter for 10 years? Will the battery last that long?

However, I see the problem, just as I get down to the last question, someone out there will think of 10 to 100 more good questions. Not to speak of all of the new plants that show up every year.
 

VaughnH

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Here is some more data from today's PAR measurements. The first is the meter readings along the length of the tank, at two distances from the bulbs: (10.25 inches is about at the substrate)
EndtoEndLight.jpg


Then, the meter readings from front to back at two distances from the bulbs:
GraphLightFrontBack.jpg


Look at how much the intensity drops off in just 6 inches!
 

orion2001

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Great work Vaughn! I can already begin to form a mental scape of the light distribution in the tank. Nothing very unexpected in your results so far except it is quite evident how much of a drop off in intensity there is with height! Btw, are all your data points at the intersection of black lines or are there any more data points in between?
 

VaughnH

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All of the data points are those on the graphs. I took about 100 data points today, so these graphs are just some of the data. The rest of it doesn't add much so I haven't tried to graph the rest.

Tomorrow I will cover the reflectors with aluminum foil, take more data. Then spray the aluminum with glossy white paint and take more data. This should add to my understanding of the effectiveness of mylar, aluminum foil and plain white paint for reflectors. I can get all of that on one graph.

One more thing - I took one measurement in my 45 gallon tank, with 110 watts of AHS light kits, at the substrate level, near the front of the tank. It was 90. I need to do a major pruning job on the tank before trying for more readings there, and it is much more difficult to get these readings, for several reasons.
 

orion2001

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I can't wait to see your results from testing out the aluminum foil and white paint. Thanks a ton for your time and effort! I've been holding out on purchasing mylar as I wasn't sure if I was willing to spend 20 bucks on a big roll of it.

Btw, I might have missed this, but are all these readings in a dry tank or a filled one?

Good luck with all the experiments.
 

VaughnH

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All of the ten gallon tank light fixture readings are out in the open air, with the fixture propped up on a wood stand. For the moment I am interested in what the light fixture can do, so it doesn't do any good to try this is water, and it is much more difficult to do it in water. (There is barely room to get my hand down into the water in a ten gallon tank.)
 

Tom Barr

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A min level for most plants is about 25-30.
Max levels are about 450-600.

Full sun = 2000.

Also, if you notice the peak in the middle of the graph when measuring the length of the tank under the bulb.

Since the source (the bulb) is a linear tube, how might this graph change if it was a MH HQI bulb?

Also, consider multiple strike angles here.

If you add 2 bulbs, one on the front and the other on the rear of the tank, how might this influence light intensity?
Measure in the middle, the front and the rear at the same depth.
It should be fairly even now.

Rotate/change the orientation of the probe also as different angles(leaves do not always point directly straight up). Then try it with a single bulb and then with 2 bulbs spaced far apart(like at the front/back of the tank example vs one in the middle or both in the middle).

You can predict the light intensity pretty well vs distance.
Now try patterns from the source of light, angels, reflectance etc and what a real plant leaf does through time.

General physical patterns are fine and do help, but biological patterns are far more interesting and relevant.

Regards,
Tom Barr
 

VaughnH

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Some times you find yourself with egg all over your face, a wet splotch on your trousers, toilet paper hanging behind you, etc. Well, this, for me, is one of those days.

Today I tested two more reflector materials. First I covered the mylar surface with plain aluminum foil, with only moderate care not to wrinkle it. Next, I covered the mylar with white paper and spray painted the paper with glossy white paint. I took PAR readings with both configurations just as I did yesterday, to prove to myself how superior the metalized mylar is - and I have been insisting to all who care to listen that metalized mylar has to be superior. Here are the results: (Ignore my tear drops on the graphs.)
ReflectorMatlComp1.jpg
ReflectMatlComp2.jpg


It is obvious how wrong I have been. Aluminum foil, even used with little care to keep the wrinkles out, is much better than the aluminized mylar. But, the real shocker is that plain old paper with a thin coat of glossy white paint is also better than the mylar.

I can either believe the PAR meter, and my eyes, which also gave the same results, or I can insist that my theoretical musings about the superiority of aluminized mylar were correct. Being a retired engineer I will go with the PAR meter results. Back to the drawing board for me!