Light Intensity in an aquarium

orion2001

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Wow! That is some eye opener! I had a sneaky suspicion about this being true beforehand since I have one of my CFL's with an aluminium foil reflector and the other with a much more reflective foil material (not mylar). It seemed like the aluminum foil side was a tad brighter but I thought it was in my head. Still, it might be that optimized reflectors (correct geometry and size for the bulbs) might still work better with Mylar as compared to aluminum/white paint.
 

VaughnH

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The difference between aluminum foil and mylar is so great I can't believe the mylar would ever come out better. If I had taken great care to keep the wrinkles out of the foil, and glued it down smoothly to the reflector, I suspect the difference would have been even greater.

I have known for years that aluminum is the best reflector material known. All optical mirrors are aluminized for that reason, but those are first surface mirrors, with only a molecular layer of silicon dioxide covering the aluminum. It follows that aluminum foil, applied carefully enough, would be a good reflector. The mylar just gives such a great mirror reflection it is hard to accept that less light is being reflected - the reflection is just truer to what is being reflected. And, as has been stated here and other places, white paint, at least reasonably glossy white paint, is a very good reflector for light. Duh....that's why it looks white!
 

joyban

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Light Intensity in an aquarium:-

Good at last we have a possibility of analyzing the real data based on simple experiment to prove the movement and intensity changes of light in Water which actually would contribute to Photosynthesis...lot of this was discussed in the "A Comparison Between Light Sources Used in Planted Aquaria " By Ivo Busko, long time ago but some serious discussion never happened, the copy of the article can be read at :-

Aqua Botanic-light bulb comparison

Also having read some chapters of "Light and Photosynthesis in Aquatic Ecosystems" second edition by John T.O. Kirk some of the following parameters do apply to light physical properties as it travels from the source to water and into it, primarily which are as:-

· A photon of wavelength 700nm from the red end of the spectrum will have less energy from a photon at 400nm from the blue end of the spectrum. where energy = (1988/ λ) X 10-19 Joules,where λ = wavelength

· Hence at 400nm the energy of a photon is 233.33% more than the energy of a photon at 700nm.(energy at 400nm = (1988/400) x 10-19 Joules = 4.97 x10-19 J and at 700nm it is 2.84 x10-19 J)

· Light travels at a velocity equal to the velocity of light in a vacuum divided by the index of refraction (n), which is typically for water n = 1.33. Water 100'C 1.31; Water 20'C =1.33335 & Water 35'C = 1.33157

· Hence the velocity in water is about 2.25x10^8 m/s. Because light travels slower in water than in air, some light is reflected at the water surface. For light shining straight down on the water, the reflectivity is (n - 1)2 / (n + 1)2.

· For freshwater, the reflectivity is 0.02 = 2% at zenith angle of incidence between 0 to 20 degrees and then it remains low increasing rapidly after 50 degrees, to 89.6% reflectance at 89 degrees of incidence angle.


See Chart below:-


Zenith Angle Of Incidence Degrees Reflectance %


0 deg 2.0% 50 deg 3.3%
5 deg 2.0% 55 deg 4.3%
10 deg 2.0% 60 deg 5.9%
15 deg 2.0% 65 deg 8.6%
20 deg 2.0% 70 deg 13.3%
25 deg 2.1% 75 deg 21.1%
30 deg 2.1% 80 deg 34.7%
35 deg 2.2% 85 deg 58.3%
40 deg 2.4% 87.5 deg 76.1%
45 deg 2.8% 89 deg 89.6%

Reflectance of Unpolarized Light from a flat Water Surface assuming that the water has a refractive index of 1.33See – Fresnel’s Equation
Hence most sunlight reaching the water surface is transmitted into the water, little is reflected. This means that light incident on the water surface in the aquarium is mostly absorbed below the water surface.

At this moment on water surface and just below the water surface light or photons are lost by absorption and scattering out of path and Gain by scattering into path. Also there is a factor of refraction and reflection of light at air –water boundary as shown below :-


lightairwaterrq2.jpg





Refraction and reflection of light at air-water boundary. (A) A light beam incident from above is refracted downwards within the water: a small part of the beam is reflected upwards at the surface. (B) A light beam incident from below at a nadir angle of 40° is refracted away from the vertical as it passes through into the air: a small part of the beam is reflected downwards again at the water-air boundary. (C) A light beam incident from below at a nadir angle greater than 49° undergoes complete internal reflection at the water-air boundary.
Ref:- (Source Page 45 Light and Photosynthesis in Aquatic Ecosystems)

Also how much light is reflected or refracted depends on the wind speed and disturbance of the water surface by wind , in case of a aquarium it can be attributed to water movement on the surface due to Air Pumps, CO2 diffuser or power heads.

The light absorbing components of the aquatic system are :-

· The Water Itself
· Dissolved Yellow Pigments
· The Photosynthetic biota
· Inanimate Particle matter

Water:-
Water though it appears colorless in small quantities is blue in colour and it absorbs light more in the region from 550nm and above quiet significantly in the red region. It has been found out that about 35% of incident light at 680nm is absorbed at 1 meter depth of water or 1 meter thickness of water. So if we were to refer the absorption Coefficients derived from the published attenuation coefficient Morel & Prieur 1977 you would see that at 380nm the value is as 0.023 , 400nm = 0.018 ; 450nm = 0.015; 500nm = 0.026; 550nm = 0.064 ; 600nm = 0.157; 630nm = 0.310; 650nm = 0.350 ; 670nm = 0.430; 680nm = 0.450; 690nm = 0.500 and at 700nm it is 0.650;
At 740nm ,760nm and at 800nm the values are as 2.38, 2.55 & 2.07


Hence Blue light is absorbed least, red light is absorbed most strongly.

Attenuation per unit distance is proportional to the radiance or the irradiance of light where x is the distance along beam, c is an attenuation coefficient and I is irradiance.

dI/dx = -cI

Fig6-17.jpg


Figure Absorption coefficient for pure water as a function of wavelength λ of the radiation. Redrawn from Morel (1974)

If the absorption coefficient is constant, the light intensity decreases exponentially with distance.

I2 = I1 exp(-cx)

where I1 is the original radiance or irradiance of light, and I2 is the radiance or irradiance of light after absorption.

Actually the contribution of water itself to the attenuation of PAR by absorption of quanta is important only above about 500nm, and where we use artificial light which have more spectrum in blue and blue green range rather than at red, yellow - red range ( a good full spectrum light may have a good ratio of red : blue though the light may be towards or around 3500K to 4000K) would mean that more red light or photons are lost compared to the blue as they travel downwards with in water, now how much of this loss is significant to an aquarium where the water depth may not be more than 18 to 24 inches ( 1 meter = 39.37 inches) is a question one has to measure and check ; hypothetically more blue light reaches the plants than red as we go downwards the depth of water from surface.

Humic, Tanin etc:-
From the point of view of photosynthesis water soluble humic substance impart yellow colour in water and this leads to absorption of light particularly at the blue end of the spectrum. Yellow material of the humic type can also be generated by decomposition of plant mater with in the water body.

So due to tanin, humic acid in water Blue light is absorbed more towards the 400nm to 550nm region and much less at and beyond (drops significantly) at 600nm region.

Particles:-
The inanimate particulate matter in water and their typical concentration does not absorb light strongly but scatters quiet intensely but studies have shown that absorption is low or absent at the red end of the spectrum and rises steadily as wavelength decreases into the blue end of the spectrum.

The Photosynthetic biota:-
The Absorption of Light by the photosynthetic pigments – chlorophylls, carotenoids , biliproteins etc contributes to the attenuation of PAR with depth. Specific absorption coefficient corresponding to 1mg of chorophyll a per unit area of 1 meter cube is as :-

At 400nm = 0.017, 450nm = 0.024 ; 500nm = 0.018 ; 550nm = 0.011; 600nm =0.007 650nm= 0.014 & at 700nm= 0.003 so it is a maximum around 440nm almost touching 0.025 and then dipping at around 575nm and again peaking around 670nm at 0.018 almost like the photosynthesis action spectrum curve.

Hence as light travels through the depth of water there is significant amount of light loss due to various factors like water depth itself, tanain humic acis, other particles , dust, and then the chlorophyll from algae & plants itself, hence a combination of these contribute to light loss in water or loss of photons in water.

Scattering of Light within the aquatic medium also contributes towards the availability of photons for photosynthesis. Many of the photons undergo scattering one or more times before they are absorbed . Scattering does not remove the photon but the photons have to travel more in a zig zag path as they get scattered and this increases the total path length traveled by them which may result in capture of the phonon by one of the components of the absorbent medium (water) as mentioned above. In addition some photons are scattered back in upward direction thus the effect of scattering is directly related to the light or photon intensity and the vertical attunation of light in water due to scattering of light by density fluctuation or by particle scattering.

Scattering of pure water is of the density fluctuation type and varies with wave length. Experimentally scattering is found to vary in accordance with 1/λ4.32 . The scattering coefficient of natural waters are invariably much higher than those of pure water. See table below:-

wavelength scattering coefficient/ meter


Pure Water

400nm= 0.058
450nm= 0.0035
500nm= 0.0022
550nm= 0.0015
600nm= 0.0011


Pure Sea Water

450nm= 0.0045
500nm= 0.0019

Marine Water

Atlantic Ocean
Sargasso Sea
440nm= 0.04
633nm= 0.023

Pacafic Ocean
Galapagos Is.
440nm= 0.08
655nm= 0.07

Fresh Water- River

Irondequoit Bay Ontario
400-700nm= 1.9-5.0

Perry - Tasmania
400-700nm= 0.27

Gulungul - Myrray Darling System
400-700nm= 5.7


Fresh Water- Lakes
Rotokakahi
400-700nm = 1.5
Rotorua
400-700nm = 2.1
D
400-700nm = 3.1



(Source Page 101 Light and Photosynthesis in Aquatic Ecosystems by by John T.O. Kirk )

Cont Next Post...
 

joyban

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Cont from previous post:-


Hence
Attenuation of light in the water column – due to absorption and scattering

Transmittance (amount of light left) = Iz/ I0 x 100
where I = irradiance,
I0 = irradiance just below surface
Iz = irrad. at depth z

Absorbency [100 x (I0 - Iz)]/I0

Fig6-18.jpg


Figure Left: Attenuation of daylight in the ocean in % per meter as a function of wavelength.
I: extremely pure ocean water; II: turbid tropical-subtropical water; III: mid-latitude water; 1-9: coastal waters of increasing turbidity. Incidence angle is 90° for the first three cases, 45° for the other cases. Right: Percentage of 465nm light reaching indicated depths for the same types of water. From Jerlov (1976).

Attenuation equation
Iz = I0 e - kz
where e = natural logarithm
k = attenuation coefficient (extinction coefficient ref- Wetzel)
characteristic for each water body and each wavelength
often converted to a linear plot by taking the log of both sides:
ln Iz = ln I0 – kz


Components of the attenuation/extinction coefficient

K_l = K_abs + K_ scattering
K = K_water + K_dissolved organics + K_particulates


K_water
- for pure water, absorption at long wavelengths dominates (>550 nm; red and IR)
- So, IR disappears in the top 1-2 m of most lakes
- Scattering at short wavelengths,
 

Tom Barr

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Bottom line is to have a standard that is biologically comparable to other aquariums.
A meter is nice because it does not have all the assumptions of the model, and there are many.

You can measure things through space and time as a plant grows, things change dramatically as the plant grows from the bottom and heads to the surface before you prune it.

You can measure this and see this with the model as well.
Still, few aquariums are the same when it comes to light and each factor that influences it.

Same for CO2.........

Thus the same for the nutrient demands........

You can assume non limiting or near/saturating light values at 450-600micromols.
Then proceed to CO2, then get a non limiting value for that, then start measuring nutrients.

That's what I did a long time ago. But.....that's just for high light.........and most folks have less............

But now I know those curves, now I can come back and revisit CO2 and light and make their curves under specific cases as well. Then get a precise feel for tanks using T5's at 1.5w/gal say............or my own tanks.............or a non CO2 tank.........or your own tank...........etc

And you also have a good predictor for if there's enough light in a certain spot in your tank, which is pretty practical if you think about it and the relevant Biological question most aquarist have.

Regards,
Tom Barr
 

Carissa

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I would love to see results for different bulb types....cf's, T8's, T5's, T12's, etc. all other things being equal. With this data along with the data on the reflector types, it should be possible to create a calculation that we could plug values into that will more easily compare apples to apples when comparing light levels from one tank to another. To be more accurate it would also need to take into account the distance from the light to the surface, and to the substrate of the tank, since this obviously has a huge effect on the total amount of light available to the plants. It wouldn't cover every variable for every tank, but it would sure beat the pants off of the wpg rule.
 

Tom Barr

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I think what Vaughn and others will find in using a PAR meter is rather simple and eluded to here:

You see many errors and assumptions you have made using a model to estimate the true values of light and how they effect plant growth/RGR's etc.

Now going out and testing a bunch of light bulbs under ideal situations is really not going to get around all those assumptions in the model.

It will give insight into the inherent issues with various light models used to estimate how much light you want without a meter etc.

But it's got it's limits.
So do not put that much faith into such news or test when you read it.

With EI, we can estimate a partial true estimation and get fairly close over a range, with light, it can be trickier, but still, the 2-4w/gal rule has not failed me yet on tanks from 10-400 gallons...........and I've set up a few in some 30+ years......

That w/gal model has issues, but the issues are not resolved by doing all these calculation and factoring in these variables because there are many many species of plants, bulb ages, reflector types etc.

It starts to become so complex and cumbersome that it no longer is useful to a newbie or the old timer. A better method is to simply have a "group meter" if you are really interested in making compoarisons and testing it for your tank specifically.

Then you have a true value and you can measure at various locations and through space and time in your tank of interest and with respect to your plant species of interest.

Folks grow an infinite(almost) combination of species together, and those are all influenced differently and have different rates of growth in response to light.

Only after getting a lot of standardized info, can you really start looking at generalizations, and that still assumes they are doing everything right with CO2/nutrients. This takes awhile.

Most do not want to go that route, they want a short cut.
But short cuts come with a price that you must pay.

LCP's like in the Egeria najas study, or the Hydrilla study or a Myriophyllum study are not really done with respect to CO2 enrichment.
LCP's tend to be even lower if you add high CO2. So even those are not quite applicable to our cases, however, Tropica did a small study that showed this with Riccia.

Other studies have shown this as well, but not down near the LCP.
Bulbs alone are not the whole picture either, the brand, the ballast, the spacing between the bulbs and configurations, the distance from the bulb, whether you use glass covers etc(how thick they are etc).

There are many things you can add to this model.
I'm not so sure doing all that is going to get you that much closer to knowing and understanding the light that the plants need and is it that = to another tank in any quantifiable manner.

To do that, you need a PAr meter with a water proof probe.

BTW, the Apogee probe is water proof:)

Regards,
Tom Barr
 

VaughnH

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Today I made some PAR readings on my light fixture for my 45 gallon tank. The set up is similar to what I used for my 10 gallon fixture:
IMG_1605.jpg


I took 21 readings at the distance from the bulbs that is about the substrate surface in my tank. The readings ranged from 97 to 125 along the length of the tank at the center, and from 75 to 96 along the front glass and back glass. Then to see how the light decreases with distance I took readings at 1.5 inch increments in distance below the plane of the bulbs. To get a comparison between this light fixture and the ten gallon fixture I interpolated my 10 gallon fixture data and plotted that data on the same graph as this data:
LightComparison.jpg


The two curves are very similar. I tried to fit a inverse square equation to the 45 gallon data and quickly realized that you can fit any curve to that data if you substitute (X + A) for (X) - using a virtual location for the lights above the real location. And, with two bulbs being involved, with the AHS bulbs being PC twin tube bulbs, you can't use just (X) as the distance. So, it still iisn't clear to me if the intensity drops off with the square of the distance, or if it drops off directly with the distance. I think it is a non-linear drop off, but not exactly an inverse square drop off, for both types of bulbs. I'm still playing with that data. It does look obvious that if you normalize the two curves, they are almost identical in the range shown.

EDIT: If you scale the 45 gallon fixture light intensity curve by the ratio of the intensities at 4.75 inches, the following is the result:
ComparisonChart.jpg


I think this demonstates that the linear bulbs in the AHS fixture lose light with distance less rapidly than the "blob" bulbs in the CFL Screw-in fixture, so at the 10.75 depth, the screw-in bulbs have lost twice as much as the linear AHS bulbs.
 

Carissa

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Tom,
I understand your point...it would be impossible to factor in all the variables that exist in reality to get something that would work out correctly and accurately, every time. It would seem that the wpg rule that people tend to rely on too heavily (like you said, people like short cuts), is pretty shot through with this data. I mean, just the material that the reflectors are made out of can change the intensity by a huge amount. Then start changing bulb types and fixture height, and again things can be completely changed. It's true that we can't eliminate all variables, that would be an exercise in futility. But if we could just take the top 4 or 5 variables and create a formula to account for those, I would think that the margin of error would be considerably less than just using two variables (watts and gallons). Then people who don't have years of experience in setting up planted tanks would have a better starting point to jump from when troubleshooting or deciding what type of lighting to buy for a new tank.
 

Tom Barr

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Well, the main variables themselves has issues, eg as Vaughn quickly saw: the reflector material alone is highly varied.

So saying you have a reflector does not say much, or that you painted it white or used mylar etc......

Likewise, saying you have PC's, well, the screw in globe type, T5's, what brand(they do vary by a huge factor BTW), linear, 96W vs 55 W bulbs......these too all vary enormously.

The point is that the while it would be nice to get a better model, and reduce it to the main 4-5 variables, these variables are variable themselves:)

Which is the point in Vaughn and other folks to try the PAr meter out and see and think about what it means.

Information is not knowledge.
You have to apply and do something with it and have some logic behind it.

My general goal is to move away from the technical stuff and apply simple models, I do like them, however, with light, it's very easy if you have the equipment to measure all sorts of interesting things and doing so is VERY EASY.
The trade off is the cost for the meter.

I compared the A&H light to AJebo once at a meeting.
You could easily visually see the difference, and the PAr meter confirmed the reading.

A&H + stock reflector(new, 6700K bulb 55w) at 4" : 450micmol
Jebo........anyone wanna guess? Same color temp, same watts, same distance and stock(Jebo's) reflector and ballast: 280 micmol.

Kim from A&H likes to show this at aquarium shows to make the point that not all PC light systems are the same.

How do you factor these in as well?
You can do it like Reef folks did, brand by brand, situation, and color temp and start with the ballast brand/model etc, bulb type, distances, decay over time, reflector type, etc.

You can say but but but............however the issue will not be put to sleep so easily, there are many things you can measure the theorize about doign and models to make.

You can also waste huge amounts of time in doing this;)
Stopping for a moment and thinking about what you want to do and set up that will be useful later... is critical.

What is really practical?

Given the time/all the different brands/lights out there, I think if folks came together and divided the labor up, and did their tank a few times and then did a new light set up and posted such back ground info, that's great.
This means folks need a meter. I do not have access to that many light systems and those that I do, are rather unique overall.

Using the meter itself is very easy, thinking about how to use it and what questions and information you want is really the issue here.

I'll likely loan the meter to some other friends after Vaughn has had his way with it.

Regards,
Tom Barr
 

pomacanthus

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Tom-

FWIW I owned a "Lux meter" back in the early 90's to record levels at different depths in my reef aquaria, but I dont qualify as a hobbyist. At that time i was a retail saltwater store owner.

So after digesting 70% of this technical thread data, and feeling very sleepy i have a really EZ question for you. Do you prefer fluorescent (regardless of type) or metal halide, or both in combination. Dont over think this question as it pertains to different planted aquaria etc. just looking for a general answer ( ie your opinion.)

Im a dinosaur saltwater guy, and a plant tank newbie (in progress) Thanks for the forum, its exactly what i was looking for.
 

orion2001

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OK, now I am thoroughly confused. After reading up on Mylar on Wikipedia (yes, I know it isn't the most reliable place for info) :
PET film (biaxially oriented - Wikipedia, the free encyclopedia)

It seems that Mylar as we know it is basically a metallized version of this PET polyester film. The mylar that we use is actually aluminum metal in PET!! Now this makes it extremely hard for me to figure out how Mylar isn't giving better/equivalent results to aluminum foil. Given the processes used to metallize Mylar, I'd assume you have very low surface roughness (which explains why it looks so much like a mirror). Also considering

The wiki on Aluminum states:

"Aluminium is one of the few metals that retain full silvery reflectance in finely powdered form, making it an important component of silver paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3000–10000 nm (far IR) regions, while in the 400–700 nm visible range it is slightly outdone by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper."

Hoppy, this is my theory. Let me know what you think of it. It could be, that in the case of your Spiral CFLs, your reflector geometry isn't optimal for something as highly reflective (next to zero diffuse reflections) as mylar leading to significantly higher light losses due to restrike. On the other hand, Aluminum foil, though not as reflective as Mylar, causes more diffuse reflections which basically lower this restrike as some of the light that would be reflected straight back into the bulb is now being scattered at different angles, bouncing around and in general providing better illumination. This would also explain why something like the white paint on paper behaved quite well since it is entirely diffuse in its reflection of light...again lowering restrike effects.

Something I had thought about a while back but never fleshed out in my head is the following idea:

For non optimal reflector geometries, there is a good chance that a completely diffuse reflector such as white paint or a partially diffuse reflector such as aluminum foil will provide better illumination than a perfect reflector like Mylar since the non optimal reflector design coupled with the perfectly reflecting Mylar surface will cause a significantly larger amount of restrike and thus, losses. On the other hand, diffuse reflectors in such a situation would cause some of this light that is being forced to "Restrike" due to the non-optimal reflector design in the case of Mylar to be scattered in other directions which wouldn't experience "re-strike". This effect would be even more pronounced in the case of Spiral CFLs since they have a geometry which lends itself to a lot of restrike. In addition it is next to impossible to design an "Ideal reflector" for Spiral CFLs. In this case, diffuse reflectors are probably the best materials to use.

I don't know if this is clear in words. I could draw a diagram to explain my idea.

I would postulate that for a T5 tube (restrike being less of an issue) and an optimally shaped reflector (see pic below), Mylar should still be more effective than aluminum foil or white paint (As Tony Gomez in the site linked below seems to believe).

reflection.jpg


This picture is from Aquarium Plants - Info Pages , interestingly the author claims that white paint is much better than aluminum, and hints that mylar would be the best.

I'd love to hear what people think about this idea.
 

orion2001

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I tried to put this reasoning into some sort of math. This is what I came up with:

X is the fraction of light emitted by bulb that restrikes the bulb for a perfect mirror material(mylar) used for a suboptimal reflector geometry.

Fraction of light emerging out of fixture for Mylar = 1-X

Now consider a diffuse reflector.

If light is incident on a diffuse surface (lets assume perfectly diffuse...meaning equal probability of reflection in all directions in a hemisphere from the surface) then a fraction of the total light will actually restrike and be reflected back into the bulb. IF we call this fraction Y, Y can be expressed as the ratio of the angle subtended by the bulb at the point on the reflector to the total angles it could be reflected at (ie 180 degrees for a hemisphere). Now this ratio Y would change depending on which point of the reflector you are at. Right above the bulb, at the point on the reflector closest to the bulb, this ratio would be the greatest (it is clear in the picture in my previous post), whereas at a point on the left or right portions of the reflector, this fraction Y, is lower. A good guess at what the mean value of Y should be, if you integrate across the whole surface might be Y=0.2-0.3. Lets assume Y=0.25.

This implies that in the case of a perfectly diffuse material you would expect 1-Y=0.75 of the total light being output by the bulb (in the upwards direction) to be available. So roughly you would get around 175% of the light as compared to if you had a perfectly black reflector.

Now if you have a perfect reflector geometry, X can be extremely small for the perfect mirror material. However the efficiency for a perfectly diffusive surface will not improve much at all since the diffuse nature will cause light to scatter in all directions. An interesting corollary is that perfectly diffuse reflectors will be very insensitive to the geometry of the reflector in terms of how efficient they are. The variability in performance of diffuse reflectors for a whole host of geometries should be pretty low.


It is very easy to see that in the case of a perfect mirror surface like Mylar, for a spiral CFL and non optimal reflector geometry, the fraction X could easily exceed this value of 0.25, and in this case a diffuse reflector would be a better option as compared to a perfectly reflecting material.
 

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pomacanthus;25324 said:
Tom-

FWIW I owned a "Lux meter" back in the early 90's to record levels at different depths in my reef aquaria, but I dont qualify as a hobbyist. At that time i was a retail saltwater store owner.

So after digesting 70% of this technical thread data, and feeling very sleepy i have a really EZ question for you. Do you prefer fluorescent (regardless of type) or metal halide, or both in combination. Dont over think this question as it pertains to different planted aquaria etc. just looking for a general answer ( ie your opinion.)

Im a dinosaur saltwater guy, and a plant tank newbie (in progress) Thanks for the forum, its exactly what i was looking for.

Hard to say.
They are very different for a number of reasons to me personally.
Aesthetics are vastly different.

I like mixing color temps, always have going way back.
This always seemed to give me a nice color I liked.
I tend to like warmer color temps than most these days.

For bring out reds, FL's, say the GE 9235K's are very hard to beat or the triton bulbs(but they only make a straight pin 21" bulb or the T12/T8 sizes), I also have a nice mix with 8800K and 5000K CSL PC bulbs, but no more(no longer made).

MH's always washed out many colors but the pearl intensity and the shimmer, greens, always seemed really pretty.

One is point source and displays a different pattern under water, whereas the FL's will do something different, point source vs the linear line of light.

Mixing the trade offs of each with each other is what I do for myself and clients.
So I prefer both.

I can run my tanks at low light, about 100 micmols down to 30-40 with the PC's, or go to the high light ranges with PC+MH and have 80 at the lower ranges/edges, up to 600 in the hot spots.

Regards,
Tom Barr
 

Tom Barr

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I suggested mylar in the past as well.

It may depend on the shape of the foil or mylar.
Foil can be bent and messed with over time, mylar stays fairly stable and doesn't crinkle(which can help or hurt you with PAR).

I made curved arcs like the reflectors using mylar.
It's been awhile since I've used it.

Regards,
Tom Barr
 

VaughnH

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Orion: First a bit of nit picking: fluorescent tubes, whether T5, T8 or T12, all emit light from the inside surface of the tube, not its centerline. The centerline of the bulb contains the mercury arc which emits UV, which strikes phosphors on the inner surface of the bulb, which absorb the UV and re-radiate the energy as visible light. So, Tony's chart you showed isn't at all accurate, and there is no optimal reflector for a fluorescent bulb.

Yes, I understand your theory about diffuse or imperfect reflection vs near perfect reflection. You may be correct, but I don't know any way that I have available to verify that. One could do an experiment with a light source that approaches being a point source, with a simple flat reflector, but the sensor would have to be very small for the experiment to work. (I think.)

The mylar I am using is, I think, aluminized on both the front and back surfaces. Neither surface is 100% metalized, but the combination comes close. (Some light goes through the metalizing through the gaps.) That is one source of inefficiency. Another source is the anti-oxidation coating on the aluminum coating, and I don't know what that is or how thick it is.

Mylar looks so "perfect" only because it gives a very nearly true reflection, so it works reasonably well as a mirror for shaving, for example. But, our eyes are extremely capable of using a wide variation in light intensity and ignoring the variatiions. So, if only 80% of the incident light is reflected the mirror effect will still seem to be perfect.

Aluminum foil, not having nearly as perfect a surface, looks like a poor reflector - it gives a distorted image. But, our eyes concentrate on that aspect and ignore the higher percentage of incident light being reflected. That is my opinion.

White paint gives pure diffuse reflection, but appears blindingly white only because such a high percentage of incident light is reflected. That reflected light is reflected at small angles off of perfect reflection, making it useless as a mirror, but the small angles are not great enough ot greatly reduce the total amount of light being usefully reflected. Again, this is my opinion.

Others have been posting that mylar isn't really very good as a reflector, and that white paint is very good. I couldn't accept that since it was so counter intuitive. Those "others" also have said that aluminum foil isn't a good reflector. I intuitively accepted that. What I think we have here is a failure of intuition.
 

VaughnH

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Below is a plot of my data yesterday for my 45 gallon tank fixture with AHS light kits, measured along the length of the fixture at the depth of the top of the substrate. I added the data for the 10 gallon fixture with the screw-in CFL bulbs for comparison.
LightIntensityLength.jpg


I think the drop off in intensity results because as you move towards the ends of the fixture the light contribution from the opposite end drops off rapidly, and there is less contribution from the shorter end in the opposite direction.

It should be clear that there is no one number that characterizes the light from a light fixture, no matter what bulbs or reflectors are used. So, we might as well just use wattage and bulb/reflector type as our descriptive "number" for such fixtures. i.e. 110 watts of AHS Bright Light kits with GE 9325K bulbs.
 

VaughnH

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Today, after extensive thinning of my plants, I managed to get a reading with the PAR meter, at the substrate, about 8" to the left of the centerline and at about the centerline from front to back. The reading was 50 - 80, depending on whether I moved an inch one way or another. This is reasonably close to the value I measured in air. I still have no way to accurately position the sensor way down in the water like that, and the remaining plants can shade the sensor enough to drop it to a 20 reading with just a small movement in either direction. Clearly one would need a tank without plants to measure the actual light intensity unaffected by the plants. I see no way to determine the distribution of intensity as I did in air.
 

VaughnH

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Today I measured the light intensity from my 55 watt AHS Bright light kits in my 45 gallon tank light fixture, for different reflector materials. To make this easier, I did the measurements with only one bulb of the two installed. First I checked the reflector with no modifications, using the same technique as I used for this fixture before. Next, I covered the AHS reflector with aluminum foil, trying to minimize the wrinkles in it. And, finally, I covered the reflector with white paper, spray painted with glossy white paint. All of the PAR measurements were taken at the fixture centerline, so the light was affected by the reflector, since that bulb was off center above the sensor.

I plotted all of the data, plus the data taken with both bulbs installed, on one graph:
AHSReflectorComparisons.jpg


Two obvious conclusions: I get about 50% more light at the center of the tank by using two light kits instead of one. And, aluminum foil and white paint are effective reflector materials, although not as good as the AHS reflectors. I think I may have misread the meter for the aluminum foil test at the closest to the bulb point. (I suspect all three readings for that location should be about the same, since almost all of the light is directly from the bulb with little from reflection.)

I'm out of ideas now for using the PAR meter to answer questions I have. But, if I think of something else I will try it.
 

Carissa

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Same tank and reflectors - different bulb types? Maybe that would be difficult without removing the fixture though....but you could do T8s and T12s on the same fixture if you had two bulbs of the same size.