Light & Depth

[pjf]

Calculations versus Observations

Calculations versus Observations

<a href=showthread.php?s=&postid=8373832#post8373832 target=_blank>Originally posted</a> by Fredfish
Thanks for the link pjf. I missed this article.

I always thought that the effects of a reflector in capturing and redirecting light into the tank would be greater than what is shown by this article.

I think that Sanjay actually separated out the effects of light loss so that after applying the 70-90% loss due to inverse law, a further 14% was lost due to absorption. That the way I read it anyway.

Sanjay also seems to think that we can gain a lot of light using a white sand bed. I wish he had quantified 'significant'.

I stand by my original statement that a lot of light is lost from the top to the bottom of an aquarium. It appears from Sanjay's article that most of this loss is due to the invers square law though.

Fred

If your interpretation of Joshi’s statement is correct, then the inverse square law will show a light loss of more than 96% as we move the photometer from just underneath the surface (6” from the lamp) to 2 feet under the surface (30” from the lamp):

• At 6”, the attenuation is 1/36 of total output.
• At 30”, the attenuation is 1/900 of total output.
• The light at 30” is 4% of the light at 6” (4% = ratio of 1/900 to 1/36).
• Subtract 14% absorption loss from the 4% and we have 3.4% remaining.
• At a 2-foot depth, only 3.4% of the light that passed through the surface remains.

This drastic loss of light intensity cannot be. No one is claiming a 96% light loss at the substrate of a 2-foot aquarium with a lamp 6” above the water. Where did we go wrong in the calculation?

The water’s surface is a lens and the inverse square law cannot be blindly applied across a refractive surface. It also cannot be blindly applied when a reflector redirects the light downwards. The R-value in the inverse square law must be re-evaluated with each refractive and reflective surface.

And we haven’t even discussed TIR yet. There are simple methods that a home aquarist can use to see if his aquarium behaving as a waveguide:

• If you have a hand-held laser pointer, you will be able to see the drastic refraction at the air-water boundary and the reflection at the glass-air interface. I don’t see much diffusion of the laser light as it makes a crisp red image on the substrate. It does not appear to be following the inverse square law. (It does but the R-value is probably miles long.)

• You can evaluate total internal reflection (TIR) in an otherwise dark room with only your metal halide lights turned on. If light “leaks” through the aquarium, you will see glitter lines on the floor or walls of your room. I see no glitter lines coming through the front and back glass of my standard 75-gallon tank. Yes, there are glitter lines leaking through the upper corners of my side glass but the light at the substrate is still much stronger than what a simple application of the inverse square law would suggest.

Bottom line: If 2 feet of water only absorbs 14% of the light, the rest of it is probably bouncing around your aquarium or getting absorbed by the substrate - inverse square law or not!

 

[hahnmeister]

The inverse square law still applies to halides with reflectors, unless you use very wide reflectors like lumenarcs.

Here's why... Although it buffers the overall effect, the light that is gained from the reflector is not as intense as the source itself. It must travel from the bulb to the reflector first, get reflected as much as possible, and much of this ends up as light at such an angle that it just reflects off the water surface when it gets there. The proof of this lies in the shadows. If you can hold your hand in the tank (lets say about 1/2 way from the top/bottom) directly under the light, and the result is no shadow, then the inverse square law does not apply. If there is a shadow, then it does. There are exceptions though. The inverse square law does not apply to smaller tanks that run large reflectors... like my 60cube with a lumenarc3... the whole cover is a reflector, and so the light is very well spread out. The proof of this is that the tank has no 'shimmer' lines. None. Which brings me to my last and final point... there are cases where halides do not act as point sources, or with varying degrees. A large tank with a little DE reflector? You bet its a point source. But its all relative. My 40B w/ a PFO mini pendant has no shimmer either, but that pendant over a 180g... lots of shimmer... and therefore the inverse square law applies more. My rule has been 'if you see glimmer lines, its a point source'.

 

[pjf]

Use a Laser!

Use a Laser!

The inverse square law describes the diffusion of light, not its absorption. Per Joshi, a 2-foot column of water absorbs only 14% of the light passing through it. Here’s how we can help the rest of the light get to the substrate:

• Parabolic Reflectors â€"œ There's no use in lighting your ceilings. Less light is reflected by the water’s surface if the light is coming from overhead. Per Joshi, “the reflectance remains low about 2-3.3% for zenith angles ranging from 0-60 degrees, and rising rapidly after it.”
• Lighting Height â€"œ Divide the width (front to back) of your aquarium by 4 and mount your lights at that height. Then the light will strike the water at less than 60 degrees from vertical. The light will be refracted by the water’s surface to less than 41 degrees from vertical. It will then strike the glass-air boundary at angles that produce total internal reflection (TIR).
• Housekeeping â€"œ Clean water and glass lessens light absorption. That includes the outside of the glass because it is the glass-air interface that is responsible for TIR.

In my rectangular aquarium with metal halide lighting, there are “glitter lines” at the substrate but no glitter lines project through the front and back glass to the floor of the room. Glitter lines do “leak” through my side glass because manufacturers tend to mount bulbs closer to the center of a fixture than to the ends. If you have tube lighting, you can test for TIR with a laser pointer.

If your aquarium is a waveguide, depth is the least of your worries!

 

[hahnmeister]

A laser wont work. The whole point of a laser is that its intensity doesnt drop off with distance like a point source of some other light does.

The best way I know of measuring how much the inverse square law applies to a given bulb is by measuring the shadows from a standard object at various depths and comparing them.

 

[pjf]

Clean Water and Mirrors

Clean Water and Mirrors

Near the surface, water disperses light less than air. This can be seen either with a laser or “by measuring the shadows from a standard object.”

The water’s surface bends light towards the vertical and hence reduces dispersion and intensity loss. The surface will refract the light closer to vertical so it appears that your metal halide pendant is farther away with no loss of intensity. To determine light dispersal through the water, you must calculate a new R-value for your pendant. Place the laser pointer on your metal halide pendant and shine it into the water until it hits the far edge of the substrate and measure the angle of the ray in the water. You may also use shadows of objects in water to determine the angle of the shadow. Use trigonometry to determine where this ray would intersect the zenith of the pendant. That location gives the new R-value for the “apparent” light source. Since this new R-value is longer than the original distance between your pendant and the aquarium, this is proof that there is less light dispersion in your aquarium than in air.

Alternatively, you may use Snell’s Law which says light striking the water at 60 degrees from vertical will be bent to 41 degrees from vertical. If your pendant is 6” above the water, then to the fish, it would appear to be 9” (= 12 * sin (90-41 degrees)) above the water with no loss in intensity. If your pendant has a perfect parabolic reflector, then it’s “laser-like” and your R-value is miles high.

Dr. Joshi states that 2 feet of water absorbs approximately 14% of the light passing through it (www.advancedaquarist.com/2005/8/aafeature). This was measured in the ocean because he alludes to “natural light at aquarium depths.” When describing the aquarium light field, he admits “Using the diffuse coefficients for the ocean waters.” Therefore, he did not account for TIR and the wave guide properties of the aquarium.

This means that although there is an absorption loss of 14% of light in a 2-foot deep aquarium, TIR can be used to focus the remaining light on a spot on the substrate. With the right aquarium walls and lighting, the light at a spot on the substrate can be just as bright as that near the surface. Here are some examples:

• Look through the side glass of your rectangular aquarium. At this angle, the front and back glass appear to be mirrors.

• Imagine a standard hexagonal aquarium with a metal halide pendant. Adjust the height of the metal halide pendant so that the light reflects off the six glass walls and focuses on the center of the substrate. This spot on the substrate receives direct light plus reflected light from 6 mirrors. This reflected light is not received near the surface.

• Imagine a spot on the substrate near the left side of a standard rectangular aquarium. This spot is the same distance from the left, front and back glass walls. It receives light from a metal halide pendant directly above it and light reflecting from 3 mirrors. Imagine a similar spot near the right side of this aquarium with a second pendant above it. These two spots receive both direct lighting and reflected lighting. This reflected light is not received near the surface.

• Imaging a line on the substrate running left to right down the center of the aquarium. Imaging tube lighting above this line. The line receives both direct light and the light reflected from the front and back glass.

All it takes is clean water and mirrors.

 

[hahnmeister]

Ok, I see what you are saying with the laser... you are using it for angle measure. I thought you were going to use it in a way where it would have to behave like regular light. Measuring angles isnt going to do much though... a laser doesnt dissapate like regular light does. Sure, it might give you some ideas, but for the most part, we already know the angles... I dont see how this would be as useful as measuring actual shadows being cast from an object at various depths. There is just so much going on that trying to measure the angle of every ray of light... well... it would take forever. You have light reflecting off of the glass, you have rocks blocking areas partially, etc.

 

[pjf]

Easy to Determine TIR

Easy to Determine TIR

<a href=showthread.php?s=&postid=8395617#post8395617 target=_blank>Originally posted</a> by hahnmeister
Ok, I see what you are saying with the laser... you are using it for angle measure. I thought you were going to use it in a way where it would have to behave like regular light. Measuring angles isnt going to do much though... a laser doesnt dissapate like regular light does. Sure, it might give you some ideas, but for the most part, we already know the angles... I dont see how this would be as useful as measuring actual shadows being cast from an object at various depths. There is just so much going on that trying to measure the angle of every ray of light... well... it would take forever. You have light reflecting off of the glass, you have rocks blocking areas partially, etc.

Here’s an easy method for determining total internal reflection (TIR):

• Leaving your metal halide lights on, turn off the room lights and close the curtain.
• If light “leaks” through the aquarium, you will see “glitter lines” on the floor or walls of your room.

If you do see glitter lines leaking through the front and back glass of your rectangular tank, simply raise your lights a couple of inches. The rule is to divide the distance between your front and back glass by four and mount your lights at least that distance above the water.

If there are glitter lines leaking through the upper portion of your side glass, it is normally because manufacturers tend to mount bulbs closer to the center of a fixture than to the ends of the fixture. This causes the light to strike the glass too close to perpendicular for TIR. Nevertheless, the light at the substrate is still much stronger than what a simple application of the inverse square law would suggest.

You don't need a laser or more lighting for your deep tank.

 

[pjf]

Laser Easier to Use than Shadows

Laser Easier to Use than Shadows

<a href=showthread.php?s=&postid=8395617#post8395617 target=_blank>Originally posted</a> by hahnmeister
I dont see how this would be as useful as measuring actual shadows being cast from an object at various depths. There is just so much going on that trying to measure the angle of every ray of light... well... it would take forever. You have light reflecting off of the glass, you have rocks blocking areas partially, etc.

Using the laser to measure light dispersion is easier than "measuring actual shadows being cast from an object at various depths." It is easier for two reasons:

1. You don't have to get your hands wet. You can determine the angle of the laser beam by sighting with a protractor on the outside of the glass tank.

2. You only have to measure one angle to determine the light dispersion in water. In my post of 10/22/06, I explained how and gave a computational example.

 

[pjf]

Uniform Lighting

Uniform Lighting

<a href=showthread.php?s=&postid=8320762#post8320762 target=_blank>Originally posted</a> by JoeESSA
Also for 6' long tank, empty, still water, single MH 0.5'x0.5' reflector scenario I gave above I think we would see a significant drop-off in light intensity with depth. Of-course the light intensity at the surface would also be a strong function of x-y location (non-uniformity). Point being, it's as Fredfish said we cannot achieve a unifrom light source above all aquariums in practice. Having said that it would be interesting to see measurements from a high power LED array above a tank. That's probably as uniform as it gets...

Per Sanjay Joshi, 2 feet of water absorbs only 14% of the light. With uniform lighting and total internal reflection (TIR) from the aquarium walls, light intensity at the substrate should be similar to light intensity just under the surface.

This uniform lighting is easily accomplished with fluorescent lighting. Modern T-5 tube arrays provide even lighting from top to bottom. Mr. Wright Huntley used fluorescent lighting to back up his pioneering essay, “AQUAMYTH #1 More Light for Deep Tanks -- NOT!” Measurements taken from tanks illuminated with fluorescents show that light intensity does not decrease appreciably with depth. (See links at the start of this thread.)

While metal halide (MH) pendants cannot provide lighting with such uniformity, they too can take advantage of the reflective aquarium walls. In a rectangular aquarium, there are two points that are equidistant from the front, back and side glass. Pendants placed above these two points will concentrate light reflecting from 3 walls on the substrate below. In longer aquariums, additional pendants can light the center. Center pendants take advantage of light reflecting off the front and back glass.

Current LED technology for reef aquaria is providing illumination that is not much more uniform than that provided by metal halide technology. The LED’s tend to be clustered in a 5 by 5 matrix. Each LED cluster is positioned in the fixture at a location normally occupied by a MH bulb. Today, the primary advantages of LED lighting include light weight, simplicity and precise lighting control. Uniformity may come later.

 

[hahnmeister]

“AQUAMYTH #1 More Light for Deep Tanks -- NOT!”

got a link to that?

I couldnt agree more though. Yet another reason why T5s burn the heck out of halides...lol.

 

[pjf]

AQUAMYTH #1 More Light for Deep Tanks -- NOT!

AQUAMYTH #1 More Light for Deep Tanks -- NOT!

The original article by Mr. Wright Huntley, entitled “AQUAMYTH #1 More Light for Deep Tanks -- NOT!” can be found at the following URL:
http://fins.actwin.com/aquatic-plants/month.9510/msg00297.html

For advocates of tube lighting, you will be pleased with the following link that measures little or no attenuation of fluorescent light intensity at aquarium depths:
http://fins.actwin.com/aquatic-plants/month.9510/msg00301.html