Light & Depth

[pjf]

There is an assumption that more light is required for deeper aquariums. However, since the tank is a waveguide, light intensity measured near the bottom of most home aquariums should be similar to that measured nearer to the surface. Has anyone used a light meter to verify this?

Here are links to a couple of short posts about this subject:
http://fins.actwin.com/aquatic-plants/month.9510/msg00297.html
http://fins.actwin.com/aquatic-plants/month.9510/msg00301.html

 

[theatrus]

Yes there have been many measurements showing the PAR available at depth is much lower than the surface.

 

[pjf]

Do you know of a link or reference to any study of PAR versus depth in an aquarium? Thanks very much!

 

[daytonians]

Basic physics. Double the distance from a light source, and the light is decreased by 75%

So, if the light has a par value of say 100 at 6 inches from the bulb. It will only have a par value of 25 by the time it reaches 12 inches. And, on a par of 12.5 at 24 inches.

This is not taking the filtering effect of water into account, so it would actually be less.

Hope this makes since.

 

[pjf]

Aquariums & Fiber Optics

Aquariums & Fiber Optics

The references I gave in my initial posting show light meter readings that indicate little variance in light intensity with depth in home aquariums. Are there other studies that can either corroborate or challenge those measurements?

The authors of the original postings believe that an aquarium tank acts like a waveguide much like optical fiber. Across distances deeper than any aquarium, measurements of light intensity in optical fiber vary little.

 

[daytonians]

Fiber optics channel light in much the same way a laser does. The light does not spread, therfore it does not decrease in intensity.

Light does spread in our tanks, and decreases in intensity as it spreads. Given the reflective nature of glass, the decrease may be less, but actually readings do not substantiate this.

I have never seen anyone do readings and find that the light wasn't less at the bottom of a tank. I would be currious to see how he took his reading.

Here is a post from our local reef club's forum. The member has a par meter and has taken reading from many members tanks.

"Most 400W lights that I've measured so far over other people's tanks (XM 10K, XM 20K, Radium, Ushio 10K) tend to be about 10" above the tank, and have a PAR level of 500 at around 6"-10" below water. That's over an area about the size of a dinner plate directly under the lamp. Another 6" below that the PAR level is about 250-300. In the lower corners of the tank (2' deep), the PAR level is around 100. "

 

[pjf]

Cite Your Source

Cite Your Source

In a way, you have helped to prove my point. If you take a near surface measurement directly under a metal halide pendant and compare that to the integral or “sum” of the measurements taken at depth, you will find that they roughly equal each other.

Near the surface, light may pass through the glass walls of the tank and be lost. Beyond a certain critical angle or depth, light is reflected back into the water. Then, the total amount of light passing through each layer of water decreases little with greater depth. On the bottom of most aquaria, the light is uniform and the intensity changes little if the tank is several inches deeper.

The two articles that I cited in my original post used tubes with reflectors to ensure a uniform light field. They found little variation in light intensity as the meter is immersed deeper in home aquaria.

When metal halide lighting is used with home aquaria, the authors found that:
(1) If the pendant is mounted high enough or if there are enough pendants, the light reaching the aquarium is uniform, similar to that produced by tubes.
(2) Near the surface, light may pass through the glass walls and be lost.
(3) With greater depth, light is reflected back into the water and the intensity decreases very slowly with depth.

In other words, if you have enough lighting to uniformly cover the bottom of a 2-foot deep tank, you will have enough for a 3-foot deep tank.

If you know of any studies that either corroborate or challenge the results of the original two articles, please post the links. Thanks very much!

 

[daytonians]

So what your saying it that the small amount of light that is measured at 2 feet of depth, will not decrease if going to 3 feet of depth. Interesting theory. It is still a small amount of light. I would like to test it.

I have to find someone with a very deep tank.

 

[pjf]

Deeper & Cheaper by the Dozen

Deeper & Cheaper by the Dozen

Here are a couple of real-life choices:

• A 20-gallon long (30”x12”x12”) versus a 37-gallon (30”x12”x22”). They have the same length & width and the same lights will mount to them but one is 10" deeper than the other.
• A 30-gallon breeder (36”x18”x12”) versus a 65-gallon (36”x18”x24”). Again, same lighting requirements but one is twice as deep and can support twice the fish.

Just find the deeper tank and make measurements at different depths. If you have uniform lighting, you don’t have to take several readings at the same depth. One reading can be representative over the entire depth.

I suggest the above choices because an aquarist may choose the shallow aquarium believing that he would have to spend more to light the deeper one. I submit that he should choose the deeper aquarium because the lighting required will be the same! (Assume clean water and same amount of coral, plants and live rock.)

Other food for thought:

• A deep aquarium is cheaper to light than a shallow aquarium of the same volume.
• A 2-story house is cheaper to build than a one-story house of the same square footage.

Again, if you know of any studies that either corroborate or challenge the results of the original two articles, please post the links. Thanks very much!

 

[daytonians]

What can I say. Actual light readings with a par meter in the water show that there is a dramatic drop in light as depth increases. It's a great theory, but doesn't work in real life.

My guess is that his meter was picking up light that was channeled down the glass itself and giving false readings. Just a theory. Measurements taken in the tank are the only way to be acurate.

 

[pjf]

Tubes and Pendants

Tubes and Pendants

Here is my guess as to why your readings differ from those of the authors of the original posts:

• The original posts described a uniform light field above the aquarium. In other words, the light intensity at the center is similar to that at the corners. This was accomplished with fluorescent tube lighting but can also be accomplished with multiple pendants.

• Your readings were made with pendant lighting which produces a “spotlight” effect. Your readings are high when taken under the pendant and lower when taken closer to the corners and at greater depth.

There are a couple of ways to get internal reflection and a uniform light field with a pendant:

1. Simply hang it high enough above the aquarium such that the corners and center of the tank have similar light intensity.

2. Look for the pendant through the glass wall. At the critical depth, you will no longer be able to see the lamp and that is where you have internal reflection. Below that depth, light intensity decreases little with greater depth. Do make sure that the water and the glass walls are clean.

 

[WarrenG]

I don't remember all the details but a person who knows much more about physics than me explained to me that a pinpoint source of light like halogen, HID, and similar disperses light (loses intensity) at a rate nearly 4x as soon (in distance from the light source) as a tube source of light like that from a fluorescent tube, and this was in air.

 

[pjf]

The Glass Tank is a Light Pipe

The Glass Tank is a Light Pipe

You are correct that a point source of light without a reflector loses its intensity by the inverse square law but this light is reflected back into the water by the glass walls of the tank. The original post states:

“The situation is only slightly different for a pendant MH hanging well above the water, in that the the total light in the tank depends greatly (by roughly 1/R^2) on the lamp-to-water distance. Once the light is inside the water, however, the propagation is still in a light pipe, and only absorption and scatter cause light to be lost. The water absorption, at normal aquarium depths, can usually be ignored, unless you are horribly addicted to overfeeding.”

In other words, once the light is reflected back into the water, greater depth does diminish the light very much.

 

[JoeESSA]

Well, you can just tell by using your eyes that the light intensity at the top of the tank is greater than at the bottom when using a point source light such as a MH bulb. However, it's a fair point. Light coming at all angles from the point source is refracted ''into" the water column as the light transitions from air to water. A substantial portion of this light will be directly transmitted from the source and directly hit objects. This initial portion of incident light that is at a steep enough angle to not hit the glass will follow 1/R^2 w.r.t the water surface. Some of the light energy is then converted to photons (light) from these objects and is transmitted through the glass - hence we see the object. Also some light at a shallow enough angle to hit the glass is reflected from the glass back into the tank, and some is lost due to transmission through the glass. The steeper the angle of incidence to the glass the more will be reflected. Given this I think it's fair to assume that the 1/R^2 law does not fully apply to the light intensity once in the water. However, at the top portion of the tank the 1/R^2 law will dominate, but deeper down the light intensity will diminish more due to absorbtion by the water itself rather than "geometrical" loss. This depends on the distance of the point source above the water. The further the point source is above the tank the more planar the wave front.

 

[JoeESSA]

All this theory about the tank acting as a lightguide, assumes that all points in the tank have a direct line of sight with all four glass walls. In practice this is far from the case. We know that, but nevertheless an interesting thread...

 

[daytonians]

OK, being a photographer, I have a camera that can be used to check the level of light that is reflected off live rock.

Tonight I tried to duplicate the experiment in the article. I turned off my MH light, leaving only my PC lights which cover the length of my tank. I then check the light level at the top parts of the live rock, which are about 8 inched from the surface. Looking for similar looking sections of live rock at deeper depth, I took more readings. In ever place that I checked, the light had dropped off significately.

At a depth of about 18 inches the light measured 1/2 of the first measurements.

The top rock measurements were at ~ 8" deep. The lights are mounted~ 10 inches about the water surface. That is 18 inches from light source.

The measurement at 18" depth would put the measurement taken 28 inches from the source.

According to the norm of light decreasing by 3/4 every time the distance is doubled, this would be about right.

 

[grupper]

If the light hits the side of the tank at an angle greater then ~49 degrees, then all of the light will be reflected. This is called total internal reflection or TIR. As pointed out, it is possible for TIR to be used to cause the light to propagate for long distances without very small losses. There are several important assumptions made here:

1) The tank must be deep. If you take the 49 degree angle with a 4' width, then you find that the tank must be at least 3' deep before all of the light meets the critical angle. (With a 4' florescent light). Note: you will get more TIR off of the front and back then you will from the sides due to the fact that the tank is usually longer then it is wide.

2) The glass must be completely clean. Any debris on the glass will decrease the reflection. In particular, if the glass is painted black or blue, you will completely loss the TIR. (Interesting note, if you hang a black or blue background with just a few microns of space between it and the glass, then you can still get the full TIR.)

3) The light must not be scattered or absorped off of other objects in the tank. Your rock work, corals, ect. will cause the light to be either scattered or absorped and will prevent the light from bouncing back and forth between the sides of your tank as it tries to get to the bottom.

Final conclusion, your hypothosis requires a clean deep tank with nothing in it except for water to work correctly. However, even if TIR does not prevent the light from decreasing with depth, it can still significantly increase the amount of light that you get at the bottom of the tank. For example, if the reflector for your light causes the light to hit the top of the water at an angle less then 48 degrees, then when that light hits the side of the tank it will go through TIR and all be reflected back into the tank. This prevents you from lighting the walls or floor of your house and allows all of the light to be used on the inhabitants of your tank.


Greg

 

[lanceleaderx]

noob question, my tank only 16'' high so i should get 150w than 250 w right ??

 

[JoeESSA]

Grupper, thanks, I was trying to remember my school optics from 2nd year physics classes, but simply too long ago (apprx. 20 years ago . Given that many portions of the tank do not have line of sight with the glass and due to the coverings you mention, seems fair that the major component of light hitting objects in the tank will be direct from the source. In the case of a point source this light will follow 1/R^2. The total light intensity may be slightly more than this due to reflection from glass...

 

[pjf]

Light goes straight to the bottom!

Light goes straight to the bottom!

Thanks for the responses! I have to sweat more now to defend my concepts. It will be a while before I can produce a physics and math defense but here’s my general argument:

• Most aquarium lighting is directed straight into the water. Light from a pendant does disperse via the inverse square (1/R^2) law. But when a parabolic reflector is used, most of that light is reflected downwards to the water with little dispersion. Light from tubes disperse with an inverse (1/R) law but with parabolic reflectors, most of that light is also reflected downwards to the water.

• When the light strikes the water, it is bent closer to the vertical and “focused” downwards with even less dispersion. In other words, most of the light will go straight to the bottom!

• The stray light that strikes the glass (or plastic) is mostly reflected back into the water by the glass-air boundary. For those with physics degrees, the water-glass boundary produces a slight change in the refractive index but it is the glass-air boundary that produces the total internal reflection (TIR).

Skim that dirty water and clean the outside of your tank!