Interesting info. I wonder what the actual difference in efficiency is between T5 and Power Compact?

Great thread! It's quite fitting that I'm reading this while waiting for my flight out of San Jose, where I've been the last couple of days attending an Optics conference. =)

Hoppy, the 1/R dependence does make sense. For those who are familiar with electrostatics, a good comparison is the expression for the electic field for charges. It has a 1/R^2 dependence for point charges but for an infinite line charge, or for a cylinder (small radius...and if looking at a point far from the cylinder...essentially approximating the infinite line charge situation) you can prove easily with Gauss's law that it will have a 1/R drop off as you mentioned.

There will probably also be some losses from reflection at the top surface of the water...more so for rippling water (more points for the light to hit the surface off normal) although I don't know if that is significant. Another thing to consider is that if the tank is filled with water, the amount of light reaching the bottom will be more than if you had a dry tank because of total internal reflection once the tank is filled with water. The diverging light would hit the tank walls and then be by and large perfectly reflected since they will be incident above the critical angle for the glass-air interface (which is why the sides look like mirrors).

Tom, thanks a lot for the links. Sanjay's article is very well written. I think it can be very informative and useful for all the planted tank folks.

With water in the tank it may be that less light escapes from the front/back of the tank, but you can easily see that a lot does escape. Just turn off the room lights and you can see the light pattern on the floor. My tank definitely loses light through the front glass.

Look over Refraction of Light and try the graphical calculator there - try water to glass and glass to air. You get this:

So, any light from in the water that strikes the front glass at an angle greater than about 41 degrees does refract so it escapes out the glass into the air, or a fraction of the light does. This tells me that the light striking the top part of the front glass does escape, while the light striking the bottom part doesn't. And, that, of course means.......ah.......(brain ran out of gas)

EDIT: I went upstairs to my aquarium and played with cards to block light striking the floor. For my tank the escaping light comes almost entirely from the top third of the front glass, with none that I can see from the bottom third.

This is very heavily dependent on the design and material of the reflectors. Since T5 bulbs approach being a line source of light they can have the best reflectors, by far. And, a smaller percentage of the bulb length is dark for a T5 bulb.

I noticed that the algae on the front glass is heaviest based on the height adjustment of my fixture. The closer to the tank, the further on the bottom the algae starts. Just look thru the side of the tank.

Mine light up my entire room when on (lol), but at least provide enough light to read by........

Great thread and lots of great info, although a lot of the more technical discussions are WAY over my head..............

I have a 10 gallon tank which seems to exist only to aggravate me. It has a pair of GE 20 watt 6500K screw-in CFL bulbs, each with a pretty good DIY reflector. If I look up at the bulb I can see almost every surface of the coil or its reflection, so I think I am getting a substantial part of the bulbs light down into the tank. But, my results show beyond a doubt that this is low light intensity.

Those screw-in bulbs are a blob source, close to a spherical source, no where near a linear source. So, their intensity will drop off proportional to the inverse square rule. My bulbs are about one inch above the water line, and 10 inches from the substrate, where glosso is struggling to grow. The inverse square rule says I only have about 1% of the intensity at the substrate as at the surface!

If I had a T5 bulb, even a 20 watt one, I would get at least as much light at the water surface and would only lose light at the substrate proportional to the inverse of the distance (not squared), so I would have 10% of the light at the substrate as at the surface, ten times as much light as with the CFL bulbs.

Now I think this is the reason for the ineffectiveness of screw-in bulbs. It isn't restrike that kills them, it is the inverse square effect.

If I can find the parts without spending too much I will modify that Perfecto hood to use a linear bulb, just to test out this idea.

Vaughn, you can borrow the meter if you want to measure things more.
It's just sitting here

Regards,
Tom Barr

PM sent!! I would love to spend a day with that.

Take a week

Regards,
Tom Barr

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