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glass over tank when using hood?
A treatise on HO T8 shoplights & dirty glass)

by rjw/ (Ronald Wozniak)
Date: Mon, 26 Oct 98

Date: Fri, 2 Oct 1998 13:49:11 -0700 (PDT)
From: jay dike <>
Subject: glass over tank when using hood?

Is it common practice to keep a glass covering on a tank even when a hood is used? It seems worthwhile to keep it in some ways since it might help prevent CO2 loss and fish loss. However, it seems like unless it is kept quite clean it could cut down on light intensity.

Also, is there a good estimate of how much intensity is gained by using a reflector around regular T-12 fluorescents? What is the better type - the white metal ones that come with shoplights or something like the Triton Britelight, which looks like it has a mirror-like finish?


Jay Dike

Jay, Here is my attempt to answer your questions:

When using shoplights over an aquarium, it is common practice to use a glass covering. The glass protects the shoplight from metal corroding condensation. For "high-end" (high priced) setups, it's more common not to use glass tops. For some reason, people believe that a glass top reduces too much light. What I'm going to suggests is that even a "dirty" glass top doesn't significantly reduce the lighting levels when you consider other sources of light loss.

I measured the light reduction entering the aquarium water on a "dirty" glass top to be about 5 percent more than a no glass top setup. (I wipe clean my three-year-old glass top once a month, but otherwise do nothing special.)

Here's a table showing recent measurements using a 55 Watt compact fluorescent lamp positioned above the aquarium water with a light meter directly below the lamp:

Cover Light Intensity
Just Above Water Surface
Light Intensity
Just Below Water Surface
No Glass Top - Open 21250 Lux 19500 Lux
Dirty Glass Top 20000 Lux 18750 Lux

Note: The precision of my "analog" Sub-Lux light intensity meter on the high range scale is about 1250 Lux. Thus, the 19500 Lux measurement was done by guessing sub-divisions. Be that as that it may, the light lost due to a dirty glass top is detectable but ,IMHO, not significantly different compared to either a clean glass top or a no glass - open top. (Remember don't confuse Lux with Lumens.)

The lamp was positioned 1 3/4 inches about the water surface using a "homebrew" reflector. I'd guess this reflector is about 60% efficient. (Efficiency here means % total light exiting the reflector/total bulb light output)

Contrast that to the unavoidable loss of light of 5 to 15% at the water surface for water with a moderate amount of surface ripple.

Now, to answer some of your other questions.

With a fluorescent bulb with no reflector located a couple inches above the water, only about 25% of the light will make it through the air/water interface. (It's a critical angle kind of thing. The critical angle for light entering water is 53 degrees. So at the most, 29% of light from a cylindrical fluorescent bulb (106 degrees/360 degrees) would be incident to the water surface. For a rough estimation, assume another 15% of 29% of the light is reflected off the surface of the water and doesn't make it into the water.) For a fluorescent bulb with no reflector that's located higher than about one half the aquarium tanks width, the light will get further reduced. As one may see, a light bulb without a reflector is not very efficient in transmitting light into an aquarium.

A good reflector should at least double the light compared to no reflector. (The reflector wraps the otherwise lost light towards the water at an angle closer to vertical and allow more light to enter the water.) About 60 to 65% percent of the light is transmitted with good LPS "aquarium stock" reflectors like the Triton Brite Lite. Light attenuation as a function of bulb height is practically zero for a good reflector since the bulbs light is focused vertically towards the water surface.

Based on some of the light modeling software I've looked at, I'd guess that the "best" computer designed reflector would transmits about 90-95% of the bulbs light, which means you get about 80-85% of the light through the water surface. An optimum reflector requires that the cross sectional area of the reflector is significantly larger (i.e. > 6) than the diameter of the fluorescent bulb. For a discussion of this see this article. Designing an optimum reflector with six T12 bulbs over a 18 inch wide tank becomes difficult as the bulbs tend to block each others light. Designing with the narrower T8 bulbs is easier.

A good link for light modeling software.

One program that does luminare analysis is "Photopia".

For fluorescent lighting, I measured and found that plain aluminum foil will produce about 10% more light than a clean white reflector. (Less light is is scattered in a silver like mirror compared to a enamel white reflector. A white reflector tends to diffuse light in all directions. Ideally, a reflector should redirect the light so that it arrives vertically to the glass or water surface. (A silver like mirror does a better job of focusing the light in a direction vertical {i.e. perpendicular} to the glass top surface and avoids the inverse distance squared light reduction people sometimes talk about.) Of course, there are better materials besides aluminum foil to line the inside of a reflector. However, please be careful of using plastics and paper for lining a reflector. Plastic or paper lined reflectors will thermally isolate the bulbs and may cause a significant temperature rise. Make sure the reflector and bulbs don't get too hot, since a bulb light output reduces with increasing temperature. Any increase in light due to the mirror like plastic may be negated due to increased bulb temperatures.

Here's my experiment comparing enamel white and aluminum foil reflector material:

Comparison between White and Aluminum Foil lined KMART Liteway Shoplight


Light Meter Sub-Lux (the light sensor was positioned about 3/8 inch above bottom)
Aluminum Foil Reynolds Wrap "Everyday Heavy Duty"
Black Cloth Actually a black "Dockers" pants leg placed between the bulbs & reflector to simulate the no reflector condition.
Bulbs New GE SPX50 T8 fluorescent 4 ft tubes
Motorola Ballasts (Used to replace the original shoplight ballasts)
Shoplight KMART Liteway 4ft shoplight Model# SL240K120. The shoplight is made for KMART by: Liteway (Bristol, PA 19007)
Wooden Blocks supports the bulbs 6 and 3/8 inches above measurement surface.
Measurement Surface Dark Blue Towel with markings as shown BELOW:

NOTE: The Shoplight fixture was centered & positioned at 6 and 3/8 inches above the <Center Line>. Data Points collected as indicated. The shoplight comes with a white enamel reflector. The center to center spacing of the two bulbs in the Liteway shoplight is 2 & 1/8 inches.

Light Intensity DATA.
Comparison between White, Aluminum Foil, & Dark Reflectors (All values are in units of Lux)
Position White Metal Aluminum Foil Black Cloth Position White Metal Aluminum Foil Position White Metal Aluminum Foil
1L 2500 2750 1500 1C 2500 2750 1R 2600 2750
2L 3750 4250 2250 2C 3600 3875 2R 3750 4000
3L 4250 4700 2625 3C 4250 4900 3R 4200 4750
4L 3750 4000 2250 4C 3700 4250 4R 3750 4000
5L 2500 2875 1500 5C 2750 2750 5R 2600 2600

Here's a table that summarizes some of what I've said so far.

Rough Estimates of Light Entering Water Using a Wide Range of Reflectors and Tops:
Type of Hood %Light Reaching Just Below Water Surface
No Reflector/Open (No Glass) top ~25%
No Reflector/Glass top ~22% (10% of 25% light loss to glass)
No Reflector/Dirty Glass top ~21% (15% of 22% light loss to dirty glass)
Typical Reflector/Open (No Glass) top 50-70%
Typical Reflector/Open - Dirty Reflector ???*
Typical Reflector/Glass top 45-63% (ditto)
Typical Reflector/Dirty Glass top 42-54% (ditto)
"Best" Reflector/No Glass top ~85%
"Best" Reflector/Glass top ~74% (ditto)
"Best" Reflector/Dirty Glass top ~72% (ditto)

* At the risk of starting a flame war, I'd think water condensation and splashing would "dirty" the light bulbs and the reflector inside a hood that has no glass or plastic between the light bulbs and the water. Also, It would seem easier to me to clean a few pieces of flat glass, rather than to keep clean all the bulbs, wires, nooks and crannies inside an open top hood. Any guesses as to how much light is LOST due to dirty light bulbs and reflectors inside an open top hood? I've heard that a dirty white hood would lose less light than a dirty chrome mirror lined hood. Also, I haven't tried it, but I would not use aluminum foil to cover the inside of an open top hood, since I would think that the aluminum would get tarnished & dull due to repeated exposure to condensation.

Besides the light over glass top or over open water, there are some other, perhaps more important sources of light loss that one needs to consider:

According to Sylvania's Engineering Bulletin 0-362 a 40 Watt lamp can loose a third (33% - 3000 lumens to 2000 lumens) of its light output when the bulb temperature increases from 100 degrees F to about 160 degrees F. I've measured temperatures in the 150 degree range inside a good LPS aquarium trade twin tube reflector. One need to consider heat removal in designing a hood. Also using an excessive number tubes can cause heat build up, and thus, increasing the bulb temperature and thereby reducing the light output. Its possible to design a three bulb hood that produces more light than a six bulb hood.

Another consideration is the light loss do to bulb aging. A loss of 10% to 15% over a year (5000 Hours) is not too unusual (as shown here). Higher operating tube temperature makes the aging rate considerably worse. How often a bulb turns on and off is another variable. The quality of the lamp ballast (cheap electronic {coil & cap}, magnetic & electronic) also effects lamp aging loss. See "Light Fixture Effects" at Aquatic Concepts.

IN SUMMARY, there are several important factors that determine how much light enters an aquarium. A dirty glass top doesn't contribute to as much light loss as one may think.

Jay, you didn't say what kind of tank you have, but as an example, this is how I would light a 75/90 gallon 48" wide aquarium. (Inspired by Erik Olson in 1994.)

Note: The Lights of America model that Erik mentions and the KMART Liteway are not the same shop light. The "Lights of America" model has a cheaper coil & cap ballast.

Setup A.
1. Glass top (cost $30US to $50US)
2. Three Liteway brand shoplights from KMART (cost $8.00 each on sale, and about $12 otherwise)
3. Six T12 bulbs of your choice. ($2 to $30 each)
4. Aluminum foil to line the inside shoplight reflectors

I found that the KMART Liteway brand shoplight had a decent magnetic ballast in it. This ballast is better than those cheapie light coil/capacitor ballasts. With this setup, you don't have to buy water resistant end caps or other moisture resistant hood materials. The shoplights reflector is only about 3 1/2 inches wide. You could enclose them inside a hood. On the other hand, without a hood, I found that the shoplights don't look that bad. They have a fairly low profile.

Another possibility is to use T8 lights as inspired by Karen Randall: ( "Lighting for a 29G", Aquatic-Plants Mailing List, Dec. 29, 1995)

Setup B.
1. Glass top (cost $30US to $50US)
2. Two Liteway brand shoplights from KMART ( $8.00 on sale, and about $12 otherwise)
3. Four T8 bulbs of your choice. ($4 to $20 each)
4. Aluminum foil to line the inside shoplight reflectors
5. Two Motorola High Output Electronic Ballasts ($30 to $40 each)

For T8 bulb & ballast selection try this article, and for Motorola ballasts: Motorola's site. I like the Motorola Ballasts in that they have these nifty "wiretrap" connectors that makes swapping out the original magnetic ballast relatively easy to do.

The HO T8 ballasts are highly efficient - up to 90 lumens per Watt. Four High Output T8 lamps provide more light output than either six T8 or six standard T12 lamps!

Here's a summary of what I measured:

Light Intensity DATA
Comparison between various ballasts, reflectors material, and hoods (All values are in units of Lux).
Pos. White
SO - T8
HO - T8
Al Foil
HO - T8
Mirrorlike Paper
HO - T8
Mag - T12
Al Foil
Mag - T12
Twin Tube
Mag - T12
1L 2200 2500 2750 3500 1400 1700 2250
2L 2650 3750 4250 4250 1900 2000 2600
3L 3250 4250 4700 5000 2250 2500 3000
4L 2650 3750 4000 4400 2000 2100 2600
5L 2200 2500 2875 3500 1600 1700 2500

*Bulb center to center spacing is 3 inches in this commercial aquarium hood.

Note: Measurements taken in open air, so light lost due to heat build up during actual use was not taken into account.

Materials used in the above comparison:

Motorola SO Motorola electronic ballast M2-RN-T8-1LL-D-120
Motorola HO Motorola electronic ballast M2-RH-T8-1LL-120
Liteway Magnetic Tar magnetic ballast that came with the Liteway shoplight
T8 bulb GE F32T8SPX50 (5000 Kelvin daylight)
T12 bulb Phillips F40/50U (5000 Kelvin daylight)
Mirrorlike Paper From local Art Store - used for posters. (The paper makes a better reflector than aluminum, but I haven't used it for aquariums because of the paper/plastic, acting as a thermal insulator, could cause an increase in bulb temperature, or perhaps fire hazard???)

Finally, I'd like to do a cost and light output comparison between Setup A and Setup B for a 75/90 gallon aquarium.

Note: 5000 hours per year is about 13.5 hours per day. Assume bulbs replaced at half their rated life.

Setup A. Three Shoplights with Aluminum Foil
Equivalent Max light output: 3 x 2500 Lux =7500 Lux
Fixed Cost
Three Liteway Shoplights 3 @ $10 each =$30
Ongoing Costs (Power is 100 Watts for each shoplight including bulbs & ballast)
Six T12 Daylight bulbs (replaced every year) 6 @ $5 each = > $30/year
Electricity ( cost at $0.05, $0.08/kWH or $0.10/kWH - kWH is kilowatt per hour)
($0.05/kWH) 3 shoplights * 0.10 kWatts * 5000 hours/year * $0.05/kWH = $75/year
($0.08/kWH) 3 shoplights * 0.10 kWatts * 5000 hours/year * $0.08/kWH = $120/year
($0.10/kWH) 3 shoplights * 0.10 kWatts * 5000 hours/year * $0.10/kWH = $150/year
Total Cost equation:
($0.05/kWH) $30 + $105/year
($0.08/kWH) $30 + $150/year
($0.10/kWH) $30 + $180/year

Setup B. Two Shoplights with Aluminum Foil & HO T8 Ballasts
Equivalent Max light output: 2 x 4700 Lux = 9400 Lux
Fixed Cost
Two Liteway Shoplights 2 @ $10 each = $20
Two Motorola HO Ballasts 2 @ $40 each = $80
Ongoing Costs (Power is 90 Watts for each shoplight including bulbs & ballast)
Four T8 Daylight bulbs (replaced every two years) 2 @ $8 each => $16/year
Electricity (cost at $0.05, $0.08/kWH or $0.10/kWH - kWH is kilowatt per hour)
($0.05/kWH) 2 shoplights * 0.090 kWatts * 5000 hours/year * $0.05/kWH = $45/year
($0.08/kWH) 2 shoplights * 0.090 kWatts * 5000 hours/year * $0.08/kWH = $72/year
($0.10/kWH) 2 shoplights * 0.090 kWatts * 5000 hours/year * $0.10/kWH = $90/year
Total Cost equation:
($0.05/kWH) $100 + $61/year
($0.08/kWH) $100 + $88/year
($0.10/kWH) $100 + $106/year

According to these calculations, Setup B will be cheaper after about a year or two of operation while the plants continuously enjoy about 25% more light. YMMV

I'd be glad to try and answer any questions on this posting.

Ron Wozniak Allentown PA, USA
AGA member

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