Lighting myths

>It would be nice if some of the old-timers would quit advising that
>more light is needed for deeper tanks. In general it isn't. I recognize
>that heavy plant growth absorbs a lot of light, and a really thickly
>planted tank may use some more Watts (NOT lumens, by the way), but
>that's the subject of another missive. The point is that it is
>certainly not the depth that calls for more light.

OK, I'm curious. Why watts, not lumens? Watts is a measure of electrical
input not light coming out. If we look only at watts, we wouldn't be
distinguishing between incandescent and fluorescent, for example.
Incandescents as we know are less efficacious--that is, put out fewer
lumens per watt.

For that matter, why are we measuring lumens instead of light level at the
target plane?--measured in footcandles.

While Wright is exploding myths, let me bring up a question about another
popular lighting myth--why do folks keep advising us to replace fluorescent
lamps every six months or so?

The reason I read in the FAQ is a drop in intensity. The engineering
studies (for typical fluorescent lamps) I've seen show that lumen output
drops a total of about 10 percent over the first 7,000 hours of operation.
At 10 hours a day, that's 700 days--or about 2 years of operation. After
that, lumen output continues dropping but at a far slower rate--totalling
about another 5 percent at the end of another 7,000 hours of operation.

Is a 10 percent drop at the end of two years sufficient to warrant
replacing lamps every six months? If so, wouldn't it be cheaper just to add
another lamp from the start and "overlight" slightly?

Greg. Tong
San Francisco, CA, USA
gtong at sirius_com

"Every infinity is composed of only two halves."