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Re: [APD] Ideal Lighting for Crepuscular Tastes

Excellent points. The bulbs can be dimmed but most ballast can't do it. However, I'd say would not say that the bulbs do exaclty fine. The dimming seems to accelerate the deterioration of the filaments, as if the arcing occurs at only one spot and so the filament wear occurs in one small location instead of being spread over the filament. 
Besides the current and cathode voltage, the current crest factor an temperature affect the filament life and how  ballast cares for these factors depends in part on bulb(s) to which it is attached.
When you say 20%-30%, you mean a 70-80% reduction in light output, right? Not bad, considering a few years ago you'd be hard pressed to find a set up that could go below about 40% output and at that a 32 W T8 would burn out at less tahn half normal life. The better dimmables, using  voltage switching go down as far as about 1% these days, but you don't get an infinite range of dimming in most set ups. You get better control with pulse width modulation or phase control and they do have other virtues but don't dim nearly as much and are a bit pricey.
In any event, getting a flourescent down below output 20% or rated output means substantially reduced energy efficiency. 20% is the current phosphor-painted glass wall for sensible fluorescent dimming. In a sense, reduced efficiency with dimming is true for incandescents too but they are so absolutely horribly awfully terribly badly ineffficient to begin with, going from 5% energy efficiency to 2% doesn't seem so bad at first glance.
Which brings up another way to do the penumbra thing on an aquarium. Us some small incandescents for the short dawn and dusk periods andthen swing over to PCs or MHs for the full-light periods.
There are lots of ways to get multiple lighting levels, some can get quite complicated and drive you batty ;-)
have plants, have fun,
----- Original Message ----
From: Bill Wichers <billw at waveform_net>
To: aquatic-plants at actwin_com
Sent: Monday, June 26, 2006 12:55:02 PM
Subject: Re: [APD] Aquatic-Plants Digest, Vol 34, Issue 24

>Just be sure that your ballast and bulbs can take dimming. Most
flourscent >ballasts and bulbs cannot.

Actually, most bulbs can handle the dimming part just fine -- the problem
is that the ballasts are unable to properly drive the bulbs at the reduced
output levels. There are lots of dimmable ballasts on the market. Lutron
(http://www.lutron.com/) makes a nice unit that can dim based on a DC
control voltage, which would allow several ballasts to be easily ganged
together and controlled with a simple circuit. For a dimmable aquarium
hood that's probably the easiest way to go. The dimmable ballasts that run
on a regular "light dimmer" usually can't dim much below around 20-30% or
so, the dimmable electronic ballasts give a fuller control range.
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