# Re: [APD] How much light? -- or - replies on bright reflections

```--- gbooth at frii_com wrote:
> Sorry, Scott, I should have sprinkled a few smileys in
> there...

No porob. I smiled when I read it. :-)
>
> anyway...
>
> Scott retorts:
> >Well, if you you get over, say, about 18" or so, even
> 2-3
> >watts can be inadequate for some of the foreground stuff
> >that's so far away from the bulb. So with tall tanks you
> >might want to use more wpg than with shorter tanks.
> That's
> >all I meant. Maybe that goes without saying.
>
> Yes, I knew what you meant. More smileys next time, I
> promise.
>
> "So far away from the bulb"??

I think I was missing a smiley that time.

>
> In all seriousness, I disagree with that. There seems to
> be a
> misconception that water depth by itself requires more
> light. I think you
> would be hard-pressed to demonstrate that, in a bare
> tank, the light
> intensity 18" below the surface is much different than
> the light intensity
> at 24".
>
> Certainly the light at 12 feet would be less than at 3
> feet in a natural
> body of water, but we aren't talking about much depth
> differential or a
> natural body of water.
>
> The commonly used "Inverse Square Law" doesn't apply very
> well here. First
> of all, it's only true with a point source in free space.
>  A single MH
> bulb comes close to a point source but FL bulbs are a
> linear source. In
> free space, FL bulbs will loose intensity proportional to
> distance from
> the bulb if you don't use a reflector. If you use a good
> reflector, the
> light is not radiating away from the bulb in all
> directions (the reasoning
> behind the Inverse Square Law) but is directed towards
> the tank. In this
> case, there are few common rules that apply.

But even with a line source, as you move away from it,
well, the light still spreads. And flo bulbs, even with AHS
reflectors are pretty diffuse lamps. Distance has got to
count for something. But that's jsut reasoning -- I never
measured it. If you did, I can't argue with that.

> Secondly, once light gets into the water, the aquarium
> walls make darn
> good reflectors, bouncing any light at shallow angles
> back into the water.
> I think someone (Wright?) likened this to a light pipe of
> sorts. Once
> light gets into the tank, a lot of it stays there.

I'll buy that. If I lift the hood (tilt the front portion
up and back after the design you show on your web site ;-)
), then single 55 watt bulb puts more light into the room
than the other several hundred that are still shining into
the tank. A similar sort of thing can be seen if I postion
the lamp or hood over a tank without water in it (without
water in the tank, I  mean, I wouldn't put water in the
hood) and then put water in it (again, in the tank, not the
hood ;-) ).  The room just doesn't seem as bright when the
tank is full of water.
>
> I had a chance to measure the lighting in a bare 29g tank
> once with a
> luxmeter. With water, of course. I found that the light
> at lower levels
> was actually a little more intense than that just below
> the surface! I
> checked that a couple of time since it was so
> counter-intuitive.

I'd expect that unless you measured directly under a bulb
both times and there was only one bulb. Otherwise, I'd
expect that you'd get the light from the others bulbs, if
there are more, if you go down a bit lower.

> IMHO, taller tanks require more light IFF there are tall
> plants that are
> blocking the light from short foreground plants.  Yes,
> the same end
> results but let's get the reasoning accurate to avoid
> confusion.

;-)

But seriously, my reasoning was just that nonshaded plants
at the lower depths tend to behave in my tanks like the
ones in the shade. But if I move them up closer to the
lights, or let them grow up closer to the lights, they
behave like brightly lit plants -- more growth, the ones
with "reddening tendencies" tend to get red, that sort of
thing.

But I'm willing to learn. ;-)

Scott H.

=====
S. Hieber

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