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# Re: Lighting, from another angle

• To: Aquatic-Plants at actwin_com (Aquatic-Plants)
• Subject: Re: Lighting, from another angle
• From: "David W. Webb" <dwebb at ti_com>
• Date: Thu, 31 Oct 1996 14:16:58 -0600
• Conversation-Id: <BMSMTP8467910277a0206807 at dsks52_itg.ti.com>

```>From: JOlson8590 at aol_com
>
>Thus far, I have seen no mention of the Law of Inverse Squares as it
affects
>lighting.  Very briefly, the intensity of light (any light) shining on a
>surface varies inversely as the square of the distance from the light to
the
>surface. This is true, regardless of the efficiency of the reflector, etc.
>(Unless, of course, you have a coherent source of light, such as a laser,
but
>they are really lousy lights for growing plants.  :-D  )
>
>This Law of Physics also applies to other sources of energy, such as radio
>waves, etc. (Anybody out there brave enough to get into explaining how
>waves and light visible to humans light are part of the same spectrum of
>energy, which at one end eventually ends up as Gamma Rays?  Not ME!!!  :-)
)

<Explanation snipped>

>The reason for mentioning this is not to stir up problems and lead to
>complicated calculations.  I just wanted to stress the fact that you want
>your light sources to be AS CLOSE TO THE SURFACE as is practical.  A
really
>good reflector can almost double the intensity at the BULB.  Aluminum foil
is
>quite good, but a good coat of white paint isn't all that bad.   You can
lose
>a lot of the effectiveness of your expensive lights by just having them a
bit
>too high.

One way to work around the law of inverse squares is to use external
parabolic reflectors to "get all (most) of the light going in the right
they do focus the light from the tube, getting you more light from the bulb
into the tank.

I recently purchased a 4-tube HF ballast and four T-8 tubes and mounting
brackets.  Now, I'm looking into a good way to set up mirrored reflectors
for the tubes to mount to.  I intend to use the tube mounting brackets to
position the tubes, and bend the polished metal (aluminum?) into a
parabolic shape so that mounting the tube brackets will place the tubes in
the focal point of the reflector.  Since the light will enter the water
with less reflection if it's at a near perpendicular angle than it would at
steep angles (I don't remember the angle, but past a certain point you get
100% reflection), a deep parabolic reflector may be a more efficient way of
getting more light into the tank than having the bulbs close to the tank
and using shallow reflectors.  To prevent loss of a percentage of light
from the bulb due to reflection back onto the bulb, an M-shaped, but
otherwise parabolic design is a good choice.

I advocate mirrored reflectors for this type of reflection.  If the goal is
to focus the light perpendicular to the surface of the water, the
non-scattering nature of a mirrored surface will be more effective than a
non-mirrored surface.

The only way to effectively measure losses due to reflectivity in this case
is to measure light intensity both above and below the surface under no
reflectors, flat reflectors, shallow parabolics, and deep parabolics.  I'm
not convinced that internal reflector tubes are the way to go, since the
phosphors in these tubes are going to get hotter and therefore less
efficient.  I suspect they may degrade faster than non-reflector tubes as
well.

I recently purchased 4 30g breeder flats (24"x24"x12"h) that I intend to
try deep parabolics on.  I'll keep people informed based on subjective
readings (I don't have a light meter).  I'm expecting to focus up to 70% of
the light from the T-8 tubes, potentially gaining a significant intensity
improvement, speculated at 3x to 4x.

If it doesn't work, I'm out the money for the metal I use for the
reflectors and the time for setting them up.  :-)

If it does work, I may be running single tube setups over my 12" wide tanks
before long.

David W. Webb

```