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# Re: Light intensity and water depth

```>From: George Booth <booth at hpmtlgb1_lvld.hp.com>
>Date: Tue, 07 Nov 1995 13:06:45 -0700
>Subject: Re: Light intensity and water depth
>
>- ----------------------------------------------------------------------------
>
>                  Light Intensity in Water
>                  ------------------------
>
>                                    Distance below surface of water (cm)
>                                       0       10      20     30     40
>- ------------------------------------------------------------------------
>1. Bulbs and fixture from (3)       16,000    8,800   7,700  6,500  5,100
>
>Notes:
>1. Measurements made in a 29T gallon tank with fresh, clean water only.
>2. Fixture positioned at back of tank and overhung the sides by 9".
>3. Measurements made below center of bulb.
>4. Bulb centerline was 5 cm above water surface.
>- ----------------------------------------------------------------------------
>
>The intensity in water was surprising at first.  From 0 to 10 cm, the
>intensity dropped by half.  Below that, the intensity dropped much
>less and linearly with distance.  I speculate that reflections from
>the sides of the glass were augmenting the intensity at lower depths
>and cannot be depended on in a real setup with plants and all.
>
>
>George

I've got an easy explanation.

A little background first.

From Wright Huntley's _AQUAMYTH #1 More Light for Deep Tanks -- NOT_
=>>> "If the lamp housing is on, or close to, the top of the tank,
the amount of light reaching the bottom does not depend on depth."

For this statement to be true, one must have =>>>
"A distributed source, like fluorescents"

What Wright's talking about is a uniform distribution of light at the top surface
of the aquarium.  Otherwise, light will continuously "mix" until the light is uniformly
distributed at some water depth depending on the width of the aquarium and how
non-unform the light was at the surface.

I'll try to explain your observations.  However, since I don't have all the details
of your setup, I'll have to make some guesses.

A 29T tank is about 12 inches wide and the your shop light produces light at the
surface that is about 4 inches wide and is located "at back of tank". look something
like this:

|   <----  8" ---->    | <-- 4" --> |
__    __
| /  \  /  \ |
||    ||    ||   <- Shop Light ( 2 Lights ? )
| \__/  \__/ |
_____________________________________
|  ~0 Lux    ~0 Lux      16000 Lux  |   <- Top of tank
|                                   |
|                                   |
|                                   |
|                                   |
|                                   |
| ~1000 Lux ~3000 Lux   ~12000 Lux  |       4 inches down
|                                   |
|                                   |
|                                   |
|                                   |
|                                   |
| ~2000 Lux ~6000 Lux    ~8000 Lux  |       8 inches down
|                                   |
|                                   |
|                                   |
|                                   |

.                                   .
.                                   .
.                                   .

|                                   |
|                                   |
|  ~5333 Lux ~5333 Lux    ~5333 Lux |    "far enough" down
|                                   |
|                                   |

Basically, all I did was redistributed 16000 Lux over a length of 4 inches to
eventually having this 16000 lux redistributed over 3 times it original length.
I course assumed that the light doesn't get attenuated in the water.
Keeping the amount of light constant per unit length (parallel to lamps) we get:

16000 Lux * 4 inches =  5333 Lux * 12 inches

Since the light was located at the back, I had to assume that the light would
spread out asymmetrically, but always keeping the total amount of light per unit
length ( 16000 * 4 ) constant.  Since I don't know the exact physical dimensions
of your fixture and lights, I had to guess-a-mate as to how the light disperses off
the shop light.

Ron Wozniak  Allentown PA, USA
```