Re: Light intensity and water depth

Here's a old posting I made concerning the subject:

Date: 26 Apr 1994 15:03:02 GMT

Dustin Lee Laurence (laurence at cco_caltech.edu) wrote:

> Grant.Gussie at phys_utas.edu.au (Grant Gussie) writes:
> >The famous inverse square law applies ONLY to a point source of light that

> In fact, the 1/r intensity rule for line sources is not terribly
> relevant either as far as fluorescent lit reefs is concerned.  1/r^2
> is more than accurate enough for considering metal halide lighting,
> which is for these purposes a small number of point sources which can
> be superimposed incoherently. 

Here's what I measured last night.  These were quick and dirty 
measurements made after consuming a lovely cocktail and as such,
probably don't qualify for the Journal of the Society of Lighting 
Engineers.  But they should be good enough to add fuel to the fire :-).  

Executive summary: Light intensity from a dual 40w shoplight is inversely
proportional to the distance from the bulb.  Triton "Enhancers" (shiny
metal reflectors) don't enhance much compared to a plain white reflector.
Light intensity in an aquarium is tough to predict. 

                  Light Intensity in Air

                                    Distance from center of bulb (cm)
                                      10       20      40     80    120
1. Sears shoplight; 1 Triton,       10,800    5,300   2,400   940   530 
   1 Coralife TriChromatic

1.5 measured at end of fixture       6,900    3,400   1,600   780   450

2. Sears shoplight, 1 Triton,       15,050    8,000   3,600  1,520  810
   1 Ultra Trilux (bulb set 1)

3. Sears shoplight, 1 Triton,       13,500    7,200   3,100  1,370  780
   1 Ultra Trilux (bulb set 2)

4. Triton Enhancer, 1 Triton,       13,300    8,400   4,100  1,630  830
   1 Ultra Trilux (bulb set 2)

1. All fixtures use magnetic ballasts.
2. All measurements made at center of fixture (except 1.5).
3. Bulbs are various ages.
4. Intensity values are lux (lumens per square meter).
5. Sears shoplight reflector is "w" shaped; Triton Enhancer is
   "sort of" hemispherical.  Enhancer seems to help a little at longer

                  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

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. 

"Gentlemen, start your theories"