Re: Aquatic Plants Digest V2 #296

> From: JOlson8590 at aol_com
> Date: Sun, 3 Nov 1996 01:28:06 -0500
> Subject: Lighting again (!!)
> Not trying to give anyone a hard time, just trying to be helpful.

Your postings are generally excellent (especially the Ballast/Theory
of Operation one from a few weeks ago) but since you insist on pushing
the Inverse Square Law thing, I must keep responding.  

One of us is promulgating incorrect information and that in itself is
NOT helpful. Using your example, a naive individual might think "Joe
Bob has four bulbs 8" from the water and his plants grow great.  This
is the same as one bulb 4" from the surface (1/2 the distance, 4X the
bightness).  I can get great growth with just one bulb and save all
that money".  George Booth doesn't think so, not bright enough.
Can't do it, one bulb.  Not.

In your first post on this subject (2" vs 12") you stated that the
difference in intensity *at the water surface* was 36:1 based on the
ISL.  However, you then went on to say that you measured a difference
of 2.5:1 at the gravel.  How do you explain this?  If there was 36
times as much light at the surface, there should be 36 times as much
at the gravel (roughly the same percentage should be lost from
reflections, etc).  There may be some difference due to differing
angles of entry of the two lamp positions, but not *that* much.

> I measured the difference between 12 inches and 2 inches, and the
> results for a single bulb are closely in agreement with the L.I.S.
> calculations.

OK, I guess it's time to put your measurement technique to the test,
since my measurement results differ wildly from yours.  Here's my data
from a posting a few years ago:

From: booth at lvld_hp.com 
Newsgroups: rec.aquaria,alt.aquaria,sci.aquaria
Subject: Re: Light Intensity and Physics 101
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 
> 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 with an Ultralife Luxmeter.  They 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 Intesity 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

3. Sears shoplight, 1 Triton,       13,500    7,200   3,100  1,370  780
   1 Ultra Trilux

4. Triton Enhancer, 1 Triton,       13,300    8,400   4,100  1,630  830
   1 Ultra Trilux

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 Intesity in Water

                                    Distance below surface of water (cm)
                                       0       10      20     30     40
1. Bulb and fixture from (3)        16,000    8,800   7,700  6,500  5,100

1. Measurements made in a 29T gallon tank with 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 surpising 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.