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Light Intensity Studies


  1. Information about lights
    by booth-at-hplvec.LVLD.HP.COM (George Booth) (Fri, 20 Nov 1992)
  2. Bogus reflectorized FL bulbs
    by () (16 May 1994)
  3. Light bulb efficiency
    by () (6 Jun 1994)
  4. Light bulb efficiency
    by (Uwe Behle) (Wed, 8 Jun 1994)
  5. Free Spectrograph Offer
    by (WRIGHT HUNTLEY) (1 Feb 1995)
  6. Sticking my neck out - again
    by (Mon, 11 Nov 96)
  7. Lux Meter Readings
    by peachdoo/ (Tue, 22 Aug 2000)
  8. lighting
    by busko/ (Ivo Busko) (Tue, 25 Jul 2000)

Information about lights

by booth-at-hplvec.LVLD.HP.COM (George Booth)
Date: Fri, 20 Nov 1992

I recently made some intensity and color temperature measurements on a
bunch of fluorescent and metal halide bulbs and would like to share
the data with you.

My goal was to determine the relative brightness of various bulbs (to
check out the claims of the vendors) and to get a feel for the color
temperature of the bulbs.  A secondary goal was to determine the
effects of age on both color temperature and brightness.

To measure intensity, I used a lux meter offered by Marine
Invertebrates, Inc.  This is advertised in FAMA and costs about $100.
I'm not sure about its absolute accuracy, but, being digital, its
resolution is quite good.  The instructions claim an accuracy of +/- 5
lux and it seems very repeatable.  I'm also not sure about how color
temperature affects its accuracy.  Its response curve peaks at about
600 nm, so there is probably some effect.

To measure color temperature, I rented a Minolta Color Meter II from a
photographic supply store in Denver.  It's primary use is to determine
the correction filters required to balance a specific film for a
specifc light source, but it also indicates color temperature in
degrees Kelvin.  I'm not sure of its absolute accuracy (and I did see
some strange results with metal halide lights), so take the numbers
with a grain of salt.  This is a neat instrument that sells for about
$700, so I feel that numbers aren't too far off for the fluorescent
bulbs.  Minolta also makes a different meter for "scientific and
industrial colorimetery", but it's not available for rental locally.

BTW, I tried to call the Minolta Customer Service Department to determine
the validity of my measurements, but the "representative was busy and
his voice-mail was full", so I couldn't leave a message.
This posting is the raw data I collected.  I have not as yet drawn any
conclusions aside from obvious things like Penn-Plax Ultra TriLux
bulbs are brighter than Triton bulbs and they have a color temperature
closer to sunlight (whether or not that is good, I'm not saying). 

Perhaps one of the net light experts could comment on the data.



Light Fixture Effects 

To determine if the light fixture and ballast affected the temperature
and intensity, 2 new 40 W Cool White bulbs (2 for $2.49) were tested
in two fixtures.  The readings were taken 8" from the bulbs, centered
over the middle of the fixture.

An "electronic" shop light ($15.99 at Ace hardware) with a small coil/
capacitor ballast circuit produced 5000 K and 5500 Lux.  This is an
"energy saving" type fixture.  My experience indicates this type of
fixture is hard on bulbs such as Tritons and shortens their life.

As a side note, the October '92 Consumer Reports tested various types
of energy saving light bulbs including compact fluorescent bulbs with
electronic and magnetic ballasts.  They found that a large number of
the bulbs with an electronic ballast had premature failures.

A "standard" Sears shop light with a normal magnetic ballast produced
5300 K and 6650 Lux.  The same fixture with aluminum foil behind the
bulbs produced 5300 K and 7900 Lux.  Also with this setup, the color
temperature ranged from 5200 K to 5500 K when the distance from the
bulb varied from 3" to 24".

I also tried to check the lights in a Coralife metal halide and
fluorescent fixture to see if an "expensive" ballast and polished
reflector did anything, but the bulb spacing was very different giving
readings which could not be meaningfully compared to the first three.
1) Cheap electronic fixtures reduce your light intensity and bulb life.
2) Cheap reflective material can increase your light intensity.

Color Temperature (single bulbs)

I had hoped to determine color temperature and aging effects of metal
halide bulbs, but the temperature meter responded very strangely to
most of the bulbs I tested, giving readings of 9000 K to 14000 K
depending on the distance from the bulb.  One older, 1000 W MH bulb
read 6200 K, but I don't have much faith in that number.  By the way,
an actinic bulb caused an "overrange" reading (above 40000K).

The local fish store allowed me to check some of the bulbs in the
store.  Most of the bulbs are about 1 year old.  I was surprised at
the temperature range of the 40 W Power-Glo bulbs; possibly caused by
differing age or fixture differences or just bad quality control - I
don't know.  I believe the higher K temperature indicates more blue,
which shows up some fish colors better.

20w Ultra TriLux     5200 K   new      (in a cheapie hood)
Phillips UltraLume   5800 K   6 months
Artic Brite          6000 K   1 year
Power-Glo            7200 K   
  "    "             7850 K   
  "    "             8700 K
  "    "             8700 K                      
  "    "             9350 K
  "    "             9800 K

A local lighting store has a light bulb comparison center with various
commercial bulbs.  The age of the bulbs was unknown.  Unfortunately,
the store was remodeling and some of the display was not working, so I
didn't get a chance to try a Chroma 75.

Warm White Delux     3460 K
Warm White           3630 K   
Regal White          3640 K   
Natural              3900 K
Spectra 35           3930 K
Spectra 30           3930 K
Chroma 50            4600 K   
Cool White           5000 K
Cool White           5150 K   
Daylight             7500 K

We have quite a few bulbs of various ages on hand.  Each was tested in
the Sears shop light.  The second bulb in the fixture was covered with
a piece of cardbard so it would not affect the readings.  The age of
the the bulb is indicated, if it was known.  All readings are 8" from
the bulb.

Agro Lite            3590 K
Wide Spectrum        3900 K
Advantage X          5300 K          5100 K in electronic fixture
Artic Brite          5850 K    new   5900 K in electronic fixture
  "    "             5700 K    
Ultra TriLux         6750 K    new
  "     "            6350 K          6050 K in electronic fixture
  "     "            6150 K
  "     "            6150 K
Triton               8150 K    new
  "                  8400 K    new
  "                  7550 K    300 hours
  "                  7550 K
  "                  7350 K    4800 hours

Color Temperature (combinations)

Combinations of bulbs in use on our various plnated aquariums were 
tested.  The average age of the bulbs is about 6 months. 

1 Power-Glo, 
1 Ultra TriLux       6550K   

1 Triton,            from 
1 GE Wide Spectrum,   6000 K   
1 Triton,              to 6300 K
1 Ultra TriLux          to 6500 K 

1 Ultra TriLux,      from 
1 Ultra Trilux,       6150 K 
1 Triton,              to 6050 K
1 Ultra Lume            to 5900 K

1 Artic Brite,
1 Ultra TriLux       6400 K

Intensity (single fluorescent bulbs)

From the local fish store (meter was 4" from the bulbs except for the
metal halide bulb): 

1000 W Metal Halide, 12" from fixture   69,000 Lux
2 Power-Glo bulbs,                      11,000 Lux
1 Power-Glo, 1 Ultra TriLux              8,200 Lux
1 Power-Glo, bad fixture                 2,400 Lux

From the local lighting store (meter was 4" from the bulbs):

Regal White          3300 Lux 
Chroma 50            3500 Lux 
Warm White           4600 Lux 
Cool White           5000 Lux

Our bulb stock (meter was 8" - twice as far - from the bulb):

Wide Spectrum        2400 Lux
Artic Brite          3750 Lux  new   2780 Lux in electronic fixture
  "    "             3200 Lux  
Advantage X          4200 Lux        3370 Lux in electronic fixture
Triton               3890 Lux  new
  "                  3820 Lux  new
  "                  3730 Lux  300 hours
  "                  3610 Lux
  "                  3300 Lux  4800 hours
Ultra TriLux         5000 Lux  new
  "     "            4720 Lux        3520 Lux in electronic fixture
  "     "            4580 Lux
  "     "            4480 Lux

Intensity (single 5500 K metal halide bulbs)

We have had our dual 175W metal halide fixture in use for over 1 year
and have tracked the intensity of 3 of the 5 bulbs in use over that
period.  The bulbs are on for 10 hours per day.

Directly below the bulb at the acrylic shield:

Bulb 1:  78,000        90 to 270 days
         72,000        320 days
         57,000        360 days

Bulb 2: 126,000        10 minutes
        110,000        4 hours
        102,000        2 days
         94,500        4 days
         90,000        13 days
         90,000        21 days
         86,500        62 days
         82,000        110 days
         72,000        285 days
         60,000        315 days

At the water surface, 13" from the bulb:

Bulb 2:   9,900 Lux    1 year old; replaced at this point

Bulb 3:  19,900 Lux    after 2 hours
         17,600        1 day (12 hours)
         17,100        2 days (22 hours)
         16,900        3 days
         16,400        4 days
         15,700        6 days
         15,700       16 days

I was very surprised at the rapid initial drop off of intensity from
the MH bulbs; I had expected a linear drop for most of the lifetime. 
The data indicates that MH bulbs should be changed once a year if
you have intensity sensitive applications.

Bogus reflectorized FL bulbs

by ()
Date: 16 May 1994
Newsgroup: rec.aquaria,alt.aquaria,sci.aquaria

I've collected more light data that should be of interest to folks
plannng on buying new flourescent bulbs.

We had a discussion some time ago about the possible effectiveness of
bulbs with built-in reflectors.  I had a chance to directly compare
the same bulb with and without a built-in ("built-on" in this case :-)

We have a month-old Coralife Trichromatic bulb with their version of a
reflector (a piece of silver tape attached to the outside).  We just
got two Trichromatics without reflectors.  I assume the bulbs
themselves are the same and Coralife adds the tape and charges $3
extra.  Before installing them, we set them up in a spare shoplight to
see what the differences might be.  We also compared them to a 1 year
old PennPlax Ultra Trilux.

For the first test, a bulb was put in the Sears magnetic ballast
shoplight with a Triton bulb in the other half.  Both the Triton AND
the reflector were covered with a black cloth to eliminate any
external reflections and light.  Two lux measurements were made.  The
first was done 3" from the bulb with the meter supported by a piece of
2 1/2" dia. PVC pipe.  This is like a "spot" measurement and should
show the intensity coming from the surface of the bulb, i.e., like
from a single spot not including integration over the length.  The
second test was done 12" from the bulb and should be indicative of the
light reaching the middle layers of the water, including the light
summation from the whole bulb (more light at the middle than the
ends).  All values are in Lux (lumens per square meter).

                                     3"            12"

Trichromatic with reflector         5400          1060

Trichromatic without reflector #1   5300          1100

Trichromatic without reflector #2   5400          1160

Ultra Trilux                        7300          1480

These results indicate that the Coralife concept of a reflector
(silver tape on the outside of the tube) doesn't do anything for bulb
intensity.  It also shows that the Trilux is much brighter than the
Coralife bulbs.  They both appear to be the same color.  Coralife
claims a "6500K" color temperature; I measured 6150K to 6750K for
the TriLux bulbs in a previous test.

For the second test, just the Triton bulb was covered by the black
cloth.  This allowed the shoplight reflector (white paint or a white
coating) to work sort of normally.  Since the other bulb was covered,
a little under half of the reflector was "out of service".  I would
expect higher measured values with a complete reflector, except in the
case of the bulb with a built-on reflector.  (The first measurements 
are in parenthesis for comparison). 

                                     3"              12"

Trichromatic with reflector         5600  (5400)    1400  (1060)

Trichromatic without reflector #1   7900  (5300)    1900  (1100)

Trichromatic without reflector #2   7500  (5400)    1820  (1160)

Ultra Trilux                       10800  (7300)    2820  (1480)

As I expected, the bulb with a built-on reflector did poorly here,
since the bulb's reflector prevented most fo the light from getting to
the fixture reflector (the bulb refector covered 1/2 the bulb; some
"side light" did get to the fixture reflector increasing the overall
intensity a little).

Bottom line: the Coralife "reflector" does nothing for bulb intensity
and actually cuts the available light if used in a fixture with a
reflector.  I see no reason whatsoever for paying extra for a bulb
with a built-on reflector.  I see no reason to get the Coralife bulb
instead of the Trilux, except perhaps for availability.

Some time ago, using a different setup, I measured a Rainbow Lifeguard
"BioLume" bulb with a built-IN reflector and was also unimpressed.
The Biolume measured 4600 Lux and a group of TriLux bulbs measured
6700 to 7800 Lux (old to new bulbs).

George Booth                 | Specialist in Freshwater Plant Tank Technology     | Keeper of Discus, Angelfish and Rainbowfish

Light bulb efficiency

by ()
Date: 6 Jun 1994
Newsgroup: rec.aquaria

Whilst browsing a thermodynamics book trying to figure out how to 
calculate the amount of heat needed in substrate heating coils to 
move x amount oif water through the substrate, I came across a bit 
of interesting trivia:

   For converting electrical energy to light energy:

      incandescent (and halogen) bulbs are 7% efficient,
      fluorescent bulbs are 20% efficient and
      sodium vapor (and metal halide) bulbs are 40% efficient. 

So I guess that means that it would take 9 40w fluorescent bulbs to 
equal 1 175w MH bulb and 10 100w incandescent bulbs to equal 1 175w
MH bulb. 

Unless, of course, the wattage rating of the bulbs means different 

BTW, I don't think you can calculate how much heat is needed to move
x amount of water through the substrate, unless your last name is
Einstein :-). 

George Booth                 | Specialist in Freshwater Plant Tank Technology     | Keeper of Discus, Angelfish and Rainbowfish

Light bulb efficiency

by (Uwe Behle)
Date: Wed, 8 Jun 1994
Newsgroup: rec.aquaria

Diane Justice ( wrote:
: In article <2t0a7n$>,  <> wrote:
: >Whilst browsing a thermodynamics book trying to figure out how to 
: >calculate the amount of heat needed in substrate heating coils to 
: >move x amount oif water through the substrate, I came across a bit 
: >of interesting trivia:
: >
: >   For converting electrical energy to light energy:
: >
: >      incandescent (and halogen) bulbs are 7% efficient,
: >      fluorescent bulbs are 20% efficient and
: >      sodium vapor (and metal halide) bulbs are 40% efficient. 
: >
: >So I guess that means that it would take 9 40w fluorescent bulbs to 
: >equal 1 175w MH bulb and 10 100w incandescent bulbs to equal 1 175w
: >MH bulb. 

:      It may be, some time in recent history, but I can show you
: factory spec on a commonly used reef fluorescent bulb that is more
: efficient then a commonly used reef MH.

: -Anthony

I seem to remember that the Lumen/watt is really highest for fluorescent bulbs.
Next are MH and SV. The possible light intensity is higher with the high pressure
lights because they illuminate a smaller area with the same power.


NAME	Uwe Behle, HP Boeblingen Instruments Division
EMAIL (internet), (packet radio)

Free Spectrograph Offer

Date: 1 Feb 1995
Newsgroup: alt.aquaria,rec.aquaria,sci.aquaria

Hi all,

This belongs on sci.aquaria, but not everyone can get it, so I am 
wasting some space to cross-post it. Non-techies can quit reading. :-)

George Booth asked if my optics background could be used to help the 
group. That got me to thinking about the small diameter screw-base 
flourescents I wrote about here a month or two ago.

The LOA units (expensive) were clearly labeled tri-phosphor. The GE 
Biaxes (much cheaper) were not identified as anything, but have seemed 
to work just as well. I wondered why.

Setting up a monochrometer and detector in the lab to run a spectrum is 
a real pain, so I used a poor-man's spectrograph to inspect lights all 
over the house (not possible with a monochrometer, anyway).  The results 
were so handy, I decided I should share the new tool with the group.

A small holographic grating, held in front of your eye, breaks up a 
distant light into a series of rainbows (it was just a 500 line/in 
grating, so about 3 orders, or rainbows,  are visible on each side of 
the original light).

Looking at a "cool white" shop light, the rainbow was quite smooth, 
except for a sharp green image of the lamp in the middle. There was a 
sharp drop off at blue (no violet) and a sharp drop at about red-orange 
at the long-wave end. The strong green mercury line, and the attenuation 
at the ends of the spectrum, were about what I expected of a broad-band 

Looking at the lights over my tanks, I got a series of relatively sharp 
lines (images). One deep red, a red-orange, a darker smear of yellow, 
green, cyan, and a rich violet. The appearance was totally different 
from the cool white bulb in the garage. Side-by-side, the $16 LOA and 
the $9 GE had exactly the same spectra. To be fair, the LOA unit could 
have its bulb replaced, and the GE was dead at end-of-bulb-life. In 
aquarium use, the ballast life may be the limiting factor anyway.

This ability to see the light structure is highly portable. You can go 
to the lamp shop or hardware store, and get a good look at what _any_ 
lamp is doing. It's so neat, I wondered how I could share it with the 
good folks on the net who have helped me so much. Maybe someone can look 
at an old tube, side-by-side with a fresh new one, and see if the 
spectrum is changing with age.

Here is my deal. You send me a self-addressed, stamped envelope, and I 
will send it back with a chunk of holographic grating that you can use 
to compare the spectrum of your lamps. I will cut it big enough that you 
can use it in front of a 35mm camera lens to photograph the spectrum, or 
build your own spectrometer with a slit and detector to move thru the 
image at the film plane.

I do not know how you guys in other countries can do this, without 
costing me a fortune in postage. I assume US stamps are hard to get in 
New South Wales. Any suggestions?

If the demand is reasonable, I can probably accomodate everyone. I have 
a huge supply of this stuff. If I get ambitious, I may write up a 
one-page manual with actual grating pitch, how to calculate the 
wavelength angle, which way to rotate for sharpest spectra etc. Send 
your questions, so I can see if the answers should be included.

The address is:

Wright Huntley
2483 South Park Lane
Santa Clara, California USA

Any legitimate students, out there, who want to build a fancier 
spectrometer can request some 50,000 line/in grating for their project. 
I can accomodate a few with 2"  (+/-) long by 1/2" wide pieces, 
sandwiched in glass. With 100,000 lines, you should get a spectral 
resolution of about 10ppm. Please give me a note describing your 
project, not that I really give a damn. This is just to assure 
legitimate requests from knowledgeable students, and prevent a deluge of 
idle requests.  Besides, you pay shipping, whatever it costs me.

"The first (and key) step to liberty is to be a good neighbor."

Sticking my neck out - again

Date: Mon, 11 Nov 96

     >Do you have a 35mm camera with a built-in light meter? One of the 
     >"older" kind, in which you manually set the shutter speed, ASA or DIN 
     >film speed, and hand-adjust the f-stops?  
     I found an approximation for converting readings from a camera's light 
     meter into foot-candles (Source:  Greenhouses--planning, installing 
     and using greenhouses.  Ortho Books, 1991).
     Set your camera to ASA 25 and 1/60 shutter speed.  Aim and focus the 
     camera so the object you want to measure fills the viewer.  Adjust the 
     f-stop until the needle indicates the correct exposure.
     If the object you are metering is WHITE, then:
     f-stop     foot-candles
     2          100
     2.8        200
     4          400
     5.6        800
     8          1,600
     11         3,200
     If the object you are metering is GREEN (like a plant leaf), then you 
     must increase the f-stop by two settings to get the correct amount of 
     light (e.g., if it reads f5.6, add two f-stops, resulting in f11 and 
     3,200 foot-candles).
     A camera's light meter only reads reflected light, so if you measure 
     from something white (e.g., 90%+ reflectivity), you are getting a 
     pretty good measure of the amount of incident light.  Green plants 
     reflect much less light (about 20%), so you must adjust your f-stop 

Lux Meter Readings

by peachdoo/
Date: Tue, 22 Aug 2000

Hello Folks:
     I finally got around using my new lux meter which I purchased from pets
wharehouse on someones recommendation from our list.  I got the $99.00
dollar model.  I did some quick measurements just to start getting a feeling
for what was happening with the florescent lighting from my Phase IV hood. 
I played with the sensor in the water and noticed that if I tilted the
sensor so that it was not at flat and level with the hood the readings
varied.  I opted to simply maneuver the sensor until I could get the highest
reading it was able to get on the lux meter.  
     I put in 4 new Hagen 20 watt bulbs.  Two of them were Power Glo and the
other two were Aqua Glo. My hood has a thin plastic shield to protect the
tubes.  I took my measurements from the surface of this plastic shielding
and moved down into the water.

80 watts of lighting in one hood.
7000 lux = #1  I put the meter up against the clear plastic shield that
protects the florescent tubes.  Phase IV is at 5.5 inches above
the water surface.

2540 lux = #2  My first reading was at 12 in. from the the plastic shield or
6.5 in. underwater.

1440 lux = #3  My second reading was at 18 in. or 12.5 in under water.

Does anyone know what values are ideal in terms of lux.  We always talk in
terms of watts.  My readings are not "scientific" because there are so many
variables that will affect the amount of light intensity that can reach the
bottom.  I took readings #2 and #3 with the hood hanging over the floor and
got readings of #2 = 3930 lux and #3 = 229 lux.

Hood suspended  5.5 inches above water surface  for a total of 80 watts.

Readings directly at plastic shield on hood   7000 lux 

Readings in the water (12 in from hood) =   2540 lux       1440 lux

Readings out of water (18 in from hood) =   3930 lux       2290 lux

I know that others have been taking readings and concerning themselves with
reflectivity in the aquarium and from the hood reflectors.  But my concern
is what kind of light intensity is needed by the plants and is minimal or
optimal for the various species. My reflectivity inside the tank I know is
diminished by algea growth on the glass.  

I took similar readings from my old florescents which I replaced.  These
Sylvannia bulbs T-8.  They were 7 months old and each one was 18 watts.  Two
of the bulbs were Growlux, one was an AquaStar, and the other a LuxlinePlus.
These bulbs were only 23-10 lux less than my new bulbs.  Pretty good
considering age and wattage differences.  So I have stored them on standby
and will see what I get for the Hagen's in 7 months.

In case someone was interested in comparing figures and setting up a better
test situation let me know.

Thanks, Diana


by busko/ (Ivo Busko)
Date: Tue, 25 Jul 2000 wrote:

> Hello Plant lovers:
>      About the lighting in tanks: a discussion fired up by Thomas Barr.  I
> have been looking for a light meter that measures in Lux or Lumens or any
> measure that lends itself to being plugged into math formulas so I can
> extrapolate, analyse and play with my set-up as I am learning and
> experimenting.  Has anyone found useful and inexpensive meters that will
> lend themselves to measuring light levels at various depths in the tank.  I

I've been playing with my SLR camera meter (an old-fashioned but very
accurate Olympus OM-2). Two problems here:

What you are looking for is a diffuse reading. High-end hand-held photographic
ligth meters can take both diffuse and direct readings. With the sensor
exposed, they take direct readings. Covering the sensor with a white 
translucent plastic hemisphere, they take diffuse readings. This is the
one you should in principle use, locating the photometer at the spot
where you want to know the lux level, and pointing it to the ligth source.

SLR through-the-lens meters take direct readings only. Still, it is
possible to use such a device to measure the lux level at a given spot
by placing a diffusive white surface target at the place where you want 
to know the ligth level, and using the SLR photometer to take a spot reading
of that surface. A tele lens migth be useful for constraining the field of 

The second problem is that the EV scale resolution of photographic
photometers is usually poor. Each f/stop or shutter speed increment
corresponds to a factor sqrt(2) in illumination level at the target, or
about 40%. I found that with my camera I can get reliable readings at a 
1/4 f/stop accuracy, which is sufficient to tell apart ligth levels that 
differ by 10%. Not a very accurate reading but enough for our purposes.

> can tell that the watts per gallon rule is much to rough based on the fact
> that the same level of wattage in one tank may look dim in one tank set-up,
> and another set-up will look very bright. Some lights will illumine in the
> yellow ranges and others will illumine in bluer ranges.  So I want equipment
> that will let me measure not only intensity but chroma (I hope that is the
> right term) as well.  This may require two different meters, I know.  Any

You migth try to use color filters on the same meter, but I wonder then how
to calibrate the readings in lux. You could instead multiply the standard
reading in lux by a conversion factor. Say you want to get the illumination 
in PAR units. Just multiply by a PAR/lumen factor, which depends only on 
the ligth bulb type. Factors for several ligth bulbs are listed in

> help will be appreciated. I am always serching the net and have come close.
> The meters average around $300.00.  I would like to find something cheaper.
> I think I should be able to use a light meter for cameras (about $35.00).  I

Make sure it can take spot and/or diffuse readings, and that the scale
can be read at a fraction of a f/stop reliabily.

> have one, but do not have any info on how to set the ASA rating and then
> convert the numbers I get into useful info that would tell me my actual
> light intensity.  I could stick the camera light meter in a baggie and

Here is a conversion table, courtesy Mike Dubinovsky:

  Aperture    Exposition time   EV   Illumination (Lux) - ISO 100/21 film
                                      External     Internal meter focvused
                                        meter         at white diffusive

  2.8        2                  2   8              2

  2.8        1                  3   17             4

  2.8        1/2                4   35             8

  2.8        1/4                5   70             15

  2.8        1/8                6   140            30

  2.8        1/15               7   250            60

  2.8        1/30               8   500            120

  2.8        1/60               9   1000           240

  2.8        1/125              10  2100           500

  2.8        1/250              11  4300           1000

  4          1/250              12  8700           2000

  5.6        1/250              13  17000          4000

  8          1/250              14  35000          8000

> submerge it to get my readings.  I can't find formulas that would let me

Don't forget to correct for the ligth absorption by the bag itself.

> extract the data I need from such readings.  My college physics book is
> useless and I have spoken to one underwater camera company who believes this
> can be done.  There has to be a better way to measure our light intensity in
> our tanks other than the crude wattage rule.

Of course, there are submersible ligth meters specifically designed for
aquarium use. Cost in the US$100 range ? Maybe the APD archives or the 
krib have info about them ?
> Help!
> Diana

- -Ivo Busko
 Baltimore, MD

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