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Re: lighting

peachdoo at excite_com 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 
view.

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 
http://www.aquabotanic.com/lightcompare.htm.

> 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
                    (c)
                                      External     Internal meter focvused
                                        meter         at white diffusive
                                                            object

  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