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Re: Lighting and Lux

Scott wrote:

Wayne Jones said, in part, re John T. Fitch's analysis of Amano's

> That article is not really plotting the lighting requirements of
> tanks
> but the efficiency of short verses long lamps. Also since the short
> lamps are less efficient he has to crowd more lamps in his fixture
> which
> results in even greater efficiency losses. If Amano was using PCs he
> would not be using so many watts on his small tanks. This watts per
> gallon rule should only apply to linear fluorescents.

I looked again at the chart it appears for all the world to plot the
watts used by Amano against the tank sizes that those wattages were
used on, by Amano.  This is not Watts per gallon?


Well yes it is labelled watts per gallon but the effect has nothing to
do with higher light levels/gallon required for smaller tanks. The light
levels per gallon are probably constant. It is the lamp efficiency that
is changing. The short fixtures just use more watts to create a unit of
light than the long fixtures.

The effect of ballast efficiency, lamp efficiency and fixture efficiency
is something that is completely ignored by the watts per gallon rule and
by this analysis. Naturally fixtures with short linear lamps will be
less efficient than long linear lamps. In some cases very much so.

There is a newish sort of rating for luminaires called LER. I think it
stands for luminaire efficacy rating. It includes the above 3
efficiencies in a single number. It is used to determine which fixtures
will save you the most electricity. It is lamp lumens x number of lamps
x fixture efficiency/input watts. The fixtures with the highest LERs are
the best. T8 lamps with electronic ballasts have the best possible LERs.
They are somewhere around 75 lumens per watt. It is not hard to find
fixtures with LERs in the 30s. It is not possible to compare fluorescent
lamp fixtures directly with MH fixtures using this method but if you
convert the lamp lumens to PAR you can do it quite easily. Lithonia has
the best web site for looking at this sort of stuff. They give you
photometric data for individual fixtures as well as which lamp spectrum
and the numbers they used to calculate the LER. Pretty facinating stuff
for me at least. The fixtures with the highest LERs are all open
fluorescent T8 systems with electronic ballasts and painted white
reflectors. MH fixtures have significantly lower LERs but those lamps
usually have more PAR per lumen so it makes up for it.

LER is not the whole picture though. You really don't need to include
the input watts unless you are trying to figure out how much electricity
is going to cost. The lumens after 40% of rated life should be used and
should be converted to PAR too. There is still the question of how
efficient a fixture is at gettting light past the water surface or to
the bottom of the tank. That question cannot be answered using luminaire
photometric data. A water filled tank would have to be included as part
of the luminaire. There is also the question of spectral efficiency.
That is how fully the plants can use a given spectrum. All in all though
LER is pretty interesting stuff. It is really just the fixture
efficiency with the effect of the tank included that is required to
complete the puzzle. There is the question of spectral efficiency but I
don't think it is a realistic goal. Nobody is ever going to do the
research on this with aquatic plants and even if they did it probably
wouldn't matter if the lamps with the greatest spectral efficiency don't
also look good. For most people the look of a tank would be the priority
rather than having the most efficient plant growing spectrum. That is
what a plant grow lamp is supposed to do but most people don't use them.

There was an article in the aquatic gardener that had actual empirical
data for converting lumen measurements to PAR. The author had measured a
number of lamps with a PAR meter and a LUX meter. I lost my copy and
cannot remember which volume it was in. Does anyone know where I can
find this data. I would like to add it to my collection.