wide range of lighting conditions. Specific guidelines for
individual corals are hard to obtain, but on the basis of
my experience measuring light levels in aquariums, the
box on page 47 provides a reasonable guide.
The light requirements of corals are usually expressed
as photosynthetic photon flux density (PPFD) and measured in micromoles/m2/sec. This unit of measurement
is quite different from the lumen measurements used to
specify light output based on human vision.
When measuring in lumens, light output is weighted
differentially, with green given the highest weight and
red and blue given much lower weights. This is expressed
as a photopic curve and defines the luminous efficiency
function. However, where photosynthesis is concerned,
all photons of light are treated equally, hence this weighting is not necessary. Unfortunately, LED manufacturers
do not provide data in terms of micromoles/m2/sec, and
it is not possible to convert from lumens to PPFD without explicit knowledge of the spectrum. There are special light meters, known as quantum meters, available
that can measure light as micromoles/m2/sec, which
basically is a measure of the number of photons falling
onto an area measuring 1 m2 in one second. Using a
quantum meter, the light spread from LEDs can be measured and compared with the requirements of the corals
to establish whether the LED light can provide adequate
light for their growth. Empirical observations seem to
indicate that if the light can generate coverage of the bottom of the aquarium at
a light level of 100 micromoles/m2/sec,
there will be an adequate light gradient
across the vertical cross-section of the
tank where corals can be placed. Corals
with a high light requirement will readily
thrive in the upper third of the tank, with
adequate light at the bottom for those
with a lower requirement.
Detailed analysis and comparison
of LED fixtures and their light output is
available online and is not the focus here.
Suffice it to say that the range of choices
available to aquarists is expanding rapidly, with options for all budgets.
Every aquarist will have to decide for
himself or herself what benefits LEDs
provide compared to other, longer established lighting technologies—or even
more radically new choices, such as plasma lighting—and whether they warrant
the purchase of LED lighting now or in
the future.
Sanjay Joshi, Ph.D., professor of industrial and
manufacturing engineering at Penn State University, University Park, Pennsylvania, will discuss
plasma lighting in a future issue of CORAL.
