A Little About
Metal Halide Lighting
The advent of
metal halide lighting, during
the eighties, within the
played a large part in making
successful reef aquariums
possible. Up until that time, it just
wasn't possible to deliver light
with the intensity and spectrum
required to maintain SPS and LPS
corals for any length of time in
reef aquariums. Today, of the
various types of light sources
generally employed in reef
halide are generally considered
to provide the highest output lighting
available - although this may be
changing with the recent advent of LED
lighting. Metal halides produce
about 90-100 lumens for each watt
lights are able to
concentrate their output into
a very small space - other
light sources, fluorescent
lights for example, produce
an even, almost diffuse, broad area of illumination.
The concentrated output of metal
halides is often referred to as
point source lighting. It is this
concentrated light that produces
the visible rippling effect that
so many reefkeepers consider to
be one of the advantages of
In addition, the concentration of
light increases the light intensity.
This combined with well designed
reflectors, allows the illumination from
metal halide lamps to reach relatively
deep into the water column.
temperature and wattage
are available in many
from 6500 Kelvin up to 20,000 Kelvin.
The temperatures most often
available are 6,500 K, 10,000 Kelvin,
12,000 Kelvin, 14,000 Kelvin, and 20,000
Kelvin. Common output values in
watts are 70W (usually only
available in double ended bulbs), 150W, 175W, 250W,
400W, and 1000W. The three most
often employed bulbs within the
hobby are 175W, 250W, and 400W.
A quick glossary
for metal halide lighting,
you'll come into contact with
quite a few technical sounding
terms. Halides, metal halide, HQI, etc.
Let's take a look at a few:
Halides - a
group of minerals containing one of the halogen elements (chlorine, fluorine, bromine, and iodine) as a building block, a salt of any halogen acid
Halide - a lamp which generates
light by passing an
electrical current from one
electrode to another through
a metal vapor, to form an
arc between the electrodes. Like most high-intensity discharge lamps, metal halide lamps operate under high pressure
and temperature. Because of this, special fixtures are necessary for safe operation.
Iodide is a
particular kind of high-intensity discharge (or HID) lighting.
Light is produced by an electrical arc
being passed through a gas. Hydrargyrum
is the Latin name for the
bulbs are available in two basic
ended (SE) bulbs
- These are
the bulbs most encountered
in the United States.
of the most important
advantages of the SE bulbs
is that the arc tube (the
light producing assembly) is
enclosed in a protective
glass envelope. This
envelope not only protects the inner components and arc tube from oxidation, it is also very
effective in preventing
harmful UV light from
escaping and possibly
harming delicate corals,
other invertebrates, and
even fish - because of this,
the vast majority of SE
bulbs need no additional UV
should be noted that
manufacturers of SE bulbs
recommend that the bulbs be
positioned so that the
"nipple", located on the arc
tube, is pointing away from
the direction the light is
needed. Using my Apogee PAR
meter, I have never been
able to verify this to be
are designed to be used with
a ceramic mogul socket
ended (DE) bulbs
DE bulbs do not have a
protective protective glass
envelope and must,
therefore, be shielded with
a piece UV proof glass
placed below the bulb.
are designed to be used with
special two piece sockets
(one piece is shown in
the pic, below).
I thought I'd mention that although
there are many claims that DE
bulbs are better than SE bulbs
in somer espects and SE bulbs are
better than DE bulbs in other
respects, I have never been able
to ascertain any appreciable
differences between the two.
All metal halide lamps require ballasts to regulate the current
flowing through the arc tube.
Ballasts also ensure that the proper voltage
is delivered to the arc. It is
important that the proper type
of ballast is used with a
particular bulb. Some metal
halide bulbs contain a third electrode to
start the arc when the lamp is first lit.
Others, such as pulse-start lamps don't contain a starting electrode,
so they require an igniter to generate a high-voltage pulse to start
combine igniter and ballast into a single package. These ballasts use high-frequency to
drive the lamps.
They have less loss than a line-frequency
(60Herz) ballasts, they are more energy efficient.
Some electronic ballasts, like
the ones I use in the Glass Reef
are "dimmable", with the output
being adjustable from approx.
70% to 100%.
what kind of MH bulb is used. No
matter what ballast is used to
drive them. A reflector is
needed to optimize and control
the light output. A well
designed reflector, properly
matched to the tank setup with
which it will be used, ensures
that a maximum of light that is
distributed in as even a manner
as possible will be achieved. In
my opinion, the choice of
reflector is just as important,
and sometimes more so, than the
choice of bulbs to be used. Be
diligent in your research. Do
not skimp when purchasing the
reflectors for your reef tank.
Buy the best you can afford.
Reflectors like the one in the
next pic were once considered
top notch. Longer!
modern reflectors (like the
LumunBright shown below) do an
excellent job of concentrating
and evenly distributing light.
The model shown below is
intended for use in
installations where the
reflector cannot be seen by
people observing the reef tank.
It is a "bare bones" model and
there has been no attempt to
make it "attractive". It is
meant to be incorporated into a
canopy that will hide it from
This is the same reflector, as above, but has been enclosed in an attractive housing. In this form,
it can be employed as a pendant,
fully visible to the viewer.
There is a downside
to metal halide lighting. They
produce much more heat than, for
instance, a T5 fluorescent
installation. That heat has to
be dealt with - cooling fans, a
chiller, or possibly both must
be considered. The
initial cost can be a shock! Metal halide fixtures are more
expensive than fluorescent systems
at first glance - although in
large T5 systems where as many
as 20 and more T5 bulbs are in
play, the cost of T5 bulbs can
far exceed that of MH bulbs in
that same sized system.
You can't use
an MH bulb forever. After about
eight months (just an average) a
slow shift in spectrum occurs as
does a steady lessening of the
amount of light produced. This
happens so slowing that we don't
notice it, but our corals do. I
make sure to use my MH bulbs for
no longer than nine months, or
Which bulbs should I use?
20 reef enthusiasts about which
MH bulbs deliver the "best"
spectrum, PAR value, and look,
and you'll get 20 different
answers. It's probably true that
the current "hot favorite" in
the hobby is the Radium 20,000
K. I tried them but found
them to be too blue. I switched
back to my 12,000 K ReefLux's.
My honest opinion is, if you
stick with one of the quality
bulbs available on the market
these days, and all other
conditions in your tank are
conducive to coral health,
you'll get great growth. No
particular bulb is the magic