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Light for Freshwater Tropical Aquariums
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Freshwater
Tropical Aquarium Fish Lighting
Part 2
The following article was written by Mr.
Richard Sexton.
June 17, 1998
While mostly aimed at aquarists, this
article is also applicable to other people interested in growing things
under artificial light.
INCANDESCENT LIGHTING
Cheap, low quality light
Incandescent lights are the ubiquitous screw-in bulbs you most likely
have lighting your home. An Incandescent bulb consists of a glass bulb,
with a tungsten filament in a near vacuum; just a small amount of argon or
krypton is present. When current flows through the filament it heats up,
and glows giving off both heat and light.
Halogen bulbs
A variation of the incandescent bulb is the halogen bulb. This is an
improvement to incandescent bulbs invented by GE in 1958 for the wing tip
navigation lights of the Boeing 707. In a regular incandescent bulb, the
tungsten filament evaporates, and over time the inside of the bulb is
coated with a fine coat of tungsten from condensed tungsten vapour. This
coating will severely limit the light output of the bulb. In a halogen
bulb, a small amount of one of the halogens (Iodine or Bromine are used)
is present and combines with the evaporated tungsten. This Tungsten Iodide
or Tungsten Bromide molecule has an affinity for the tungsten filament,
and returns there and splits. The tungsten from this molecule returns to
the filament while the halogen returns to the atmosphere inside the bulb.
This process does not work unless the bulb jacket is at least 200 degrees
Celsius. This is why halogen lamps are so hot, and must be taken into
consideration for aquarium applications. Halogen lamps are 25-30% brighter
than regular incandescent bulbs. The halogen cycle, as it is called, takes
place in a very small capsule, as it is easier to maintain the high
temperature required for the halogen cycle to operate in a smaller space.
This capsule is placed inside another glass capsule which serves as the
bulbs outer casing and although is still plenty hot, is not as hot as 200
degrees Celsius.
Output spectra is biased towards the red
The output spectrum of incandescent light, halogen or regular, is
biased heavily toward the red. Non halogen bulbs have a colour temperature
of 2700K, while halogen bulbs have a colour temperature of 3000K - they
are a slightly more whitish light. Both have a CRI of 100. A diagram of
the spectra looks rather like a triangle, starting with almost no output
in the green and rising at an almost linear rate to the far red and infra
red. Although incandescent bulbs are very inefficient, they are a very
good source of near and far red light which is certainly very important.
They are sometimes used as supplements in systems which are deficient in
the red end of the spectra.
Efficiency
The two great disadvantages to incandescent lights are their
inefficiency - you don't get a lot of light compared with how much energy
you put apply. One saving grace in this respect is that the efficiency
increases proportionally to the wattage, for example a single 100 watt
bulb is much brighter than two 50 watt bulbs. The energy that does not get
converted to light is wasted by being given off as heat. All but the
smallest wattage bulbs can generate an awful lot of heat, and this must be
taken into consideration. Another point to consider is, because the heat
is so great, a splash of water on a hot bulb can shatter it.
Halogen bulbs are more efficient than "regular" incandescent
bulbs by virtue of remaining brighter, longer; they still give off 95% of
their initial light output at the end of their lives, which are about
twice as long as regular incandescent bulbs. They are also more expensive.
The great advantage of non halogen bulbs is of course their extreme low
cost for initial purchase, and of course their great availability; you can
buy them anywhere. Halogen bulbs are on the average 5 to 10 times as
expensive as their non halogen counterparts and can usually be found at
larger hardware stores. Since their primary market is yuppie track
lighting they are usually found as spot or flood lights. Of potential
interest to aquarist is the low voltage bulbs used in some track lighting
systems. Operating as 12V, these bulbs are quite small and would be good
to use a supplemental light augmenting a fluorescent setup. They are also
the cheapest of halogen bulbs. While I have seen them at $30 each in fancy
designer light stores, I have also seen them in Price Club at 3 for $12.
Sylvania makes a series of bulbs called Capsylite that come in
"regular" bulb shapes plus the large parabolic reflectors
sometimes used to illuminate the outside of houses. Osram makes a large
array of different shapes and sizes, most of which look like the vacuum
tubes. They are probably the most useful to aquarists because of their
smaller size and wide range of wattages; from low power bulbs all the way
up to 150 watts. They are however not cheap and can be quite a challenge
to find somewhere that stocks them.
Longevity
Incandescent bulbs have a lifespan of about 1000 hours. Halogen bulbs
have a life of about 2000 hours. One interesting personal note here;
although regular incandescent lights are rated at 1000 hours, we've all
had some bulbs that seem to burn on forever. The Guiness book of world
records lists the longest lasting light bulb as being an incandescent bulb
in a fire house in, I believe Boston that is some 70+ years old; it is
never turned off, which is a key point. This is why your parents always
gave you hell for flicking the lights on and off really quickly, the wear
on the filament from having current suddenly shot through it is quite
great. If you'll notice, most bulbs fail when turned on, not in the middle
of operation, or when they are turned off. The halogen bulbs I have
throughout my home seem to be on a timer; when 2000 hours is up *poof*,
they expire. I curse them out, do a rough calculation and come to the
conclusion that their 2000 hours just expired.
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