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An
Article by George J. Reclos
Lighting is a key issue in
fish keeping – I am sure most of you know that. Your particular setup
may require too much or too low lighting, special color lighting (color
temperature), special type of lighting (e.g. need for deep penetration),
even lighting at special angles. All those elements can make a tremendous
difference in what you and your visitors see when they look at your
display aquarium and may also have a great impact on the well being of the
animals and plants you keep in there. Of course, all those elements, when
not explained in detail, create a much bigger confusion which forces
hobbyists to opt for an “average” solution. What’s worse, there are
no rules of thumb and those that exist should not be used - another reason
for more confusion. There are some
principles though. Let’s start by saying that the
lighting requirements are dependent on the animals, plants and
invertebrates which live in your tank and not on the size / shape of the
tank.
Indeed, there are
several issues related to lighting: Type of light source, amount of light
(in Lumens or Lux but not Watts), light color (more
correctly: light temperature measured in degrees Kelvin), quality of light
(CRI), duration of light and many more. Yes, things are not simple at all,
when it comes to lighting. No wonder there is a whole chapter about it in our Physics books. I will try to make this issue as
simple as possible but not oversimplify it. The aim of this article is to
provide enough information for the hobbyist and enable him to choose the
right lighting equipment because he knows what he is looking for. The
article will have succeeded its goal if more and more hobbyists visit
their petshops asking for lamps with specific characteristics instead of
relying on what is stock (or what the shop works with a greater profit
margin). Of course, this is not going to be an “easy reading” article.
It couldn’t be – and it shouldn’t be !
Light color (temperature of light
source). Our eyes (and of course the eyes of our fish and our plants’
photosynthesizing elements) are “calibrated” to perform in ambient
sunlight. Subsequently, our eyes interpret this light as white light. As all of us know,
“white” light simply doesn’t exist. In simple words, “white” is
not a color but a combination of many wavelengths (colors), which form a spectrum
(see Glossary at the end of this article). What we perceive as “white
light” is actually the average of the solar spectrum. This spectrum
comprises roughly from six basic colors (and all their hues since the
spectrum is continuous) : Red,
Orange, Yellow, Green, Blue and Violet.
Those colors are listed in ascending frequency order (or descending
wavelength, thus the Violet has the shortest wavelength and the highest
frequency - hence higher energy). The visible wavelengths range from 760
nm (deep red) to 380 nm (violet) – see Diagram
1.
Diagram
1. Full spectrum of the visible light. Left: Visual light is a tiny
part of the electromagnetic wave spectrum. Right: This is the spectrum of
sunlight. As you can see, the spectrum is continuous, while not all colors
are emitted with the same intensity. However, all animals living on the
surface of the earth or very shallow waters (sea or lakes) have light
sensors (eyes or other organs) “calibrated” for this spectrum.
Any wavelength longer
than 760 nm is in the infra red (IR) region while anything shorter than
380 nm is in the Ultra Violet region (UV). One very common
misunderstanding occurs when we discuss the “temperature” of the light
and the “warmth” of it. To the human eye, the “warm” colors are
the ones at the red end of the spectrum while the “cool” ones are at
the bluish end of it. However, when we refer to light sources, things are
exactly the opposite. Sunlight white usually has a temperature of
5000-5500 K while light sources rated at higher temperatures contain more
and more blue in them (10000 – 20000 K) – see Diagrams 2 and 3 and Table 1.
Diagram
2. A full spectrum, low temperature light source. Note that the red /
yellow region is enhanced while the emission in the blue region is
reduced. This resembles the emission spectrum of an incandescent lamp.
Diagram
3. A full spectrum, high temperature light source. The red, green and
blue regions resemble that of the sun. Compare this spectrum to the
“bands” of the spectrum emitted by the full spectrum aquarium tubes
(photos 2 and 3).
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Light
Source
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Temperature
(degrees Kelvin)
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Candle
flame
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1800°
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Incandescent
lamps
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from
2500° to 3050°
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Fluorescent
warm white lamps
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3000°
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Fluorescent
cool white lamps
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4100°
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Daylight
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6500°
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Midday
sunlight
North
Sky light
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5500°
7500o
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Cloudy
sky
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7000°
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Clear
sky
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from
10000° to 30000°
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Table
1. The Kelvin ratings of some common light sources. Reading it will give
you a good idea of what this “Kelvin” rating means.
Of
course, “white” is something relevant. We see the light coming from an
incandescent lamp and the light coming from a normal fluorescent tube (the
“cool white” ones) and we interpret both of them as “white”,
especially when we have nothing to compare them against. However, if you
take a picture of those two light sources on a regular (daylight) film,
you will see that the incandescent lamp is yellow while the fluorescent
tube is green – see Diagram 2 and
4.
Diagram
4. The cool white fluorescent tubes found in most homes and buildings seem
to be white while actually they emit a big amount of green light, which
makes them unsuitable for the planted tank. This can be captured on
daylight film.
This difference may not be
crucial when reading a book, but it is very crucial for the well being of
your corals or freshwater plants which need the energy of the red and
especially the blue rays. Moreover, it is perfectly possible to create
“white” light by mixing the three basic colors (red, green and blue).
This creates a white light, although in this case the spectrum is not a
continuous one but one which shows three peaks (or bands). This is the
case in most good quality lamps. It goes without saying that, depending on
the mixture of the colors a different result is obtained. Thus, the cheap
household fluorescent tubes create a “green” cast with a narrow
spectrum, the “full spectrum” lamps create a bright white color, while
the so called “actinic” lamps have a very narrow band in the blue
section of the spectrum.
Some other factors that you
should know (and perhaps use them to work for you) is that the shorter
wavelengths (greens and blues) are less scattered in the water (and the
air) therefore more penetrating while the red, orange and yellow rays are
far more scattered (which is the typical answer to the question “why is
the sky during the sunset orange?”). So, in an ideal, full spectrum
lamp, more green and blue will reach the bottom than reds and yellows.
Your plants will definitely appreciate that and the colors of the fish
will be enhanced. However, you will “feel” that there is less light in
your tank than actually is. In contrast, the red – yellow – green
color creates the impression of more light in your tank.
Moreover, the human
eye “receives” different colors in different ways. If you had to take
a look at monochromatic light sources (only one color emitted) of the same
intensity, then you would think that yellow is brighter than the rest and
blue the dimmest. Keep in mind that the human eye is more sensitive to the
green color (hence the more green emitted by the normal tubes). Apart from
that, red creates the “alert” feeling, orange the “warmth” feeling
and blue the “cold” one. Thus we have two factors that make visual
estimation almost impossible : One thing is what we see
and another thing is how much
light (and what type of light)
reaches our plants, corals or fish.
Light
is very important to the well-being of many organisms we keep in our
tanks. Thus some of the organisms we use to keep in our tanks depend on
light for their feeding needs. Corals, for instance, use the products of photosynthesis
(food and oxygen), provided from the symbiotic zooxanthellae that live within their tissues. Zooxanthellae, on the
other hand, use the by-products of corals with come in the form of carbon
and wastes rich in Nitrogen and Phosphorus as their food source.
Light
is also essential for the function of color pigments of some organisms,
while some other organisms use light to manage vitamins and/or minerals,
useful for the constructing or maintaining structures of their skeletons.
Another
critical fact for the right lighting of an organism is the angle that the
specimen is placed in the tank. Some organisms prefer direct light, while
some require that the light will “hit” them not directly from above
but at an angle. Some others prefer to get the light obscured by shades.
As already pointed, the species you will keep will determine the kind of
light you need.
Continued
in next page ..
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