Figure 1: Inverse Square Rule. The area of the coverage of light goes from ~12.57 units at 5 units from the light source to ~50.28 at 10 units or 4 times the area.
The subject of lighting is quite complex and confusing. There are a number of different options available for lighting and it is not easy to decide which is the most appropriate. While some generalisations can be made, it is helpful to understand The Properties of Light as well as something about the Underwater Lighting Conditions the planned inhabitants will have experienced in the wild.
For a Fish Only tank, the decision is quite easy. Fish require enough light to see and that is all. This can easily be achieved with a few normal output fluorescent tubes. The choice of tubes is fairly arbitrary and you should choose what you find appealing. See: Fluorescent Lamps for a list of inexpensive lamps that can be used.
Most reef tanks are used to house organisms that require light for photosynthesis. (See Photosynthesis for a summary on the subject.) The organisms we are most interested in are animals with photosynthetic symbionts. Generally, the lighting requirements of these animals correlate with the depth from which the animals are normally found. Those found in shallow water, normally require high intensity lighting. Those found in deeper water can make do with much lower intensity of lighting.
When planning the lighting for a new tank, the needs of the organisms with the highest lighting requirements should be considered first. If you can provide for these organisms, it is relatively easy to provide areas of lower light intensity through the use of caves, overhangs or the placement of organisms away from the lamps.
As discussed in the article on photosynthesis linked above, the most important aspect of light for organisms that rely on photosythesis is the amount of light they receive over time. For aquarists, this translates to a combination of light intensity and duration. That is, the duration that the light is provided is almost as important as the intensity of that light. Lower light intensity can to some degree be compensated for by longer duration.
Light Output versus Light Intensity
It is important to understand the distinction between light output and light intensity. Light intensity can appear quite complex, especially when comparing different light sources. It is often confused with the light output of a lamp.
Light output is the specification of the total amount of light put out by a light source in all directions. For example, a fluorescent tube send light out right around the tube and the light doesn't just leave the tube perpendicular to the tube. Our Sun is spherical and light radiates in all directions but only some of the light on one side reaches the Earth.
Light intensity is the measure of the amount of light hitting a certain area. (Note that there are different terms such as illuminance and irradiance which describe different aspects of light intensity.)
It may be easier to consider an analogy. If you were to place 40 kg of sand into an aquarium 30x30x30 cm, the depth of the sand would be close to 30 cm. If you placed the same amount of sand in an aquarium that was 60x60x60 cm and spread it evenly over the bottom, the depth of the sand would be less than 8 cm. In a 90x90x90 cm aquarium, the same sand, spread evenly, would only be around 3 cm deep. With the 40 kg of sand in the 60x60x60 cm aquarium, if you were to create a mound in the middle of the tank, the depth would be more than 8 cm in the middle and much less at the edges. Light output is like the 40 kg of sand and light intensity is like the depth. With the exact same light output (amount of sand) you can have different resulting light intensities (depth of sand). To make the analogy even more accurate, it should be assumed that some of the sand was spilled before it made it into the tank, in the same way that not all the light put out by the lamps on our aquaria actually makes it into the aquaria.
As shown above, the area over which the light is spread affects the intensity of the light in that area. The further you move from a light source, the greater the area over which the light is spread and so the lower the intensity. This is best shown using a flashlight in a dark room. Turn the flashlight on and hold it a few centimetres from a light coloured wall. You will see a relatively small but bright patch of light. Now move the flashlight until it is a few metres from the wall. The same amount of light is now spread over a larger area and it is not as bright. The light output of the flashlight has not changed, but the intensity of the light has.
The effect of distance on light intensity will vary depending on the type of light source which I will cover in the specific types of lighting available for our aquaria.
Light output is usually provided in lumens which is a unit of luminous flux. The unit Lux is often used to measure light intensity (illiminance) and 1 lux is 1 lumen per square metre. That is, a lux is one lumen of light spread out over an area one metre by one metre.
Photosynthetically Available Radiation (PAR) is a measure of irradiance and is basically a measure of the number of photons of light hitting an area over time. At noon in the tropics, the irradiance at the ocean surface is around 2000 μE.m-2.s-1. One Einstein (E) is equal to 6.0225 × 1023 photons. So, 2000 μE.m-2.s-1 means 1,204,500,000,000,000,000,000 photons of light hitting an area one metre by one metre every second.
There are basically two types of lighting that are useful for marine tanks: fluorescent and metal halide.
Fluorescent lamps are available in a number of different form factors, different output levels and provide different spectra. Metal halide lamps are available in different sizes, different fittings and different spectra. While light is light and so the two different types produce essentially the same light, the two different types distribute the light differently which affects the light intensity at different points in aquaria.
I will discuss metal halide lighting first as it is somewhat simpler with respect to the distribution of light.
A metal halide lamps is know as a point source as the light effectively radiates from a single point. Note that while the actual "point" may be larger than what might be considered as a point, the properties of the lamp over the distances being measured approximate to a point source. You may find it interesting that the Sun, which has a diameter of 1.4 million kilometres, is considered a point source of light by virtue of the fact that is is around 150 million kilometres away.
Being a point source, a metal halide lamp is subject to the inverse square rule which describes the relationship between distance and intensity. If the distance from point source is doubled, the area over which the light is spread is quadrupled and so the intensity is reduced to one quarter. If the intensity at 5 cm was 1000 (the units are not important), at 10 cm the intensity would be 250. Figure 1 shows the relationship graphically.
Figure 1: Inverse Square Rule. The area of the coverage of light goes from ~12.57 units at 5 units from the light source to ~50.28 at 10 units or 4 times the area.
Last updated: November 7, 2004