Ptiotobiology

Photobiologists, scientists who study the interactions between living organisms and light, measure light intensity in terms of photon irradiance. Simply put, this is a measure of the number of light-energy units (photons) falling on a given area in a given period of time. The standard measurement is a unit of energy (microeinsteins per square centimeter per second). Why use such a measurement? Because energy drives living systems. For photosynthetic organisms, the amount of light energy reaching them determines how well the vital engine of photosynthesis runs. This situation can be compared to the relationship between the accelerator pedal (light intensity) and the speed of a car engine (photosynthesis). Press down on the pedal (increase the light intensity) and the engine revs up (rate of photosynthesis increases). Just as a car runs best at moderate as opposed to very slow or very high speeds, there is an optimum level of "acceleration" for photosynthesis. Photobiologists have learned that the total number of photons (intensity) falling on a given organism is more important than the energy level (wavelength) of the individual photons.

How then, is the aquarist going to insure that this energy requirement is met? Natural daylight is the best lighting source, but it is an impractical one for most of us today. To utilize natural daylight for aquarium lighting, a greenhouse or special window arrangement is required in most climates. More and more advanced reef enthusiasts are experimenting with the use of natural sunlight in such situations to enhance growth rates or coloration of stony corals, but these systems almost invariably require both supplemental lighting and a water chiller. Even in the most northerly regions, an aquarium placed in direct sunlight can overheat. Aquarists should avoid placing the aquarium in a sunny window, as seasonal fluctuations in temperature in such a location will make maintaining the correct water temperature a challenge. Ar tificial lighting, for most home situations, is the better choice, being more controllable, predictable, and programmable for the most convenient viewing period.

Providing Sufficient Light Intensity

Before installing a lighting system, one must make certain that the resulting levels of illumination over the aquarium will be sufficient. Simple approaches are given below, but many aquarists enjoy the challenge of planning their own lighting schemes. Ideally, one could carry out direct measurements in the tank with an underwater lux meter. This is more difficult to do accurately than one might think, even assuming one has the rather expensive instrument available.

One way to estimate intensity is to rely upon the old-style method of measuring intensity in lumens and irradiance in lux (lumens per square meter). Over tropical seas, irradiance can exceed 150,000 lux, far more intense than any aquarist can hope to achieve with artificial light. Fortunately, 10,000 lux is sufficient for most aquatic organisms, although more than this is certainly not harmful. Thus, to determine the light intensity required, first calculate the surface area of the tank in square meters. One square foot equals 0.093 square meters. So the surface area of a 55-gal-lon tank (about 4 square feet) is about 0.37 square meters. Multiplying this value by 10,000 lumens per square meter yields 3,700 lumens for the minimum amount of light that must reach the organisms in the tank.

How can one determine which lamps will provide this much light? Major lighting manufacturers provide this data in the specification sheets they publish for every type of lamp they make. For example, according to the Phillips Lighting catalog, one of their 40-watt Ultralume fluorescent lamps (F40T12/50U) has an initial lumen output of 3,300 lumens. Therefore, two of these over a 55-gallon tank

Chapter Four 107

should provide plenty of light. Unfortunately, the specified lumen output of the lamp does not accurately represent the light intensity that can be expected in actual use. All lamps decrease in intensity with use, so the average lumen output over the life of the lamp, not the higher initial lumen output, must be taken as the starting point. This varies with different lamp types, but is usually somewhere around 60% of the initial output. That would be 1,980 lumens for the Ul-tralume lamp in this example. Two Ultralume lamps would therefore have an average output of 3,960 lumens. It still seems like a high enough output for the 55-gallon tank in our example, but other factors must be considered.

Up to this point, we have assumed that all of the light emitted from the lamps will reach both the water surface and the bottom of the tank, but the laws of physics are against us. In order for the total output of the lamps to reach the surface, we would need a reflector over the lamps that was 100% ef ficient, casting all of the light output onto the tank. Of course, no such perfect reflector exists. Estimates of reflector efficiency, coupled with allowances for reflection from the water surface and other factors, reveal that, at best, roughly 50% of the light emitted from the lamps will reach the tank. That means we will actually need four of the 40-watt lamps in this example to provide a total of 3,960 lumens at the water surface.

In order for the light to reach from the surface to the bottom of the tank, we must next take into account a principle of physics called the Inverse Square Law. This states: "The intensity of light falling upon an object decreases in proportion to the square of the distance between the object and the light source." For our purposes, this can be translated, "If you double the distance between the lamps and a photosynthetic organism growing in the tank, you'll need four times as many lamps in order for the organism to continue to grow at the same rate." It would be difficult, therefore, to provide too many of the 40-watt Ultralume lamps, but the minimum would be four for a 55~gallon tank, 20 inches deep, to provide reasonably bright light. As we will see, even this is insufficient for many reef organisms.

Aquarium and Fish Care Tactics

Aquarium and Fish Care Tactics

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