Measuring Light Intensity

The intensity of a light source reaching any surface is measured in units of lux. Natural bright sunlight produces about 70,000-80,000 lux, although much of this is lost by the time it reaches aquatic plants. The lux requirement of aquarium plants varies between roughly 300 and 6,000 lux, depending on the species. Plants such as Anubias and Cryptocoryne spp., which are often found in shaded streams, require less light than plants found on or above the water surface and in open, shallow areas, such as dwarf Echinodorus and Myriophyllum. Lux can be measured by a photographic luxmeter, although measuring lux readings in the aquarium to ensure correct light intensity is unnecessary. It is much better to start by looking at the output of various light sources.

Output of light

The output from a light source is measured in lumens. (Lux is a measure of lumens per square meter.) If the optimum output of light is achieved, taking Into account losses of light, it should be possible to ensure that the plants receive the correct amount of illumination. As a rough guide, a standard, rectangular, planted aquarium will require around 30-50 lumens per liter of water.

Light efficiency

The efficiency of a light source can be measured by the amount of lumens produced per watt. An artificial light source uses electricity (watts) and converts it into light and heat. A fluorescent tube never gets very hot, and can be touched when in use. This is because most of the electricity used is converted into light, making a fluorescent tube very efficient. On the other hand, a 60-watt household incandescent tube will get much hotter and produce less light than one 60-watt fluorescent tube (or two 30-watt tubes). However, this does not mean that the most efficient light source is always the best one. For larger and deeper aquariums, a number of fluorescent tubes may be needed to produce enough output

(lumens). Each of these fluorescent tubes requires costly starter units and space above the aquarium. Before long, the cost and practicality of fluorescent tubes may become unwarranted and you should consider alternative light sources.

Although less efficient than fluorescent lamps, because they produce more heat and less light per watt, mercury-vapor or metal-halide lamps (halogen) lamps are a good choice. Their much higher wattage ensures a higher light output and higher light intensity. Initially these lamps may seem expensive, but compared to the equivalent light output of fluorescent tubes, they are actually considerably cheaper.

Useful definitions

Lumen: Unit of luminous energy, historically the light from one candle. Lux: Unit of illumination representing one lumen falling on one square meter. Watt: Unit of electrical power.

Adaptation Plants Strong Light

Left: Plants that are highly adapted to different light conditions can nevertheless live in the same areas in nature. This tropical lily requires strong light, while the plants shaded beneath it need only moderate light.


the surface of plant leaves. Suspended debris In the aquarium water can cut down the amount of light reaching the plants quite considerably, but is easily removed using a fine floss medium in the filter. Cover glasses and condensation trays can screen out up to 30% of the light output from a given source. Plastic condensation trays are particularly bad in this respect because they quickly discolor and are difficult to clean. If possible, use glass covers and clean them on a weekly basis to reduce any significant loss of light. The light from fluorescent tubes is given off in all directions and much is lost as it is absorbed by the aquarium hood or released through the aquarium glass. Using specially designed reflectors, you can angle the light so that it is only directed into the water and not outward. Reflectors can increase the effectiveness of fluorescent tubes by up to 40%.

Choosing lighting

What we can conclude from looking at different types of light is that the best type of light depends on the function it is required to perform. Household incandescent lights are inefficient and produce a lot of heat, but are very cheap and ideal for domestic use. Fluorescent tubes are very efficient and relatively cheap if used in small numbers, and ideal for smaller planted aquariums. The less efficient, but higher output/intensity lamps are ideal for larger planted tanks. Remember that it is often better to provide too much light rather than too little, although far too much light will be damaging. When choosing the correct lighting for a particular system, the four main factors to consider are:

1. Efficiency (output in relation to power consumption)

2. Output/Intensity

3. Initial cost

4. Light spectrum

Fluorescent lighting

Fluorescent tubes emit light by electrically charging a gas contained within the tube. The light produced by the gas is mostly in the invisible areas of the spectrum, but the fluorescent coating on the inside of the tube converts this into visible light. By altering the chemical coating on the inside of the tube, the spectrum of light emitted can be changed, so fluorescent tubes can be designed for specific purposes and to emit specific colors. Fluorescent tubes designed for aquarium plant growth often produce a red-yellow or red-violet-blue color, which, although ideal for plants, may give the aquarium a slightly garish look. To remedy this, full-spectrum tubes can be added to balance the color output.

Fluorescent tubes are the most widely used method of lighting aquariums,

How light is lost in the aquarium

Particles in the water will soak up a large amount of light.

Some light is lost through ^ the aquarium glass.

Light Intensity And Plants

Large-leaved and tall plants take up light in the upper areas of the aquarium and reduce the light reaching smaller and/or lower plants.

Plants toward the bottom of the aquarium receive only a small proportion of the emitted light.

Large-leaved and tall plants take up light in the upper areas of the aquarium and reduce the light reaching smaller and/or lower plants.

Light from a fluorescent tube is emitted in all directions.

Particles in the water will soak up a large amount of light.

Some light is lost through ^ the aquarium glass.

Condensation trays must be kept clean to reduce light absorption.

Some light is refracted and/or reflected from the water surface.

Reflectors help to redirect light into the aquarium.

As light passes through the water, its spectrum is altered and intensity is reduced as it is absorbed and converted into heat energy.

mainly because they are very efficient, use little electricity, and are relatively cheap when used in small numbers. Most fluorescent tubes will last up to two years before they start to flicker and eventually become useless. However, their light output drops considerably within the first year of use, so the tubes become less effective and useful for plants unless they are changed at least once a year. Fluorescent tubes are by far the best lighting solution for smaller or shallow tanks, but for deeper or larger tanks or for plants that require intense lighting, there are other alternatives.

Although light is measured in lumens and lux (see page 60), fluorescent tubes vary in the intensity and output of light depending on the chemical coating used inside the tube and are rarely labeled in lumens or lux. Fluorescent tubes can be adequately rated for most purposes by the wattage of the light tube rather than its output of light. In aquariums that are 15 in (38 cm) deep or less, use 1.5-2 watts of light for every 4 liters (about 1 gallon) of aquarium water.

Metal-halide lamps

Metal-halide, or halogen, lamps provide intense, high-output light via a tungsten filament. They are ideally suited to deeper aquariums, with a water depth of 24 in (60 cm) or more. Suspended at least 12 in (30 cm) above the aquarium to allow ample ventilation, a single unit will illuminate approximately 2 ft2 (1,800 cm2) of surface area. (An aquarium 24 in/60 cm long and 12 in/30 cm wide.) Halogen lights are usually available in 150-watt or

250-watt versions; a 150-watt light should provide a suitable output for most aquariums. (250-watt versions are better suited to marine invertebrate aquariums, where demand for light from corals is higher.) You may need more than one lamp for aquariums longer than 42 in (107 cm). Halogen lights are initially the most costly method of lighting, but provide the best output for demanding aquarium plants.

Mercury vapor lamps

Like halogen lamps, mercury vapor lamps are suspended above the aquarium and provide a high-intensity light that is able to penetrate deeper water than fluorescent tubes. For aquariums with a depth of 18-24 in (45-60 cm), mercury vapor lamps

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The COMPLETE guide to Aquariums

The COMPLETE guide to Aquariums

The word aquarium originates from the ancient Latin language, aqua meaning water and the suffix rium meaning place or building. Aquariums are beautiful and look good anywhere! Home aquariums are becoming more and more popular, it is a hobby that many people are flocking too and fish shops are on the rise. Fish are generally easy to keep although do they need quite a bit of attention. Puppies and kittens were the typical pet but now fish are becoming more and more frequent in house holds. In recent years fish shops have noticed a great increase in the rise of people wanting to purchase aquariums and fish, the boom has been great for local shops as the fish industry hasnt been such a great industry before now.

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    How much lux need measure in bottom aquarium for plants?
    9 years ago
  • swen
    How to measure light intensity for a water plant?
    7 years ago
  • Carol
    How much lumens for aquarium plant?
    7 years ago

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