Light Transmissivity in Water

Natural light, in other words sunlight, passes through the atmosphere and reaches the ground. We recognize light as brightness and color. Although light is absorbed and decreases in the atmosphere, it decreases only by a few tens of percent while traveling 1,000 km of the vast atmospheric layer. In contrast, light decreases greatly in water. It decreases to about 50% when it goes through a mere 20 m of purified water. In lakes, even if water clarity is very high, light decreases by 90% in 20 m. This high rate of decrease in water explains why aquatic plants grow only in relatively shallow areas. It is well known that the light reduction rate varies largely depending on wavelength. As seen in the graph below, red light has the highest reduction rate. It decreases to about one half at a water depth of 30 cm. In contrast, blue light decreases only by 0.5 % in 1 m of water. To put it plainly, red light does not reach deep into water and the deep is a world in which only blue colors remain. This is the reason why the photographs and video images taken underwater appear bluish. What this indicates may be that aquatic plants utilize light in the blue spectrum to photosynthesize in water.

This graph shows the light reduction rate depending on wavelength of light. Red light decreases to about one half even at a water depth of 30 cm.

Plant Grow Light

Decades ago, fluorescent plant grow lights were commonly used for aquatic plant layouts. This type of lamp contains a lot of red light that plants use for photosynthesis. It used to make an aquarium appear reddish. While this lamp is used to grow terrestrial ornamental plants and vegetables in green houses, it is not very effective on its own for growing aquatic plants in an aquarium due to the aforementioned reason. Since a plant grow light lamp has a low light intensity, and a large amount of its light is in the red spectrum, its light appears dim due to the high rate of decrease of the red light in water. Short foreground plants in particular do not grow well under it. As a result, combining it with a normal color lamp became a common practice later on. Actually the use of a plant grow light for an ornamental fish aquarium seems to have started in the first place for the reason that a red gold fish looks very pretty under it. There was a time that every aquarium light fixture was equipped with a plant grow light before aquatic plant layouts became as prevalent as today. While it made a red fish appear pretty, it had a big problem of making the green color of aquatic plants appear dull.

In Search of Beautiful Green

A common household daylight fluorescent lamp replaced the plant grow light later. Brightness and color improved greatly, but something was still missing. After all, we needed a lamp developed specifically for the light that aquatic plants in an aquarium need. As a result, a high color rendering “NA Lamp” that was developed for growing aquatic plants was introduced. The growth of aquatic plants was the foremost concern in the development of this lamp. It became possible to deliver an adequate amount of light by including a large amount of blue spectrum light with a high transmissivity in water. Short aquatic plants such as foreground plants also grow well under this lamp. This blue light happens to be an optimum wavelength that aquatic plants utilize for photosynthesis. Consequently aquatic plant growth improved dramatically. Another ability that the NA Lamp sought was “to render beautiful green color.” So we focused on color temperature and CRI. Color temperature is a measure of the color of light. The redder the light is, the lower the color temperature. The bluer the light is, the higher the color temperature. Color temperature is expressed in Kelvin (K). For example, the green color of aquatic plants in an aquarium appears yellowish and unhealthy under a less than 5000 K light. Around 10,000 K aquatic plants appear bluish and artificial. The green color of aquatic plants appears differently when the color temperature is varied. We settled on a 7, 000 to 8,000 K color temperature range by searching the color temperature under which aquatic plants look most natural. This range is used for the NA Lamp. Another unit that indicates the appearance of an object is color-rendering index (CRI). Simply put, it indicates how natural the color of an object appears. Light with a high color temperature is generally considered to have a poor color rendering ability. However, the NA Lamp has a superior color rendering ability, and the colors of red plants and fish look good and the entire aquarium appears bright and natural under this light. A new green phosphor technology was adopted to bring out the beautiful green color of aquatic plants. With this, the gradation of green hues is enhanced and the depth created by overlapping leaves is displayed clearly. When a prototype NA Lamp illuminated a Nature Aquarium aquatic plant layout for the first time, it created such a huge impact that it is not an exaggeration to say that we sensed the arrival of a new era. The characteristic of the lamp for aquatic plants that started out from a fluorescent tube is inherited by the new ADA metal halide lamp with a stronger light intensity as well.

The Development of a Metal Halide Lamp for Aquatic Plants

The concept of the NA Lamp is applied to the current metal halide lamps. In order to develop a metal halide for aquatic plants, it was also extremely important to improve earlier metal halide lamps that had a poor color rendering ability. The color of the light emitted from a lamp is adjusted by varying the composition of various metal halides in the metal vapors contained inside the lamp. A new technology that defies conventional wisdom is used to create the light that brings out the beautiful green color of aquatic plants and accentuates the clarity of water. Thus a metal halide lamp “NAMH – 150W” that is suitable for aquatic plant aquarium, and “NAG-150W – Green” that enhances the appearance of green were born.

The Merit of a Suspended Light

The mainstream of light fixtures in the past was the type that was directly placed on an aquarium. A pendant type fixture opens up the space above an aquarium and allows a view from above. Viewing aquatic plants and fish from above the water surface allows another way to enjoy an aquarium with a totally new sensation by making one feel as if looking into a natural river. In addition, it enables an open top aquarium from which driftwood and emersed grown leaves of aquatic plants protrude and gives flexibility to a layout. Day to day maintenance is easier and it is less bothersome to put a hand into an aquarium quickly to remove dead leaves as well as to feed fish and add fertilizers. Suspending a light fixture creates a distance between the water surface and a lamp. It reduces light intensity and therefore requires stronger lighting equipment. This led to the development of a metal halide lamp for aquatic plants.

The graphs show the distribution of light on the surface of an aquarium tank, W90xD45xH45(cm), with different types of light fixture suspended 30cm away from the water surface. In case of pendant style light fixture, the water surface is the brightest near the center area, and the intensity drops toward the outside of an aquarium. At the corners where the light level is much weaker, it should be considered to arrange shade loving plants. Since the emission area of Grand Solar I and II is larger than that of Solar I and II, their light is distributed on the water surface evenly. Although the distribution of light on the water surface becomes more even as placing a light fixture further away from the top surface of an aquarium, the overall intensity of light decreases. For an aquarium tank,W90xD45xH45(cm), the light intensity of Solar II may not be strong enough. However, if you place Solar II 10cm away from the water surface of a smaller sized tank, W60xD30xH36(cm), you will find positive outcomes.

The Distance for the Water Surface

Solar series light fixtures have a small window from which metal halide light is emitted. Since the light is emitted in a certain angle, the light fixture has to be installed at a distance from the water surface to illuminate an entire aquarium. When the entire top surface of an aquarium is covered by a fluorescent light fixture, fluorescent bulbs illuminate an aquarium evenly. In the case of metal halide lighting, the water surface is the brightest near the center of the light fixture, and the intensity drops toward the outside of an aquarium. In other words, the light level is different at various parts of an aquarium. For example, let us consider a case in which a Solar I is installed above a standard 90 cm aquarium. In order to provide adequate amount of light for a W 90 cm x D 45 cm area, the light fixture must be suspended at least 30 cm away from the water surface. If it is any closer, light does not reach the ends of the aquarium and the aquarium appears dim. How far the light fixture needs to be located depends on the purpose. If the fixture is 30 cm away, it illuminates the entire aquarium. Since the center area of the aquarium is especially bright, this location is suitable for light loving plants such as stem plants. If the fixture is placed 40 cm away, the light is somewhat weaker, but its level would not hinder the growth of aquatic plants. Since this allows extra space between the water surface and the light fixture, the distance is good not only for growing aquatic plants under water but also for growing emersed leaves of the aquatic plants. If the fixture is placed 50 cm away, the light level inside the aquarium is much weaker, but shade loving plants such as Cryptocoryne and ferns should have no problem. With this placement, it is possible to grow emersed grown leaves of a larger variety of Echinodorus or those of a tall standing variety of stem plants since an even larger space is available above the water. As discussed here, the distance from the water surface can be varied depending on the type of aquatic plants or a layout.

Light in the Natural Habitat of Aquatic Plants

When light enters water, light bounces off the water surface. The angle of reflection of light depends on the entry angle. In nature, the amount of light that enters water varies as the position of the sun changes during the day. Around noon when the sun is in its highest position, the amount of light that enters water is high; the amount of light decreases depending on the time of day due to reflection. In small rivers in a tropical rain forest, the light levels in the morning and in the evening are further limited by surrounding trees and emersed plants. Therefore the time that aquatic plants can get an adequate amount of light is limited to a short time in midday. Aquatic plants photosynthesize vigorously at a burst during this short period in order to grow. Grand Solar I is a light fixture than can recreate this in an aquarium. It consists of two compact fluorescent bulbs and one metal halide bulb. The change in light levels in nature can be recreated by turning on and off the two different types of bulbs separately by timers. For example, while fluorescent bulbs are turned on for 10 hours a day, the metal halide bulb can be turned on for 4 to 6 hours during that time. It makes it possible to turn on the relatively weak light of fluorescent bulbs for morning and evening and the intense light of metal halide in midday to allow plants to photosynthesize vigorously.

The Decreasing Light Level of a Lamp

Both fluorescent and metal halide lamps are articles of consumption. Their light levels decrease while they are in use. If a fluorescent bulb is turned on for 8 to 10 hours a day repeatedly, its brightness rapidly decreases in half a year. In a layout with aquatic plants that particularly love strong light, the growth of aquatic plants may slow down or their colors may dull. The bulbs should be replaced with new ones every half year as a guideline. The light intensity of a metal halide lamp decreases as well. It should be replaced after 4,000 to 5,000 hours of lighting.