01-09-2011, 01:24 PM
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Your question on CRI [colour rendering index or colour rendition index] is one I cannot fully answer, simply because CRI has rarely been referenced in any of my research into aquarium plants and lighting. Diana Walstad [Ecology of the Planted Aquarium, p. 178] mentions it in passing, and says that she chooses a fluorescent tube with a CRI of 80-100 and/or a colour temperature within the 5000K to 7000K range. She notes that plants respond best in growth within these ranges, and studies are cited as evidence.
Kasselmann and Hiscock, two other eminent authorities on the subject, state, as does Ms. Walstad, that aquatic plants photosynthesize best under red and blue light, and red is especially crucial. As we know, white light such as from the mid-day sun is composed of different wavelengths and each has its specific colour--the rainbow we see is created by water droplets breaking up the light into its various wavelengths visible as the different colours. These wavelengths are measured in nanometers (nm), billionths of a meter. Our eyes see light between 380 and 700 nm, what we term the visible spectrum. Blue light occurs at the lower end, 400-500 nm, while red light occurs at the higher end, 650-700 nm. Aquatic plants prefer light around 700 nm for the best photosynthesis (growth), and to a lesser extent blue (around 450 nm). You can read more on this in Hiscock [Encyclopedia of Aquarium Plants, pp. 56-63] and Kasselmann [Aquarium Plants, English edition, pp. 45-52] as well as Walstad.
To photosynthesize, plants use chlorophylls and these require specific light colours. I am attaching a graph showing the use of light by aquatic plants, and you will note the high use is in the red and blue with almost none in the green. You will probably see the higher blue and wonder why I mention red as more important; this is due to the light absorption by water; red is absorbed first, close to the surface, whereas blue light, being "stronger" wavelengths, penetrates deeper. Aquatic plants can adapt to this, at least somewhat.
This is why the so-called "aquarium" and "plant" lights are high in red and blue, producing a purplish or ghoulish hue in the aquarium. More important though than this unattractive (to me anyway) colour, is the fact that these tubes are also considerably less intense in their light. The "daylight" tubes solve both these problems.
Algae is different, in that it can use green light too, or just blue, which is why it is so important to provide the best light spectrum for our plants so that algae does not take advantage. For instance, when fluorescent tubes age and begin to emit too little light for the plants, algae will always increase.