This might be overwhelming if you are new in planted tank, and/or you are not opto-engineering graduate. Please slowly read through and digest this article.

Light Spectrum
Most people choose lighting solely based on Kelvin temperature of a bulb. Even me myself, i also refer to 10000k T5HO bulb as best represent sunlight.

Visible light is on a scale in nanometers (radiated wavelength) from 400nm to 700nm. Plants can only utilize light that is absorbed. The blue zones and red zones are most beneficial to plants. Green plants appear green because it is the reflected light.

Most of fluorescent light bulb in market does not indicate what wavelength in nanometer the bulb is actually emitting. If you want to optimize plant leaf development (blue light) and stem elongation and color (red light). you need light in both the blue and red spectra for photosynthesis. Sunlight peaks in the blue spectrum at 475 nm. As light passes thought water the intensity decreases. The shorter wavelength blue light penetrates water better and more quickly than red, which is slower and absorbed more quickly.

Chlorophyll, the photosynthetic pigment used by plants traps blue and red light but is more efficient with red light at 650nm – 675nm. Blue is used at the same rate as red because it is more available for reasons mentioned above.

For green plants the lighting peaks that are most important:
Chlorophyll-a: 430nm/662nm
Chlorophyll-b: 453nm/642nm
Carotenoids: 449nm/475nm

Lux is lumens/square meter. It defined in terms that are meaningful to human perception of light – not plants. They stress the amount of energy in the green band to which humans are most sensitive – not plants. Lumen is a measure of flux, or how much light energy a light source emits (per unit time). The lumen measure is define in such a way as to be weighted by the (bright-adapted) human eye spectral sensitivity.

Green algae and green plants use the same pigment for photosynthesis (chlorophyll a/b & carotenoids). So, light that helps one helps other. The algae that are different are the blue-green algae (cyanobacteria), which contain Phycocyanin and absorb light heavily in the low 600nm (orange-red), which is unfortunately present in most standard fluorescent.
The tanks’ appearance can be compensated (balanced) with blue light and some green light for brightness to the human eye. Strong blue light will cause plant growth to be more compact and bushy and will also tend to promote algae growth. So remember to balance 2/3 red to 1/3 blue light emissions.

Source: Aquatic Plant Central.

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