Originally Posted by jochemspek
thank you Diana, for your kind reply! I understand that lowering the water increases ir/uv light access to the plants, but I must admit I really enjoy the bubbles travelling up and having no visible watertable for the moment. I'll see how the plants do in the coming days and if growth is too slow, I'll lower the level as you suggest. (I completely believe you that it will make water changes easier : )
@hoppycalif, thanks for your reply as well!. My reasoning about your measurement is as follows: both uv and ir radiation get absorbed by glass. Even though the visible part of the spectrum may increase because of reflection, the useful part of the spectrum for plants decreases (rapidly) with higher watertable. Even if by total internal reflection both ir and uv radiation make it to the bottom, this will be only under a shallow angle, and will definitely be offset by the (dramatic) loss of radiation through water absorption. see http://www.koppglass.com/blog/optica...lass-interact/
Did you measure the total lux or the intensity of ir and uv separately? I'm interesting to hear what the distribution is of those wavelengths.
Plants use light in the range of 400-700 nanometers, from near ultraviolet to near infrared. Very little radiation outside of that range is usable by plants. Light reflection from the glass sides of an aquarium occurs for that whole range also. The reflection occurs at the outside face of the glass, making it vital to keep both sides of the glass as clean as possible if you want maximum light intensity. I used a PAR meter, which covers almost all of the 400-700 nanometer range, and a little bit more. I noted as I moved the sensor from the glass towards the center of the tank, the intensity dropped significantly, instead of increasing as I was used to seeing with typical fluorescent lights. (I use a LED light panel, which spreads the light over about a 120 degree cone, so a lot of light hits the glass.)
Water does not significantly absorb light until the depth of the water is around a meter, the absorption of light isn't at all significant at 25 cm, for example. The drop in intensity with distance in an aquarium is just the result of the cone of light causing the area, the energy is in, to get larger proportional to the square of the distance the light travels, no matter what medium it travels through.