Water depth & light penetration

[Malarky]

I am interested whether there is a general rule for light drop off through a (clear) water column unobstructed by shading plants, floating or otherwise. I have inherited an established tank 47 cm deep (40 x 80 cm long and wide), and it will be initially no-CO2, minimal fish, with a strong emphasis on easy plants. Is light drop off a consideration with fairly low light levels, or will whatever light source I choose be consistent through the entire water column? Hope that makes sense.

 

[John q]

Hiya mate, yes the further away you go from the light source there will be a fairly steep drop off in par values. If you look through these posts there's loads of par data available for a number of lights and you will soon see how the numbers fall once we get to 40 ~ 50cm away from the light source.

Product Review - ADA | WASMAS | ZETLIGHT | TWINSTAR | CHIHIROS | LED PAR DATA [The power of light]

Some food for thought... [edited for easy reading 14.03.2017] In some discussion's with Nuno we thought would be nice to have some data about LED's. Remember that this data is not 100% scientific, but we tried to keep the same test conditions for all fixtures. In the worst scenario, the...

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Try not to get to caught up in the numbers per se in these posts, I'm just trying to give you an idea of how they diminish over distance.

Thankfully in a low tech tank you don't need mega high lights, in fact lots of people end up getting in trouble thinking that's the case.

If the tanks already come with a light just use that and see how things pan out, if you go out to purchase one try and get a fixture that can be dimmed, you easy going plants will thank you for it.

@oreo57 Might be able to give you a more scientific answer as to how water effects drop off and if there's a a general rule numbers wise.

 

[oreo57]

I am interested whether there is a general rule for light drop off through a (clear) water column unobstructed by shading plants, floating or otherwise. I have inherited an established tank 47 cm deep (40 x 80 cm long and wide), and it will be initially no-CO2, minimal fish, with a strong emphasis on easy plants. Is light drop off a consideration with fairly low light levels, or will whatever light source I choose be consistent through the entire water column? Hope that makes sense.

This gives a picture of light drop off vs depth.
One thing though it is fairly linear in "tank ranges" i.e. 1/2 par at 2x distance .
Many quote the "inverse square rule" where 2x distance is 1/4 the intensity.
Due to tank/ water refraction and reflections it does 't hold well.
So really it is like quasi-logarithmic or quasi- linear.??????
I toyed with coming up with some rule of thumb for led lights based on wattage .
Had minimal utility.
The premise was based on typical .2 to .5w leds and the native 120 degree lensing.
At 45cm (18") depth par is around equal to wattage of the light.
As promising as it was and even with the limited parameters stated above it broke down with the quality of the diodes. Not totally unexpected but broader than expected. From .8 x to 1.5x wattage.

YMMV..

 

[Malarky]

Thanks for the guidance, much appreciated.

 

[FISHnLAB]

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