# Lighting distance from water surface?



## madlan (15 Jul 2013)

Hi all,

How much of an effect does having the lighting close to the water surface make on PAR?
I'm using some LED strips that are waterproof - I could easily mount them 1" above the water, or attach them to the shelf above which is about 8" above the water surface...


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## Ian Holdich (15 Jul 2013)

The closer to the water the stronger the Par.


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## madlan (15 Jul 2013)

Are we talking a massive difference for 8" ?


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## dw1305 (15 Jul 2013)

Hi all,


madlan said:


> Are we talking a massive difference for 8" ?


Depends upon the beam angle. Light intensity (and therefore PAR) declines as a square of the distance from the source.






If you have a narrow beam angle light intensity will decline more slowly (the light spread will be over a smaller angle), but if you have a wide angle PAR will fall much more swiftly.

cheers Darrel


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## madlan (15 Jul 2013)

Thanks Darrel 

Each meter has 120 x 14 Lumen Leds (1680 Lumens per meter), I will be having two strips across a 120cm tank.
So  240cm = 4032 Lumens over the tank.

The tank is only 30cm deep, (20 when water line and substrate are taken into account. I'll also be growing only mosses, what do you think?

I can't see any beam angle info on the website:
120  Led / 1680 Lumen 15.6 W  Cool White 6000k Led Tape (1 Metre) IP67


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## madlan (15 Jul 2013)

Apparently the angle is 120' (I emailed the supplier linked above)


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## dw1305 (15 Jul 2013)

Hi all,


madlan said:


> Apparently the angle is 120' (I emailed the supplier linked above)


The light intensity will decline pretty rapidly as you raise the lights due to the wide beam angle. You will have to have a play, but  I'd be tempted to try them low down to the water surface, if the moss yellows I'd raise them.

cheers Darrel


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## madlan (15 Jul 2013)

Have you ever used LED strips before? I use them on some dart frog setups, they seem to grow terrestrial plants quite well.

Do you consider 4032 lumens over a 30cm deep 180 litre tank low lighting?

I'm sticking the strips to some steel channelling, these can just sit across the tank without issue. (heat is minimal)


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## madlan (17 Jul 2013)

Just ordered the lighting so I'll let you know how it goes.


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## gmartins (1 Aug 2013)

These are some data I got. The source lights are two 18W PCs... it's different but it should illustrate the point.





cheers

GM


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## flygja (1 Aug 2013)

Hi Darrel. Does your chart consider breaking the water surface as well? Have a look at this YouTube video. PAR is around 490-500 but once it breaks the water's surface it drops to 350-400.


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## dw1305 (1 Aug 2013)

Hi all,


flygja said:


> Hi Darrel. Does your chart consider breaking the water surface as well? Have a look at this YouTube video. PAR is around 490-500 but once it breaks the water's surface it drops to 350-400.


 No it doesn't, but the figures sound about right. Light intensity will definitely decline as the water depth increases, but I think that the effects of the water will depend on all sorts of factors, one would be the refractive index and the angle of the light beam, another would be internal reflection inside the tank, and a third the clarity of the water.

cheers Darrel


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## niru (2 Aug 2013)

dw1305 said:


> Hi all,
> 
> Depends upon the beam angle. Light intensity (and therefore PAR) declines as a square of the distance from the source.
> 
> ...




Hi Darrel

I have seen this 1/distance^2 explaination many times in the hobby. The above diagram is true and valid for a point source only. In our tanks we all have tubes or led panels, never point sources. As a physicst I know that for such cases the intensity falls as 1/distance, i.e inverse of distance. Hence the fall is less drastic than expected.

Perhaps we ought to consider this effect as well?

niru

Sent from my GT-N7000 using Tapatalk 2


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## Spnl (3 Aug 2013)

You might find this link useful as well
 PAR vs Distance, T5, T12, PC - New Chart
It comes from a very recent thread which contains one of those brilliant contributions from Ceg. 
Annoyingly I can't find the actual thread, but this link that Ceg gave was so interesting I saved it.

4000 lumens low light??? A 2 foot T5HO would typically put out about 1700 lumen.

There will undoubtedly be some loss of PAR with water depth as red light is rapidly absorbed, but in 30cm I doubt it is very significant.

Cheers


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## Marcel G (5 Aug 2013)

Or you can look here:
Přírodní akvárium

I have some PAR measurements there.


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## dw1305 (5 Aug 2013)

Hi all,


niru said:


> As a physicst I know that for such cases the intensity falls as 1/distance, i.e inverse of distance. Hence the fall is less drastic than expected.


Yes that is true, and our lights aren't point sources, which is actually why I prefer tubes for growlights over metal halides etc. If you think about the light coming inthrough a window, you still have the same effect, but the larger the window the more light enters the room.You also have to take the effect of the reflector into account, I've got some SON-T growlights with parabolic reflectors and they have an even light fpootprint over quite a large range of distances. The actual calculation for an LED array would have to treat each LED as a point source, and you would end up with a mosiac of light intensities as the beams moved away from the source and intersected.

Looking on the bright side I've just purchased a PAR meter for work <PAR Sensors - Skye Instruments>, so I'm interested to see what it shows.

cheers Darrel


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## flygja (7 Aug 2013)

Thanks Darrel, good info.


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