# Working out light intensity at a new height Twinstar



## shangman (16 Dec 2021)

Hiya all,

I have a Twinstar EA 900cm, which prior to today sat on the legs it came with, at 10cm above the water's surface. I had found that I get reasonable growth and kept most algae at bay at 40%.

I bought a hanging kit (from Liti Aquaria, looks great!! did a DIY copper pipe job to hang it from the shelf), and have just hung it up higher so I can see more of the surface, and so the light has better spread. My giant piece of wood previously was in the way fo the light so the back was in shadow, whereas now it's quite illuminated, which I think my echinodorus will enjoy! 

The lights are now 21cm above the surface. Can anyone help me work out what % the light should be on now it is higher up, to give the equivalent light of 40%? I am just absolutely rubbish at maths!! 

For today, while I worked out the height I had it on 100%, and at the new height it seems a bit too bright. I could try it at 70% or 80%, but I'd rather ask you guys in case you can help me prevent an algae outbreak 





​Might need to make myself some sort of DIY shader now too lols! Looks great from far away but hurts my eyes a bit when I'm closer atm. Really looking forward to growing lots of emergents now though


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## oreo57 (16 Dec 2021)

shangman said:


> Hiya all,
> 
> I have a Twinstar EA 900cm, which prior to today sat on the legs it came with, at 10cm above the water's surface. I had found that I get reasonable growth and kept most algae at bay at 40%.
> 
> ...


Inverse sq rule says 2x the distance 1/4 of the light.

Doesn' t account for glass reflections/refractions ( mostly gains) or light spill outside the tank ( losses) 

That said l don' t see 100 % as an issue BUT and a big but, only a guess.


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## MichaelJ (16 Dec 2021)

shangman said:


> Hiya all,
> 
> I have a Twinstar EA 900cm, which prior to today sat on the legs it came with, at 10cm above the water's surface. I had found that I get reasonable growth and kept most algae at bay at 40%.
> 
> ...


Hi @shangman  I love this tank! I wish I could do this... Anyway, from the picture it does look a tad bright to me - not overly though - very hard to say from a picture obviously.... I would just lower the intensity down a bit and perhaps add a bit more pennyworth  (not that I am giving you aquascaping advice here)

Cheers,
Michael


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## The grumpy one (16 Dec 2021)

oreo57 said:


> Inverse sq rule says 2x the distance 1/4 of the light.
> 
> Doesn' t account for glass reflections/refractions ( mostly gains) or light spill outside the tank ( losses)
> 
> That said l don' t see 100 % as an issue BUT and a big but, only a guess.


The way I see it is Inverse square rule is for a point source. This is a bar source so the light along the bar axis still (except for the end LEDs) gets to the tank.
Hence 2 x distance is 1/2 of the light. 80% plus some losses, guess 3%
Set it to 83%. Does that look the same brightness.
You could look at some of your old pics and see what shutter speed and f stop was. Then up the light till you get the same shutter speed and f stop for the same frame size as the original pic.


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## Maf 2500 (16 Dec 2021)

The distance from the light fixture to the bottom (or middle) of the tank hasn't doubled though, only the distance to the surface has. Distance to the substrate has increased by maybe 20% or so. Personally I would take it steady, starting at 60 or 70% for a time and seeing if more is needed by how the plants respond.


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## oreo57 (16 Dec 2021)

The grumpy one said:


> The way I see it is Inverse square rule is for a point source. This is a bar source so the light along the bar axis still (except for the end LEDs) gets to the tank.
> Hence 2 x distance is 1/2 of the light. 80% plus some losses, guess 3%
> Set it to 83%. Does that look the same brightness.
> You could look at some of your old pics and see what shutter speed and f stop was. Then up the light till you get the same shutter speed and f stop for the same frame size as the original pic.


Yea falloff is fairly linear within certain ranges.



83 % is certainly in range. I was assuming much more than 3 % spill.
20cm height and 120 degree beam angle throws a spot size of 69cm in diameter





						LED Lighting Beam Angle Calculator
					

A simple calculator that determines spot size by a light's beam angle




					store.marinebeam.com
				




I was attempting to downplay the "rule" It is used (incorrectly) too often.








						Lighting an Aquarium with PAR instead of Watts
					

Before Any Discussion of Aquarium Lighting Can Proceed, First We Have to Debunk the Myth About “watts Per Gallon” Being a Measurement of Light Intensity.when the Only Practical Source Of…




					cflas.org
				




Watts per gallon isn't totally dead either


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## Maf 2500 (16 Dec 2021)

oreo57 said:


> 20cm height and 120 degree beam angle throws a spot size of 69cm in diameter


Yes it is true there will be a fair amount of  light spill at this height. Lots of variables - eg. how much light does the blue background reflect back intio the tank? I think there is no way of easily working it out scientifically and good old intuition followed by trial and error is needed. Fuzzy logic.

Edit: it looks like the light unit is nearer to the back of the tank than the front? Which, if true, would be a better from the light spill perspective.


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## shangman (17 Dec 2021)

What a fabulous discussion! I only understand half (ok, maybe less than half) of what you guys are saying but it's very appreciated!! 



oreo57 said:


> That said l don' t see 100 % as an issue BUT and a big but, only a guess.


Thanks!! I've had lots of problems with hair algae before so want to avoid that (and clado trying to take over again), so 100% makes me a lil nervous.



MichaelJ said:


> Hi @shangman  I love this tank! I wish I could do this... Anyway, from the picture it does look a tad bright to me - not overly though - very hard to say from a picture obviously.... I would just lower the intensity down a bit and perhaps add a bit more pennyworth  (not that I am giving you aquascaping advice here)
> 
> Cheers,
> Michael


Haha the pennywort agrees, I pull out huge piles of it every 3 weeks. A good plan, will let it take over and see what happens. The only annoying thing is that I have a few nice rare plants at the top that I never see, alas. The shrimps love though.




Maf 2500 said:


> Yes it is true there will be a fair amount of  light spill at this height. Lots of variables - eg. how much light does the blue background reflect back intio the tank? I think there is no way of easily working it out scientifically and good old intuition followed by trial and error is needed. Fuzzy logic.
> 
> Edit: it looks like the light unit is nearer to the back of the tank than the front? Which, if true, would be a better from the light spill perspective.


The light set bang in the center of the tank front-to-back, I think it just looks that way cos I took it from a high angle. At the back, there is a 10cm gap between the tank and the wall so I can plug the extension cords in. The wall incidentally is a satin finish so it does reflect a bit of light! 


Today I set it to 75% as the high point for the majority of the day, to see what happens there. It definitely looks brighter than it used to, but I think this could be because the spread is much better now, so there are just less shady spots (though still plenty for the fish to enjoy). Also, the cardinals look FABULOUS in the higher light, so it's nice to have for a while. The last 2 hours are 50% - 0 so gives it a nice gloomy period too.

Will let you guys know how it goes, if the dreaded algae attacks or (hopefully) all the plants just grow amazingly suddenly. It honestly looks so much cooler than I thought it would, really brings it into the room as a feature now. The light isn't so bright on the emergents like it looks here


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## shangman (16 Jan 2022)

Update:

It's been at 75% for a month and everything's been working very well, I haven't had any extra algae, and all the plants in the tank seem to be benefiting from the wider scope of the light - notably the plants at the back which used to do badly are doing much better now. The surface plants are particularly pleased and are growing super healthily and vigorously and I have to cut them back a lot. Big thanks for your advice, I personally like the light at this level of intensity (not too bright), but I think it could easily take 85% or higher without any problems. 

I really recommend raising your light, the whole tank is better lit, the plants are happier and seeing the whole surface is SO GOOD, it really makes the tank feel more of a part of the room. Admittedly I did make a cardboard half-pipe that goes over the light so I don't hurt my eyes when looking at it, but it looks fine. Will make a pretty version soon.


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