Total internal reflection in acrylic tanks?

[razorskiss]

I'm planning a nano cube (18" acrylic) and was doing some doodling thinking about light placement. I have a kessil A360x I plan to use and was debating the value of using a narrow angle reflector and positioning the light further up away from the surface of the tank to reduce spillover and get a high par spot where I could potentially try introducing SPS in the future if the fancy struck me (which I'm guessing it almost certainly will at some point).

This site lists the refractive index of acrylic in the blue range as n= ~1.5:

Refractive Index of Acrylic, Acrylate, Lucite, Perspex, Plexiglass for Thin Film Thickness Measurement

Affordable thin film thickness measurement systems from the world sales and technology leader.

www.filmetrics.com

which is only slightly lower than the typical range for glass (n=1.51 - 1.52).

So my back-of-the-envelope geometry gives the half angle of light coming out of the light as 60 degrees (since the full angle is ~120 degrees), passing through the air-water interface refracts that to ~ 40 degrees. Changing the perspective to the water/acrylic interface means that at the widest light would hit the surface at 50 degrees from the normal which is a decent bit less than the critical angle of 64 degrees for that interface, so most of the light would just pass out of the glass minus whatever surface reflection occurs.

Kessil offers a narrow angle reflector with a full angle of 55 degrees, refracting through the seawater to a half angle of 20 degrees, thus hitting the water acrylic surface at 70 degrees for the normal, now well above the critical angle.

I'd have to mount the light a fair bit higher to cover the whole tank effectively, but that + $30 for the reflector seems worth it for keeping more of the light inside the tank, the internal reflections should generate more even lighting from different angles, and there should be less annoying light spill out of the tank.

I can't be the first person to think down these lines, so can anyone point out any disadvantages of taking this approach that I may have missed? The main issue I can think of is that the internal reflection angle is quite shallow so most of that light would probably just hit the substrate? Also I guess this would create a halo effect where there's a band of extra light within a few inches of the perimeter of the tank's bottom.

 

[blaxsun]

Honestly, it's really one of those aesthetic things you'll probably have to try and play around with to find the desired visual result.

 

[ying yang]

Ha ha if this is a joke thread,hats off to you,you've pulled out all the stops with lots if numbers and stuff that flew right over my head ha ha. ^_^

If isn't a joke,I have no way of even knowing if all your maths is right ir wrong or what lol


Again if a joke nice one

Also whitish sand reflects light ,what % and what angle I have no clue

 

[razorskiss]

Ha ha if this is a joke thread,hats off to you,you've pulled out all the stops with lots if numbers and stuff that flew right over my head ha ha. ^_^

If isn't a joke,I have no way of even knowing if all your maths is right ir wrong or what lol


Again if a joke nice one

Also whitish sand reflects light ,what % and what angle I have no clue

Sorry, not a joke

Writing it out like this is a lot harder to understand than a drawing, but I was too lazy to upload a sketch and my drawing would probably be too messy to read anyways...

The basic principle comes from Snell's law:

Snell's law - Wikipedia

en.m.wikipedia.org

Which let's you calculate how much light bends when going between different materials (like from air into water or water into acrylic/glass).

If light hits a boundary like that nearly 100% of it bounces off instead of going through, this is how fiber optics work. So hanging the light high up and using the kessil reflector to focus the light enough to get total internal reflection should be more efficient and light up the rest of the room less (since more light can stay inside the tank vs leaking out by bouncing off the walls)

But like with most things, probably just have to try it and see how it works/looks.

 

[dwest]

Unless you keep your glass really clean, I think much of the light will be absorbed by green or coralline algae (at least mine is on the back and sides of the tank). So, while the internal reflection can come into play, I don’t think it is the major factor in a real tank.

I keep my kessils wide angle because I hang them low inside my canopy.

 

[razorskiss]

Unless you keep your glass really clean, I think much of the light will be absorbed by green or coralline algae (at least mine is on the back and sides of the tank). So, while the internal reflection can come into play, I don’t think it is the major factor in a real tank.

I keep my kessils wide angle because I hang them low inside my canopy.

That's a great point!

 

[dwest]

That's a great point!

I was a physics teacher for my second career so I really appreciate a Snell’s law reference though!

 

[Jedi1199]

I'm planning a nano cube (18" acrylic) and was doing some doodling thinking about light placement. I have a kessil A360x I plan to use and was debating the value of using a narrow angle reflector and positioning the light further up away from the surface of the tank to reduce spillover and get a high par spot where I could potentially try introducing SPS in the future if the fancy struck me (which I'm guessing it almost certainly will at some point).

This site lists the refractive index of acrylic in the blue range as n= ~1.5:

Refractive Index of Acrylic, Acrylate, Lucite, Perspex, Plexiglass for Thin Film Thickness Measurement

Affordable thin film thickness measurement systems from the world sales and technology leader.

www.filmetrics.com

which is only slightly lower than the typical range for glass (n=1.51 - 1.52).

So my back-of-the-envelope geometry gives the half angle of light coming out of the light as 60 degrees (since the full angle is ~120 degrees), passing through the air-water interface refracts that to ~ 40 degrees. Changing the perspective to the water/acrylic interface means that at the widest light would hit the surface at 50 degrees from the normal which is a decent bit less than the critical angle of 64 degrees for that interface, so most of the light would just pass out of the glass minus whatever surface reflection occurs.

Kessil offers a narrow angle reflector with a full angle of 55 degrees, refracting through the seawater to a half angle of 20 degrees, thus hitting the water acrylic surface at 70 degrees for the normal, now well above the critical angle.

I'd have to mount the light a fair bit higher to cover the whole tank effectively, but that + $30 for the reflector seems worth it for keeping more of the light inside the tank, the internal reflections should generate more even lighting from different angles, and there should be less annoying light spill out of the tank.

I can't be the first person to think down these lines, so can anyone point out any disadvantages of taking this approach that I may have missed? The main issue I can think of is that the internal reflection angle is quite shallow so most of that light would probably just hit the substrate? Also I guess this would create a halo effect where there's a band of extra light within a few inches of the perimeter of the tank's bottom.

The problem is not reflection, it is absorption.

Once you put ANY light source on a tank, you trigger algae growth. It is easy to say, "I will clean the glass diligently every single day". It is quite another to actually do it.

As soon as you add water to the tank, you provide a surface for algae to attach. Even a film that is transparent to the naked eye, will impact the reflective light you are discussing. Given a couple days to grow, you will completely eliminate any advantage from the reflections.

In my own opinion, it is better to use lights that have the best power to punch through the water, plus the spread to light as much of the tank in one shot. Reflected light is a bonus, and a short lived one at that.

 

[Chrisv.]

The problem is not reflection, it is absorption.

Once you put ANY light source on a tank, you trigger algae growth. It is easy to say, "I will clean the glass diligently every single day". It is quite another to actually do it.

As soon as you add water to the tank, you provide a surface for algae to attach. Even a film that is transparent to the naked eye, will impact the reflective light you are discussing. Given a couple days to grow, you will completely eliminate any advantage from the reflections.

In my own opinion, it is better to use lights that have the best power to punch through the water, plus the spread to light as much of the tank in one shot. Reflected light is a bonus, and a short lived one at that.

+1 I clean my glass like once a month. I don't normally notice that my glass is all that dirty. When I clean it, all my corals get pi ssed off due to the massive bump in light from internal reflection. I now actively try to not get the glass "too clean" when I clean it, and I only clean the front.

 

[razorskiss]

Sweet, thanks everyone for the stimulating discussion. I think I'll still try it out both ways while cycling the tank to see if there's a big aesthetic difference in light leakage just from the narrow vs wide emission angle, but I have a much better idea of things to look out for now

 

[Chrisv.]

Also, that kessil will have ZERO problem growing plenty of basic sps, including acropora. You don't need to worry about making a hot spot. I am very happy with the sps growth I get from my kessil a360we at 50% power (granted, very close to the water with corals close to the surface.

 

[Jedi1199]

+1 I clean my glass like once a month. I don't normally notice that my glass is all that dirty. When I clean it, all my corals get pi ssed off due to the massive bump in light from internal reflection. I now actively try to not get the glass "too clean" when I clean it, and I only clean the front.

I clean my glass weekly. I notice a haze on day 2, and a full reduction of visibility on day 4. Since my work week is 4 days, I usually clean the glass on my Saturday morning (which is Thursday).

 

[Jedi1199]

Bottom line here is that even the most dedicated reefer, is not going to clean the glass on a daily basis to the point where you will maximize reflective potential from your lights. Realistically, we are talking several times a day as the algae will repopulate the cleaned surface almost immediately, thus reducing the benefit of reflections.

Weather you clean the glass daily, weekly, monthly or whatever, the result is the same. The reflective light available to the tank from the interior surface of the glass is minimal and should NOT, in my opinion, be considered as an element of your tank lighting system.

 

[ying yang]

Sorry, not a joke

Writing it out like this is a lot harder to understand than a drawing, but I was too lazy to upload a sketch and my drawing would probably be too messy to read anyways...

The basic principle comes from Snell's law:

Snell's law - Wikipedia

en.m.wikipedia.org

Which let's you calculate how much light bends when going between different materials (like from air into water or water into acrylic/glass).

If light hits a boundary like that nearly 100% of it bounces off instead of going through, this is how fiber optics work. So hanging the light high up and using the kessil reflector to focus the light enough to get total internal reflection should be more efficient and light up the rest of the room less (since more light can stay inside the tank vs leaking out by bouncing off the walls)

But like with most things, probably just have to try it and see how it works/looks.

Aw ok,good interesting stuff in that article,takes me back to college when studying my gas exams,looks all googly gook until get explained how to do it then becomes much easier.
Good luck with working out your height off light and using physics to try solve it.
I know my tanks glass is super reflective and when look in from the side the other sides off glass just look like mirrors,and my copperband used to fight his reflection alot ,then decided to clean sides of tank glass none to very less ,just quick once over and just clean front panel properley,which helped and he stopped chasing himself at sides then,but often after lights out he goes to one side of tank where TV on wall right next to tank and chases his reflection still, so have resorted to placing a blanket over the tank as tv lights on and usually Mrs wants a lamp on little table on which I feel stupid doing but less stressful for the fish so feel it needs doing.

Anyway goodluck