anyone use glass mirrors for MH reflectors?

[rsteagall]

I'm about to build a hood w/MH and PC's or VHO's. I was wondering if anyone has ever used glass or acrylic mirrors for the reflectors inside their canopy.

[Adam]

rsteagall,

Glass mirrors aren't as efficient as you might think. The light has to pass through the glass of the mirror.... twice (in and out), so the effect is similar to having double the thickness of glass between your lamp and tank.

Acrylic is marginally better since it is clearer than glass, but it probably would melt from the heat of MH's.

www.aquatictech.com sells reflective aluminum for DIY projects.

HTH

Adam

[beaniebeagle]

when growing plants in greenhouses, mylar reflects 90-98% of light

a regular mirror is actually less than 90%

there are some acrylic paints that reflect 98%+ reflectance that dont get yellow under UV, I don't know how they will react with saltwater or with the fish

would check out some gardening/greenhouse or growing under lights forums

[rsteagall]

Interesting. I didn't know mirrors were so inefficient. I'll look into using other reflective materials.

Without creating another thread.... How do you guys feel about this retrofit system?

http://cgi.ebay.com/Dual-250-Metal-Halide-Reef-Aquarium-Retro-Kit_W0QQitemZ7740716101QQ

or possibly

http://cgi.ebay.com/36-Metal-Halide-VHO-Retro-kit-new-light-with-bulbs_W0QQitemZ7740690461QQ


Thanks.

[Dennis7]

I use the PFO parallel reflectors, polished aluminum. I think they work great.

[hahnmeister]

According to lighting engineers, the best material is mirror polished aluminum.

[a4twenty]

we're not talking about collecting light for the hubble telescope here. i would stay away from using mirrors to reflect light as they create hot spots, and can be blinding. mylar works great for reflecting light, but considering it is only slightly more efficient than white paint ( 2 - 5 % ) and can pose a fire risk, it's not worth it ( also can create hot spots if it's not laid flat ). your going to have to paint or seal the inside of your canopy anyway, so just use flat white paint. they say that flat white paint reflects 90% of light and diffuses much better than silvery materials.

[rsteagall]

Not to discard what you said about flat white paint.... but would any experts like to chime in? What is best for reflecting light from a canopy back into an aquarium?

[BeanAnimal]

certain white paints are as reflective (or more so) than some shiny metals.

The surface of the reflector has a lot to do with how the light is scatterd as well.

A few percent does matter because the light that strikes the tank is mostly reflected. The biggest hurdle is getting the light to reflect into the tank on as few bounces as possible. This is where the SHAPE of the reflector comes in. The shape of the reflector has EVERYTHING to do with the efficiency of the lighting system. Light leaves the bulb in all directions. It has to be reflected and focused onto the area you want to use it. Light that spills out of the area is wasted. In addition lets look at the "bounce" of the light. Lets say your reflective material is 90% efficient. Light that take 2 bounces to be directed to where it needs to go loses 10% of 100% which is 90 and then loses 10% on the second bounce. The light is now 81% as bright as it was when it left the bulb envelope. In other words you have lost 19% of the original light already. Using the same bulb and different reflector that is say 87% reflective.... In this case you have lost 25% of the original light as compared to the first reflector.


Now remember that those number are not for the whole bulb, just those rays of light that shoot off at that particular angle. If you examine engineered reflectors you will notice that the bulb is placed high up in the reflector. This is so that light that comes off of the bottom half of the bulb strikes a reflecting surface and then the tank, instead of spilling out at a wide angle.

In other words, the best refectors are engineered to focus the light where it is needed in a specific shape. The reflective material is important, but the shape is more important.

Bean

[MiddletonMark]

Totally agree with Bean.

The difference between a flat reflector, and curved [and various styles] is large, and striking.

Take a look at Sanjay Joshi's research on reflectors [papers, discussions here] as it's incredibly instructive, and IMO will suprise you with the difference between reflectors.

Don't go with glass, and IMO don't cheap out.
You're paying for the power to run the bulb - get every photon you can to your corals. IMO, it's all about efficiency - and trust me, going from a flat white reflector to PFO parallel blew me away - visually far more light, far better distribution - went from dark corners to even light across the tank.

Just my opinon :)

[jmkarcz]

I'm not an "over grown drip", but I just ripped som 3" pvc pipe into some 1/3 round pcs, and used that as my reflector. I'll try to find a pic... I was happy. And it cost about 1$.

Jason

[BeanAnimal]

The pipe is not really the greatest shape, but it is better than a flat or odd shaped hood. In the end as long as your happy with your creation, that is all that matters.

[BeanAnimal]

I also hope that PVC pipe is for fluorescent type bulbs, as PVC and high heat (halides) could spell disaster.

[fppf]

You can get glass mirrors that are +95%. They are mainly used in stage lighting and solar collectors. Glass mirrors are prefered in solar collectors because aluminum will oxidize and loss efficiency over time and any coatings will do the same. The glass mirrors will hold over time. The main reason there used in stage lighting is to reduce heat. The bigger lights are so intence at the output where aimming mirrors are placed low efficiency mirrors over heat and crack.

[MiddletonMark]

Good reading on reflectors can be found here:
http://www.advancedaquarist.com/issues/mar2003/feature.htm

Of note is the final measurements on the bottom, where he compares a painted white flat reflector to the other reflector #'s shown. [getting at the efficiency of that]

[areze]

what do you guys think a mirror is?

the glass is just giving it substance. glass is still clear. its the backing that is reflecting anything at all; and the backing is only as reflective as whatever polished film its made of.

Id bet the high output mirrors are just using starphire glass. but its a mute point for this. you'd be amazed how shiny you can make metal. think of a polished metal car rim(not a chrome rim, but they look the same)

[Adam]

fppf,

Specialized optical mirrors are scary expensive. And as an esoteric extension of your point, there are mirrors called "cold mirrors" and dichroic mirrors that are designed to reflect certain wavelengths and transmit certain wavelengths.

In the case of "cold mirrors", they reflect visible light but infrared passes through. Imagine how much energy could be saved on chillers if only a fraction of the heat from lamps was reflected into the tank!

Unfortuantely, the cost of such mirrors is scary prohibitive.

areze,

I think many posters in this thread have made it clear that they understand exactly what a mirror is. In addition to the fact that the reflective coatings probably come in a range of grades... as you yourself said, so does the glass. Even starfire glass is not perfectly clear, so some light is lost as the light passes through.

Adam

[fppf]

Yes there is a whole slew of special mirros. They can make a mirror or filter depending who you ask to pass or reflect just about any wavelength you want.

I never said they where cheap. But the mirrors used on solar arrays are not horrible $$$. To get the high numbers they use really thin glass with a silver backing. Most of them are only 1-2 mm thick. Its the silver backing that gets ya.

[BeanAnimal]

Most high efficiency mirrors are front coated and used in a sealed invironment. Mirrors that are coated or put behind a glass binder are at a disadvantage, no matter how thin the coating or glass. There are at leat 2 additional angles of incidence that are parasitic to the amount of light reflected to it's final destination. Differetent materials have different critical angles but because both surfaces of the coating are not perfectly parallel planes, some light is bound by the mirror and bounces back and forth between the coating and the protector. Yet more light is scattered off at angles other the angle of incidence because of the uneven surfaces.

This thread has kinda gone from useful to musings about mirrors. Furthermore, a specialzed high efficiency mirror is useless for our purposes. It can not be shaped to maximize the focus where we need it. Remember we are trying to get the most out of a round bulb that radiated in all directions. We ARE NOT trying to transmit a shaft of focused light from point A to point B

The bottom line is that you need to choose a properly designed reflector to maximize the light from your selected material. In our hobby or in our reality there are very few choices to choose from. Polished metal, white or colored paints, and mylar or other flexible plastics are about the only options.

[Adam]

Bill,

True, true!!

Thanks for bringing this back to the point.

Anyone who wants to beat this horse any more really must read the work by Sanjay Joshi that is referenced above in this thread. You will quickly see that nit-picking over 1-2% of reflectivity is nothing compared to the 10's of percents that can be had or lost by changing the reflector design.

As for Bill's point about materials choices... when it comes to the reflectors themsleves, polished metal really is the only choice because of the heat involved. Mylar/reflective plastics and paints may be suitable for canopy interiors though.

Adam

[sttroyiii]

No matter what you do, I wouldnt use anything that doesnt scatter the light somewhat. Mirrors will make a hot spot for each mirror you use, putting a perfect image of that plasma burning in the tube onto your livestock. You need to use something that will give you an even amount of light dispersion IMO.

[spslover123]

<a href=showthread.php?s=&postid=6679347#post6679347 target=_blank>Originally posted</a> by sttroyiii
No matter what you do, I wouldnt use anything that doesnt scatter the light somewhat. Mirrors will make a hot spot for each mirror you use, putting a perfect image of that plasma burning in the tube onto your livestock. You need to use something that will give you an even amount of light dispersion IMO.


But isnt there already a "perfect image" from straight from the bulb to the coral? Just sayin=)

[BeanAnimal]

The surface of the water acts somewhat to diffuse the light. In all honest you don't want to "scatter" the light before it gets to the tank. You actually want to focus the light into the desired shape and then diffuse it from there. There is a big difference between light being "scattered" and light being "diffused". In the first case, the scattered light may or may not make it into the tank. In the second case, the light is directed towards a diffusor that spreads the light over the desired surface area.

The problem we run into is that for our purposes, anything more than a simple well engineered reflector is cost prohibitive with regards to the amount of return. You could build a hood with a very focused beam of light, using primary surface coated reflectors and state of the art optics to focus the light on the exact footprint of your corals. You may gain 15% or so in efficiency, but you would spend exponentialy more in terms of setup and maintenance costs. It is easier to just add a higher wattage or more lamps.

Again this converstation is way out of the realm if what is needed or feasible for out tanks.

[olemos]

I am now in the process of building a DIY lumenac reflector, it is made out of aluminum sheeting and I was going ti re-surface the interior with mirror.
What am I not understanding here ? How can mirror not reflect more light then the average polished aluminum?
I understand that you may have more heat but isn’t that because there is more light being emitted? and is so isn’t that what we are trying to achieve? emit the most light we can from a bulb to the reef.
Please I’m not bashing any opinions, I’m just trying to understand, I already have the reflectors made and don’t want to have the work of cutting all the mirror, if it is not going to be good.

[BeanAnimal]

olemos, I think you need to re-read my posts (and those from Adam and Middletown and areze etc) in this thread and then do a bit of research on reflector design. I will cover the general points 1 more time, but you seem to have missed the entire context of this thread.

A "mirror" like you are talking about, is made of glass with a metal coating on the BACK side of it. The light must travel through the glass, strike the coating, then travel back through the glass. In this process many things happen that degrade the quality and amount of light that is reflected. A metal mirror (ex. polished aluminum) with no "glass" dos not have these problems. Again we are not trying to bounce a focused beam of light from point A to point B or transmit a perfect image from point A to point B. We are concerned with moving a maximum amount of light from 3D space A to flat plane B.

The shape of the reflector is very important in regards to how much light is directed towards the tank. A "glass mirror" is a flat surface, and NOT the best or most efficient at transmitting the light from a 3 dimensional cylinder (a bulb) to a rectangular plane (the surface of your tank). An ideal reflector would be designed using a computer to determine the best shape, size and angles to transmit the most light to the area needed. remember the bulb radiates in all directions. Remember that light bounces off of an object at the same angle it strikes it. We want the light going as straight down as possible when it leaves the reflector, not at a steep angle where it will spill out of the sides of the tank, ove r the tank or bounce off the surface of the water.

There is a lot more to talk about but it is not worth quibling about and. Lets put this another way. The "luminarcs" themselves are a design compromise for many reasons but likely are also optimized for their cost. To build a "knock off" that is not an EXACT copy is likely going to be even more of a compromise. Adding glass mirrors that are less efficient than polished aluminum will degrade the efficiecny even more. So in the end you may have what looks somewhat like a luminarc but performs 15-20% less efficient in terms of the amount of light transmitted to your tank.

[reverendmaynard]

What about 180o output bulbs? Do they actually work to direct all/most of the light to the open half?

[BeanAnimal]

I have not looked at the reflectors inside of these bulbs, but they would be bound by the same laws of physics that a regular reflector would be. If the are FLAT then they scatter the light to the sides at a pretty steep angle and still need a proper external reflector to guid the light to where it is needed. If they are shaped internal reflectors, then they reflect the light in a much tighter pattern and an external reflector maynot be need, as the internal is doing the same thing as the external. albiet in a tigher final focus.

It would be interesting to measure the output pattern of some of these bulbs (Sanjay may have already).

[reverendmaynard]

<a href=showthread.php?s=&postid=6686310#post6686310 target=_blank>Originally posted</a> by BeanAnimal
I have not looked at the reflectors inside of these bulbs, but they would be bound by the same laws of physics that a regular reflector would be. If the are FLAT then they scatter the light to the sides at a pretty steep angle and still need a proper external reflector to guid the light to where it is needed. If they are shaped internal reflectors, then they reflect the light in a much tighter pattern and an external reflector maynot be need, as the internal is doing the same thing as the external. albiet in a tigher final focus.

It would be interesting to measure the output pattern of some of these bulbs (Sanjay may have already).

Thanks for the reply. Yes it would be intersesting.

The ones I have seen have something like aluminum foil tape on the non output side, so it's shaped to the outside of the bulb, not flat. There might be an additional reflector inside the bulb itself...

[sttroyiii]

<a href=showthread.php?s=&postid=6681854#post6681854 target=_blank>Originally posted</a> by spslover123
But isnt there already a "perfect image" from straight from the bulb to the coral? Just sayin=)

Nope, think about how you dont see an image of the sun on the ground. But if you use a mirror reflecting the sun you get a very bright spot on the ground. The light naturally spreads out from a light source or bulb giving a somewhat even amount of light in all directions. A mirror reflects exactly what it sees, which is a very bright spot in the middle of a bulb.

[BeanAnimal]

You dont see an image of the sun on the ground, but if you look into you see the same thing that the mirror sees. Looking into the sun however does not change the amount of light that was striking you, so that analogy is a poor one.

A mirror simply takes light from one place and moves it to another. The spot on the ground is brighter because you are directing extra light to it. This is what all reflective surfaces do.

"mirrors" don't create hot spots, they simply don't spread the light over the space that you need it. Any poorly shaped reflector has the same effect.

Placing a "mirror" above the bulb will direct extra light downwards, but will also spill a lof of light that strikes it from the sides of the bulb.

Remember light does not "rise" and a bulb radiats light in all directions. Light bounces off of a mirror in the same direction it strikes it (for the most part).

You may want to get out a pen and paper and make a few drawings so that you can see what is going on. Draw a round circle (the end of the bulb) and draw a flat line a few inches above it (the mirro). Now draw light raws radiating from the bulb in all directions. Every line that strikes the mirror needs to have a reflection line that leaves the mirror from same point but opposite angle it struck the mirror at.

Bean

[sttroyiii]

"You dont see an image of the sun on the ground, but if you look into you see the same thing that the mirror sees. Looking into the sun however does not change the amount of light that was striking you, so that analogy is a poor one."

That doesnt really make sense to me, even substituting the word that is missing with 'mirror' and 'sun'. I never said anything about looking into the sun, only reflecting it. If a mirror reflecting the sun is directed at you, and you are in the sun, it changes the amount of sunlight you get right? If you want to see an eclipse, you can use a mirror to bounce an image of the sun coming in your window onto your ceiling/wall. I did this last year. If you are in the light of a MH bulb, and then you use a mirror to reflect an image of the bulb on you as well, you are getting more light right? It should be an image of what the bulb looks like...white hot plasma inside a glass bulb, but only the bright plasma will make 'light image' (you wont see the bulb relfected). What you look at doesnt change how much light you receive, I agree. Mirrors wont create a hot spot, but they can reflect them, in a sense making more. I think we are just describing the same thing differently, because your right, they dont spread the light much. I just dont think the sun is a poor analogy because a MH bulb is like a tiny sun. The only difference is the sun is so far away so you get an image that is more true, where a bulb close to a mirror will spread the light more. I think I would just use the shiny side of aluminum foil for a cheap route, or go with a polished aluminum relfector for a little more money. I like the idea of just using white paint as mentioned before, it would be a soft light, but that would probably make most people feel like they didnt do enough. Good surface agitation would keep the hot spots for focusing in one spot too long as well.

[olemos]

Ok, lets say we have two reflectors of the same “shape” and with a identical “light bulb”.
reflector #1- is made of a reasonable polish aluminum
reflector #2- is made with mirror
If you put a light meter under them, would any of t hem show difference in light intensity?

[BeanAnimal]

sttyroyiii the point is that the problem is not the material used to reflect the light, it is the shape of the reflector.

The difference between a mirror, polished metal, aluminum foil, white paint and any other surface is what determines the amount of light reflected off of the surface, and the direction the light travels once it is reflected.

THE IMPORTANT POINT is the we need to put the light where we want it AFTER IT LEAVES THE BULB. Light leaves the bulb in all directions, but we need to re-route that light to our tanks surface. The more light you have to work with, the better off you are.

The major problem is bending or forming the "reflective material" into a shape that will transfer as much light as possible into the area we want. DIY is not the way to go here, and that was the point of most of this discussion.

If you take a 100% reflective surface and put it over one bulb, and a 50% reflective surface and put it over a second bulb, which bulb will put more light over your tank? You can't answer the question until you look at the shape of each reflector. the 100% material may put less light over the tank if the reflector is not as well designed as the one made out of 50% reflective material.

I surely hope that you understand this, as it has been the major point of this thread.

Light does not randomly disipate or "fall" into the tank. It has to be reflected there. A white painted hood will help compared to a black hood, but in no way are you getting anywhere near the light you would with an engineered reflector.

Bean

[BeanAnimal]

You need to understand that a mirror and a piece of poloshed metal ARE THE SAME THING.

olemos it simply depends on which material has a higher pure reflectivity and less loss due to refraction or scattering. The "mirror" will likely lose simply because the light has to travel through glass at least twice to get where it is going.

In reality each "ray" of light will have to travel through the glass of the mirror several times as it bounces around the reflector on it's way to the tank.

The "glass" keep the polished metal from dulling. However both the glass surface and a polished surface are subject to the same "dirt" and other debris.

We could talk a lot about how "dirt" effects front or back coated mirrors and the light that is projected from them... but it is pointless with regards to our use.

Bean