# Spectral Loss Through Glass



## awohld

My current tank has the standard glass that the light sit on. Does anyone know what wavelenghts and how much light glass absorbs. Some glass naturally absorbs UV think.

I'm thinking of buying the light fixtures from AH Supply and I'll be making my enclosure with glass on the bottom.


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## DonaldmBoyer

You are correct. All glass will reflect all UV rays. As applied to aquarium lighting, glass will reflect the upper end of the spectrum (the blues and violets) the most, in the order of around 10-20% depending on thickness. 

That is why I never use glass tops on my tanks. There really is no benefit, unless you have very "jumpy" fish.


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## plantbrain

I measured 6-8% loss through the 3/16" glass using a PAR meter.

Regards, 
Tom Barr


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## BryceM

Some glass does allow UV light to pass through, but it generally isn't used by plants anyway. The % of light that is able to pass through greatly depends on the type of glass used. There is a huge variation available. If you want to do an internet search do a google search under 'greenhouse light transmission'. I looked into this extensively when I was trying to decide what type of cover to use. I finally settled on lucite, a rather soft, non-resilient but incredibly clear polymer. For a given thickness it allows more light to get through (at least until it gets scratched). Is is cheap, light, easy to cut and doesn't chip or break. Just don't use ammonia based cleaners (windex) on it or it will haze over.

Personally I prefer a tight cover to reduce evaporative losses and to keep fish in and kids/cats/pennies/flashlights/etc out. Over the years I've lost tons of fish as they tried to escape 'over the wall'. My congos are always banging their heads into the cover when they rush up to eat.

Probably the most important reason is to keep the water away from the electricity. If you use a DIY light kit I STRONGLY suggest that you go to great lengths to keep water out of the fixture. Even medium-sized fish can splash quite a lot. Also be very careful about how and where you put your electrical connections. UL doesn't allow lighting fixtures over aquariums without covers for a reason. It really sucks when your hobby kills you.


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## DonaldmBoyer

Actually (no offense guaiac boy...you did help me in the past), plants do use UV in photosynthesis to split apart hydrogen from oxygen in water. They need it to drive the energy cycle. Not that aquarium lights produce UV rays, but this is usually compensated by the blue wavelengths found in aquarium lighting, so the plants use the blues instead of the UV.

Now you perked my curiosity, Guaiac.....I thought that NO glass allowed UV to pass through it; hence, you don't get sunburns sitting by a window reading, driving a car on a sunny day with the windows rolled up, etc. What types of glass allow UV to pass through? Like I said, I am strictly curious and not trying to be sarcastic! Edu-ma-cate me, my man!


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## BryceM

I did a google search for 'UV glass transmission' and got all kinds of info. "It was on the internet -- it must be true!" There is one used in laser applications called fused silica that allows almost 100% to get through. Glass used for sunglasses typically only allows 3-5% of UV through. Some automobile glass actually allows 30-40% through. The cheap stuff that aquarium lids and run-of-the mill pane glass windows are made of is usually in the 5-30% range. Sapphire glass (used in many custom aquariums due to its superior optical clarity - almost as good as acrylic) allows quite a bit of UV through.

Kind of like everything else - not so simple when you look closely.


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## BryceM

awhold,

quick answer - use glass, or don't, it doesn't really matter. There is absolutely so little precision in our "wpg" measurements anyway that it is useless to fret about a few %. Watts per gallon is an exceedingly poor measure of light. Its just the one were stuck with. At 3 wpg, small tanks (<20 gal) probably don't have enough light and large tanks (>100 gal) probably have too much.


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## misterchengmoua

see now you guys got me thinking...should i use a cover or not?...hmmm


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## DonaldmBoyer

guaiac---no kidding! I guess you learn something new every day......I stand corrected!

mister--as guaiac said, it probably doesn't matter, then, if you use a cover or not. As I said before, I don't. But as Guaiac implied in the earlier thread, BE CAREFUL!!! You don't want those lights falling into your tank as your doing maintenance if they are plugged in especially!!!!!!!


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## plantbrain

Huh? This is wrong:



> Actually (no offense guaiac boy...you did help me in the past), plants do use UV in photosynthesis to split apart hydrogen from oxygen in water. They need it to drive the energy cycle. Not that aquarium lights produce UV rays, but this is usually compensated by the blue wavelengths found in aquarium lighting, so the plants use the blues instead of the UV.


UV does not help growth.
See references below.
PAR is 400-700nm, that is visible photosynethically active radiation.

Water is split with a wavelength of 680nm in PSII.
Any and all light energy is either near this range or shorter(higher energy) in wavelength that is used for PAR.

You do not need blue light or UV to split water.

Water is not split with any other nm range. 
Not UV, not IR.
It must lose energy to split the water at the 680nm range. IR is too low, so it will never be able to do so. 700nm helps PS1, but does not split the water molecule.

So you could perhaps have some energy loss at say at 350nm, and the tranfer will lose energy and get to 680nm, but you will not get better growth in doing so and perhaps/(Very likely) some damage to the plant.

http://uvb.nrel.colostate.edu/UVB/uvb_plant_intro.html

http://bssv01.lancs.ac.uk/psi/research/uv_radiation.html

http://www.fao.org/docrep/W5183E/w5183e09.htm

Graph of Absorption of Nucleic acid:

http://www.ciesin.org/docs/001-503/fig4.gif

http://agron.scijournals.org/cgi/content/abstract/96/6/1562

That actually has primary ref's so you can trust that more than so and so's site.

You'll note the graph shows why UV-B is so nasty
UV-A is less harmful.

But there is little to show that it helps, actually nothing I've found suggest in any general way it does anything beneficial, rather, virtually said it had a negative effect on plant growth.

Stick with PAR for growing plants.

You can avoid suicidial fish: build a canopy without the light/glass cover etc.
Thinner plastics are much better than glass for light transmission.

Regards, 
Tom Barr


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## DonaldmBoyer

....and thank you plantbrain for quoting me, though you are right. Actually, I went back and researched it...UV seems to be helpful when it comes to pollination by insects to some "above water" plants.

Guess I should stay out of this forum for a while.


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## awohld

plantbrain said:


> Stick with PAR for growing plants.
> 
> Regards,
> Tom Barr


I didn't know what PAR means so I looked it up on this forum. I read that it stands for Photo Active Radiation.

But what you mean by that if it doesn't include UV? Or am I confused.


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## BryceM

I think what he's trying to say is forget the UV, go for visible light, select a fluorescent in the 5000-8000K range, cover it - or don't, and watch the plants grow.


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## plantbrain

No, ask simple, even seemingly silly questions, questioning the basics are some of the best questions.

As far as UV light, it matter which type, the UV-B as you note on the figure ref I gave is very good at absorption by DNA, which is why UV sterilizers are useful.

UV-A maybe have little effect and may influence negtaive relationship to growth.

We do not really want UV light in our tanks, but PAR.
Hohoho!

Regards,
Tom Barr


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