Does a mirror reflect UV light?

GreyWolf · Dec 31, 2003

Well now that I am closer to doing my first alternative process printing (cyanotype) I thought it is time to consider building a UV light source. I have been pricing out fluorescent bulbs and began to wonder if I should line the inside (sides) of the box with mirror like material.

So ... does UV light reflect from a mirror? Is the extra effort worth the end result in my building plan?

In the interim (after I complete the sink install), what could I use (aside from natural sunlight) as a cheap UV light source for 4x5 contact printing?

Kind Regards

efikim · Dec 31, 2003

White paint (or naturally white surface) would probably be a better choice - mirrors generally reflect less light than white surfaces.

Ed Sukach · Dec 31, 2003

There are two (2) distinct types of "mirrors". One is the garden variety rear-surfaced (reflective surface in back of glass - the kind on the wall in the bathroom) where light must pass through the glass twice (once going to the reflective surface, once returning). "Ordinary" window glass absorbs Ultra Violet light very effectively ... therefore much of it will be lost (actually, converted to heat).

The other type of mirror in a "First Surface" mirror, where the reflective surface has no glass to cover it: the light will be reflected without passing through *any* glass to absorb it. These will reflect UV - and most every other wavelength of light very efficiently.

The drawback to First Surface mirrors is their delicacy... to maintain flatness of the reflective surface, it must be *thin* - therefore, they are vulnerable to *any* slight contact. The acids from a normal fingerprint will penetrate immediately, destroying the coating. Cleaning them without damaging them is a definite problem. Before anyone starts ... yes, I know about "1/4 wave quartz overcoating" .... Expensive, isn't it ...!

Another possibility might be plastic. Acrylics are used a LOT for optical components and do not attenuate UV much (like 93% - 97% transmission) . The problem here would be availability ... I have *no* idea where to get "rear-surfaced" acrylic mirrors.

glbeas · Dec 31, 2003

Actually since the proposed use of the mirror is not an optical path per se but a reflector then a sheet of polished metal would be a durable and efficient front surface mirror. If you can't find a source for some good stuff try aluminized mylar or maybe cut up an old chrome plated ferrotype tin. Sheet aluminum is the easiest to use but tends to dull quickly unless coated to prevent oxidation.
FWIW I've used those thin foam aluminized insulation sheets that builders put under siding as a cheap and quick reflector and can be cut up and assembled as needed. The uneven surface tends to diffuse the light a bit better than a smooth one would.

efikim · Dec 31, 2003

and white paint would diffuse even better, and reflect more light.

glbeas · Dec 31, 2003

efikim said:
and white paint would diffuse even better, and reflect more light.

Of course your right about that, but not all white paints are really bright and I have no information to make me assume they all reflect UV the same. My humble opinion if using white paint, then flat white titanium would probably come closest in efficiency. Even the whitest paint has some absorbtion.
If I were going for maximum output a highly mirrored surface would be a better bet.
If I were going for ease of construction and cost factor the paint would win hands down.

efikim · Dec 31, 2003

glbeas said:
Even the whitest paint has some absorbtion.

and the brightest mirror has more absorbtion than the whitest paint. I no longer have the figures, and can't remember where they were, but the difference was significant. Of course the figures for UV may be different.

efikim · Dec 31, 2003

to get back to Grey Wolf's other question:

GreyWolf said:
In the interim (after I complete the sink install), what could I use (aside from natural sunlight) as a cheap UV light source for 4x5 contact printing?

While trying to make a usable digital negative, I've been using a Phillips home solarium (for tanning your face - stupid idea!) as a reasoble source:

4 12" tubes in a reflector, that covers an A4 page evenly at a distance of a few inches. Was aboutt £15 (about 25 US $) from a home shopping catalogue.

efikim · Dec 31, 2003

... and I've remembered where I found the reflectances (in a book on building greenhouses).

I remembered it wrong, so my apologies!

white paint reflectance 80-90%
glass mirror 80-90%
polished aluminium 60-70%
aluminium paint 60-70%
stainless steel 55-65%

Eric Rose · Dec 31, 2003

Hey come on don't you have of those old black lights from your hippie days in the 60's and 70's?

glbeas · Dec 31, 2003

efikim said:
... and I've remembered where I found the reflectances (in a book on building greenhouses).

I remembered it wrong, so my apologies!

white paint reflectance 80-90%
glass mirror 80-90%
polished aluminium 60-70%
aluminium paint 60-70%
stainless steel 55-65%

Thanks for the info, any idea what polished chrome would be?

efikim · Dec 31, 2003

No, that's all the book has; sorry.

ThomHarrop · Dec 31, 2003

Polished chrome would probably be a bad idea because it is too specular and would make your exposures uneven. Any specular surface would most likely do the same because of the inverse square law. Best to use something more diffuse. If you are using multiiple tubes you should have plenty of UV anyway.

Dave Mueller · Jan 2, 2004

I checked the Edmund Industrial Optics catalog, in the mirror section. The aluminum coatings are the most efficient at UV, down to about 350nm. Gold was worst, dropping off sharply below 650nm. These are all for first surface mirrors, since as others have pointed out normal glass absorbs UV. Their UV lenses are made from "fused silica". They used to sell a white reflective coating paint, but I couldn't find it in their catalog. That would be your best solution, since it would diffuse as well as reflecting it. I think you would have too many hot spots with a mirror. If you want to go the mirror route, get a standard first surface mirror and don't worry about exotic coatings. I bought an 8x10" piece once from a glass store, they sold it for people who build kaleidoscopes (so it was cheaper than a "scientific" mirror

sanking · Jan 2, 2004

The information posted here about reflection of light from certain surfaces has been quite interesting. However, to return to the original question that prompted the thread, I agree with Mike Finley's first response, i.e. a coating of white paint on the inside of a UV bank of tubes would be more effecteive than mirrors, and probably about as effective as any other coating.

I have made and used a number of UV tube banks. Initially the issue of internal reflection was a subject of considerable interest for me. Eventually what I found was that it makes virtually no difference at all whether you provide internal reflection or not, and i f so plain white paint is as good as anything. However, even if you just leave the inside unpainted (assuming it is a light wood wuch as pine or fir) I seriously doubt that you would be able to detect any loss of printing speed compared to any other reflection method you devise.

Dave Mueller · Jan 2, 2004

I just remembered Ed Buffaloe's excellent site, www.unblinkingeye.com. Check the article from Sandy King on building a UV source:
http://unblinkingeye.com/Articles/Light/light.html
I gave it a quick glance and didn't see any mention of reflectors behind the tubes.

GreyWolf · Jan 5, 2004

Thanks everybody for sharing all that you know. Thanks to Sandy for bringing the practical answer into the thread. This is good news that plain white paint is as good as anything else. I really did not want to get into attempting to line the box with mirror like material.

As an aside.....

I caught Sandy's answer to a question on ABC Pyro on Tuan's site. I just want to say that we are indeed blessed not only to have Sandy's wealth of knowledge shared here, but the crown jewel is that he has the wisdom and maturity to temper his responses with a great deal of practical common sense.

A personal trait I fear that is often sorely lacking from some of the answers I have seen throughout the years. (of course nobody here comes to mind) I cannot sing my praises loud enough in respect for Sandy and how an answer like that has moved him very high on the ladder in regards to being listened to and believed when he offers a candiate opinion.

Kind Regards,

Does a mirror reflect UV light?

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