Business opportunity: you can now make stuff which only passes vitamin D wavelength

Stuff includes: umbrella, awning, shirts, hajab, hat, mirror, flexible plastic sheet . . . .

  • Might be patentable, especially plastics – go for it ! !

Materials can include

  • tan-through cloth – which passes both UVB and UVA – so you get a tan
    Investigating suppliers as of Dec 2, 2012
  • plastic (flexible or rigid) having a thin coating of silver - perhaps vacuum deposited.

Wikipedia Spectra for mirror coatings

Silver (Ag) is almost perfect: It passes UVB, but adsorbs UVA, visible, and Infrared

The above was from another page on VitaminDWiki: Vitamin D from the sun thru a special window or heat shield

There are various ways to put silver onto thin materials, such as vacuum deposition.

Dec 2, 2012 I measured various materials around the house with a UVB meter and found that most plastic bags and wax paper are almost transparent to UVB.

As you will see below, many untreated plastics breakdown when exposed to UV. This might mean that the “stuff” you make from plastics might have a lifetime of a year or so. Teflon would be great however – as it is not affected by UV while being transparent to UV. But, of course, it is very hard to get something (like silver) to stick to Teflon. Perhaps there could be a ‘’rough teflon” or a teflon-like material.

Nice information about plastics, teflon, etc. and UV

Question about UV transmission by plastic 2004, Answer by Jim Swenson

It really depends. Get yourself a lot of clear plastics to try, because some will go through, some won't at all, some will depend on thickness, and some won't. Beware of scattering and purpose-added UV-blocking dyes. Find out the wavelength dependence of your UV meter and your UV source (the sun?).

Most plastics that are not clear will block most or all of the UV. Even clear colored dyes usually absorb some UV. And anything that is flat-colored or opaque doesn't just absorb light, it scatters light. Try looking at the shadow of a piece of wax paper. It absorbs almost nothing, but most of the light gets bent or reflected somewhere else, so little of the light goes straight through, so it has a fine shadow, especially if it is far from the ground.

To be able to test thin sheets of these things that scatter, you may want to tune up your measurement methods to allow a small amount of scattering. Largely this means putting the plastic sheet right up close against the entrance hole of your UV meter. If the percentage transmission decreases when the plastic backs away from the meter, then it's scattering loss not absorption loss. Having a pre-scattered source of UV light is another way to tolerate scattering. The sun in a clear sky gives mostly direct, unscattered UV light. The blue sky all around contributes some scattered UV light. A cloudy but hot day, the kind when they warn you about sunburn, gives only scattered UV. So wax paper will give a different transmission percentage on a cloudy day than a clear sunny day, especially if the paper is 1 inch from your UV meter instead of right on it.

I know the formula for absorption loss vs. thickness. But I think the formula for scattering loss vs. thickness is different, when the scattering gets high, and I do not know how it goes. Maybe it would be a good goal to measure a percentage-transmission curve for that and try to find a formula that fits it.

Home Depot sells plastic sheets of Lexan (poly-carbonate) which are printed with claims of blocking UV. Try this stuff. You might find it blocks only 90%, depending on your UV meter. I do not think it depends on thickness, though. I think they take a clear plastic that transmits most UV, then they soak into one face a "UV dye" that absorbs only UV, just like the PABA or other active chemical in sun-screen lotions. Even says on the cover sheet: "this side towards the sun". This is the side with the UV-blocking dye, less than 0.1mm deep. A lot of plastic products that are supposed to live outdoors have UV-absorbing dye in them, otherwise the UV light sailing right through all day occasionally cuts a long chain molecule and the plastic eventually browns and cracks. Sometimes they call this "UV inhibited" instead of "UV dyed" or "UV blocked".

Most clear plastics transmit UV if they are chemically pure. The chemically pure plastics that block UV usually block just a little blue light as well, and therefore have a faint yellow or brown tint. I doubt you will find many of those around.

But I could list specific plastics.

Pure Plexiglass ("PMMA", poly-methyl-meth-acrylate) transmits most of the UV that will give you a suntan. Clear poly-styrene plastic is chemically simple, just C's and H's (Carbon and Hydrogen), and no big electron clouds. So it transmits UV better.

Poly-ethylene is even simpler, and will transmit even farther into the UV. But it always has scattering, always looks cloudy or milky (translucent).

DuPont's Teflon (TM) has only Carbon and Fluorine atoms, and transmits so far into the UV that scientists have difficulty getting UV lasers to cut it. But you have seen that it is really white ("PTFE" type), not just cloudy like poly-ethylene, and this kind of

DuPont's Teflon (TM) has very strong scattering. If it is too white to see through, you will get a low percentage transmission in the UV too. Then there are clearer types ("PFA" type) which are only a little cloudy. They transmit well too. Good thing is, almost never does someone bother to put UV-blocking dye in Teflons.

Go ahead and throw wax-paper into your study. The wax is chemically similar to the poly-ethylene above, plus there is paper, cellulose, (-HCOH-) chains, which also transmit most UV. It has some scattering, of course. Any clear colored plastic, including clear fluorescent orange or green, is a good thing to try. Many dyes that block visible colors will miss the UV a little bit, so the plastic would then transmit a moderate percentage of UV and depend on thickness. I'm not sure whether reddish, greenish, or bluish dyes are more like to miss the UV.

Drug-store mineral oil is a good thing to put between multiple sheets of plastic, so the surface-reflections do not add up. When you stack up 10 layers of thin clear sheet, sometimes it looks really shiny, silvery. One sheet was just clear! Oil in between will prevent multi-surface reflections from building up. Mineral oil will not dissolve your UV meter, or hardly any plastics. The oil is C's and H's just like polyethylene, but it is liquid because the molecules are shorter and/or smaller. So it really transmits UV. Mineral oil usually contains a fraction of a percent of an additive for "anti-oxidant" purposes. This anti-oxidant probably also absorbs UV, but maybe not enough of it to completely block your UV. Percent transmission versus mineral-oil depth is a nice little project all by itself, and if the resulting curve shows that it only absorbs 20% in 1 cm, then you have proved that it transmits >98% when you use less than 0.1 cm to wet a bunch of plastic sheets together.

I guess, from your language, that you have a UV photometer of some kind. Really good. But UV is not just one color, it is a range of colors that some other imaginary species can see. So try to find out what wavelengths your UV meter "sees". Include it in your report, so people can know well what you tested. Remember that there is visible blue on one side of it and farther UV on the other side of it. And sometimes it might be measuring the sum of two different colors, with two different absorption rates in the plastic.

Thin plastic, such as a plastic bag is OK , but not mylar (from the web)

Opportunity: use a tanning bed to produce just vitamin D;
Can get vitamin D without getting a tan by having a thin plastic sheet which has a very thin coating of silver

  • Useful for those who do not want darker skin, but want more vitamin D (and do not want to take supplements)
  • This could initiate the use of "tanning beds" in by people with dark skin anywhere around the world.
    Including in the tropics - though the sheet should be used during the middle of the day for free.
  • Allow longer time in the bed since the customer would not be getting the burning rays of the UVA, and thus could get more vitamin D per session
    UVB rays do not burn as much as UVA rays
  • Increase use of Vitamin D beds by those who do not want wrinkled skin (UVA/UVB?)
  • Might even get around some of the crazy laws in different countries - which focus on the spectra of light in the bulbs, not the spectra that the customer gets
    Example of crazy law: Apparently in Feb 2013 Sweden will no longer allow tanning bed operators to mention vitamin D production in the advertisements

UV Properties of Materials by Zeus is attached at the bottom of this page

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