There’s a discussion on another message board.l There’s a comic strip with a character who can appear human, but is actually a supernatural creature. Among other abilities she can see into the infra-red and ultra-violet.
Is it possible to write a sign in letters that only she can read? That is, the paint reflects light in IR or UV that she can see, but which is totally invisible to humans. Invisible meaning that it is exactly the same shade as the background, not that it is transparent. White letters on a white background, the letters reflect UV, the background doesn’t, and humans can’t see any difference.
Note - I know there are security marker pens that leave invisible marks that become visible under UV light. That is not what I want. The letters must be visible to her under normal daylight, and invisible to humans under any light.
It probably does not exist because it would not be very useful (given that humans cannot see these wavelengths), but I do not see why it should not be possible to make such stuff.
If it is going to work on more than one color-shade of background, I guess it would have to be transparent to visible light.
I’m not in a position where I can google the links for you right now, but yes, such paint can trivially be made. In fact, probably many of our existing, “normal” paints, reflect some of the UV and/or IR spectrum.
I don’t understand this part. While the paint is in the can, it doesn’t HAVE any background. What does it look like then?
I see two ways of what you’re asking: [ul]
[li]The paint reflects IR/UV, and absorbs all others. That would look black to humans.[/li][li]The paint reflects IR/UV, and lets other colors go straight through. That would look transparent to humans.[/li][/ul]
You can buy UV marker pens and inkpads very cheaply. The ink is invisible (transparent) under normal light. Of course, it’s not paint, but it looks just the same as UV paint…
I’m not quite sure why you specified this part:
Why does it have to be the same shade as the background, rather than transparent? If you want white letters on a white background, just paint the whole thing white then draw the letters with UV ink/paint.
The second way amounts to a mirror that’s reflective in the infrared or ultraviolet spectrum but transmissive in the visible spectrum. It’s possible to design an optical coating that behaves like this, but it probably wouldn’t be a paint. I’d expect you could do so using the same vapor deposition process used to make other optical coatings.
Surely the point of those is that they fluoresce (in visible wavelengths) under intense UV illumination. That is not what Peter Morris wants. He wants stuff that a creature with UV (or IR) sensitive eyes could see (in normal daylight, presumably), but we could not.
I think we usually call it varnish. (I would not be surprised if many varnishes actually do what you want, even though not designed for the purpose.)
Technically, you are correct. I suppose my error was when I used the word “reflect”. There’s probably a better word to use.
Suppose the OP didn’t want IR/UV paint, but wanted GREEN paint. This is a paint which absorbs all the other wavelengths, and what does it do to the green wavelength, if not “reflect”" it?
But even a layer of transparent paint, or ‘varnish’ as **njitt **puts it, would still be visible to humans, as an area more shiny than the rest.
You know the test for colour blindness. A number or a letter made of coloured dots is put on a background of coloured dots. People with normal vision can spot the difference between the two colours, and read the letters. To people with colour blindness, the two colours look the same, and the letters are invisible.
I want something like that, with the difference being visible only to those who can see IR or UV.
There certainly are such paints (especially if you’ll allow painting them on a black background). Canadian currency actually uses the inverse of this – there are black bars on the currency which are invisible in the infrared. They’re mixed in with other black bars that aren’t, so it’s a clever anti-counterfeiting measure.
You need two paints that look the same in visible wavelengths, but look different in IR or UV. It would be challenging to make them identical across all visible wavelengths, but doable. Two paints that were both blue would be a different set than two paints that were both red.
You say you don’t want transparent paint, but if you had two transparent paints, one with different IR/UV properties than the other, then you’d paint the entire area, part with one, and part with the other, and the whole area would be shiny.
Also varnishs can have different finishes - they are not all “shiney.” A flat or semi-gloss varnish would not be noticably different in reflectivity than the paint it was applied over.
I agree, and will point out it might be slightly less challenging if you only cared about them looking identical under a single illumination condition (e.g. outside daylight).
Doing it with a coating that’s transparent in the visible range is probably harder.
And then there’s the slightly separate question of whether the super-being’s visual receptors are going to be sensitive enough to distinguish the two different shades in a particular lighting condition.
If the super-being has a fourth color receptor, narrowly peaked at the UV frequency where the two shades are maximally different, and the light source has a lot of UV (say, sunlight), then we’re good.
But if the super-being’s receptor isn’t narrowly tuned (so it picks up a lot of purple along with the UV), the UV difference between the shades might get drowned out by the identical blue colors in the two shades (if they both reflect a lot of blue).
And then, no matter what their receptors are like if the light source doesn’t have much UV or IR, there’s no way the superbeing can tell the difference between the shades. Just like in single-colored light, humans can’t distinguish two shades that they could tell apart in whiter light.
I don’t think it is terribly difficult at all. Many organic compounds absorb well in the UV, but not in the visible. Napthalene does this. Most organics will also absorb in the IR, but what part of the IR is important. If you want NIR, it might be more difficult than mid IR. It’s still just a matter of finding the compound with the right absorb acne spectrum.
Flowers already do this. To a human eye, a dandelion is all yellow, but to a bee, there is a dark spot in the middle. Many flowers have a similar dark spot in the middle that we can’t see. Here is a very nice collection of ultraviolet photographs illustrating the phenomenon. The dark areas contain pigments called “dearomatized isoprenylated phloroglucinols (DIPs)” (PDF source via a Cornell research team) which also act as a deterrent to unwanted insects. (Apparently, these DIPs are in hops, and show up in beer as a bitterant and preservative.)
You could add this pigment to unpigmented binder (acrylic gel medium, linseed oil, gum arabic, etc.), or add it to a paint. I wonder what the working properties would be.
On the other end of the spectrum, infrared photography looks cool, but does not reveal any obvious “hidden world” effects like UV. I imagine you could make a paint that would “fluoresce” from visible light down to IR, but a human would notice that area as being darker, I think.
There may be current military uses for such paints – for identifying objects through IR-sensitive night vision goggles, for example, or cameras that see into the UV range.
