[casaccia]

Does anyone know of any liquids that absorb infrared light but not visible light? In other words, something that looks clear but appears dark or black when viewed in infrared. Specifically I'm looking for something that can be made from a common substance or is easily attainable. Help is greatly appreciated! Thanks in advance.

[billnotgatez]

casaccia -

would you consider using ultraviolet instead

[Arkcon]

If you wanted a solid, I'd just send you to Edmund Scientific, and have you buy a hot mirror.  They will sell to anyone:http://www.edmundoptics.com/products/displayproduct.cfm?productid=1492

Since you want a liquid, I can give you an easy one -- anyone who does an IR spectrum knows that water is a strong absorber of near infrared.  For what an image would lokk like, here's an example:  http://egsc.usgs.gov/isb/pubs/factsheets/fs12901.html

[casaccia]

Thanks for your suggestions, but I think I need to explain. I am looking for something like invisible ink, but using the infrared spectrum, not ultraviolet. What I have done is modified a digital camera to capture only infrared light, and I would like to be able to write something that is only visible through the camera.

[fledarmus]

Ah, then you actually want a solid or a gum that absorbs IR and can be dissolved in an appropriate solvent. If it is a liquid, it won't stay where you put it on the paper.

[casaccia]

Got any ideas?

[Arkcon]

What wavelengths does your infrared lamp emit?  If its just a heat lamp, you may not find the information on the box label.  But you may find the answer by checking with the manufacturer.  Once we know the wavelength range, we can start the task of finding ink that is transparent at visible wavelengths, but not infrared.

[casaccia]

The thing is, I'm not using an infrared lamp. I'm just using infrared light from the Sun. I'm not sure, but would the wavelength to look for be the one which penetrates the atmosphere most?

[Arkcon]

Well, let me ask a different question -- how are you filtering out the other wavelengths the sun provides.  Specifically, the visible ones, but also the UV.  You're looking for something that shows up, only on a bright warm sunny day, but is likewise invisible under say, classroom fluorescent light? 

Hmmm....tricky.

See as the wavelengths get longer, traveling along the spectrum from gamma rays, X-rays, UV rays, violet visible light, red light,  infrared to microwaves and radio waves, they have less energy, and they're less likely to interact with matter, especially chemicals.  You can see that from my list above -- which do we avoid tissue exposure to?  Infrared light will be overwhelmed by shorter wavelengths.

[fledarmus]

@Arkcon - he's doing this photographically by using a filter to remove the UV and visible radiation, then (digitally? for viewing on a computer screen?) changing the IR radiation to something in the visible spectrum for viewing. What is needed now is a distinction between the IR absorption of the ink and the IR absorption of the paper it is written on. And of course, it would have to be photographed under lighting conditions that provide at least some IR radiation, like direct sunlight.

I would find a good book on IR spectroscopy and look at compounds that show very strong, broad absorbances in the IR spectrum. There are also ways of calculating the wavelength of light absorbed by organic compounds depending on the conjugation in the molecule. You should be able to find a dye molecule that will absorb in the wavelengths you are looking for.

[casaccia]

Fledarmus, you are correct. Arkcon, I'm filtering out visible light using six layers of "Congo Blue" filter gel.

[Arkcon]

Ah, sorry, I didn't carefully read that bit. So, you've assuming the Cong Blue gel is adequate  -- I can play along.  How did you modify the digital camera to respond only to IR?  Do you have a specific wavelength rating for the filter there?  Or did you do the online hack that I'd heard about, where you filter the visible out with a piece of overexposed film negative.  Because in that case it will be harder to figure out.  You may have to call the camera manufacturer, and ask them to try to give you some detector info.

http://hackaday.com/2005/03/14/make-an-infrared-webcam/

[casaccia]

I removed the visible light filter from the camera and replaced it with the Congo Blue. See http://home.comcast.net/~zachstern/toyir/toyir.html for the process. Also have a look at http://amasci.com/amateur/irgoggl.html. I made the goggles described before modifying the camera. The figures at the bottom of the page seem to indicate that the Congo Blue filters pass mostly light of frequencies above 700nm.

[Arkcon]

I think near infrared dye should be easy to find, that's pretty much how supermarket scanners work.  You can use trial and error, I'd start with black waterproof marker (the Sharpie^TM brand for example.)  I probably should have suggested trail and error sooner.  Heh.

[billnotgatez]

I am wondering if a solution with a refractory like alumina would work or some other infrared absorbing compound.