Exploring the World of Infrared Ink: Is It Real and How Does It Work?

[Cosmobrain]

(I think Chemistry is the best forum to post this)

My question is... does infrared ink exist? Also, If you paint your room with a near infrared "color", would you be able to see it differently in a picture taken by a average camera? (just like you can see the IR light in a remote control through a camera).

Thanks.

 

[Simon Bridge]

My question is... does infrared ink exist?

Yes.
http://www.maxmax.com/airinks.htm
Did you try googling for "infrared ink"?

Also, If you paint your room with a near infrared "color", would you be able to see it differently in a picture taken by a average camera? (just like you can see the IR light in a remote control through a camera).

Depends on the color and the camera.

 

[Cosmobrain]

Yes.
http://www.maxmax.com/airinks.htm
Did you try googling for "infrared ink"?

Depends on the color and the camera.

I did. But the reason why I decided to start the thread is because it doesn't seem to be very common. I have never seen it anywhere. Probably because it is pretty much useless, but it still could be used as a way to kids to play, writing stuff in infrared then finding a way to read it. I have never seen such thing.

I still am not quite sure what it would look like. Apparently it looks transparent. However, I thought it would look black, because it absorbs all the visible light. Now I want a IR pen.

cb

 

[Simon Bridge]

I still am not quite sure what it would look like. Apparently it looks transparent.

Did you look at the videos?

However, I thought it would look black, because it absorbs all the visible light.

A pigment does not have to absorb all other wavelengths.
If it is transparent all visible wavelengths but reflects IR it would work the same.

The usual ink actually absorbs a range of non-visible light, transparent to a wide range, and re-emmits in IR.
The effect is flourescent - like when you shine UV on some materials they glow a distinctive color.

 

[Yanick]

It really depends on what you consider to be IR ink. The only model I'm aware of regarding ink and colors is the Subtractive Color Model (http://en.m.wikipedia.org/wiki/Subtractive_color) which is all about the frequencies which hit the ink, which ones are absorbed and which ones are bounced back.

The site linked to above shows that the IR emission from those inks are really more about a fluorescence effect, which gets you an answer but maybe not what you where hoping for. IR is fairly weak radiation and many (most) molecules/compounds will absorb it and transition to higher vibrational, translational and maybe even electronic energy states.

Just throwing some stuff out there.

 

[Simon Bridge]

It really depends on what you consider to be IR ink.

Definitions are important.
I was using the definition that the stuff I find in my fountain pen or a printer ink cartridge is "ink" ... so "infrared ink" would have similar properties.

Blue ink would be any fluid that let's you write in blue... by analogy...

Then I checked my concepts by finding out what was being marketed and/or researched under the label "infrared ink". It's not exactly a scientific term.

The only model I'm aware of regarding ink and colors is the Subtractive Color Model (http://en.m.wikipedia.org/wiki/Subtractive_color) which is all about the frequencies which hit the ink, which ones are absorbed and which ones are bounced back.

That model only refers to the visible light ... and there is an "additive color model" that you must surely have heard of also.
http://www.widescreenmuseum.com/oldcolor/additive-subtractive.htm
...better reference than wikipedia:
http://hyperphysics.phy-astr.gsu.edu/hbase/vision/subcol.html

A basic discussion of colors and how they work:
http://www.fi.edu/color/color.html

Basically, everything will absorb some light, reflect or scatter some, and transmit some.
The proportions change with the wavelength.
When the effect is to produce a striking effect to the eye, the substance is said to have a color.

So colors are mostly about visible light. But knowing that visible light is part of the same phenomena as radio waves and gamma rays means we can extend the concept of color to include any electromagnetic wave. But if we do that, then our ideas about what counts as a pigment need to be adjusted too.

A surface has a color if it strongly scatters light in a well defined range of wavelengths.

Common inks scatter a wide range of wavelengths of visible light, but absorb one or more narrow bands in that range. Outside the visible spectrum you can get a wide range of behaviors - but pretty much all common inks are transparent to high wavelengths, absorb infrared well, but are also transparent to radio.

You can get data on the absorbtion spectra of different pigments.

 

[Yanick]

So colors are mostly about visible light. But knowing that visible light is part of the same phenomena as radio waves and gamma rays means we can extend the concept of color to include any electromagnetic wave. But if we do that, then our ideas about what counts as a pigment need to be adjusted too.

This is mostly what I wanted to get at, in addition to the fact that the color models used for inks and such (subtractive) as opposed to that used for monitors and TV's (additive) rely on the "filtering" of light by absorption (or subtraction) of the incident light. Our perception of the color is then the stuff that was not absorbed.

This is the picture I really wanted to find, from Simon's hyperphysics link: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/filter.html#c1

This principal, which explains inks, is fundamentally different from the IR inks which you linked to which rely on fluorescence to emit the IR frequency of light. I was just trying to make that distinction and to point out that almost every compound will have absorption in the IR region since the region of the EM spectrum induces transitions in the low lying energy levels corresponding to vibration/rotations at the molecular level. See IR Spectroscopy for a more detailed description, I suck at QM.

So trying to find ink which can be called IR ink working on the principle of the subtractive color model would likely be a bit difficult, though not impossible. That last bit is conjecture by the way.

 

[Simon Bridge]

You could use the subtractive color approach to get an IR image via filters.

But for pigments, the subtractive model is only for visible light. That's why the link I gave you only shows visible colors in it. It does not explain inks, it explains how colored inks can be mixed to make other colors.

i.e. in the subtractive model: Yellow+Blue=Green, but in the additive model Yellow+Blue=White

You seem to be asking for a substance that absorbs all wavelengths except a small range in the infra-red.
(Or whatever color you want.)

No pigment works like this. Blue ink does not absorb every single wavelength that hits it that isn't blue. If it did, it would get hot.

Objects that absorbs every wavelength are called "blackbodies" and they actually radiate light in a continuous spectrum. If they absorbed everything but blue, they's still radiate a continuous spectrum. When something absorbs light, it gets hot. It radiates according to how hot it is.

To make a black pigment, you have to get it to absorb as much visible light as you can, are re-radiate the energy outside the visible spectrum: usually this is infra-red. i.e. colors work like they do because the surface does not absorb every other wavelength.

Regular colors are only the way they are if you isolate a narrow range of wavelengths (the visible spectrum).

Considering that everything warm glows in infra-red, the main issue with making an IR ink is fitting the part of the definition that says you have to be able to write stuff with it. This means it has to be visible against the general IR background, which is why the fluorescent inks are the way to go.

 

[Yanick]

You could use the subtractive color approach to get an IR image via filters.

I guess so, I'm not an ink expert.

You seem to be asking for a substance that absorbs all wavelengths except a small range in the infra-red.
(Or whatever color you want.)

I never said this, I said:

Our perception of the color is then the stuff that was not absorbed.

which is technically correct without getting into the specifics of mixing wavelengths and perception of color etc. I'm aware of the complementary color concept in the subtractive color model, I'm just not using the technical jargon and maybe that is where you don't like my explanations.

Once the matter/light interaction has been completed we can analyze it similarly to the additive model. In other words, we start with white light and use some dye to subtract some wavelengths away. Our perception of the resulting photons is the same as using a source of light which only emits those wavelengths. I'm well aware that we can make a blue perception without using a narrow band of ~450-500nm light.

No pigment works like this. Blue ink does not absorb every single wavelength that hits it that isn't blue. If it did, it would get hot.

Sure they do! See Chlorophyll. Strong absorption of red and blue photons leads to a perception of green leaves. And tying into my above statement, is the same as having a lamp emitting those wavelengths.

I do understand where you are coming from in terms of printing/painting etc. I also can see your point regarding making an IR ink, but my original point still stands. An IR ink which produces IR radiation by a fluorescence phenomenon is fundamentally different than the phenomenon which produces the black on the printed page. I guess you can consider it an academic or useless distinction, but I figured we're on a science forum so we may as well think about things like these.

 

[Simon Bridge]

Sure they do! See Chlorophyll. Strong absorption of red and blue photons leads to a perception of green leaves. And tying into my above statement, is the same as having a lamp emitting those wavelengths.

This example illustrates my point. I was concerned about a common misunderstanding.

Chlorophyll does not absorb every single wavelength incident on it.
Red and blue are not the only parts of the EM spectrum that chlorophyll absorbs - they are just the wavelengths strongly absorbed which are also part of the visible spectrum.

Thus, there is no reason to presume that a pigment with a non-visible spectrum color should be black to the eye.

An IR ink which produces IR radiation by a fluorescence phenomenon is fundamentally different than the phenomenon which produces the black on the printed page.

We are in agreement there.

The trick therefore is to work out how to get an IR color by selective absorbtion of part of the spectrum...

 

[armar]

Simon, do you know if there are any inks that aren't visible to the human eye, but are visible to an iphone camera/silicon chip?

 

[Simon Bridge]

ifaik: phone cameras are optimized to work in the visible range. For a specific camera, you'd have to get the frequency response curves off the manufacturer. Good luck with that.

otoh: semiconductor arrays can be made which react to most frequencies we may be interested in.
"silicon chip" is a very broad term, while, by comparison, "iphone camera" is a very narrow one.

 

[armar]

Thanks so much for the response. I'm trying to find some type of ink invisible to the eye but visible to an iPhone camera. As I understand it, the iPhone camera can interpret light that our eyes can't see.

 

[Ygggdrasil]

ifaik: phone cameras are optimized to work in the visible range. For a specific camera, you'd have to get the frequency response curves off the manufacturer. Good luck with that.

otoh: semiconductor arrays can be made which react to most frequencies we may be interested in.
"silicon chip" is a very broad term, while, by comparison, "iphone camera" is a very narrow one.

The CCD in most digital cameras (as well as mobile phone cameras I'd assume) is sensitive to IR, but manufacturers install a filter in the optical path to block out IR light before it reaches the detector. It is possible to modify a digital camera to take IR images by removing the IR filter and replacing it with one that blocks visible wavelengths, though I'm not sure you'd want to do this to an expensive iPhone.

 

[armar]

Right, modification of the phone won't work. I'm trying to develop a sunscreen that parents can apply to children and be able to see with their phone but not be able to see with normal eyesight. Are there other ink pigments that might work instead?

 

[Simon Bridge]

As I understand it, the iPhone camera can interpret light that our eyes can't see.

Where does this understanding come from: please provide a reference so I know what you are referring to.

The CCD certainly responds to frequencies outside the visible spectrum, it's just that a camera is more than a CCD array.

I'm trying to develop a sunscreen that parents can apply to children and be able to see with their phone but not be able to see with normal eyesight.

... so they could easily pick their kids out in a crowd?

Consider: the product becomes very popular so just about everyone uses it. A parent loses a child and holds their camera up to a crowd to look for them ... what do they see?

 

[armar]

Here is the foundation that I'm working from. Please let me know if it is incorrect: http://gizmodo.com/282689/kameraflage-images-only-visible-through-a-digital-camera

No, the application would be to make sure that the sunscreen was applied evenly and thoroughly and that, when the child goes in water, it is reapplied appropriately to protect the child.

 

[DaveC426913]

No, the application would be to make sure that the sunscreen was applied evenly and thoroughly and that, when the child goes in water, it is reapplied appropriately to protect the child.

Yeah, I got the idea as soon as you mentioned it. Very clever.

Note, BTW, that using the term "ink" is probably narrowing your options and responses. A pigment that could be mixed into a fluid like sunscreen probably gives you more options.

 

[Simon Bridge]

Fair enough - also see:
http://www.internationalskeptics.com/forums/showthread.php?t=159310
... you'd have to experiment, and also read the patent carefully.

 

[armar]

Thanks again. Yes, infrared reflective material seems to be close to what I need to find.