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hiddent IR message - can anyone test this? Answered

I am just wondering if this particular idea works. Just get a mirror(or maybe even better a retro reflective surface for camera flashes) and using a black marker... write a message on it After that Place a infrared passing filter on top of the message... it shall look black Now take a photo of it with flash inside a dark room... and also a photo of the object in a very sunny day outside. (keep note to take it from different angles.) Once you done that... tell us if it works! :) Another idea to see if it works is to replace the mirror with white paper... perhaps it may better scatter the sunlight for daytime use.


Black marker ink is transparent to IR light... I actually use it as a visible light filter for IR projects on clear plastic.... to try it, just color on a small piece of clear plastic or plastic wrap and put in front of your TV remote and change the channels...

...he says after I research and write my post. I gotta start having the page in a second tab so I can check for posts while I'm composing.

Alright then, how can one really do a "hidden message?" Is there an IR-absorbing paint or ink that one can first write down a message with, then cover the surface over with an IR-transparent paint, so it looks blank to the eye but will show up with a (DIY?) camera?

lol I've been thinking same as you forgesmith.
Retro reflective over a filter with something in between.

However I am going to test using a IR filter with normal paper underneath, that has been marked with a black pen.

Offhand I'd say it probably won't work. It depends on what is supposed to receive the infrared light.

If it's a photo with good old film, I'd think it wouldn't work as normal film picks up visible light, although there are some IR film types out there, say for scientific use. And for that sunny day test, there'd be so much excess IR whatever image was possible would likely get washed out.

For a digital camera, there's a grey area. While it was said that at least one model could pick up infrared, for a x-ray vision peekaboo effect, generally they don't. And there may still be washout. However, a CCD detector can receive IR, in the near-infrared range just outside of human vision. There are many ( one, two, three, four...) instructions out there for converting both cameras and even webcams for IR images. Note they don't really see heat, that's far-infrared long-wavelength for which special cameras are used.

There's another issue too. Your infrared-passing filter (which could really be just a piece of cardboard) is blocking visible light. Well, the marker, being black, would take in the light and give off some heat (IR), with sunlight being the best source as a digital camera flash may be too weak. Without that visible light, depending on what type of IR you're trying to view, it would depend on whether marker is transparent or at least partially opaque to that IR, which would yield whatever contrast you could get to distinguish it from the surface. It likely would only work in near-infrared, as marker shouldn't do much to block heat.

Yes, use another reflective surface. Using a mirror, or something with a strong specular refection is a terrible idea--what are you reflecting? The sky makes a lousy IR light source for a reflective image (it's normally dark) and if you're carrying an IR lamp (LEDs), then you're either reflecting the light source in a direction the camera can't see, or you're reflecting it directly back into the camera (ever take a flash photo into a mirror?)

You need the 'scattering' of non-specular reflections.

Note they don't really see heat, that's far-infrared long-wavelength for which special cameras are used...

Your infrared-passing filter (which could really be just a piece of cardboard) is blocking visible light.

I believe the OP was referring to the IR spectrum commonly used in IR film. As such, a normal Kodak Wratten #87 filter works fine, and effectively blocks most visible light.

Something similar on the 'msg' might work, but remember that any light reflecting off the surface travels through the filter twice...

But yeah, if using digital the integrated IR filter should be removed from the camera.

Talk about seeing heat, Boy that would be a real treat if some one would post a Ible on how to make a FLIR for under the $4000 tag they have right now...

And you could do an Infrared subtractive process (( I don't know how it works but they do it on the US $5 bill and there is huge areas that don't show up under IR light... http://en.wikipedia.org/wiki/Image:FiveDollarBillInfrared.jpg )) Just a though ...