Hello everyone.

Well, to add to my recent Windows nightmare and the fact our new camera turned out to be broken, more odd stuff is going on.

I have two of AlexP’s lasers.  If we sit them on top of some frosted acrylic and put a cam directly underneath, the blobs are incredible.  However, if we point the cam at a mirror, the blobs just vanish.  We can adjust to barely see them, but then everything is so bright we get one big blob.

Why would a mirror kill the IR?  It’s just bloody barmy.

A.

Actually, lots of mirrors do this. Just because they reflect visible light, does not mean they will reflect infrared due to differences in the way different materials interact with different frequencies of light. You can however buy special mirrors that are made especially to reflect IR as well as visible light. I can’t remember which is which but they are either called ‘hot’ mirrors or ‘cold’ mirrors. One allows IR to pass through and one reflects it. Might cost a bit more though, so probably worth experimenting with other ‘normal’ mirrors and seeing if you can find one that does the job.

Good luck!

Hot mirrors allow for visible light to pass through and they reflect IR… i dont know about cold mirrors but i assume they do the opposite

Taha - 23 July 2008 03:50 PM

Hot mirrors allow for visible light to pass through and they reflect IR… i dont know about cold mirrors but i assume they do the opposite

Oops, my bad. I guess I misunderstood what hot and cold mirrors are about! Still I think that the mirror you have is allowing some IR to pass through or be diffused. I suppose you could test by shining IR at it and putting your camera the other side. Don’t forget that unless you have a front surface mirror then the light has to go through a layer of glass before it is reflected anyway, and that no mirror is perfectly reflective…

Glass attenuates IR 40% - 50%. So to reflect off a mirror, it has to pass through twice the thickness of the galss! This is why conventional mirrors don’t work when you aim the camera at them to see the table surface. I had the same problem, did some research and discovered this info.
A frontside mirror may do better because the IR reflects off the metal without having to pass through glass first!
Try a frontside mirror and see if it helps. You can make your own by removing the paint from the back of a conventional mirror with a chemical paint stripper.
The side you strip is the new front of the mirror.