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I noticed that human body conducts red visible light quite well. A hand placed over a powerful red lamp seems to be semi-transparent while over any other color it does not.

Possibly the light conductivity in near infra-red range is even better.

So my question is why (infra-)red light is not used for diagnostics instead of harmful X-rays?

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up vote 7 down vote accepted

Although IR light penetrates tissue much more easily than visible (much lower absorption), it is still scattered quite quickly (within a few hundred microns from the surface). This makes it impossible to form an image with it in the same fashion as with x-rays. However, it does find use in medicine with techniques such as optical coherence tomography and two-photon microscopy.

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Also in pulse-oxygenation monitors. – Colin K Jan 16 '12 at 3:46

The absorption of IR lights are still more than x-rays which decrease the resolution of the imaging process. It is usually used in surface imaging in medicine.

On the other hand, if we increase the IR in order to increase the resolution the temperature will be the problem.

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