Frustrated Total Internal Reflection

Under "ordinary conditions" it is true that the creation of an evanescent wave does not affect the conservation of energy, i.e. the evanescent wave transmits zero net energy. However, if a third medium with a higher refractive index than the low-index second medium is placed within less than several wavelengths distance from the interface between the first medium and the second medium, the evanescent wave will be different from the one under "ordinary conditions" and it will pass energy across the second into the third medium. (See evanescent wave coupling.) This process is called "frustrated" total internal reflection (FTIR) and is very similar to quantum tunneling. The quantum tunneling model is mathematically analogous if one thinks of the electromagnetic field as being the wave function of the photon. The low index medium can be thought of as a potential barrier through which photons can tunnel.

The transmission coefficient for FTIR is highly sensitive to the spacing between the high index media (the function is approximately exponential until the gap is almost closed), so this effect has often been used to modulate optical transmission and reflection with a large dynamic range. An example application of this principle is the multi-touch sensing technology for displays as developed at the New York University’s Media Research Lab.

Applications

• Optical fibers, which are used in endoscopes and telecommunications.
• Rain sensors to control automatic windscreen/windshield wipers.
• Another interesting application of total internal reflection is the spatial filtering of light.^[2]
• Prismatic binoculars use the principle of total internal reflections to get a very clear image
• Some multi-touch screens use frustrated total internal reflection in combination with a camera and appropriate software to pick up multiple targets.
• Gonioscopy to view the anatomical angle formed between the eye's cornea and iris.
• Gait analysis instrument, CatWalk,^[3] uses frustrated total internal reflection in combination with a high speed camera to capture and analyze footprints of laboratory rodents.
• Fingerprinting devices, which use frustrated total internal reflection in order to record an image of a person's fingerprint without the use of ink.
• Flashlights lenses.^[4]

Source: Wikipedia

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