Editing Total internal reflection (section)

== Applications ==
{{Further|Prism (optics)#Reflective·prisms}}

'''[[Optical&nbsp;fiber]]s''' exploit total internal reflection to carry signals over long distances with little attenuation.<ref>Jenkins & White, 1976, pp.{{nnbsp}}40–42.</ref> They are used in [[telecommunication cable]]s, and in image-forming fiberscopes such as [[colonoscopy|colonoscopes]].{{r|rudd-1971}}

In the '''catadioptric [[Fresnel lens]]''', invented by Augustin-Jean Fresnel for use in [[lighthouse]]s, the outer prisms use TIR to deflect light from the lamp through a greater angle than would be possible with purely refractive prisms, but with less absorption of light (and less risk of tarnishing) than with conventional mirrors.<ref>Levitt, 2013, pp.{{nnbsp}}79–80.</ref>

[[File:Porro binocular.jpg|thumb|'''Fig.{{tsp}}14''':{{big|&nbsp;}}Porro prisms (labeled 2 & 3) in a pair of binoculars]]<!-- THE 283px WIDTH OF THIS IMAGE HAS BEEN FOUND BY TRIAL AND ERROR TO GIVE GOOD LEGIBILITY OF THE LABELS '2' AND '3'. -->

Other '''[[Prism (optics)|reflecting prism]]s''' that use TIR include the following (with some overlap between the categories):<ref>Jenkins & White, 1976, pp.{{nnbsp}}26–7 (Porro, Dove, 90°{{nnbsp}}Amici, corner reflector, Lummer-Brodhun);{{tsp}} Born & Wolf, 1970, pp.{{nnbsp}}240–41 (Porro, Koenig), 243–4 (Dove).</ref>
* '''[[erect image|Image-erecting]] prisms''' for [[binoculars]] and [[spotting&nbsp;scope]]s include paired 45°-90°-45° [[Porro&nbsp;prism]]s (Fig.{{nnbsp}}14), the [[Porro–Abbe prism]], the inline Koenig<ref>Born & Wolf, 1970, p.{{hsp}}241.</ref> and [[Abbe–Koenig prism|Abbe–Koenig]] prisms, and the compact inline [[Schmidt–Pechan prism]]. (The last consists of two components, of which one is a kind of [[Bauernfeind prism]], which requires a reflective coating on one of its two reflecting faces, due to a sub-critical angle of incidence.) These prisms have the additional function of folding the optical path from the [[objective&nbsp;lens]] to the [[prime&nbsp;focus]], reducing the overall length for a given primary [[focal&nbsp;length]].
* A '''prismatic [[star diagonal]]''' for an astronomical [[telescope]] may consist of a single Porro prism (configured for a single reflection, giving a mirror-reversed image) or an [[Amici roof prism]] (which gives a non-reversed image).
* '''[[Roof prism]]s''' use TIR at two faces meeting at a sharp 90° angle. This category includes the Koenig, Abbe–Koenig, Schmidt–Pechan, and Amici types (already mentioned), and the roof [[pentaprism]] used in [[single-lens reflex camera|SLR&nbsp;cameras]]; the last of these requires a reflective coating on one {{nowrap|non-TIR}} face.
* A '''prismatic [[corner reflector]]''' uses three total internal reflections to reverse the direction of incoming light.
* The '''[[Dove prism]]''' gives an inline view with mirror-reversal.

'''Polarizing prisms''': Although the Fresnel rhomb, which converts between linear and elliptical polarization, is not birefringent (doubly refractive), there are other kinds of prisms that combine birefringence with TIR in such a way that light of a particular polarization is totally reflected while light of the orthogonal polarization is at least partly transmitted. Examples include the [[Nicol prism]],<ref>Born & Wolf, 1970, pp.{{nnbsp}}690–91.</ref> [[Glan–Thompson prism]], [[Glan–Foucault prism]] (or "Foucault prism"),{{r|nave-prisms}}<ref>Jenkins & White, 1976, pp.{{nnbsp}}510–11 (Nicol, Glan–Thompson, "Foucault").</ref> and [[Glan–Taylor prism]].{{r|archard-taylor-1948}}

'''[[Refractometer]]s''', which measure refractive indices, often use the critical angle.<ref>Buchwald, 1989, pp.{{nnbsp}}19–21; Jenkins & White, 1976, pp.{{nnbsp}}27–8.</ref>{{r|wollaston-1802a}}

'''[[rain sensor#Automotive sensors|Rain sensors]]''' for automatic [[Windscreen wiper#History|windscreen/windshield wipers]] have been implemented using the principle that total internal reflection will guide an infrared beam from a source to a detector if the outer surface of the windshield is dry, but any water drops on the surface will divert some of the light.{{r|hella}}

'''Edge-lit [[light-emitting diode|LED]] panels''', used (e.g.) for [[backlight]]ing of [[liquid-crystal display|LCD]] computer monitors, exploit TIR to confine the LED light to the acrylic glass pane, except that some of the light is scattered by etchings on one side of the pane, giving an approximately uniform [[luminous emittance]].{{r|gourlay-2015}}

<!-- THE WIDTH OF THIS IMAGE MATCHES THE PREVIOUS ONE: -->[[File:Total Internal Reflection Fluorescence Microscopy.svg|thumb|'''Fig.{{tsp}}15''':{{big|&nbsp;}}Operation of a "trans-geometry" TIR fluorescence microscope: (1){{nnbsp}}objective, (2){{nnbsp}}emission beam {{bracket|signal}}, (3){{nnbsp}}immersion oil, (4){{nnbsp}}cover slip, (5){{nnbsp}}specimen, (6){{nnbsp}}evanescent wave range, (7){{nnbsp}}excitation beam, (8){{nnbsp}}quartz prism.]]

'''Total internal reflection microscopy''' (TIRM) uses the evanescent wave to illuminate small objects close to the reflecting interface. The consequent scattering of the evanescent wave (a&nbsp;form of frustrated TIR), makes the objects appear bright when viewed from the "external" side.{{r|ambrose-1956}} In the ''[[total internal reflection fluorescence microscope]]'' (TIRFM), instead of relying on simple scattering, we choose an evanescent wavelength short enough to cause [[fluorescence]] (Fig.{{nnbsp}}15).{{r|axelrod-1981}} The high sensitivity of the illumination to the distance from the interface allows measurement of extremely small displacements and forces.{{r|axelrod-2001}}

A '''[[beam splitter|beam-splitter]] cube''' uses frustrated TIR to divide the power of the incoming beam between the transmitted and reflected beams.{{r|harvard-ftir}} The width of the air gap (or low-refractive-index gap) between the two prisms can be made adjustable, giving higher transmission and lower reflection for a narrower gap, or higher reflection and lower transmission for a wider gap.<ref>Hecht, 2017, p.{{nnbsp}}138.</ref>

'''[[optical modulator|Optical modulation]]''' can be accomplished by means of frustrated TIR with a rapidly variable gap.{{r|astheimer-et-al-1966}} As the transmission coefficient is highly sensitive to the gap width (the function being approximately exponential until the gap is almost closed), this technique can achieve a large [[dynamic range]].

'''Optical [[fingerprint]]ing''' devices have used frustrated TIR to record images of persons' fingerprints without the use of ink (cf.&nbsp;Fig.{{nnbsp}}11).{{r|harrick-1962}}

'''[[Gait analysis]]''' can be performed by using frustrated TIR with a high-speed camera, to capture and analyze footprints.{{r|noldus-catwalk}}

A '''[[gonioscopy|gonioscope]]''', used in [[optometry]] and [[ophthalmology]] for the diagnosis of [[glaucoma]], ''suppresses'' TIR in order to look into the angle between the [[iris (anatomy)|iris]] and the [[cornea]]. This view is usually blocked by TIR at the cornea-air interface. The gonioscope replaces the air with a higher-index medium, allowing transmission at oblique incidence, typically followed by reflection in a "mirror", which itself may be implemented using TIR.{{r|bruce-et-al-2016|gat-gon}}

Some [[Multi-touch#Optical|'''multi-touch''']] interactive tables and [[Interactive whiteboard|whiteboards]] utilise FTIR to detect fingers touching the screen. An infrared camera is placed behind the screen surface, which is edge-lit by infrared LEDs; when touching the surface FTIR causes some of the infrared light to escape the screen plane, and the camera sees this as bright areas. [[Computer vision]] software is then used to translate this into a series of coordinates and gestures.

{{clear}}