In
optics
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
, an index-matching material is a substance, usually a liquid, cement (adhesive), or
gel, which has an
index of refraction that closely approximates that of another object (such as a lens, material, fiber-optic, etc.).
When two substances with the same index are in contact, light passes from one to the other with neither
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in ...
nor
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
. As such, they are used for various purposes in science, engineering, and art.
For example, in a popular home experiment, a glass rod is made almost invisible by immersing it in an index-matched transparent fluid such as
mineral spirits.
In microscopy
In
light microscopy, oil immersion is a technique used to increase the
resolution of a
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...
. This is achieved by immersing both the
objective lens
In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elem ...
and the specimen in a transparent oil of high
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
, thereby increasing the
numerical aperture of the objective lens.
Immersion oils are transparent oils that have specific optical and
viscosity
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the inte ...
characteristics necessary for use in microscopy. Typical oils used have an
index of refraction around 1.515. An oil immersion objective is an objective lens specially designed to be used in this way. The index of the oil is typically chosen to match the index of the microscope lens glass, and of the
cover slip
A microscope slide is a thin flat piece of glass, typically 75 by 26 mm (3 by 1 inches) and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is mounted (secured) on the slide, and then b ...
.
For more details, see the main article,
oil immersion. Some microscopes also use other index-matching materials besides oil; see
water immersion objective and
solid immersion lens A solid immersion lens (SIL) has higher magnification and higher numerical aperture than common lenses by filling the object space with a high- refractive-index solid material. SIL was originally developed for enhancing the spatial resolution of o ...
.
In fiber optics
In fiber optics and
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, an index-matching material may be used in conjunction with pairs of mated connectors or with mechanical splices to reduce signal reflected in the guided mode (known as return loss) (see
Optical fiber connector
An optical fiber connector joins optical fibers, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so light can pass. Better connectors lose very little light due ...
). Without the use of an index-matching material, Fresnel reflections will occur at the smooth end faces of a fiber unless there is no fiber-air interface or other significant mismatch in refractive index. These reflections may be as high as −14
dB (i.e., 14 dB below the optical power of the incident
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
). When the reflected signal returns to the transmitting end, it may be reflected again and return to the receiving end at a level that is 28 dB plus twice the fiber loss below the direct signal. The reflected signal will also be delayed by twice the delay time introduced by the fiber. The twice-reflected, delayed signal superimposed on the direct signal may noticeably degrade an analog
baseband
In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable int ...
intensity-modulated
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syst ...
signal. Conversely, for digital transmission, the reflected signal will often have no practical effect on the detected signal seen at the decision point of the digital
optical receiver
Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or ...
except in marginal cases where bit-error ratio is significant. However, certain digital transmitters such as those employing a
Distributed Feedback Laser may be affected by back reflection and then fall outside specifications such as Side Mode Suppression Ratio, potentially degrading system bit error ratio, so networking standards intended for DFB lasers may specify a back-reflection tolerance such as −10 dB for transmitters so that they remain within specification even without index matching. This back-reflection tolerance might be achieved using an optical isolator or by way of reduced coupling efficiency.
For some applications, instead of standard polished connectors (e.g. FC/PC), angle polished connectors (e.g. FC/APC) may be used, whereby the non-perpendicular polish angle greatly reduces the ratio of reflected signal launched into the guided mode even in the case of a fiber-air interface.
In experimental fluid dynamics
Index matching is used in liquid-liquid and liquid-solid (
Multiphase flow
In fluid mechanics, multiphase flow is the simultaneous flow of materials with two or more thermodynamic phases. Virtually all processing technologies from cavitating pumps and turbines to paper-making and the construction of plastics involve so ...
) experimental systems to minimise the distortions that occur in these systems, this is particularly important for systems with many interfaces which become optically inaccessible. Matching the refractive index minimises
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in ...
,
refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
,
diffraction and rotations that occurs at the interfaces allowing access to regions that would otherwise be inaccessible to optical measurements. This is particularly important for advanced optical measurements like
Laser-induced fluorescence,
Particle image velocimetry and
Particle tracking velocimetry Particle tracking velocimetry (PTV) is a velocimetry method i.e. a technique to measure velocities and trajectories of moving objects. In fluid mechanics research these objects are neutrally buoyant particles that are suspended in fluid flow. As th ...
to name a few.
In art conservation
If a sculpture is broken into several pieces,
art conservators may reattach the pieces using an adhesive such as
Paraloid B-72 or
epoxy. If the sculpture is made of a transparent or semitransparent material (such as glass), the seam where the pieces are attached will usually be much less noticeable if the refractive index of the adhesive matches the refractive index of the surrounding object. Therefore, art conservators may measure the index of objects and then use an index-matched adhesive. Similarly, losses (missing sections) in transparent or semitransparent objects are often filled using an index-matched material.
In optical component adhesives
Certain optical components, such as a
Wollaston prism
A Wollaston prism is an optical device, invented by William Hyde Wollaston, that manipulates polarized light. It separates light into two separate linearly polarized outgoing beams with orthogonal polarization. The two beams will be polarized a ...
or
Nicol prism, are made of multiple transparent pieces that are directly attached to each other. The adhesive is usually index-matched to the pieces. Historically,
Canada balsam was used in this application, but it is now more common to use
epoxy or other synthetic adhesives.
References
*
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Fiber optics
Optical materials