Sagnac interferometer

An interferometer with incoming light going through two coils of optical fiber. In one fiber the light travels clockwise and in the other fiber it travels counterclockwise. Rotation of the coils causes a phase shift in the combined output measured by the detector.


Saturation usually indicates a point near a transition in an interaction where the increase of one parameter no longer increases another parameter. For example, a decrease of the absorption coefficient of a medium, near some transition frequency, when the power of the incident radiation near that frequency exceeds a certain value.

(v.) - 1538, probably from adj. (early 15c.), from L. saturatus, pp. of saturare "to fill full, sate, drench," from satur "sated, full," from PIE base *sa- "to satisfy." Originally "satisfy;" meaning "soak thoroughly" first recorded 1756. Saturation bombing first recorded 1942.


A device used to examine, read, monitor, or image something that utilizes the interaction between the object to be imaged and electromagnetic radiation, particles (often charged), magnetic fields, or sound waves (ultrasound). A scanner can convert a paper drawing or photograph into pixels on a display screen. Scanners are also used to relay information in optical data processing. 2) A device for sensing recorded data, such as a supermarket bar code. 3) A device that automatically measures or checks a process or condition and may initiate a desired corrective action by means of switching.

Scanning Electron Microscope (SEM)

A microscope, with a resolution better than one billionth of a meter (1 nanometer), that uses a focused beam of electrons. The electrons are focused on the sample to be imaged and interact with electrons in the sample, producing signals that can be detected and imaged. The produced signals contain information about the sample’s surface structure and composition.


A physical process where light, sound, or particles are forced to deviate from a straight trajectory. It occurs when light encounters a rough surface, causing it to be sent off in many different directions, and does not change in frequency.

To learn more about scattering click on the link to the Biomedical Optics pamphlet.

12c., possibly a northern Eng. variant of M.E. schateren (see shatter), reflecting.

Schmidt prism

This prism reverts the image while deviating the line of sight by forty-five degrees.


1) The extent or area that a subject matter deals with or is relevant to. 2) A shortened term for anything ending in “scope” such as microscope, telescope, or oscilloscope.

(1) "extent," 1534, from It. scopo "aim, purpose, object," from L. scopus, from Gk. skopos "aim, target, watcher," related to skopein "behold, look, consider," skeptesthai "to look at." Originally "mark to shoot at," sense of "distance the mind can reach, extent of view" first recorded c.1600. (2) "instrument for viewing," 1872, abstracted from telescope, microscope, etc., from Gk. skopein "to look."


1) The ability to detect slight changes. 2) The ratio of the output to the input in a detector.

1392, from M.Fr. sensitif, from M.L. sensitivus "capable of sensation," from L. sensus, pp. of sentire "feel perceive." Meaning "easily affected" first recorded 1816. Sensitize first recorded 1856, originally in photography.

Silicon dioxide (silica, SiO2)

A white or colorless crystalline compound found in quartz, sand, flint, and other materials. Commonly used in optics to produce, for example, lenses.

From Silica 1801, Mod.L., from L. silex (gen. silicis) "flint, pebble."Silicon 1817, coined by British chemist Thomas Thomson from silica, patterned on boron, carbon, etc. Silicone coined 1860s on the same plan. Silicon Valley for the Santa Clara Valley south of San Francisco first attested 1974, from the silicon chips used in computers, watches, etc.

Slide projector

An optical projection device used to view photographic slides. Light passes through a transparent slide and creates an enlarged image onto a screen for viewing.

Solid State Lighting (SSL)

A new technology for lighting homes, schools and streets using light emitting diodes (LED). SSL consumes less electric power and lasts much longer than traditional incandescent and fluorescent lamps.

To learn more about Solid State Lighting click on the link to the Solid-State Lighting pamphlet.


Sound Navigation And Ranging is the process of listening to specific sounds to determine where objects are located. In active sonar, a sound is transmitted and the listener uses its echo to locate objects. In passive sonar, the listener uses the sounds emitted directly from the source of the sounds.

To learn more about SONAR click on the link to the Echolocation pamphlet.

Sound source

Whatever object makes the sound. All of these are sources of sound: two hands clapping together, a person speaking or singing, a submarine echolocating, a radio playing, birds chirping, ocean waves crashing on the beach.

To learn more about sound source click on the link to the Echolocation pamphlet.

Sound waves

Vibrations of air molecules that travel through air carrying energy with them. Sound waves can also travel through water and solids, but cannot travel in empty space where there are no molecules to vibrate.

To learn more about sound waves click on the link to the Acoustics pamphlet.


1) The person, place, or thing that originates or creates something. 2) A physical source of radiation.

1346, from O.Fr. sourse "a rising, beginning, fountainhead of a river or stream," fem. Noun taken from pp. of sourdre "to rise, spring up," from L. surgere "to rise."

Speckle pattern

An intensity pattern produced by mutual interference of a set of wavefronts. Speckle patterns have very bright and dark spots. One can observe speckle patterns when looking at stars through Earth’s atmosphere and when shining a laser at a small angle onto a rough surface. Some examples of applications are measuring blood flow and blood oxygen levels.

1440, spakle, probably related to O.E. specca "small spot, speck," or from a related M.Du. or M.H.G. word.


Related to or produced by a spectrum.


An instrument that separates an incoming wave’s component frequencies and captures the information on a sensitive material. There are many types of spectrographs. For some, the dispersing medium may be a prism or a diffraction grating. Some special-purpose spectrographs are equipped with tiny photodetectors situated in the spectrum at the positions corresponding to the lines of elements whose presence is to be determined.


1) A device that decomposes light into its constituent wavelengths. In this sense, water droplets floating in the air form a large-scale spectrometer that produces a rainbow. 2) A kind of spectrograph in which some form of detector, other than a photographic film, is used to measure the distribution of radiation in a particular wavelength region.

To learn more about spectrometers click on the link to the Solid-State Lighting pamphlet.


An instrument for measuring spectral transmittance or reflectance.


An instrument used to disperse usually visible light into its component wavelengths, for determining or measuring the resultant spectrum.


1) The decomposition of light into its constituent colors or wavelengths. Rainbows show us the spectrum of the sunlight. Sunlight consists of a large range of wavelengths, but our eyes can only see a small range of wavelengths that we see as different colors (violet to red). 2) Any signal that can be measured and/or decomposed along a range of values, with the spectrum containing all the components between the two extreme points of the range.

To learn more about Solid State Lighting click on the link to the Solid-State Lighting pamphlet.

1611, "apparition, specter," from L. spectrum "appearance, image, apparition," from specere "to look at, view." Meaning "band of colors formed from a beam of light" first recorded 1671. Spectroscope is from 1861.

Speed of sound

The speed at which sound travels. This is very important for scientists who study sound. In air sound travels 343 meters in 1 second (767 miles per hour), but in water sound travels 1500 meters in 1 second (3350 miles per hour). Compare these speeds to cars traveling on the highway at 65 miles per hour.

To learn more about the speed of sound click on the link to the Echolocation pamphlet.


An instrument that allows viewing separate images as would be seen from the left eye and from the right eye. By viewing these together, a sense of depth is obtained, creating a three-dimensional image.


1) To hinder or prevent passage of. 2) Optical stops limit the light bundle from entering an optical device.

O.E. -stoppian (in forstoppian "to stop up, stifle"), a W.Gmc. borrowing from V.L. *stuppare "to stop or stuff with tow or oakum." Plugs made of tow were used from ancient times in Rhine valley. Sense of "bring or come to a halt" (1440) is from notion of preventing a flow by blocking a hole, and is only in Eng.