Photonics Dictionary: D

A telescope similar to the customary Cassegrain telescope, but having a primary mirror that is ellipsoidal and a secondary that is spherical.

A condenser that forms a hollow cone-shaped beam of light with its focal point in the plane of the specimen. If it is used with an objective having a numerical aperture lower than the minimum...

A disc contained within an electronic cell counter for regulating light transmission.

The transmission of light by a condenser to observe either very small particles or very fine lines with a microscope.

A technique whereby the sample is illuminated by a hollow cone of light larger than the acceptance angle of the objective, so that only scattered light is seen, revealing any irregularities of the...

A photomicrographic recording technique that utilizes dark-field illumination to render an image of an object having a refractive index much like that of the medium holding it. The object may be...

The concept of the de Broglie wavelength exploits the wave-particle duality of quantum physics by associating all matter (of all sizes) with wavelike properties. The de Broglie wavelength states that...

A device used in ultrasonic imaging to measure the velocity and attenuation of compressional waves in a transparent liquid by virtue of acoustic waves set up in the liquid. The wavefronts serve as a...

The standard unit used to express gain or loss and relative power levels. The decibel (dB) = 10 log (P2/P1).

The condition in which two or more energy states are identical.

The distortion created because the different frequencies of a signal have different propagation velocities through a medium.

A device used to delay transmission of a signal for functions such as memory loops, sequential processing or built-in testing. The delay can be achieved by coiling long lengths of coaxial cable or...

The interval between direction of signal to a light-emitting diode and attainment of 10 percent output current in the photodetector.

Description of a material, such as a water-soluble salt, that will continue absorbing moisture from the surrounding atmosphere until it dissolves into a liquid.

The term delta error (delta-E) generally refers to the difference or change in error between two values or states. The word "delta" is often used in mathematics and science to represent a change or...

A compact, folded version of a Dove prism, made of high-index glass with a silvered base and used for image rotation.

The distance, on either side of the object plane focused on, through which satisfactory image definition can be obtained. For the special case of an imaging system with lens axis perpindicular to the...

A chemical solution that changes the silver salts (latent image) of exposed photographic film into black metallic silver (visual image).

An evacuated, double-walled container for storing liquids at low temperatures. Often made of glass, the vessel resembles a Thermos bottle.

A telescope that corrects dispersion and spherical aberration through the use of one or more lenses, usually smaller than the objective lens, that are positioned at a distance from the objective lens.

Exhibiting the characteristic of materials that are electrical insulators or in which an electric field can be sustained with a minimum dispersion of power. They exhibit nonlinear properties, such as...

A shallow, fine-pitch diffraction grating having a precise dielectric overcoating that experimentally has absorbed wavelength-specific incident light that is polarized in the P-plane.

A high-reflectance coating consisting of alternating layers of quarter-wave film of a higher refractive index and lower refractive index than the substrate. Such coatings can be made very specific to...

A number that indicates the magnitude of the shift in a solid of positive and negative charges in opposite directions when a voltage is applied across the solid.

A crystal that is characterized by its relatively poor electrical conductance.

A waveguide made up of a dielectric material, such as plastic or mica, in a cylindrical form, through which the waves travel.

A lens made up of a dielectric material that is capable of influencing radio waves much in the same way an optical lens influences light.

A variation in propagation delay caused by differences in group velocity among modes of an optical fiber. Also called multimode group delay, this effect is commonly due to imperfections or any...

An instrument that can preselect intervals, often in increments of 1, 10 or 100 ns, for the generation of electronic pulses by means of front-panel switches. Also called a time-delay generator.

In astronomy, the relation that states that the intensity of a star's visible radiation is inversely proportional to the square of the distance from which it is viewed.

A single wavelength of light that matches the color of a given sample when combined in suitable proportions with white light and a suitable adjustment of intensity.

Spectrophotometry in which radiation of two separate wavelengths, usually one in an absorption band and the other not, pass through the specimen. The resulting data allows correction for attenuation...

For a detector, the amount of time alotted for the observation of one location.

An optical correlator incorporating a binary phase-only spatial light modulator made from an iron garnet magneto-optic solid crystal and used in target recognition systems.

A value used to designate the relative sensitivity of a detector. The higher the D* value, the better the detector.

A Dammann grating is a type of diffractive optical element (DOE) used to create an array of equally spaced, uniform-intensity spots or beams. It is named after its inventor, Dr. Herbert Dammann. The...

An organizational method used in optical design computer programs. The technique produces one number (merit function) to measure the state of correction of an optical system. This number, which is...

Continuous conversion of oscillatory energy into heat, relative to time or distance.

The ability of the human eye to adjust itself to low levels of illumination.

The decay of an electrostatic charge image resulting from long exposure to the dark.