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Glossary of Optics Terms

Centration – Used to define the deviation between the mechanical and optical axis on spherical surfaces.  Optics with poor centration will suffer from beam deviation as light passes through the medium.

Dispersion (Vd) – As different wavelengths of light will bend or “refract” at different angles when entering an optical medium, dispersion defines the degree to which the separate wavelengths will diverge.  In optics, dispersion is characterized by a material’s Abbe Value (Vd) where a lower value denotes a greater degree of dispersion.

Flatness – An optical specification used to define the surface accuracy of any optic having a flat (non-radii) surface.  Typically, flatness is measured using an interferometer and quantified in terms of waves (λ) or fractions of a wave.

International Glass Code – A six digit code corresponding to an optical material’s refractive index (1st three digits) and Abbe Value (2nd three digits).

Irregularity – A specification used to define the consistency of an optical surface in terms of its uniformity.  Irregularity is specified in terms of waves (λ) or fringes (1 fringe = ½ wave) and measured using an interferometer.

Power – An optical specification used to measure the accuracy of spherical surfaces and defined in terms of waves (λ) or fractions of a wave.  Power is tested using an interferometer or through the use of a calibrated test piece with a known spherical curvature.

Refractive Index (Nd) – The ratio by which the speed of light will decrease when entering a new medium when compared to its speed in a vacuum.  It defines the degree to which light will bend or “refract” in a given optical material.

Transmitted Wavefront Distortion (TWD) – Transmitted wavefront distortion, is a specification used to measure the degree to which an optic changes or “distorts” the light traveling through a given medium.  Typically, TWD is measured using an interferometer and quantified in terms of waves (λ) or fractions of a wave.