Glossary of Optics Terms

Aberration – Failure of a mirror, refracting surface, or lens to produce exact point-to-point correspondence between an object and its image

Additive manufacturing – Is the process of creating three dimensional shapes through the process of building single layer at a time. It is guided by CAD (computer aided design) where a 3D model 

Asphere – Or Aspheric lens is a lens whose surface profiles are not portions of a sphere or cylinder. A single aspheric surface can perform the work of several spherical surfaces reducing the cost and size of an optical component. 

Bevel – is where two surface planes meet that edge is flattened to ease the transition of the two planes, generally at 45º. Also called chamfer or safety bevel.

Calcium Fluoride (CaF2) – optical crystal that has a high transmittance for optical systems that use ultraviolet/infrared regions. It has a fairly high heat capacity and a moderately high melting temperature among non-glass optical ceramics. Common uses are spectroscopy, semiconductor processing, thermal imaging.

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.

Fringes – “Optical fringe” generally refers to the patterns of light and dark bands or lines that result from the interference or diffraction of light. This phenomenon is often observed when light waves interact with each other, creating regions of constructive and destructive interference.

One common example is in interferometry, where light waves are superimposed to create interference patterns. Interferometers use optical fringes to measure various properties such as distance, wavelength, and surface irregularities.

Fused Silica - or Borosilicate Glass is a hard, high purity glass with high use temperature and near zero thermal expansion, and so is extremely thermal shock resistant. Uses include inspection for machine vision, high resolution SWIR imaging, as well as, remote sensing and metrology.

Germanium (Ge) - Known for its high refractive index and excellent transmission in the mid-infrared range (2-14 µm), germanium is widely used in lenses and windows for thermal imaging systems. Germanium has the highest refractive index of commonly available IR-transmitters and has low optical dispersion. 

Hyperbolic Mirror – is an optical component of choice for precisely and accurately focusing light in a telescope from one focal point to the other focal point. Both hyperbolic and parabolic mirrors are used in a simple Cassegrainian Telescope. A parabolic mirror has the property of directing all light rays incident on it to its focus whereas a hyperbolic mirror provides a wider field of view.

Infrared – the part of the invisible spectrum that is contiguous to the red end of the visible spectrum and that comprises electromagnetic radiation of wavelengths from 800 nm to 1 mm.

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

Knoop Hardness - The indentation hardness of optical glass is determined with the aid of the micro hardness tester. One face of the specimen with the necessary thickness is polished. The diamond indentor is formed rhombic so that the vertically opposite angle from two axes is 172 °30' and 130 ° respectively. The load time is 15 seconds, the load is 0.98 N. The glass specimen is indented at 5 places. Knoop hardness can be computed with the following equation:

Knoop Hardness =1.45l F/l²
F : Load (N) : l Length of longer diagonal line (mm)

Laser –  a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. L.A.S.E.R is and acronym for "light amplification by stimulated emission of radiation.

Near-Infrared – is the region closest in wavelength to the radiation detectable by the human eye. Mid- and far-infrared are progressively further from the visible spectrum from 780 nm to 2500 nm.

Magnesium Fluoride (MgF2) - non-glass optical ceramics with wide transmission ranges, from the ultraviolet through the infrared, making them suitable for various optical components, including lenses and windows.

Micrometer: A micrometer, also known as a micron, is a unit of length in the metric system. One micrometer is equal to one millionth of a meter. In terms of SI units, it is represented as 1 × 10^-6 meters.

• 1 meter = 1,000,000 micrometers
• 1 centimeter = 10,000 micrometers
• 1 millimeter = 1,000 micrometers

Photon – an elemental particle representing a quantum of light or other electromagnetic radiation. A photon carries energy proportional to the radiation frequency but has zero rest mass and always moves at the speed of light within a vacuum.

Plano-Convex – are optical lenses with a positive focal length. They are ideal for light collimation or for focusing applications utilizing monochromatic illumination, in a range of industries including industrial, biomedical, industrial robotics, high-performance imaging and defense applications. A plano-convex lens has one convex surfaced the second, plano (flat) surface should face the desired focal plane.

Polished edge – The edges of a optic are polished smooth resulting in a sleek appearance and shiny or glossy finish. 

Potato chip effect - An informal term referring to an optical fabrication where a glass substrate size exceeds a 5:1 ratio and it not structural staple to maintain consistent flatness. The optical component can begin to distort in a shape resembling a potato chip wave.  

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.

Ruby – A pink to blood-red colored naturally occurring gemstone, but also can be artificially created. In optics it is generally known to produce red light when at 694-nanometer-wavelength.

Sapphire – Sapphire glass is a synthetically produced crystal second only to diamond in its hardness, making it extremely durable. It is both highly transparent to wavelengths of light between 150 nm (UV) and 5500 nm (IR) near-infrared (0.15-5.5 µm), and is extraordinarily scratch-resistant. 

Silicon (Si): Effective in the near to mid-infrared range (1-8 µm), silicon is frequently used for lenses, windows, and prisms in IR optics, especially in the 3-5 µm range.

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.

Ultraviolet (UV) – beyond the violet range in the visible spectrum, electromagnetic radiation with a wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays.

Wave: an interferometer uses the wave properties of light as its diagnostic tool, it is fitting that opticians specify the flatness of a surface in terms of waves. For example, on a plano surface, the flatness may be defined as one wave. As most modern interferometers use the 632.8nm laser wavelength, a surface requiring one wave accuracy may have no more than 632.8nm of distance between the highest peak and lowest valley across the usable clear aperture.

Wavelength – the distance, measured in the direction of propagation of wave, between two successive points in the wave that are characterized by the same phase of oscillation. It is typically measured in units such as nanometers (nm) or micrometers (μm).

Witness Sample – is used for optical materials coatings. It is a sacrificial piece that allows for testing thin film coatings. Witness samples are first put into a coating chamber to ensure the coating process is accurate before moving forward production or prototype optics.

Zinc Selenide (ZnSe): This material offers good transmission from the visible range up to about 15 µm in the infrared. It is commonly used in CO2 laser optics and IR imaging systems.

Zinc Sulfide (ZnS): Transparent across a broad spectrum (0.4-12 µm), zinc sulfide is used in multispectral applications and often serves as a window material in IR systems.