Glossary of Terms


This glossary of terms contains important definitions and abbreviations used in the eSource Optics web site.


analytical quality (AQ) optics: standard quality optics typically polished to a surface finish or quality ranging from 60/40 to 80/50 scratch/dig and a surface figure of Lambda/4 to 3-5 waves depending on the optic type.  This quality of optics are mainly for low power laser or light source applications.  Also see Surface Finish & Surface Figure.

angle of incidence (AOI): The angle formed between a ray of light striking a surface and the normal line (the line perpendicular to the surface at that point).

angstrom (Å): A unit of measure equal to 10-10 meter. 10 angstroms = 1 nanometer.

antireflection coating (AR): Dielectric thin film coating applied to an optical material to increase the transmittance by reducing the surface reflectance.  

bandwidth: Spectral width of wavelengths transmitted by a bandpass filter. Usually specified as the width at 50% of peak transmittance. See also FWHM.

bandpass filter: Optical filter that allows transmittance over a particular wavelength range while restricting the transmittance of wavelengths above and below this range. Also known as interference filter.  Also see Optics Tutorial.  

BK7 Glass: Borosilicate crown optical glass with low bubble content and high homogeneity (same optical properties throughout material).  BK7 is an excellent visible to NIR material for many optical components. 

blocking: The degree to which unwanted wavelengths are restricted from passing through a bandpass filter.

broadband AR coating: A coating designed to provide increased transmittance and reduced reflectance over a very wide (broad) band of wavelengths. 

calcium fluoride (CaF2):  Calcium Fluoride is a cubic single crystal optical material that exhibits good transmission from VUV to IR wavelengths (~140nm to 8.0 µm).   CaF2 is a relatively soft material sensitive to mechanical and thermal shock. 

central wavelength: Wavelength at the midpoint of the FWHM.  The central wavelength and the peak wavelength coincide in filters exhibiting a symmetrical band shape.

centration: In a perfectly centered lens, the mechanical axis (defined by the ground outside the diameter of the lens) is made to be exactly coincident with the optical axis (defined by a line through the centers of curvature of the optical surfaces).

chamfer: A ground beveled edge on an optic. Used to prevent chipping and/or to allow mechanical fit.

clear aperture: The usable area, within published specifications, of an optic.

crystalline quartz:  Crystal Quartz exhibits good transmission properties from ~ 147nm Vacuum UV (VUV) to ~ 2.5um in the Infrared.  Crystal Quartz is a very hard crystalline material with low thermal expansion coefficient.  Crystal Quartz is a grown crystalline form of Silicon Dioxide (SiO2) optical material.

cylindrical lens: A lens with one or more surfaces formed like a cylinder.  A Plano-Convex cylindrical Lens will focus light in only one dimension. Cylinder lenses can transform a point of light into a line image. Cylinder lenses magnify in one dimension and can be used for stretching images.  Also see Optics Tutorial.  

dielectric: A material that does not readily conduct electricity. Used in vacuum-deposited layer form to construct interference filters and other thin-film optical coatings.  

dielectric coating: High-reflectance or low-reflectance coating composed of alternating layers of nonconducting inorganic materials with higher and lower indices of refraction than the substrate.  Generally produced via electron beam gun, resistive heating or sputtered evaporation (deposition).  

electron-beam (EB) gun evaporation: The process of thin-film deposition whereby electrons boiled off a heated cathode are used to melt the coating material(s). Multisectioned crucibles can be used to apply many materials in one coating process.

equilateral prism: A dispersing type prism typically used for wavelength separation applications.  The three sides are of equal length.  A light ray is twice refracted passing through the prism with total deviation a function of prism material index of refraction, and hence wavelength. 

evaporated coating: Thin precisely controlled layers of solid material(s) deposited on a substrate after being vaporized under high-vacuum conditions.

focal length: The distance between the last surface of a lens, or the front surface of a curved mirror, and the focal plane.

fused silica: See UV grade fused silica. 

FWHM.: Full width at half maximum. The width of the transmitting band of an optical filter, measured at 50% of the peak transmission. See also bandwidth.

index of refraction: The ratio (at a given wavelength) of the speed of light in a vacuum to its velocity in a refractive material.

infrared (IR): The long wavelength Invisible portion of the electromagnetic spectrum between ~2.5 µm and 14.0 µm.

laser quality (LQ) optics: Precision quality optics typically polished to a surface finish or quality of 10/5 scratch/dig and a surface figure of Lambda/10; mainly for medium to high power laser or imaging applications.  Also see Surface Finish & Surface Figure.

magnesium fluoride (MgF2):  Magnesium Fluoride is a birefringent crystal optical material that exhibits high transmission from VUV to IR wavelengths (~110nm to 6.0 µm).   MgF2 is a relatively hard material resistant to mechanical and thermal shock.  MgF2 is typically oriented along the optical axis to avoid birefringent effects and provide maximum transmission.  

micron (µm): A unit of measure in the metric system equal to 10-6 meter. Also abbreviated µ or µ.  Usually used in the measurement of near-infrared (NIR) to far-infrared wavelengths of light (~1.0 µm to 14.0 µm)

multilayer coating: Coating made up of several layers of materials with alternating high and low refractive indices. Varying combinations will produce a variety of coating properties.

nanometer (nm): Unit of length in the metric system equal to 10-9 meter. Usually used in the measurement of vacuum ultraviolet (VUV) to near-infrared (NIR) wavelengths of light. One nm = 10 angstroms.

narrowband AR coating: An AR coating designed to provide increased transmittance and reduced reflectance over a very restricted (narrow) band of wavelengths.

near infrared (NIR): The long wavelength Red to Invisible portion of the electromagnetic spectrum between ~700 nm and 2500 nm (2.5 µm).

neutral density filter: A light filter that decreases (attenuates) the intensity of light without altering the relative spectral distribution.

optical window: Typically flat polished glass or fused silica materials. Generally used for view ports or sealing and or protecting other components within an optical assembly, instrument or laser.  Also see Optics Tutorial.     

parallelism: The angular wedge difference between two opposing optical surfaces. Usually measured as arc minutes or arc seconds.

passband: The range of wavelengths transmitted by an optical filter.

peak transmittance: The maximum transmittance achieved within the passband.

peak wavelength: The wavelength of maximum transmittance within the passband. Differs from the central wavelength only in filters exhibiting asymmetrical band shapes.

plano-concave lens: A lens with one flat (plano) surface and the other surface curved inward (concave).  Typically used to diverge collimated incident light.  Often used to expand light or increase focal lengths in an optical system.  These lenses have negative focal lengths.   

plano-convex lens: A lens with one flat (plano) surface and the other surface curved outward (convex).  Used as a simple imaging lens to focus, collect and collimate light. 

radius (of curvature): The measurement of a curved surface as applied mainly to lenses.   

reflection: The return of light from a surface with no change in its wavelength(s).

right angle prism: A type of 45-90-45 degree prism typically used to bend a beam of light through a right angle with the surfaces forming the 90 degree angle acting as the transmitting faces.  The most common type of prism for many applications.  Also see Optics Tutorial.  

spherical lens: A lens with one or more surfaces formed like a sphere.  Spherical lens surfaces are curved to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.  Also see Optics Tutorial.  

substrate: The underlying material to which an optical coating is applied.  Typically an optical material such as BK7 Glass or UV Fused Silica.  Also see Optics Tutorial.  

surface figure: The accuracy with which an optical surface conforms to its intended shape. Usually measured with an interferometer or reference test plate by forming an interference pattern through variance of the optical path length across the surface. Specifications are usually presented in fractions of a wavelength, typically at the helium/neon laser line of 632.8 nm.  Also see Optics Tutorial. 

surface quality (or finish): Allowable cosmetic flaws in an optical surface by comparison to reference standards of quality. Usually made up of two types of standards defining long defects (such as scratches) and round defects (such as digs & pits).  Also see Optics Tutorial. 

transmittance: The admission of light into a medium with passage of this light through the medium without reflection back to the source or absorption in the medium.

ultraviolet (UV): The short wavelength Violet to Invisible portion of the electromagnetic spectrum between ~400 nm and ~200 nm.

uv grade fused silica: Synthetic form of silicon dioxide made from SiCl4  ,which transmits light over a very broad range of wavelengths from ~ 170 nm to 2.5 µm.  UV grade fused silica has excellent thermal stability and is used as a UV to NIR material for many optical components.  Also see Optics Tutorial. 

vacuum ultraviolet (VUV): The ultra short wavelength Invisible portion of the electromagnetic spectrum where all air absorbs (requiring a vacuum or purged measurement system, hence VUV) between ~200 nm and ~110 nm.  

visible (VIS): The region of the electromagnetic spectrum that can be seen by the human eye. Generally considered to be the ~400 nm to ~700 nm wavelength portion of the spectrum.

wavelength: The physical distance that light travels in one wave cycle. All electromagnetic energy is transmitted (or travels) in waves.

window:  See Optical Window.  Also see Optics Tutorial.


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