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The loss of light as it passes through a material, generally due to its conversion to other energy forms (typically heat)


(anti-reflective) A coating layer with a thickness exactly equal to one quarter of the optical wavelength. The light which is reflected directly on this surface will negatively interfere with the portion of light that is reflected from the boundary face between the thin coating and the glass substrate. For AR-coatings, metal and dielectric materials can be selected.


denotes the wavelength range of a light source. Wideband light appears white, narrow-band light appears colored.

Borofloat® 33

A borosilicate glass from SCHOTT AG which is produced using a micro-float process (special float glass) and features high temperature resistance, good surface quality (floated surfaces) and outstanding optical properties (homogeneity and transmission).

Borosilicate Glass

A high temperature and chemically resistant glass, containing a minimum of 5% boric acid, with a very small thermal expansion coefficient manufactured according to ISO 3585. Only strong lyes, hydrofluoric acid and concentrated phosphoric acid are capable of eroding surfaces of this glass type.


Hollow spaces inside of glass material, typically, of circular cross-section, sometimes forming as a consequence of glass manufacturing. Bubbles are marked in accordance with ISO 10110 standard requirements.

Calcium Fluoride

A cubic monocrystal which is grown using a high-vacuum process to yield good transmission properties for operation from UV to IR. Because of its superior transmittance in the UV-range down to 170 nm and the absence of double refraction, CaF2 is perfectly suited for optics transmitting in the DUV-range.

Cell (cuvette)

A glass or quartz vessel with a precise light path (path length) used to hold liquids, solids or gases for various types of quantitative and qualitative spectroscopic measurement.


designates a method to apply very thin layers of metal onto carrier material, allowing single-layer coating and multiplelayer coating. Notably in the optics industry coating is used to achieve changes in the optical properties of surfaces and boundary faces. Incident light is partly reflected, transmitted or refracted at these surfaces. Coatings are applied in order to obtain a defined transmittance and reflectance. Typically, thin metal layers (e.g. aluminum, silver or gold) are used to convert a transparent substrate into a mirror, or dielectric layers (metal oxides and fluorides) are selected as anti-reflection or reflection coatings, or as beam splitting layers, including any combination of these layers. Further applications of thin layers are surface finishing and surface protection, isolation from heat and cold, electric conductivity and storage of information.


Colorless highly transparent crown glass from SCHOTT DESAG featuring high transmission in the ranges of visible light, IR-light and UV-light.


designates the dependence of the refractive index of an optical medium on the color of light.


also referred to as “deep-UV”, covers the wavelength range below 200 nm.


generally describes the output of radiation energy, i.e. the emission of energy stored in or added to matter in the form of optical radiation. Emission is the reverse process of absorption.


Extreme Ultra-Violet (light) Ultra-violet light that is not visible to the human eye and has an extremely short wavelength. Visible light exhibits wavelengths of between approx. 800 nm (red) and approx. 400 nm (blue), Ultra-violet light has a wavelength smaller than 400 nm. In lithography, EUV has a wavelength of approx 13 nm.

Filter Glass

is distinguished by selective absorption within the range of optical wavelengths. Only certain types of filter glass that work within the visible spectral range show colored light transmission.


may characterize the surface smoothness of a glass object or the process to achieve this level of smoothness. During a firepolishing process, the surface of glass is thermally smoothened without any resulting change in optical properties.

Fused silica

Glass made of quartz


is an amorphously set, non-crystalline substance that is typically generated by melting.


Quartz glass from Heraeus Quarzglas GmbH.


A crucial prerequisite for optical glass, homogeneity describes a state, in which the optical and compositional properties of a substance are uniform through its entire material volume. It is expressed as the maximum variance in refractive power within a given piece of glass.

HR-Coating (high reflectivity)

means one or more strongly reflecting layers that cause superposition and intensification of reflected radiation in order to obtain increased reflectance. Both metal and dielectric layers can be selected as HR material.

HR-Coating (high reflectivity)

means one or more strongly reflecting layers that cause superposition and intensification of reflected radiation in order to obtain increased reflectance. Both metal and dielectric layers can be selected as HR material.


All kinds of local faults in material, typically of a circular crosssection, including stringy knots, small stones, sand and crystals. They are marked in accordance with ISO 10110 standard requirements.

Infrared Radiation

The invisible portion of the electromagnetic spectrum that lies between approximately from 780 nm to 1 mm. For better classification, this type of radiation is subdivided into: Near Infrared, Middle Infrared and Far Infrared.


describes the amount of variance in the refractive index within the volume of an optical component. It is defined as the difference between the maximum and the minimum value of a component’s refractive index.


A phenomenon which is typical of transversal waves (direction of oscillation and direction of propagation are mutually orthogonal). Transversal waves include electromagnetic waves and, hence, also light. Two or more superimposing waves of identical wavelength and identical polarization state will add to or compensate each other.


Instrument that employs the interference of light waves to measure the accuracy of optical surfaces. Essentially, the various types of interferometers rely on the same principle: two or more beams of light are routed through separate optical paths (via mirrors and/or semi-transparent plates) to be recombined at the end. Depending on the amount of difference in optical path length, an interferential pattern will show. This pattern can be rings or stripes.


Refer to “Infrared Radiation”

Irregularity (variance)

describes the amount of variance of spherical surfaces against an ideal ball shape.


is looked upon - by physics - as an electromagnetic wave that propagates (in vacuum) at the speed of light. If the wave travels inside a medium, its propagation will be determined by a specific material constant (composite refractive index). There are two magnitudes characterizing electromagnetic waves: the wavelength (which determines the color of light) and the radiation power (which determines the power which will be transported by the wave per unit area of surface). Optics from Hellma Optik GmbH Jena operates from 157 nm to the infrared range.

Monochromatic Light

means light of a particular wavelength. Laser light is of monochromatic composition. Other artificial light sources and sunlight are polychromatic.

Neutral Density Glass, Neutral Density Filters

Designed to test a photometer for absorption accuracy in the visible spectral range. Featuring a transmittance that is largely uniform and independent of the wavelength, they are also referred to as gray filters. Neutral density filters reduce the intensity of light without altering the relative spectral distribution of the energy.

Optical Glass

A glass that, during manufacture, is carefully controlled with respect to composition, melting , heat treatment, and other processing, to render its optical characteristics, such as its index of refraction, dispersion, transmittance, spectral transmittance, homogeneity, freedom from birefringence, permanence, etc., sufficient for its optical application.

Optical Path Length

Equals, in a medium of a given refractive index, the product of this refractive index and the geometrical path length that was traveled in that medium, thus representing the path which light would travel in the same time if in a vacuum.

Optical Radiation, Optical Spectrum

covers the following spectral ranges: ultraviolet radiation, light (visible) radiation and infrared radiation. Ionizing radiation follows beyond the range of short wavelengths and millimeter waves beyond the range of long wavelengths.


indicates the angle between two surfaces.


Technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection amplification and the detection by optical components and instruments, lasers and other light sources.


is defined as the amount of tolerance which a flat manufactured surface (e. g. by milling or grinding) is allowed to have to qualify as yield (graded according to ISO 10110 standard).


designates the selection of a particular vibration direction of light. With light waves, the vibration direction of electric field strength is orthogonal to a wave’s direction of propagation (transversal or cross wave). Most light sources emit light without a preferential specific direction of vibration. Polarizers can be used to sort out a particular plane from a mixture of included vibration planes; in this case one obtains a linearly polarized electromagnetic wave.

PV-Value (peak to valley)

is defined as the difference between the maximum and the minimum distance of two surfaces. It should, however, be noted that a PV-value allows no conclusion on the distribution of identified faults over a given area.


Return of radiation by a surface, without change in wavelength. The reflection may be specular from a smooth surface, diffuse from a rough surface or from within the specimen, or mixed, a combination of the two.

Reflection (diffuse)

occurs with rough boundary surfaces. Light, even though it may be initially directed, will be retro-reflected in many directions upon reaching a reflecting boundary surface.

Reflection (regular)

occurs where the law of reflection is fulfilled: a reflected and an incident beam form identical angles with the perpendicular of incidence, i.e. the normal to the point of incidence on a reflecting surface. Accordingly, these angles are referred to as angle of reflection and angle of incidence. The incident beam, the reflected beam and the perpendicular of incidence are all located in a common plane, the so-called plane of incidence.


describes the phenomenon of light changing its direction of propagation as it passes from one optical medium to another, crossing the boundary surface between the two media at any angle other than a right angle where the two media have different refractive indexes.

Refractive Index (n)

indicates the strength with which a light beam will be deflected on transferring from a vacuum to a particular medium. It is defined as the relationship of the speed of light in a vacuum to that in the other medium and depends on the wavelength of light, temperature and pressure. Usually, its value is indicated for 20 °C and 1013.3 mbar.

Registration Error (surface shape non-conformity)

designates the distance between a spherical or plane optical surface being tested and the corresponding theoretical reference surface (test glass), measured in a direction orthogonal to the theoretical reference surface which, according to definition, extends parallel to the surface under test, marked by one PV value and one RMS value. It is indicated in units of the wavelength (e. g. λ/4, λ/10) and specified in the ISO 10110 standard.

RMS-Value (root mean square)

describes the mean square deviation of a surface under test against an ideal surface. Because it integrates an area related portion, it provides an important clue to the quality of optics.

Roughness (or: coarseness)

denotes the amount of non-flatness (variance) in the height of a physical surface.


describes the intensity distribution of electromagnetic radiation versus the wavelength. The area of interest for industrial photometry comprises light waves, i.e. visible light, ultraviolet light and infrared light. A prism- or grid-generated spectrum is able to visualize the spectral colors of selected light, including anything from short-wave violet to blue, green, yellow, orange and onto long-wave red light.

Straylight, Light Scattering

occurs when a previously straight beam of light is deflected from its path by an obstacle in an otherwise homogeneous optical medium, which results in a change of optical properties. This physical process is called scattering of light at particles. Light scattering gives rise to opaqueness.


are inhomogeneities of small spatial extent, typically appearing as thread-like or cord-like zones inside a glass volume. These can be visually perceived due to their refractive index that differs from that of the surrounding material. They are marked in accordance with ISO 10110 standard requirements.


is a synthetic fused silica type from Heraeus Quarzglas GmbH.

Surface Defects

Small imperfections within the critical optical area caused by improper handling during or after the manufacturing process, for example, scratches, digs or surface spots that lack final finish-polish quality on the glass surface. They are marked in accordance with ISO 10110 standard requirements.

Surface Quality

refers to the profile of an optical surface und provides a global statistical feature.

Total Internal Reflection

occurs within an optical medium if the angle of incidence, i.e. the angle under which a beam of light hits a boundary surface is greater than the critical angle for total internal reflection.


designates a medium’s transmissivity for optical radiation, i.e. that part of the incident radiant flux which emerges without any change in frequency after transiting a sample volume. Transmission is quantitatively described by the term “spectral transmittance” representing that portion of an incident radiant flux or stream of light which completely transits a transparent component, thus delivering a measure for the “transmission quality” (example) of a type of glass.

Transparency (Transmittance)

Expressed in physical terms, transparency is the capability of matter to allow electromagnetic waves to pass through fully or partially. Transparency is an optical property of a material. It must be distinguished from mere transmission of light, as occurs, for example, with milk glass, with only diffuse translucent light coming through so nothing distinct can be perceived (translucency).

Ultraviolet Radiation

(also abbreviated as UV-radiation) represents the short wave sub-range of optical radiation. UV-radiation covers the wavelength range from 100 nm to 380 nm.


describes the resistance to motion of liquids or molten stock. The opposite of viscosity is brittleness (brittle substances break immediately after the elastic range limit).

Visible Spectrum

is the part of the optical spectrum that comprises the wavelength range of electromagnetic radiation from 380 nm to 780 nm. This range is generally referred to as light. It is the only range where the human eye is able to “see” electromagnetic radiation.


A wavefront is understood to mean any surface in a propagating electromagnetic wave, whose individual points share a common vibrational state (phase).

Wavefront Deformation

occurs when a wavefront is incident on a surface, taking on the shape of this surface and continuing to travel with this shape. Accordingly, wavefront deformation is understood as the variance of an actual wavefront against the nominal wavefront.

Wavelength (λ)

Electromagnetic energy is transmitted in the form of a sinusoidal wave. The wavelength is the physical distance covered by one cycle of this wave; it is inversely proportional to frequency.


A material that does not expand in size when material temperature increases.


is an inorganic non-porous brand of glass ceramics from SCHOTT AG. Consisting of one phase of finely distributed nanocrystallites and one residual glass phase, it provides unique features (extremely low coefficient of thermal expansion, good material homogeneity, chemical resistance, long-term stability and almost non-varying mechanical properties). Zerodur® is fabricated by controlled volume crystallization (ceramization).

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