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Then There Was Light…
The development of fibre-optical systems has caused a small revolution in chemical analysis. The technology makes it possible to carry out photometric measurements not only under laboratory conditions with cells, but through the development of fibre-optical probes, analysis has moved directly to the process measurements. Continuous measurements can be made in situ without sampling. This allows for better control of ongoing processes with much less effort.

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Absorbance, spectral A(λ)
Previously referred to as extinction E(λ). The internal spectral absorbance is the logarithm to the base ten of the reciprocal of the spectral internal transmittance τi (λ). The reading displayed by commercially available photometers usually is the absorbance as measurable variable because it is proportional to the concentration according to Lambert-Beer's law. But it must be noted that the photometer actually measures the Spectral internal transmittance. The logarithmic conversion produces a variably high resolution of the absorbance for different transmittance values. The resolution is particularly good in the high transmittance range. For this reason, in practice the range of 0 up to 0.6 A (corresponds to 100 down to 25 % τ) is preferred.
Absorption A
Attenuation of the radiant flux when optical radiation is passed through a clear medium. Part of the incident light is transmitted to the molecules so that the emergent ray has less energy than the incident ray. The amount of absorption follows Lambert-Beer’s law.
Absorption coefficient, relative, spectral K;(λ)
Previously referred to as extinction coefficient ελ, is a substance constant used to calculate the concentration of the dissolved substance from the spectral absorbance A according to Lambert-Beer's law. It is a function of the wavelength and is determined by the absorption spectrum analysis.
Absorption coefficient, spectral α(λ)
Previously referred to as extinction modulus mλ, is the quotient of the internal absorbance A(λ) and the light path of the medium traversed. The unit is m-1.
Absorption factor, spectral
The spectral absorption factor is the ratio of the entire radiant flux absorbed in an optical medium (e.g. glass) to the incident radiant flux at a certain wavelength. Spectral internal absorbance
Absorption spectrum
Plotting of the absorption spectrum (absorbance as a function of the wavelength) enables the determination of the wavelengths at which the substance concerned absorbs the light energy. The intensity of absorption is used to calculate the concentration according to Lambert-Beer´s law.
Absorption spectrum analysis
Widely used physicochemical analytical method for detecting and determining the concentration of organic and inorganic substances which measures the intensity of the absorption in relation to the wavelength.
Anti-reflection coating
is used to reduce light losses due to reflection at boundary surfaces. During the additive process thin layers are applied to the glass surface, so that a stronger extinction of the light reflected at the boundary surfaces is achieved by interference.
Aperture
is the limitation of the area through which the light bundle passes through the cell. It is important that only that light reaches the detector of the photometer that has traversed the sample. The problem of measured value error due to stray light, which presents a particular problem for semi-micro, micro, and ultra-micro cells, is solved by using black material for the lateral walls surrounding the measuring aperture.
Attenuation
of optical fibres D(λ) is ten times the logarithm to the base ten of the radiant flux ratio between the surface of entrance and the surface of emergence of optical fibres (D(λ )=10 lg (Ωeλ)in /(Ωeλ)ex).
Bandwidth
Wavelength range of a light source. Wideband light appears white, narrowband light coloured.
Borofloat
is a borosilicate glass produced by a floating process. Besides good thermal characteristics, specified in the standard ISO 3585 “Borosilicate glass 3.3” which is also met by the laboratory glasses Duran and Pyrex, Borofloat shows excellent optical properties (homogeneity and transmission) as well as very good fire-polished surfaces.
Borosilicate glass
In addition to silicon dioxide, it also contains a considerable quantity of boron trioxide. The glass properties are positively influenced by adding B2O3.
Cell
Cell Container in which liquids take on an exactly defined form making quantative optical measurements possible.
Centre height
With regard to a cell, the vertical distance, measured from the outer edge of the cell base up to the centre of the aperture.
Chipping
In most cases, it is a shell-shaped chip on a polished edge, which must absolutely be avoided when manufacturing directly fused cells. Chippings reduce the quality of fused edges and can produce undesirable light scatter.
Coefficient of expansion, thermal
is the relative change in length of a rod as it is heated by 1° C (or 1 K). Among the different glasses, borosilicate glass distinguishes itself by a low coefficient of expansion of α = 3.3 x 10-6K-1 (Duran, Pyrex, Borofloat). Even lower values can be achieved with doped quartz glass (Corning ULE) and glass ceramics (SCHOTT Zerodur).
Colorimetry
Quantitative analysis method based on comparing the absorption of the solution being analysed and the solutions of known concentration (reference standards).
Colour
Colour or colouration of liquids in the sense of photometric analysis designates their ability to alter the spectral composition of the visible light. As a result of the absorption of certain wavelengths of the light passing through it, the liquid exhibits the corresponding complementary colour. Spectrum. In spectral analysis, the interest is focussed on the colouration produced by dissolved substances.
Compact cell
Flow-through cell, the connection of the tubes to the cell being accomplished by means of screw connections.
Critical angle for total internal reflection
The angle at which a ray of light from an optically denser medium strikes a boundary surface to a medium with lower refractive index in order to be refracted along the boundary surface. At smaller angles, the incident ray undergoes refraction into the less refractive medium, at greater angles, it experiences total internal reflection back into the denser medium.
Cytometer cell
is used for measurements in a flow cytometer, typical dimensions of the capillary of a cytometer cell are a square cross section of 0.25 x 0.25 mm. All surfaces – even the internal surfaces – are polished.
Detector
Electronic component used to measure light intensity Photometer.
Differential refractometer cells: Refractometer cells
Refractometer cells are used to determine the refractive index and have two prismatic chambers. One chamber is filled with a liquid of known refractive index, the other chamber with the liquid of unknown refractive index which is to be determined. The incident ray of light experiences a deflection and the difference between the two refractive indices can be calculated from this angle.
Differential refractometer cells: Refractometer cells
Refractometer cells are used to determine the refractive index and have two prismatic chambers. One chamber is filled with a liquid of known refractive index, the other chamber with the liquid of unknown refractive index which is to be determined. The incident ray of light experiences a deflection and the difference between the two refractive indices can be calculated from this angle.
Dual-beam method
Photometric concept in which two beams (measurement and reference beam) are used, compare single-beam method Photometer.
Duran
Brand of the company SCHOTT Glas, Mainz with standard characteristics as per ISO 3585. Duran is a pure borosilicate glass with reduced sensitivity to temperature changes.
Dye-laser cell
Is used for a dye laser to receive the dye solution in which the laser radiation is generated. The cell is located inside the optical resonator.
Emission
generally designates the emission of radiant energy, that is the release of energy stored in material or imparted to it, in the form of optical radiation. Emission is the opposite of absorption.
Extinction coefficient ελ
Earlier designation for relative spectral absorption coefficient K(λ).
Extinction E(λ)
Earlier designation for spectral absorbance A(λ). Attenuation of the radiation by absorption and scatter (scattered light) as it passes through the material.
Extinction modulus mλ
Earlier designation for spectral, absorption coelcient α(λ).Previously referred to as extinction coefficient ελ, is a substance constant used to calculate the concentration of the dissolved substance from the spectral absorbance A according to Lambert-Beer’s law. It is a function of the wavelength and is determined by the absorption spectrum analysis.
FEP
Fluorinated ethylene propylene, extremely corrosionand solvent-resistant synthetic material used for tubings.
Fibre buffer
consists of one or several materials and is used to mechanically isolate the optical fibre.
Fibre bundle
Bundle of several optical fibres that are not interconnected.
Fibre cladding
That part of the optical fibre surrounding the fibre core and having a lower refractive index than the core.
Fibre core
Central region with high refractive index of an optical fibre in which the light is guided.
Fibre core diameter
Diameter of the central region of an optical fibre which can be distinguished from that of the fibre cladding by the higher refractive index.
Fibre diameter
Diameter of the optical fibre consisting of fibre core and fibre cladding.
Fibre optics
Branch of optics dealing with the transmission of light and images by fibre-optic elements (optical fibre).
Fibre-optic cable
Fibre bundle or individual fibre with extruded protective plastic buffer.
Filling volume
is the sample quantity that must be introduced into a cell having a small measuring chamber in order to fill up the aperture completely
Filter glasses
distinguish themselves by selective absorption in the optical wavelength range. Only some of these glasses appear coloured because the filtering effect lies within the visible spectral range (visible spectrum).
Fire polish
refers to both the smoothing of a glass object's surface developed during shaping under the effect of heat and also the process used to achieve this smoothing. Pressed or pulled glass objects are for example “fire polished”.
Flatness
indicates which deformations are allowed for a plane wave being reflected or transmitted by an uneven surface.
Flow cytometer
These devices help to count and measure cells or particles as they flow through a capillary because of the interaction with (laser) light. Qualitative and quantitative conclusions on the type and quantity of the particles can be drawn from the measurement of absorption, fluorescence, and scatter (scattered light
Fluorescence
Form of luminescence of solid, gaseous or liquid substances that, following excitation with light, give off part of the absorbed energy in the form of longer wavelengths.
Fluorescence measurement
Photometric measuring principle based on detecting the light emitted as a result of fluorescence. The excitation light passes through the sample containing the fluorescent substance. This substance converts part of the incident light of wavelength λ into fluorescent light of a greater wavelength λ1. The light created this way radiates in all directions and can be measured with a detector e.g. orthogonal to the axis of the incident light. For low concentrations, the radiant flux of fluorescent light is proportional to the concentration of the fluorescent substance. The main reasons for the use of fluorescence measurement are higher selectivity and sensitivity compared with absorption measurement.
Focussing, hydrodynamic
designates a flow technique used with flow cytometers through which the cells of a cell suspension will be focussed in one line, i.e. the channel axis of a cytometer cell.
Glass
is an inorganic product of fusion that is cooled and solidified without noticeably crystallising (“solidified melt”).
Glass fibres
Thin filaments (diameter less than 100 µm) made of glass. They are drawn from the melt and because of the extreme chilling conditions prevailing during their production their properties differ from those of compact glass. In particular, their tensile strength is very high.
Glass, optical (Optical glass)
Optical glass is a clear glass with specified values of the refractive indices. It should be as free as possible from bubbles, knots, stones, striae, and stress.
Holmium oxide glass filter
Slightly yellowish filter, is used to check the wavelength accuracy in the ultraviolet and visible spectral range of a spectral photometer.
Homogeneity
Important prerequisite for optical glasses. It is expressed as the maximum variation of the refractive index nd within a piece of glass.
Index of refraction (Refractive Index)
The refractive index “n” indicates how strongly a ray of light is refracted when passing from vacuum into the medium concerned. It is defined as the ratio of the velocity of propagation in vacuum and in the medium. It depends on the wavelength of the light, the temperature and the pressure. Usually, its value is indicated for 20 °C and 1013.3 mbar.
Index of refraction Refractive Index
The refractive index “n” indicates how strongly a ray of light is refracted when passing from vacuum into the medium concerned. It is defined as the ratio of the velocity of propagation in vacuum and in the medium. It depends on the wavelength of the light, the temperature and the pressure. Usually, its value is indicated for 20 °C and 1013.3 mbar.
Infrared radiation
also abbreviated to IR radiation, is invisible electromagnetic ? radiation in the long-wavelength range of the optical spectrum. The wavelength range of IR radiation is between 780 nm and 1 mm. For further identification, this radiation is divided into near infrared (NIR), mid and far IR. Infrared radiation causes heat sensations, therefore it is often also called heat radiation.
Infrared range Infrared radiation
also abbreviated to IR radiation, is invisible electromagnetic radiation in the long-wavelength range of the ? optical spectrum. The wavelength rangeof IR radiation is between 780 nm and 1 mm. For further identification, this radiation is divided into near infrared (NIR), mid and far IR. Infrared radiation causes heat sensations, therefore it is often also called heat radiation.
Interference
is a characteristic phenomenon of transverse waves (the direction of oscillation is perpendicular to the direction of propagation). Transverse waves include electromagnetic waves and therefore also light. Two or more superimposed waves of the same wavelength and the same polarisation add or compensate each other, depending on the phase position and amplitude of the electric field strength.
Internal transmittance
The spectral internal transmittance is the ratio of the radiant flux emerging from the optical medium to the radiant flux penetrating the medium at a certain wavelength. In contrast to the spectral internal transmittance, the reflection losses occurring at the measuring cell windows must be eliminated here. In practice, this is done by comparison with a measurement of the empty cell.
Internal transmittance, spectral
Ratio of the transmitted light to the incident light at a certain wavelength. Besides the spectral absorption factor (previously referred to as extinction), the spectral internal transmittance is indicated by photometers.
Internal transmittance, spectral T(λ)
Ratio of the transmitted light to the incident light at a certain wavelength. Besides the spectral absorption factor (previously referred to as extinction), the spectral internal transmittance is indicated by photometers.
IR radiation (Infrared radiation)
also abbreviated to IR radiation, is invisible electromagnetic radiation in the long-wavelength range of the optical spectrum. The wavelength rangeof IR radiation is between 780 nm and 1 mm. For further identification, this radiation is divided into near infrared (NIR), mid and far IR. Infrared radiation causes heat sensations, therefore it is often also called heat radiation.
Lambert-Beer's law
Named after the two scientists Johann Heinrich Lambert (1728-1777) and August Beer (1825- 1863), states the correlation between the absorbance A, the light path traversed, and the concentration of the absorbent substance. “The absorption is proportional to the number of absorbent molecules and to the light path traversed”. The proportionality constant is called relative spectral absorption coefficient K(&lambda) and is a substancespecific function of the wavelength. In the appropriate concentration range and if using monochromatic light, Lambert-Beer's law is reliable with great accuracy.
Light
In physics, light is regarded as an electromagnetic wave propagating (in a vacuum) with the velocity of light. If the wave passes through a medium, its propagation is determined by a material constant (the complex refractive index). Two variables characterise the the wave per unit area). The wavelength of visible light ranges from 400 nm (violet) to 700 nm (red).electromagnetic wave: the wavelength (determining the colour of light) and the radiant flux (determining the energy transported by the wave.
Light beam cross-section
Cross-section of the concentrated light in the sample compartment (where the cell is) of a photometer.
Light path
Lambert-Beer's law states that the concentration of a sample can be calculated from the absorption value if the light path is known. This is why cells must be produced with a very high accuracy of light path or the individual light path of a cell must be determined later and must be confirmed by a test certificate.
Light, monochromatic (Monochromatic light)
Light consisting of waves of one wavelength only is called monochromatic. One then speaks of a pure spectral colour. The light from the sun and from artificial light sources (exception: laser) is polychromatic. It contains waves of different wavelengths.
Luminescence
General term for certain cases of light emission. If the excitation is caused by light, it is called photoluminescence. This is the case if molecules are excited by the absorption of optical radiant energy and emit radiation when they return to lower energy levels. Fluorescence: Immediately extinguishing luminescence. Phosphorescence: Luminescence with noticeable persistence.
Macro cell
Either a rectangular cell with an interior wider than 5 mm or a cylindrical cell with a diameter of the interior larger than 8 mm. The macro cell has largely gained acceptance as the standard cell.
Measuring chamber volume
The cell volume being traversed by the light.
Measuring range
Part of the scale range of a photometer in which the guaranteed conditions of accuracy shall be valid.
Micro cell
Either a rectangular cell with an inside width up to 2 mm or a cylindrical cell with a diameter of the interior up to 5 mm.
Minimum bend radius
Smallest possible radius that an optical fibre may be bent without being damaged.
Neutral glass (Neutral glass filter)
filters are used to check the absorption accuracy in the visible spectral range of a photometer. They show a largely wavelength-independent constant transmission in a limited spectral range; other designations are neutral glass or neutral density glass.
Neutral glass filter
filters are used to check the absorption accuracy in the visible spectral range of a photometer. They show a largely wavelength-independent constant transmission in a limited spectral range; other designations are neutral glass or neutral density glass.
Neutral glass filters
filters are used to check the absorption accuracy in the visible spectral range of a photometer. They show a largely wavelength-independent constant transmission in a limited spectral range; other designations are neutral glass or neutral density glass.
NIR (infrared radiation range)
also abbreviated to IR radiation, is invisible electromagnetic ? radiation in the long-wavelength range of the optical spectrum. The wavelength range of IR radiation is between 780 nm and 1 mm. For further identification, this radiation is divided into near infrared (NIR), mid and far IR. Infrared radiation causes heat sensations, therefore it is often also called heat radiation.
Optical Fibres
are able to guide optical radiation by means of total internal reflection or continuous refraction because of their design.
Optical glass
Optical glass is a clear glass with specified values of the refractive indices. It should be as free as possible from bubbles, knots, stones, striae, and stress.
Optical light path
In the field of optics, the optical light path (optical thickness) is defined as the product of refractive index and geometrical thickness of the optical medium.
Optical radiation (Optical spectrum)
The optical spectrum comprises the spectral ranges ultraviolet radiation, light (visible radiation), and infrared radiation. The short wavelengths are followed by the ionising radiation, the long wavelengths by millimetre waves.
Optical spectrum
The optical spectrum comprises the spectral ranges ultraviolet radiation, light (visible radiation), and infrared radiation. The short wavelengths are followed by the ionising radiation, the long wavelengths by millimetre waves.
Optical waveguide
Dielectric waveguide consisting of a fibre core made of optically transparent material having a low attenuation (mostly quartz glass) and a fibre cladding made of optically transparent material having a lower refractive index than the core. It is used to transmit signals using light waves.
PEEK tubes
are short connecting pipes made of Poly- EtherEtherKetone which are glued into flow-through cells. PEEK is a chemically resistant, pressure resistant synthetic material with good mechanical characteristics. It is often used as an alternative to stainless steel.
Photodegradation (Solarisation)
mainly is the designation for the permanent or sometimes also the reversible discolouration of certain glasses under the influence of radiation with a high UV percentage. This discolouration is combined with a reduction of the transmission in the transmission range.
Photometer
In the broadest sense, a photometer is any device used to measure light intensity. With reference to industrial photometry, it is the general term for instruments that measure absorption, fluorescence, and scattered light intensity. In all cases, the objective is to expose the medium in question to light and then to measure the intensity of the light resulting from the corresponding phenomenon.
Photometry
Measurement of light intensities using photometers. In the field of analysis, a distinction is made between the measurement of absorption, of scatter (scattered light) and of fluorescence.
Polarimetrically measured
Polarimeter cells and cells for the measurement of circular dichroism must be free from stress birefringence. This is checked by measuring the empty cell in a polarimeter. Cells with a rotation of the polarisation plane of less than 0.01° are designated as stress-free.
Polarisation
For light waves, the direction of oscillation of the electric field strength is perpendicular to the direction of propagation of the wave (transverse wave). Most light sources emit light with no preference for a certain direction of oscillation. Polarisers help to separate a certain oscillation plane from the mixture of different planes. The result is linearly polarised electromagnetic waves.
Polychromatic light
Light consisting of several wavelengths. The light from the sun and artificial light sources (exception: laser) is polychromatic. The spectral distribution (spectrum) indicates the relative intensity of the individual wavelengths present. The wavelength range is called bandwidth: Wideband light appears whitish, narrowband light coloured (monochromatic light).
Primary standard
The standards offered by Hellma to check photometers are measuring devices certified in compliance with ISO 9001:2000 that must be traceable to international or national primary standards. Such primary standards can be bought (from the NIST) or calibrated (by the PTB).
Prism cells
are used to measure the refractive indices of liquids. The deflection of a ray of light as it traverses a prism depends on the refractive index of the prism material. For this reason, these cells are designed as hollow prisms.
PTFE
PolytEtraFluorEthylene is resistant to almost all chemicals. Operational temperature range from –200 °C to +300 °C. The surface is not adhesive.
Pyrex
is a glass manufactured by the company Corning, USA and like Duran, is a pure borosilicate glass having a low sensitivity to temperature changes. It conforms to the standard ISO 3585 like Duran.
Quartz glass
consists of SiO2. Its density is approximately 2.2 g/cm3, refractive index 1.460. Worth mentioning is its low coefficient of expansion. Its high thermal shock resistance is attributable to this. The high-temperature softening point of quartz glass enables the production of devices that withstand temperatures of up to 1400 °C. Further useful characteristics of quartz glass are a high internal transmittance between approx. 200 to 4000 nm, good resistance to chemicals, and low electric conductivity. A disadvantage of quartz glass is its very difficult machinability.
Radiation range, near infrared (NIR)
Infrared radiation also abbreviated to IR radiation, is invisible electromagnetic radiation in the long-wavelength range of the optical spectrum. The wavelength range of IR radiation is between 780 nm and 1 mm. For further identification, this radiation is divided into near infrared NIR), mid and far IR. Infrared radiation causes heat sensations, , therefore it is often also called heat radiation
Radiation range, visible (Visible spectrum)
is a part of the optical spectrum. It comprises the wavelength range of electromagnetic radiation from 380 up to 780 nm. This range is generally called light. The human eye is only able to “see” electromagnetic radiation in this range.
Reflectance, spectral (Spectral reflectance)
The spectral reflectance is the ratio of the reflected radiant flux to the incident radiant flux at a certain wavelength.
Reflection
is the abrupt change in direction when radiation strikes the boundary surface between two different media without a change of the radiation's wavelength. A distinction is made between regular reflection and diffuse reflection.
Reflection, diffuse
Diffuse reflection means that light, even if it is directed at the reflecting boundary surface, is reflected in many directions.
Reflection, regular
For regular reflection, the law of reflection applies: the reflected ray and the incident ray form equal angles being measured from the normal on the reflecting surface at the point of incidence. These angles are correspondingly designated as angle of reflection and angle of incidence. Incident ray, reflected ray, and normal lie on one plane, the so-called plane of incidence.
Refraction of light
is the phenomenon that light changes its direction of propagation as it passes from one optical medium to another if it does not pass the boundary surface between the two media vertically and the two media have different refractive indices.
Refraction of light
is the phenomenon that light changes its direction of propagation as it passes from one optical medium to another if it does not pass the boundary surface between the two media vertically and the two media have different refractive indices.
Remission (Reflection, diffuse)
Diffuse reflection means that light, even if it is directed at the reflecting boundary surface, is reflected in many directions.
Scale range
Range of measured values (smallest up to largest measurable value) that a photometer can cover. In most cases it is divided into measuring ranges.
Scatter losses
In addition to the absorption of radiation when traversing an optical medium, such as glass, scatter (scattered light) can also cause an intensity reduction of the transmitted radiation.
Scattered light
In an optically homogeneous medium, the light progresses in a straight line. Any change of the optical properties by an obstacle will deflect the ray of light from its path. This physical process is called scattering of light by particles. Light scattering is the cause for turbidity.
Scattered light
In an optically homogeneous medium, the light progresses in a straight line. Any change of the optical properties by an obstacle will deflect the ray of light from its path. This physical process is called scattering of light by particles. Light scattering is the cause for turbidity.
Scattered light measurements, cells for
Usually, these are cylindrical cells that have to meet high requirements as regards the existence of scratches, bubbles, and striae.
Scattered light measurements, cells for
Usually, these are cylindrical cells that have to meet high requirements as regards the existence of scratches, bubbles, and striae.
Semi-Micro cell
Either a rectangular cell with an inside width of 2 to 5 mm or a cylindrical cell with a diameter of 5 to 8 mm.
Single-beam method
Photometric concept in which only one light source and only one detector is used ( dual-beam method, photometer).
Solarisation
mainly is the designation for the permanent or sometimes also the reversible discolouration of certain glasses under the influence of radiation with a high UV percentage. This discolouration is combined with a reduction of the transmission in the transmission range.
Solarisation
mainly is the designation for the permanent or sometimes also the reversible discolouration of certain glasses under the influence of radiation with a high UV percentage. This discolouration is combined with a reduction of the transmission in the transmission range.
Spectral reflectance
The spectral reflectance is the ratio of the reflected radiant flux to the incident radiant flux at a certain wavelength.
Spectrally measured
On request, cells can be put together in a set in which all cells, measured while empty, show the same transmittance value measured at a certain ? wavelength. Because of the validity of Lambert- Beer's law, the light paths of sets of cells with small light paths (below 1 mm) should be as similar as possible.
Spectrum
Distribution of the intensity of electromagnetic radiation as a function of wavelength. The interesting range for industrial photometry comprises the light waves, i.e. the visible, the ultraviolet, and the infrared light. Spectra produced by prisms or gratings make the spectral colours of the used light visible, starting from shortwave violet and on through blue, green, yellow and orange to long-wave red.
Stray light (extraneous light)
is the light in a photometer striking the receiver and producing a measuring signal without traversing the sample. Sometimes, a distinction is made between stray light which e.g. traversed the thick lateral walls of a semi-micro cell and extraneous light which originates from an external light source and often reaches the receiver by means of reflection at the walls. Both terms are also often summarised under the designation scattered light.
Stray light (extraneous light)
is the light in a photometer striking the receiver and producing a measuring signal without traversing the sample. Sometimes, a distinction is made between stray light which e.g. traversed the thick lateral walls of a semi-micro cell and extraneous light which originates from an external light source and often reaches the receiver by means of reflection at the walls. Both terms are also often summarised under the designation scattered light.
Striae
are in most cases threadlike defects in the glass that are apparent because of their different refractive index.
SUPRASIL
is a synthetic quartz glass manufactured by Heraeus Quarzglas GmbH.
Surface defects
Scratches and holes as well as nonpolished spots on the glass surface.
Total internal reflection
Reflection within a medium if the angle of incidence on the boundary surface of a ray of light is larger than the critical angle for total internal reflection.
Transmission
Transparency of a medium to optical radiation. Designates the portion of the incident radiant flux that emerges again after traversing the sample. Transmission is quantified by the spectral internal transmittance.
Transmittance (Transparency)
Also called transparency. Property of liquid, solid or gaseous substances that enables light to pass through them partially or totally. The spectral internal transmittance is the ratio of transmitted radiant flux to the incident flux.
Transparency (Transmittance)
Also called transmittance. Property of liquid, solid or gaseous substances that enables light to pass through them partially or totally. The spectral internal transmittance is the ratio of transmitted radiant flux to the incident flux.
Triangular cells
are used for fluorescence measurements. The cross-sectional area is triangular in shape. The excitation beam hits the hypotenuse area while the fluorescent light is detected at right angles to the incident ray of light.
Turbidity
Reduction of the optical range in a transparent medium by scatter radiation and absorption caused by optically denser inclusions, e.g. particles. This phenomenon is quantified by the turbidity value.
Turbidity measurement
A measuring procedure to determine the concentration of suspended particles in a liquid medium. This procedure does not measure the intensity of the scattered light, but the intensity decrease of the primarily radiated light.
Turbidity value
The turbidity value quantifies the qualitative phenomenon of turbidity. The objective of the turbidity measurement is to obtain information on the concentration of scattering particles.
Tyndall scatter
“The intensity of light scattered at the particles is proportional to the number of particles in a unit volume and thus also proportional to their concentration.”
Ultra-high vacuum
Pressure of less than 10-10 mbar.
Ultra-micro cell
A rectangular cell the interior of which is designed so that with a light path of 10 mm a measuring volume of less than 160 µl results. These cells fit into any standard cell holder.
Ultraviolet radiation
Also abbreviated as UV radiation, is the short-wave range of optical radiation. UV radiation extends on a wavelength range between 100 and 380 nm.
UV light guide
Light guide made of UV-transparent materials
UV radiation (Ultraviolet radiation)
Also abbreviated as UV radiation, is the short-wave range of optical radiation. UV radiation extends on a wavelength range between 100 and 380 nm.
VIS radiation
Alternative designation for visible radiation Visible spectrum.
VIS range (Visible spectrum)
is a part of the optical spectrum. It comprises the wavelength range of electromagnetic radiation from 380 up to 780 nm. This range is generally called light. The human eye is only able to “see” electromagnetic radiation in this range. the dependence of the shear stress acting in a body as it is deformed on the deformation speed.
Viscosity
or tenacity of a body is the property characterising the dependence of the shear stress acting in a body as it is deformed on the deformation speed.
Vycor
is a type of glass produced by the company Corning, USA. It contains 96% SIO2- and is manufactured using a special process. It has a low thermal expansion and is used for the visible range and the NIR range.
Wave number
is the reciprocal of the wavelength, indicating the number of wavelengths per cm.
Wavelength
The distance between two equal, adjacent points of a wave at a certain point in time is called wavelength.
Wavelength
The distance between two equal, adjacent points of a wave at a certain point in time is called wavelength.

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