ISO 18115-3:2022
(Main)Surface chemical analysis — Vocabulary — Part 3: Terms used in optical interface analysis
Surface chemical analysis — Vocabulary — Part 3: Terms used in optical interface analysis
This document defines terms for surface chemical analysis in the area of optical interface analysis including ellipsometry, Raman spectroscopy and nonlinear optical techniques as well as general optical terms.
Analyse chimique des surfaces — Vocabulaire — Partie 3: Termes utilisés dans l'analyse des interfaces optiques
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 18115-3
First edition
2022-06
Surface chemical analysis —
Vocabulary —
Part 3:
Terms used in optical interface
analysis
Analyse chimique des surfaces — Vocabulaire —
Partie 3: Termes utilisés dans l'analyse des interfaces optiques
Reference number
© ISO 2022
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 General terms . 1
3.2 Terms related to properties of light. 5
3.3 Terms related to optical properties due to interactions with media . .12
3.4 Terms related to ellipsometry . 18
3.5 Terms related to Raman spectroscopy . 20
3.6 Terms related to nonlinear optical technique terms . 25
Bibliography .31
Index .33
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 201, Surface chemical analysis,
Subcommittee SC 1, Terminology.
A list of all parts in the ISO 18115 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
Optical spectroscopies and surface chemical analysis, in general, are important areas which involve
interactions between people with different backgrounds and from different fields. Those conducting
optical spectroscopy on surfaces can be materials scientists, chemists, physicists or biologists and
might have a background that is primarily experimental or primarily theoretical. Those making use of
the data and results extend beyond this group into other disciplines.
ISO 18115-1 extend from the techniques of electron spectroscopy and mass spectrometry to general
spectrometry terms and X-ray analysis. The terms covered in ISO 18115-2 relate to scanning-probe
microscopy.
This document covers terms used in optical spectroscopies. This includes terms related to general
terms, properties of light and optical properties of materials. In terms of techniques, there is a focus on
terms related to Raman spectroscopy, ellipsometry and nonlinear optical techniques.
The wide range of disciplines and the individualities of national usages have led to different meanings
being attributed to particular terms and, again, different terms being used to describe the same concept.
To avoid the consequent misunderstandings and to facilitate the exchange of information, it is essential
to clarify the concepts, to establish the correct terms for use, and to establish their definitions.
The terms are given in alphabetical order, classified under 3.1 general terms, 3.2 properties of light,
3.3 optical properties of materials, 3.4 ellipsometry terms, 3.5 Raman spectroscopy terms and 3.6
nonlinear optical technique terms. The terms in each clause are not always mutually exclusive and
some terms placed in one clause can equally belong in another.
v
INTERNATIONAL STANDARD ISO 18115-3:2022(E)
Surface chemical analysis — Vocabulary —
Part 3:
Terms used in optical interface analysis
1 Scope
This document defines terms for surface chemical analysis in the area of optical interface analysis
including ellipsometry, Raman spectroscopy and nonlinear optical techniques as well as general optical
terms.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1 General terms
3.1.1
background signal
signal present at a particular position, energy, mass or wavelength due to processes or sources other
than those of primary interest
3.1.2
CCD detector
semiconductor device that converts light into an electrical signal
Note 1 to entry: When a photon is absorbed by the detector, a single electron is released. Electrodes covering
the chip surface hold these electrons in place in an array of wells, or pixels, such that during exposure to light, a
pattern of charge builds up that corresponds to the pattern of light.
3.1.3
compensator
retardation plate of fixed or variable optical path length difference used for introducing a light path
difference between two beams or to compensate the optical path length that can cause unwanted
dispersion or time-delay
Note 1 to entry: See also retardation plate/wave plate (3.1.34).
[SOURCE: ISO 10934:2020, 3.1.27, adapted]
3.1.4
confocal optical microscopy
optical microscopy in which, light is suppressed from out-of-focus planes using one or more pinholes
such that only light from a confocal volume is detected
Note 1 to entry: An image of an extended area is formed via scanning.
Note 2 to entry: The confocal principle leads to improved contrast and axial resolution by suppression of light
from out-of-focus planes.
3.1.5
confocal volume
effective volume that is in focus around a point in the object which gives rise to the detected signal or
image in confocal microscopy
[SOURCE: ISO 10934:2020, 3.3.10.8, modified — The phrase “detected signal or” has been added.]
3.1.6
depth of field
region where the sharpness of the edges of the images reaches a pre-set optimum
[SOURCE: ISO 26824:2013, 8.16, modified — "the" has been replaced by "a" prior to "pre-set optimum".]
3.1.7
depth of focus
axial depth of the space on both sides of the focal plane, within which the image or signal appears
acceptably sharp, while the positions of the object plane and of the objective are maintained
Note 1 to entry: The method to determine when the image or signal is acceptably sharp depends on the microscopy
or spectroscopy method. For example, in confocal Raman microscopy, the depth of focus can be determined as
when the signal does not decrease by more than 87 % (1/e ) compared to the maximum signal exactly at the
object position.
[SOURCE: ISO 19262:2015, 3.68, modified — Note 1 to entry has been added.]
3.1.8
diffraction grating
set of regularly repeating structures which, when illuminated, produce, by reflection or transmission,
maxima and minima of intensity as a consequence of interference
Note 1 to entry: These maxima and minima vary in position according to wavelength. Radiation of any given
wavelength may thus be selected from interference pattern allowing the grating to be used for producing
monochromatic light.
[SOURCE: ISO 10934:2020(en), 3.1.42, modified — "diffraction" has been deleted as a consequence and
Note 1 has been reworded slightly.]
3.1.9
dipole moment
vector quantity describing the separation of electric charges where the direction is from negative to
positive charge
Note 1 to entry: When an atom or molecule interacts with an electromagnetic wave, it can undergo a transition
from an initial to a final state of energy difference through the coupling of the electromagnetic field to the
transition dipole moment. When this transition is from a lower energy state to a higher energy state, this results
in the absorption of a photon. A transition from a higher energy state to a lower energy state results in the
emission of a photon.
3.1.10
edge filter
optical filter that rejects light above or below a specific wavelength but transmits light outside that
criterion
Note 1 to entry: Depending on whether the transmitted part contains the longer or shorter wavelengths, the
edge filter is called a long wave pass (LWP) or a short-wave pass (SWP) filter, respectively.
Note 2 to entry: In Raman spectroscopy, an edge filter is used to reject Rayleigh scattering (3.3.38) but permit
measurement of either Stokes (3.5.25) or anti-Stokes (3.5.1) Raman scattering.
Note 3 to entry: In reality, edge filters have a narrow transition width. Edge filters with a very narrow transition
width are also available and are known as razor edge filters.
3.1.11
fluorophore
molecular entity that emits fluorescent light when excited by a specific range of wavelengths of light
3.1.12
goniometer
instrument that either measures an angle or allows an object to be rotated to a precise angular position
3.1.13
half-wave plate
half-wave compensator
optical device which alters the polarization state of light travelling through the device by π
3.1.14
Jones matrix
two by two matrix that is used to represent the operation of an optical element such as a polarizer on
the polarization state of light
Note 1 to entry: Fully polarized light is represented by a Jones vector (3.2.17).
3.1.15
lock in amplifier
type of amplifier that extracts a signal from a complex waveform at the same frequency as that of a
second carrier wave
3.1.16
monochromator
optical device that transmits a light beam with a certain wavelength within a wider range of
wavelengths available at the input
Note 1 to entry: The bandwidth is defined by the spectral purity (3.1.33)
3.1.17
neutral density filter
filter having uniform absorption throughout the range from near ultraviolet to near infrared radiation,
thus reducing the light intensity without altering spectral distribution
[SOURCE: ISO 6196-6:1992, 06.01.13]
3.1.18
notch filter
optical filter that attenuates light with a specific narrow frequency range while passing all other
frequencies unalterated
Note 1 to entry: In Raman spectroscopy, a narrow notch is used to reject Rayleigh scattering (3.3.38) but permit
measurement of both Stokes (3.5.25) and anti-Stokes Raman scattering (3.5.1).
3.1.19
numerical aperture
NA
product of the refractive index of the medium in which the lens is working, n, and the sine of one-half of
the angular aperture of the lens, θ
Note 1 to entry: The numerical aperture is given by NA = nsinθ, where 2θ is the full angular aperture of the lens.
[SOURCE: ISO 18115-2:2021, 5.93, modified — Note 2 has been deleted.]
3.1.20
objective lens
combination of several lenses in a common mounting which, together with the focusing lens, projects a
real reversed image of the object in the image plane
[SOURCE: ISO 9849:2017, 3.2.20]
3.1.21
optical modulator
optical device that imposes modulation on a light beam
Note 1 to entry: This modulation can be for example in phase, frequency or amplitude.
Note 2 to entry: Examples include electro-optical modulator or photo acoustical modulator.
3.1.22
peak height
distance between the peak maximum and the background
Note 1 to entry: The method used to determine the background should be carefully considered and specified.
[SOURCE: ISO 7941:1988, 4.3.2, modified — Reworded slightly.]
3.1.23
peak shape
form of a spectral feature that can typically be described by a mathematical function and parameters
such as spectral position, height, and width
Note 1 to entry: Examples of the mathematical function include Gaussian, Lorentzian, PearsonVII and Voigt
functions.
3.1.24
peak width
width of a peak at a defined fraction of the peak height
Note 1 to entry: Any background subtraction method used should be specified.
Note 2 to entry: The most common measure of peak width is the full width of the peak at half maximum (FWHM)
intensity.
Note 3 to entry: For asymmetrical peaks, convenient measures of peak width are the half-widths of each side
of the peak at half maximum intensity. Other parameters that can be measured are skewness, the amount and
direction of skew or departure from horizontal symmetry and kurtosis which is a measure of how tall and sharp
a peak is.
3.1.25
photobleaching
loss of optical fluorescence (3.2.15) in a fluorescent molecule due to overexposure with irradiating light
3.1.26
photodetector
device that converts light into an electrical signal
Note 1 to entry: Examples include photodiode, photomultiplier, CCD and CMOS.
3.1.27
photomultiplier tubes
photomultipliers
PMTs
electronic device for amplifying and converting light pulses into measurable electrical signals
Note 1 to entry: They can be used for the collection of, for example, confocal Raman, CARS (3.5.3), two photon
fluorescence, TPEF (two photon excitation fluorescence) (3.2.46) and second harmonic generation (3.6.27).
[SOURCE: ISO 772:2011(en), 1.163, modified — Note 1 has been added.]
3.1.28
polarizer
material which only transmits the component of a light wave which is oscillating in a particular
direction
[SOURCE: ISO 23713:2005, 3.2, modified]
3.1.29
quarter-wave plate
optical device which changes the polarization state of light travelling through the device by π/2
3.1.30
selection rules
set of restrictions governing the allowedness of transitions of a system from one quantum state to
another
Note 1 to entry: The selection rules may differ according to the technique used to observe the transition for
example between infrared spectroscopy and Raman spectroscopy.
3.1.31
silicon diode detector
photodiode that converts the light into electrical current
Note 1 to entry: These types of detectors can be used for SRS (3.5.24) and it is a type of photodetector (3.1.26).
3.1.32
solid angle
Ω = A/r
where
A is the area of the included surface of a sphere in a cone with its apex at the centre of the sphere;
r is the radius of the sphere
Note 1 to entry: The solid angle is the three-dimensional angle, e.g. the cone of light from a point source.
Note 2 to entry: Solid angles are expressed in steradians (sr).
[SOURCE: ISO 80000-3:2006, 3-6 and notes adapted from ISO 4007:2018, 3.4.13
3.1.33
spectral purity
indication of the monochromaticity of a given light sample
3.1.34
wave plate
retardation plate
optical device generally consisting of a piece, or pieces, of optically anisotropic material with plane
faces, to produce a specific polarization state change of the light when travelling through the device
Note 1 to entry: Waveplates are constructed out of a birefringent material (for example quartz, mica, or certain
polymers), for which the index of refraction is different for light linearly polarized along one or the other of two
certain perpendicular crystal axes.
3.2 Terms related to properties of light
3.2.1
airy disc
central spot of light in the diffraction pattern of a point light source
3.2.2
bandwidth
range of frequencies within a given band
Note 1 to entry: A common way to calculate bandwidth is to use the full width at half maximum. It is typically
measured in hertz.
3.2.3
beam diameter
diameter of an electromagnetic beam along any specified line that is perpendicular to the beam axis
and intersects it.
Note 1 to entry: The beam diameter can be defined in several ways, such as full-width at half-maximum (FWHM),
1/e, 1/e or 4σ based on the measured intensity as a function of lateral distance.
Note 2 to entry: This usually refers to a beam of circular cross section, but not necessarily so it can be, for
example, elliptical in which case the orientation of the major and minor axis needs to be specified.
3.2.4
beam divergence
angular measure of the increase in beam diameter or radius with distance from the optical aperture or
antenna aperture from which the beam emerges
Note 1 to entry: As the wavevector, k (3.2.49) is a vector it is dependent on both the spectral purity and angular
divergence of a source.
3.2.5
candela
luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency
540 × 10 hertz and that has a radiant intensity in that direction of 1/683 W per steradian (a unit of
solid angle)
Note 1 to entry: Candela (cd) is the unit of luminous intensity in the International System of Units (SI).
3.2.6
chromaticity
property of a colour stimulus defined by its chromaticity coordinates, or by its dominant or
complementary wavelength and purity taken together
Note 1 to entry: Chromaticity coordinates specifies a colour regardless of its luminance.
[SOURCE: ISO 9241-302:2008(en), 3.3.9, modified — Note 1 to entry has been added.]
3.2.7
circular polarization
polarization state in which, at each point, the electric field of the wave has a constant magnitude, but its
direction rotates with time at a steady rate in a plane perpendicular to the direction of the wave
Note 1 to entry: Circularly polarized light can be produced by passing linearly polarized light through a quarter-
wave plate at an angle of 45° to the optical axis of the plate.
Note 2 to entry: As the electric field can rotate clockwise or anti-clockwise as it propagates, circularly polarized
waves exhibit chirality.
3.2.8
coherence
characteristic of a beam of electromagnetic radiation where there is a deterministic (not random) phase
relationship between each pair of points in the beam
Note 1 to entry: There are two types of coherence; spatial coherence (3.2.40) and temporal coherence (3.2.45)
[SOURCE: ISO 11145:2018, 3.11.1, modified — Note 1 has been added.]
3.2.9
coherence length
propagation distance in a dispersive medium over which an electromagnetic wave maintains a specified
degree of coherence
Note 1 to entry: Practically, it is used for quantifying the degree of temporal coherence (3.2.45) as the propagation
length (and thus propagation time) over which coherence degrades significantly.
3.2.10
colour temperature
temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given
stimulus
[SOURCE: ISO 9241-6:1999, 3.5]
3.2.11
depolarization
act of randomizing the polarization of an electromagnetic wave
Note 1 to entry: A depolarizer is the device used to depolarise light regardless of the input wave. In reality a
depolariser will produce a pseudo-random output.
3.2.12
diffraction limit
maximum spatial resolution achievable for an optical system, governed by diffraction phenomena
Note 1 to entry: The Abbe diffraction limit, is defined as λ/ (2 NA) in which λ is the wavelength of the illuminating
light and NA is the numerical aperture (3.1.19).
3.2.13
elliptical polarization
polarization state in which, the electric field vector describes an ellipse in any fixed plane intersecting,
and normal to, the direction of propagation
Note 1 to entry: An elliptically polarized wave may be resolved into two linearly polarized waves in phase
quadrature, with their polarization planes at right angles to each other. Circular and linear polarization can be
considered to be special cases of elliptical polarization.
Note 2 to entry: As the electric field can rotate clockwise or anti-clockwise as it propagates, elliptically polarized
waves exhibit chirality.
3.2.14
extinction coefficient
imaginary part of a complex refractive index of a material which describes the amount of attenuation
when the electromagnetic wave propagates through the material
3.2.15
fluorescence
phenomenon in which absorption of light of a given frequency by a substance is followed by the emission
of light at a lower frequency (longer wavelength) from the excited states with the same degeneracy
Note 1 to entry: Generally, the emission is from singlet excited state to singlet ground state.
Note 2 to entry: In the case of multiphoton fluorescence, the emitted light may be of a shorter wavelength.
3.2.16
frequency
reciprocal of the period
Note 1 to entry: The unit of frequency is the hertz (Hz), which corresponds to one cycle of periodic motion per
second.
[SOURCE: ISO 2041:2018, 3.3.33]
3.2.17
Jones vector
two by one matrix that describes the polarization state of light where the two parts represent the
amplitude and phase of the electric field in the x and y directions
Note 1 to entry: An optical element is represented by a Jones matrix (3.1.14).
3.2.18
irradiance
power of electromagnetic radiation incident on a surface per unit surface area
−2
Note 1 to entry: The SI unit is watt per square metre (Wm ).
Note 2 to entry: This can also be expressed as the incident flux per area incident on a given surface.
[SOURCE: ISO 29464:2017, 3.6.7, modified — Note 1 has been modified. Note 2 to entry has been added.]
3.2.19
linear polarization
plane polarization
polarization state in which, the electric field vector is confined to a given plane along the direction of
propagation
Note 1 to entry: The two orthogonal linear polarization states that are most important for reflection and
transmission are referred to as p- and s-polarization.
3.2.20
lumen
SI derived unit of luminous flux (3.2.22), whereby one lumen is the luminous flux emitted within a unit
solid angle (one steradian) by a point source having a uniform luminous intensity of one candela
Note 1 to entry: If a light source that emits one candela of luminous intensity uniformly across a solid angle of one
steradian, the total luminous flux emitted into that angle is one lumen (1 cd·1 sr = 1 lm).
Note 2 to entry: Lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of
visible light emitted by a source.
3.2.21
luminescence
emission, of optical radiation by atoms, molecules or ions in a material, which for certain wavelengths
or regions of the spectrum is in excess of the radiation due to thermal emission from that material at the
same temperature, as a result of these particles being excited by energy other than thermal agitation
[SOURCE: IEC 60050 845:1987, 04-18, modified — Reordered the text of the definition.]
3.2.22
luminous flux
quantity derived from the radiant flux, by evaluating the radiation in accordance with its action upon
the CIE standard photometric observer
[SOURCE: ISO 4007:2018,3.4.4]
3.2.23
luminous intensity
quantity of visible light that is emitted in unit time per unit solid angle
Note 1 to entry: The candela (one of the SI base units, abbreviation cd), is the unit of luminous intensity.
Note 2 to entry: The unit for the quantity of light flowing from a source in any one second (the luminous power, or
luminous flux) is called the lumen.
3.2.24
monochromatic radiation
radiation consisting of only a single wavelength, or of only a very narrow band of wavelengths of which
the central wavelength is quoted
[SOURCE: ISO 10934:2020, 3.1.123.2]
3.2.25
optical constants
quantities that describe the optical behaviour of a substance for a specified wavelength
Note 1 to entry: Typical properties are refractive index (3.3.43), absorption coefficient (3.3.1), or reflectivity
(3.3.41).
3.2.26
period
smallest interval of time for which a periodic function repeats itself
Note 1 to entry: If no ambiguity is likely, the fundamental period is called the period.
[SOURCE: ISO 2041:2018, 3.3.32]
3.2.27
phosphorescence
photoluminescence (3.2.28) delayed by storage of energy in an intermediate energy level
Note 1 to entry: The emission is generally from triplet excited state to singlet ground state.
[SOURCE: ISO 17724:2003, 53]
3.2.28
photoluminescence
luminescence (3.2.21) caused by absorption of optical radiation
[SOURCE: IEC 60050 845:1987, 04-19]
3.2.29
photometry
method for the measurement of light, in terms of electromagnetic radiation weighted by the human
eye's response
Note 1 to entry: This response changes with wavelength, and to an extent, from person to person. Internationally
agreed standard observer functions are therefore used in order to provide a consistent measurement base for
photometry.
Note 2 to entry: The word 'luminous' is used to indicate that measurements have been made using a detection
system (called a photometer) that has a spectral response similar to that of a human eye.
3.2.30
photon
particle representing a quantum of light or other electromagnetic radiation that carries energy
proportional to the radiation frequency, has zero rest mass and travels at the speed of light in a vacuum
3.2.31
polarizability
ratio of induced dipole moment of a material to the electric field that induces it
Note 1 to entry: Polarizability is a measure of how easily an electron cloud is distorted by an electric field.
2 −1
Note 2 to entry: The SI units of polarizability are C m V .
3.2.32
polarizability tensor
tensor used to describe the polarizability (3.2.31) of anisotropic media (3.3.3)
3.2.33
polarization
property of transverse waves that specifies the geometrical orientation of the oscillations with respect
to the propagation vector
Note 1 to entry: For light, polarization is normally defined by the direction, or time-dependent direction, of the
electric field vector.
3.2.34
power spectral density
magnitude of a signal in the frequency domain expressed in terms of the power per unit frequency as a
function of frequency
Note 1 to entry: For optical signals the units of power spectral density are typically W/Hz.
3.2.35
p-polarized light
linear polarized light that has an electric field polarized parallel to the plane of incidence
3.2.36
propagation matrix
mathematical matrix that is used in optics to analyse the propagation of electromagnetic waves through
a stratified medium
3.2.37
radiance
radiant intensity of an element of the surface, at a point on a surface and in a given direction, divided
by the area of the orthogonal projection of this element on a plane perpendicular to the given direction
−1 −2
Note 1 to entry: The unit of radiance is the watt per steradian per square metre (W·sr ·m )
[SOURCE: ISO/TS 19101-2:2018, 3.30, modified — Note 1 has been added.]
3.2.38
radiant flux
power emitted, transformed or received in the form of radiation
Note 1 to entry: radiant flux is expressed in watts (W)
[SOURCE: CIE 17.4-1987]
3.2.39
radiometry
set of techniques for characterizing the distribution of electromagnetic radiation's power
3.2.40
spatial coherence
characteristic of a beam of electromagnetic radiation to have a degree of phase correlation between
different spatial points in the beam at the same moment in time
[SOURCE: ISO 11145:2018, 3.11.3]
3.2.41
spectral irradiance
irradiance of a surface per unit frequency or wavelength
Note 1 to entry: Spectral irradiance of a frequency spectrum is measured in watts per square metre per hertz
−2 −1
(W·m ·Hz ), while spectral irradiance of a wavelength spectrum is measured in watts per square metre per
−3
metre (W·m ).
3.2.42
spectral radiance
radiance of a surface per unit frequency or wavelength
Note 1 to entry: Spectral radiance in frequency is measured in watts per steradian per square metre per hertz
−1 −2 −1
(W·sr ·m ·Hz ) while spectral radiance in wavelength is measured in watts per steradian per square metre,
−1 −3
per metre (W·sr ·m ).
3.2.43
s-polarized light
linear polarized light that has an electric field polarized perpendicular to the plane of incidence
3.2.44
Stokes vector
4 x 1 column matrix which characterizes possible states of polarization of a quasi-monochromatic or
monochromatic transverse-electromagnetic wave
3.2.45
temporal coherence
characteristic of a beam of electromagnetic radiation to have a degree of phase correlation between
different moments in time at the same spatial point in the beam
[SOURCE: ISO 11145:2018, 3.11.2]
3.2.46
two photon fluorescence
two photon excitation fluorescence
TPEF
fluorescence that arises from the excitation by the simultaneous absorption of two photons
Note 1 to entry: This uses the simplest form of multi-photon absorption (3.6.14) such that a single fluorescent
photon is emitted for every 2 photons being absorbed.
[SOURCE: ISO 10934:2020, 3.3.29.1.1]
3.2.47
wavelength
λ
distance in the direction of propagation of a sinusoidal wave between two successive points where at a
given instant in time the phase differs by 2π
[SOURCE: ISO 80000-3:2006, 3-17]
3.2.48
wavenumber
σ
inverse of the wavelength (λ) of electromagnetic radiation
[SOURCE: ISO 19702:2015, 3.6]
3.2.49
wavevector
k
vector in reciprocal space describing the direction of propagation of electromagnetic wave and equal in
magnitude to 2π/ λ, where λ is the wavelength (3.2.47)
Note 1 to entry: In different fields of study, the wave vector is taken as 2π/λ or as 1/λ for crystallography. For
electromagnetic waves the former is usually used.
[SOURCE: ISO 18115-1:2013, 7.36, modified — The definition has been modified to apply to more
general waves.]
3.3 Terms related to optical properties due to interactions with media
3.3.1
absorption coefficient
coefficient that indicates the degree of absorption of light per unit length in a certain medium and
depends on the wavelength/energy of the incident wave
Note 1 to entry: In practice, the absorption coefficient indicates how deep into a material light of defined
wavelengths will get before being absorbed.
Note 2 to entry: Attenuation coefficient is defined in ISO 18115-1:2013, 4.33.
3.3.2
absorption edge
sharp discontinuity in the absorption spectrum of a substance that occurs at wavelengths where the
energy of an absorbed photon corresponds to an electronic transition or ionization potential
3.3.3
anisotropic media
media in which electromagnetic waves behave differently depending on which direction the
wave is propagating
Note 1 to entry: Crystals are examples of anisotropic media. Here, the polarization field is not necessarily aligned
with the electric field of light.
3.3.4
anomalous dispersion
dispersion (3.3.16) characterized by a decreasing index of refraction in the medium, as the frequency of
the propagating light increases and the wavelength decreases
Note 1 to entry: A medium will have anomalous dispersion where the real part of permittivity decreases with
frequency.
Note 2 to entry: Anomalous dispersion occurs in regions in which the material absorbs light.
Note 3 to entry: For a wave group, a related term is negative group-velocity dispersion which is sometimes
referred to as anomalous dispersion. This is strictly speaking not correct. Negative group-velocity dispersion
is characterized by a short-wavelength or high frequency pulse traveling faster than a long-wavelength, or low
frequency pulse.
3.3.5
birefringence
property of a material which causes incident light waves of different polarizations to be refracted
differently by the material
Note 1 to entry: Lyot filter is a type of optical filter that uses birefringence to produce a narrow bandpass of
transmitted wavelengths.
[SOURCE: ISO/IEC 10885:1993, 4.3.3, modified — Note 1 to entry has been added.]
3.3.6
blackbody radiation
intensity and spectral distribution of the optical and infrared power emitted by a completely absorbing
material at a uniform temperature
3.3.7
Brewster angle
angle of incidence at which light with a particular polarization is perfectly transmitted through a
transparent dielectric surface, with no reflection
Note 1 to entry: When unpolarized light is incident at this angle, the light that is reflected from the surface is
perfectly polarized.
3.3.8
Cauchy coefficients
constant parameters in Cauchy’s equation (3.3.9) that characterize the index of refraction as a function
of the wavelength for transparent materials
3.3.9
Cauchy's equation
empirical relationship between the index of refraction and wavelength of light for transparent materials
3.3.10
complex refractive index
refractive index for a medium with absorption expressed as a real and an imaginary (absorption) part
Note 1 to entry: The complex refractive index can be expressed mathematically as n = n + iƘ where i is the
square root of –1; Ƙ is the imaginary (absorption coefficient) part of the refractive index; n is the real part of the
refractive index. For an anisotropic medium, n is a tensor.
[SOURCE: ISO 13320:2020, 3.1.5 modified — Wording from ISO 80000-7:2008, 7-5 has been included.]
3.3.11
critical angle
angle between the incident beam and the specimen surface, at which the reflectivity is at the first point
of inflexion
Note 1 to entry: In practical cases, the critical angle is often taken as the angle at which the reflected intensity
has fallen to 50 % of that in the total external reflection condition. The error here is generally small. It is also the
angle between the incident beam and the specimen surface above which the transmitted electric field becomes
real. Its value depends on the energy and material properties.
Note 2 to entry: The critical angle for a given specimen material or structure can be found by using simulation
software, or by calculation where refractive indices of the two mediums are known.
3.3.12
dichroism
property of some anisotropic materials of having different absorption coefficients (3.3.1) for light
polarized in different directions
Note 1 to entry: Dichroism is in most cases related to the linear polarization directions, however there is also
circular dichroism, where the difference in optical properties occurs for different circular polarization direction.
3.3.13
dielectric constant
relative permittivity
material property given as the ratio of the permittivity of the material to the permittivity of the vacuum
Note 1 to entry: For isotropic materials this is a unitless dimensionless quantity.
3.3.14
dielectric function
optical property of a material where the complex dielectric constant depends on the frequency and
wave vector of the field
Note 1 to entry: This is generally a complex function.
3.3.15
dielectric tensor
dielectric function in anisotropic medium expressed as a 3 × 3 tensor
3.3.16
dispersion
change in phase velocity of a wave group as a function of its wavelength (or frequency) when passing
from one medium to another which causes a separation of the monochromatic components of
electromagnetic radiation
Note 1 to entry: The refractive index of a transparent material is the same as the phase velocity divided by the
speed of light in a vacuum, thus dispersion can also be defined as the variation in refractive index of a medium
which causes a separation of the monochromatic components of electromagnetic radiation.
Note 2 to entry: There are two types of dispersion, normal dispersion (3.3.27) and anomalous dispersion (3.3.4).
[SOURCE: ISO 10934:2020, 3.1.46, modified — "complex" has been replaced with "electromagnetic" and
Note 1 added has been added.]
3.3.17
elastic scattering
interaction between a photon and matter, where the total kinetic energy is conserved, as a result, the
energies of the incident and scattered photons are identical
3.3.18
electric susceptibility
dimensionless proportionality constant that indicates the degree of polarization of a dielectric material
in response to an applied electric field
3.3.19
emissivity
ratio of the radiance emitted by a radiant source to the radiance that would be emitted by a black body
radiant source at the same temperature
Note 1 to entry: Emissivity is dimensionless.
[SOURCE: ISO 13943:2017, 3.89, modified]
3.3.20
Fresnel coefficients
coefficients that give the amount an electromagnetic wave is reflected or transmitted from or through
a dielectric for transverse electric field and transverse magnetic field radiation
Note 1 to entry: Since electromagnetic waves are transverse, there are separate coefficients in the polarization
directions perpendicular to and parallel to the plane of incidence.
3.3.21
Fresnel equations
formulae that calculate the ratio of the reflected and transmitted electric field amplitude to the initial
electric field for electromagnetic radiation incident on an interface between different optical media
Note 1 to entry: The formulae assume the interface between the media is flat and that the media are homogeneous.
The incident light is assumed to be a plane wave, and effects of edges are neglected.
3.3.22
group velocity
speed and direction at which the amplitude of an overall envelope shape of a group of waves propagates
in a medium
Note 1 to entry: Compare with phase velocity (3.3.29).
3.3.23
inelastic light scattering
interaction between a photon and matter, where the total kinetic energy is not conserved, as a result,
the energy of the incident and scattered photons are not identical
3.3.24
isotropic media
medium in which electromagnetic permeability and permittivity are uniform in all
directions
3.3.25
Kramers-Kronig relationship
bidirectional mathematical relations, connecting the real and imaginary parts of a dielectric function
based on the principle of causality
Note 1 to entry: These relations are often used to calculate the real part from the imaginary part or vice versa.
Note 2 to entry: All functions used to model vibrational spectra obey the Kramers-Kronig transforms as long as
the real parts are even functions of wavenumber and the imaginary parts are odd functions of wavenumber so
that the Kramers-Kronig transforms are equivalent to the Hilbert transforms.
3.3.26
nonlinear susceptibility
tensor that describes the nonlinear response of material system upon the strength of an applied optical
field
Note 1 to entry: When this is a second order term it describes processes such as second harmonic generation
(SHG) (3.6.27) and a third-order nonlinear optical susceptibility describes processes, such as third-harmonic
generation (3.6.34) and the intensity-dependent refractive index.
3.3.27
normal dispersion
dispersion characterized by an increasing index of refraction in the medium, as the frequency of the
propagating light increases and the wavelength decreases
Note 1 to entry: A medium will have normal dispersion where the real part of permittivity increases with
frequency. Compare with anomalous dispersion (3.3.4).
Note 2 to entry: Normal dispersion occurs in most transparent media.
Note 3 to entry: For a wave group, a related term is positive group-velocity dispersion which is sometimes
referred to as normal dispersion. This is strictly speaking not correct. Positive group-velocity dispersion is
characterized by a long wavelength or low frequency pulse traveling faster than a short wavelength or high
frequency pulse.
3.3.28
permittivity
mathematical scalar or tensor that describes the degree of polarization of a dielectric material in
response to an applied electric field
3.3.29
phase velocity
speed and direction at which the phase of any individual wave of a given frequency in a group of waves,
propagates in a medium
Note 1 to entry: 1 to entry: Compare with group velocity (3.3.22).
3.3.30
phonon
quantum mechanical description of an elementary vibrational motion in which a periodic elastic la
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