SIST EN 16981:2021

SLOVENSKI STANDARD
01-december-2021
Nadomešča:
SIST-TS CEN/TS 16981:2017
Fotokataliza - Slovar izrazov
Photocatalysis - Glossary of terms
Photokatalyse - Glossar der Begriffe
Photocatalyse - Glossaire de termes
Ta slovenski standard je istoveten z: EN 16981:2021
ICS:
01.040.25 Izdelavna tehnika (Slovarji) Manufacturing engineering
(Vocabularies)
25.220.01 Površinska obdelava in Surface treatment and
prevleke na splošno coating in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 16981
EUROPEAN STANDARD
NORME EUROPÉENNE
October 2021
EUROPÄISCHE NORM
ICS 01.040.25; 25.220.01 Supersedes CEN/TS 16981:2016
English Version
Photocatalysis - Glossary of terms
Photocatalyse - Glossaire de termes Photokatalyse - Glossar der Begriffe
This European Standard was approved by CEN on 18 July 2021.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 16981:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 7
1 Scope . 8
2 Normative references and notes . 8
3 Terms and definitions . 8
Bibliography . 55

European foreword
This document (EN 16981:2021) has been prepared by Technical Committee CEN/TC 386
“Photocatalysis”, under WG 1 “Terminology”, the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by April 2022, and conflicting national standards shall be
withdrawn at the latest by April 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes CEN/TS 16981:2016.
In comparison with the previous edition, the following technical modifications have been made:
— Change of the Scope: “The glossary lists a consistent set of definitions to be used in standards on
photocatalysis for their consistency and connection with the scientific literature”.
— Change to Clause 2: Paragraphs were updated:
"Normative references and notes
There are no normative references in this document.
Most of the definitions reported in this document are a sub-set of the IUPAC definitions in
photocatalysis and radiocatalysis [1]. Some other definitions, in particular for the photocatalytic
rate and reactors, are taken from a dedicated work [2].
The technical specifications for the apparatus and physical values for irradiation conditions to be
used in the standards are reported in a separate Technical Specification [3].
For the magnitudes implying energy or photons incident on a surface from all directions, the set of
symbols recommended by the International Organization for Standardization (ISO) [4] and
included in the IUPAC “Green Book”, and by the International Commission on Illumination [5] are
adopted. This has been done primarily to comply with internationally agreed-upon symbols.”
— Clause 3: introductory wording and definitions were updated:
“ISO and IEC maintain a generic terminological databases for use in standardization, which could
complement this dedicated Glossary, at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
The arrangement of entries is alphabetical, and the criterion adopted by the IUPAC has been
followed for the typeface used: italicized words in a definition or following it indicate a cross-
reference in the Glossary.
SI units are adopted, with some exceptions, prominently in the use of the molar decadic absorption
coefficient, ε, with common units dm3 mol–1 cm–1 and a mole of photons denoted as an einstein.
As recently the definition of the SI units was established in terms of a set of seven defining
constants, including the Avogadro number, the mole (symbol: mol) is the base unit of amount
(number) of substance.
Functional dependence of a physical quantity f on a variable x is indicated by placing the variable in
parentheses following the symbol for the function; e.g. ε(λ). Differentiation of a physical quantity f
with respect to a variable x is indicated by a subscript x; e.g. the typical spectral radiant power
quantity Pλ = dP/dλ. The natural logarithm is indicated with ln, and the logarithm to base 10 with
log.”
The following definitions were deleted:
— amalgam lamp (before 3.17),
— back electron-transfer (before 3.20)
— bioluminescence (before 3.25),
— charge-transfer (CT) absorption (before 3.35),
— charge-transfer (CT) complex (before 3.36),
— charge-transfer (CT) state (before 3.37),
— circular dichroism (CD) (before 3.43)
— charge hopping(before 3.31),
— circular dichroism (CD) (before 3.43),
— current yield see photocurrent yield(before 3.49),
— dielectric (before 3.59),
— differential quantum (before 3.60),
— diode light emitting (LED) (before 3.61),
— driving force (before 3.63),
— driving force (for electron transfer) (before 3.64),
— electron-transfer photosensitization (before 3.69),
— emissivity see emittance(before 3.73),
— excitation transfer see energy transfer (before 3.77),
— flash photolysis(before 3.85),
— FWHM(before 3.95),
— germicidal lamp(before 3.97),
— hypsochromic shift(before 3.103),
— inner-filter effect(before 3.106),
— inner-sphere electron transfer(before 3.107),
— interferometer(before 3.113),
— Lambert–Beer law (before 3.116),
— Lambert law (before 3.117),
— LED(before 3.119),
— light-emitting diode (LED) (before 3.121),
— low-pressure mercury lamp (arc) (before 3.123),
— medium-pressure mercury lamp (arc) (before 3.127),
— mercury–xenon lamp (arc) (before 3.129),
— multiphoton process(before 3.132),
— OLED (before 3.135),
— OPA (before 3.137),
— OPO (before 3.138),
— optical multichannel analyzer (OMA) (before 3.140),
— optical parametric amplification process (before 3.141),
— optical parametric oscillator (OPO) (before 3.142),
— optoacoustic spectroscopy(before 3.143),
— photo-assisted catalysis(before 3.148),
— photohydration(before 3.166),
— photon emittance(before 3.174),
— photopolymerization(before 3.185),
— quartz–iodine lamp(before 3.194),
— radiant energy fluence rate(before 3.201),
— reactor CSTR(before 3.210),
— reflection factor (before 3.214),
— reflectivity(before 3.215),
— self-absorption (before 3.219),
— self-quenching (before 3.220),
— sensitizer (before 3.221),
— sensitization (before 3.222),
— singlet oxygen (before 3.223),
— singlet state(before 3.224),
— solvent shift(before 3.226),
— spectral radiant energy, Qλ(before 3.238),
— spectral sensitization(before 3.244),
— tungsten-halogen lamp(before 3.248),
— wolfram lamp(before 3.259),
The following definitions were updated:
— 3.11 actinic.
— 3.12 actinism.
— 3.18 attenuance filter
— 3.28 Brewster angle
— 3.64 extinction coefficient
— 3.65 Fermi level
— 3.71 fluorescence spectrum
— 3.92 Fourier-transform spectrometer
— 3.93 Fourier-transform spectroscopy
— 3.95 mercury lamp
— 3.100 organic light-emitting device
— 3.102 phosphorescence
— 3.103 photoacoustic spectroscopy
— 3.113 photocuring
— 3.116 photodynamic effect
— 3.140 photoreaction
— 3.142 photosensitization
— 3.143 photosensitizer
— 3.145 quantum efficiency
— 3.152 radiant energy fluence
— 3.158 rate of photon absorption
— 3.159 reaction rate
— 3.160 reactor batch
— 3.163 red shift
— 3.165 RGB color model
— 3.191 UV dose
The following definitions were added:
— 3.137 photonic unit conversion
— 3.161 reactor continuous Stirred-Tank
— 3.162 reactor plug flow
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Introduction
Photocatalysis is a very efficient advanced oxidation technique which enables the production of active
species following light absorption by the photocatalyst, such as bound/free hydroxyl radicals (∙OH),
hydroperoxyl radicals (∙OOH) and other ROS, conduction band electrons and valence band holes,
capable of partly or completely mineralising/oxidising the majority of organic compounds. The most
commonly used photocatalyst is titanium dioxide (TiO ). Photocatalysts can be used in powder form or
deposited as thin films on different substrates (glass fibre, fabrics, plates/sheets, etc.). The objective of
standardization is to introduce test standards for evaluation of the performance of photocatalysts
(including photocatalysis and photo-induced effects). These standards mainly concern tests and
analysis methods, and require a common language.
A common language for standards, disclosed to a wide audience and referring only to the operational
protocols and to their outcomes, is needed for a consistent set of standards and the connection with the
scientific literature. This glossary will take into account existing glossary of terms and literature
definitions used in photocatalysis and photochemistry. Because in photocatalysis numerous properties
are difficult to be evaluated, in this Glossary and in related standard norms the report of properties
depending on some physical-chemical properties and model parameters, like the number of active sites,
the mechanisms of adsorption or kinetic mechanisms of photocatalytic reactions is avoided.
Safety statement
Persons using this document should be familiar with the normal laboratory practice, if applicable. This
document does not address safety problems, if any, associated with its use. It is the responsibility of the
user to establish appropriate safety and health practices and to ensure compliance with any regulatory
conditions.
Environmental statement
It is understood that some of the material described in this document may have negative environmental
impact. As technological advantages lead to better alternatives for these materials, they will be
eliminated from this document to the possible extent.
At the end of the test, the user of this document will take care to carry out an appropriate disposal of the
wastes, according to local regulation.
1 Scope
The glossary lists a consistent set of definitions to be used in standards on photocatalysis for their
consistency and connection with the scientific literature.
2 Normative references and notes
There are no normative references in this document.
Most of the definitions reported in this document are a sub-set of the IUPAC definitions in
photocatalysis and radiocatalysis [1]. Some other definitions, in particular for the photocatalytic rate
and reactors, are taken from a dedicated work [2].
The technical specifications for the apparatus and physical values for irradiation conditions to be used
in the standards are reported in a separate Technical Specification [3].
For the magnitudes implying energy or photons incident on a surface from all directions, the set of
symbols recommended by the International Organization for Standardization (ISO) [4] and included in
the IUPAC “Green Book”, and by the International Commission on Illumination [5] are adopted. This has
been done primarily to comply with internationally agreed-upon symbols.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain a generic terminological databases for use in standardization, which could
complement this dedicated Glossary, at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
The arrangement of entries is alphabetical, and the criterion adopted by the IUPAC has been followed
for the typeface used: italicized words in a definition or following it indicate a cross-reference in the
Glossary.
SI units are adopted, with some exceptions, prominently in the use of the molar decadic absorption
3 –1 –1
coefficient, ε, with common units dm mol cm and a mole of photons denoted as an einstein. As
recently the definition of the SI units was established in terms of a set of seven defining constants,
including the Avogadro number, the mole (symbol: mol) is the base unit of amount (number) of
substance.
Functional dependence of a physical quantity f on a variable x is indicated by placing the variable in
parentheses following the symbol for the function; e.g. ε(λ). Differentiation of a physical quantity f with
respect to a variable x is indicated by a subscript x; e.g. the typical spectral radiant power
quantity P = dP/dλ. The natural logarithm is indicated with ln, and the logarithm to base 10 with log.
λ
3.1
absorbance
A
logarithm to the base 10 (linear absorbance) of the incident (prior to absorption) spectral radiant power,
P divided by the transmitted spectral radiant power, P :
λ
λ
 
P
λ
 
ATλ = log =−log λ
( ) ( )
 
P
λ
 
Note 1 to entry: T(λ) is the (internal) transmittance at the defined wavelength. The terms absorbancy,
extinction, and optical density should not be used. When natural logarithms are used, the napierian absorbance is
the logarithm to the base e of the incident spectral radiant power, divided by the transmitted spectral radiant
P
λ
power, P :
λ
 
P
λ
 
A λ = ln =−lnT λ
( ) ( )
e
 
P
λ
 
Note 2 to entry: These definitions suppose that all the incident ultraviolet, visible, or infrared radiation is either
transmitted or absorbed, reflection or scattering being negligible. Attenuance should be used when this
supposition cannot be made.
Note 3 to entry: In practice, A is the logarithm to the base 10 of the spectral radiant power of ultraviolet, visible,
or infrared radiation transmitted through a reference sample divided by that transmitted through the investigated
sample, both observed in identical cells.
Note 4 to entry: In common usage, A is given for a path length of 1 cm, unless otherwise specified.
Note 5 to entry: Traditionally, (spectral) radiant intensity, I , was used instead of spectral radiant power, P ,
λ λ
now the accepted term.
Note 6 to entry: The wavelength symbol as a subscript for P and in parenthesis for T and A can be omitted.
However, the wavelength should be specified for which the value of the particular property is reported.
Note 7 to entry: Same as internal optical density, which is a term not recommended.
Note 8 to entry: See also absorption coefficient, absorptance, attenuance, Beer–Lambert law, molar absorption
coefficient.
3.2
absorbed spectral photon flux density
absorbed photon flux density
number of photons of a particular wavelength, per time interval (spectral photon flux, number basis,
q , or spectral photon flux, amount basis, q ) absorbed by a system per volume, V
p,λ n,p,λ
−1 –4 –1 –3 –1
Note 1 to entry: On number basis, SI unit is s m ; common unit is s cm nm . On amount basis, SI unit
–1 –4 −1 −3 –1
is mol s m ; common unit is einstein s cm nm .
− Aλ − Aλ
0 ( ) 0 ( )
   
q 1− 10 q 1− 10
p,λ n,p,λ
   
   
Note 2 to entry: Mathematical expression: on number basis, on
V V
amount basis, where A(λ) is the absorbance at wavelength λ and superscript 0 (zero) indicates incident photons.
Note 3 to entry: Absorbed (spectral) photon flux density (number basis or amount basis) is used in the
denominator when calculating a differential quantum yield and using in the numerator the rate of change of the
number, dC/dt, or the rate of change of the amount concentration, dc/dt, respectively.
Note 4 to entry: The term rate of photon absorption is increasingly used.
3.3
absorbed spectral radiant power density
absorbed radiant power density
spectral radiant energy per time interval (spectral radiant power, P ) absorbed by a system per
λ
volume, V
–4 –3 –1
Note 1 to entry: SI unit is W m ; common unit is W cm nm .
− A(λ)
0 
P 1-10
λ
 
 
Note 2 to entry: Mathematical expression: where A(λ) is the absorbance at wavelength λ and
V
superscript 0 (zero) indicates incident radiant power.
3.4
absorptance
a
fraction of ultraviolet, visible, or infrared radiation absorbed, equal to one minus the transmittance (T),
i.e., (1–T)
Note 1 to entry: The use of this obsolete term, equivalent to absorption factor, is not recommended.
Note 2 to entry: See also absorbance.
3.5
absorption
absorption of electromagnetic radiation
transfer of energy from an electromagnetic field to a material or a molecular entity
Note 1 to entry: In a semiclassical fashion, this transfer of energy can be described as being due to an
interaction of the electric field of the wave with an oscillating electric dipole moment set up in the material or
molecular entity. This dipole moment is the result of the perturbation by the outside field, and its oscillation
frequency ν is given by the difference ΔE of the energies of the lower and upper state in the absorbing material or
molecular entity, ΔE = hν. When the frequency of the oscillating dipole moment and the frequency of the field agree,
a resonance occurs and energy can flow from the field into the material or molecule (an absorption occurs).
Note 2 to entry: When energy flows from the material or molecule to the field, stimulated light emission occurs.
Note 3 to entry: The oscillating electric dipole moment produced in the material or molecular entity has an
amplitude and direction determined by a vector M , known as the electric transition (dipole) moment. The
if
amplitude of this moment is the transition moment between the initial (i) and final states (f).
3.6
absorption coefficient
a(λ), α(λ)
absorbance, A(λ), divided by the optical pathlength, l:
 
A()λ 1 P

λ
a(λ ) log 

 
ll P

λ
 
where a is the linear decadic absorption coefficient and
P
and P are, respectively, the incident and transmitted spectral radiant power.
λ
λ
When napierian logarithms are used
 
P

λ
 
αλ( ) a(λ )ln10 ln

 
lP

λ
 
where α is the linear napierian absorption coefficient.
–1
Note 1 to entry: Since absorbance is a dimensionless quantity, the coherent SI unit for a and α is m ; the
–1
common unit is cm .
Note 2 to entry: See also absorptivity, molar absorption coefficient.
3.7
absorption cross-section
σ
linear napierian absorption coefficient, α(λ), divided by the number of molecular entities contained in a
volume of the absorbing medium along the ultraviolet, visible, or infrared radiation path:
 
αλ
( ) 1 P
λ
σλ ln 
( )
 
C C⋅l P
λ
 
where
C is the number concentration of molecular entities (number per volume), l is the optical pathlength,
P
and and P are, respectively, the incident and transmitted spectral radiant power.
λ
λ
2 2
Note 1 to entry: SI unit is m , common unit is cm .
Note 2 to entry: The relation between the absorption cross-section and the molar (decadic) absorption
coefficient, ε(λ), is α(λ) = ln10 ε(λ)/N with N the Avogadro constant. A conversion formula in common units is:
A A
2 −21 –1 3 –1
σ(λ)/cm = (3,8236 × 10 /mol)  [ε(λ)/ mol dm cm ]
Note 3 to entry: See also attenuance, Beer–Lambert law.
==
==
==
3.8
absorption factor
fraction of ultraviolet, visible, or infrared radiation absorbed by a system
–A(λ)
f(λ) = 1 – T(λ) = 1 – 10
with
T(λ) the transmittance and A(λ) the absorbance at a particular wavelength λ.
Note 1 to entry: This term is preferred to absorptance.
Note 2 to entry: The wavelength symbol can be omitted for f, T, and A. The wavelength should be specified for
which the value of the particular property is reported.
Note 3 to entry: For A(λ) << 1/ln10, f(λ) approximately A(λ) ln10.
3.9
absorption spectrum
plot of the absorption coefficient against a quantity related to photon energy, such as frequency ν,

wavenumber ν , or wavelength λ
Note 1 to entry: The plot of the absorbance against a quantity related to photon energy is discouraged, unless
the actual concentration of the species is unknown.
3.10
absorptivity
absorptance divided by the optical pathlength
Note 1 to entry: The unit length shall be specified.
Note 2 to entry: The use of this obsolete term is not recommended.
Note 3 to entry: For very low attenuance, i.e. for A(λ) << 1/ln10, it approximates the linear absorption
–A(λ)
coefficient, within the approximation [1 – 10 ] approximately A(λ) ln10.
3.11
actinic
chemical changes produced by radiant energy especially in the visible and ultraviolet parts of the
spectrum
Note 1 to entry: Applied to, resulting from, or referred to actinism.
3.12
actinism
chemical changes on living and nonliving materials caused by optical radiation especially in the visible
and ultraviolet parts of the spectrum
3.13
actinometer
chemical system for the determination of the number of photons integrally or per time interval
absorbed into the defined space of a chemical reactor
Note 1 to entry: This name is commonly applied to systems used in the ultraviolet and visible wavelength
ranges.
Note 2 to entry: For example, solutions of potassium oxalatoferrate(III), K [Fe(C O ) ] (among other systems)
3 2 4 3
can be used as a chemical actinometer. Bolometers, thermopiles, and photodiodes are physical devices giving a
reading of the radiation impinging on them that can be correlated to the number of photons detected as well as to
the number of photons entering the chemical reactor. Detailed information on chemical actinometers and
measuring systems can be found in CEN/TS 16599:2014.
Note 3 to entry: See also spectral sensitivity.
3.14
action spectrum
plot of a relative biological or chemical photoresponse (=Δy) per number of incident (prior to
absorption) photons, vs. wavelength, or energy of radiation, or frequency or wavenumber
Note 1 to entry: This form of presentation is frequently used in the studies of biological or solid-state systems,
where the nature of the absorbing species is unknown.
Note 2 to entry: It is advisable to ensure that the fluence dependence of the photoresponse is the same
(e.g. linear) for all the wavelengths studied.
Note 3 to entry: The action spectrum is sometimes called spectral responsivity or sensitivity spectrum. The
precise action spectrum is a plot of the spectral (photon or quantum) effectiveness. By contrast, a plot of the
biological or chemical change or response per absorbed photon (quantum efficiency) vs. wavelength is the
efficiency spectrum.
Note 4 to entry: In cases where the fluence dependence of the photoresponse is not linear (as is often the case in
biological photoresponses), a plot of the photoresponse vs. fluence should be made at several wavelengths and a
standard response should be chosen. A plot of the inverse of the “standard response” level vs. wavelength is then
the action spectrum of the photoresponse.
Note 5 to entry: See also excitation spectrum, efficiency spectrum.
3.15
AM 0 sunlight
solar irradiance in space just above the atmosphere of the earth on a plane perpendicular to the
direction of the sun (air mass, AM, zero)
Note 1 to entry: Also called extraterrestrial irradiance.
Note 2 to entry: See also AM 1 sunlight.
3.16
AM 1 sunlight
solar irradiance at sea level, i.e., traversing the atmosphere, when the direction of the sun is
perpendicular to the surface of the earth
Note 1 to entry: Also called terrestrial global irradiance.
Note 2 to entry: See also AM 0 sunlight.
3.17
attenuance
D
logarithm to the base 10 of the incident spectral radiant power, P , divided by the transmitted spectral
λ
radiant power, P
λ
 
P
λ
 
D(λλ)= log =−logT
( )
 
P
λ
 
where
T(λ) is the transmittance.
Note 1 to entry: Attenuance reduces to absorbance if the incident beam is only either transmitted or absorbed,
but not reflected or scattered.
Note 2 to entry: See also Beer–Lambert law, depth of penetration.
3.18
attenuance filter
neutral-density filter
filter that reduces or modifies the transmitted radiant power of all wavelengths of light equally, giving
no changes in hue of color rendition
Note 1 to entry: The term neutral-density filter is preferred.
Note 2 to entry: Detailed information on filters can be found in CEN/TS 16599:2014.
3.19
bandgap energy
E
g
energy difference between the bottom of the conduction band and the top of the valence band in a
semiconductor or an insulator
Note 1 to entry: See also Fermi level, conduction band, valence band.
3.20
bandpass filter
optical device that permits the transmission of radiation within a specified wavelength range and does
not permit transmission of radiation at higher or lower wavelengths
Note 1 to entry: It can be an interference or a coloured filter.
Note 2 to entry: See also filter. More detailed info on filters can be found in CEN/TS 16599:2014.
3.21
bathochromic shift
shift of a spectral band to lower frequency (longer wavelengths) owing to the influence of substitution or
a change in environment (e.g., solvent)
Note 1 to entry: It is informally referred to as a red shift and is opposite to a hypsochromic shift.
3.22
Beer–Lambert law
Lambert-Beer law
Beer–Lambert–Bouguer law
relation of the absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic
medium with the absorption pathlength, l, and the concentration, c, or (in the gas phase) the pressure of
the absorbing species
Note 1 to entry: This law holds only under the limitations above, for low light radiant power and for absorbing
species exhibiting no concentration or pressure dependent aggregation. The law can be expressed as
 
P
λ
A(λ) log  ελ( )⋅⋅cl
 
P
λ
 
or
−−Aλ ε λ cl
00( ) ( )
PP 10 P 10
λλ λ
P P
where the proportionality constant, ε(λ), is the molar (decadic) absorption coefficient, and and are,
λ λ
–3
respectively, the incident and transmitted spectral radiant power. For l in cm and c in mol dm (M), ε(λ) will
3 –1 –1 –1 –1 2 –1 3 –1 –1
result in dm mol cm (M cm ), a commonly used unit. SI unit of ε(λ) is m mol (10 dm mol cm ).
Note 2 to entry: Spectral radiant power shall be used because the Beer–Lambert law holds only if the spectral
bandwidth of the ultraviolet, visible, or infrared radiation is narrow as compared to spectral linewidths in the
spectrum.
Note 3 to entry: See also absorbance, attenuance, extinction coefficient.
3.23
biphotonic excitation
simultaneous (coherent) absorption of two photons (either same or different wavelength), the energy of
excitation being the sum of the energies of the two photons
Note 1 to entry: Also called two-photon excitation.
Note 2 to entry: This term is sometimes also used for a two-step absorption when the absorption is no longer
simultaneous.
3.24
biphotonic process
process resulting from biphotonic excitation
Note 1 to entry: See also multiphoton process.
3.25
bleaching
loss of absorption or emission intensity
Note 1 to entry: The definition applies only on in photochemistry.
3.26
blue shift
informal expression for hypsochromic shift
==
= =
3.27
Brewster angle
θ
B
unique angle (θ ) at which the reflected waves are all polarized into a single plane when an unpolarized
B
planar electromagnetic wavefront impinges on a flat dielectric surface
1/2
Note 1 to entry: Expression for Brewster angle: θ = arctan (n / n ) = arctan (ε / ε ) where n and n are
B 2 1 2 1 2 1
the refractive indices of the receiving surface and the initial medium, respectively, and ε and ε are the relative
2 1
static permittivities (formerly called dielectric constants).
Note 2 to entry: For a randomly polarized beam incident at Brewster angle, the electric fields of the reflected
and refracted waves are perpendicular to each other.
Note 3 to entry: For a wave incident from air on water (n = 1,333), glass (n = 1,515), and diamond (n = 2,417),
the Brewster angles are 53, 57, and 67,5 degrees, respectively.
3.28
charge recombination
reverse of charge separation
Note 1 to entry: In using this term, it is important to specify the resulting electronic state of the donor and
acceptor.
3.29
charge separation
process in which, under a suitable influence (e.g., photoexcitation), electronic charge moves in a way
that increases (or decreases) the difference in local charges between donor and acceptor sites
Note 1 to entry: Charge recombination reduces (or increases) the difference.
Note 2 to entry: Electron transfer between neutral species is the most common example of charge separation.
The most important example of charge recombination is electron back transfer occurring after photoinduced
charge separation.
Note 3 to entry: The term is often used in photocatalysis as the generated species in the bands of the
semiconductor are charged (electrons and holes).
3.30
charge shift
process in which, under a suitable influence (e.g., photoexcitation), electronic charge moves without
changing the absolute value of the difference in local charges between the original donor and acceptor
sites
Note 1 to entry: Prominent examples are the electron transfer reversing the charges in a system composed of a
neutral donor and a cationic acceptor or of a neutral acceptor and an anionic donor.
3.31
charge-transfer transition
CT transition
electronic transition in which a large fraction of an electronic charge is transferred from one region of a
molecular entity, called the electron donor, to another, called the electron acceptor (intramolecular CT)
or from one molecular entity to another (intermolecular CT)
Note 1 to entry: Transition typical for donor-acceptor complexes or multichromophoric molecular entities.
Note 2 to entry: See also charge-transfer absorption.
3.32
chemiluminescence
luminescence arising from generation by a chemical reaction of electronically excited molecular entities
from reactants in their ground electronic states
3.33
chromophore
part of a molecular entity consisting of an atom or moiety in which the electronic transition responsible
for a given spectral band above 200 nm is approximately localized
Note 1 to entry: In practice, this definition is extended to a part of a molecular entity in which an electronic
transition responsible for absorption in the ultraviolet region of the spectrum is approximately localized as well as
to a part of a molecular entity in which a vibrational, rotational, or bending transition responsible for absorption in
the infrared region of the spectrum is approximately localized.
3.34
CIELAB color model
color-opponent space with dimension L for lightness and a and b for the color-opponent dimensions
(L*a*b* color space), based on nonlinearly compressed CIE XYZ color space coordinates
Note 1 to entry: LAB is now more often used as an informal abbreviation for (L*, a*, b*) colour space (or
CIELAB). See also ISO 11664–4:2008 (E)/CIE S 014-4/E: Joint ISO/CIE Standard: Colorimetry-Part4: CIE 1976
L*a*b* Colour Space (2007).
Note 2 to entry: Unlike the RGB and CMYK colour models, LAB colour is designed to approximate human vision.
Note 3 to entry: See also RGB.
3.35
color scale
series of ordered numbers that represents observable gradations of a given attribute or gradations of a
combination of attributes of color perception
Note 1 to entry: See CIELAB, RGB.
3.36
conduction band
vacant or only partially occupied set of many closely spaced electronic levels resulting from an array of
a large number of atoms forming a system in which the electrons can move freely or nearly so
Note 1 to entry: Term usually used to describe the properties of metals and semiconductors.
Note 2 to entry: See also bandgap energy, Fermi level, valence band.
3.37
conversion spectrum
plot of a quantity related to the absorption (absorbance, etc.) multiplied by the quantum yield for the

considered process, against a suitable measure of photon energy, such as frequency ν, wavenumber ν , or
wavelength λ
Note 1 to entry: See also action spectrum, efficiency spectrum, spectral effectiveness.
3.38
conversion
chemical conversion
relative amount of substance converted in a chemical reaction
N −−Nc c
η
Nc
0 0
where
N is the number of molecules or on a chemical basis the moles and c is the (entering in or initial)
concentration (subscript 0) and flowing out or transient in the reactor. In a batch reactor
(see Reactor batch), for a reactant A that is converted according to a first order rate r = k c, the
η=1−−exp( kt )
conversion . Typically, the conversion is evaluated under steady state in CSTR of
PFR reactors (see photocatalytic reactor)
Note 1 to entry: Symbols are arbitrary, often η or X are used.
3.39
charge transfer
CT
electronic transition in which a large fraction of electronic charge is transferred from one region of an
entity, called the electron donor, to another, called the electron acceptor
3.40
cut-off filter
optical device that only permits the transmission of radiation of wavelengths longer or shorter than a
specified wavelength
Note 1 to entry: Usually, the term refers to devices that transmit radiation of wavelengths longer than the
specified wavelength.
Note 2 to entry: See also cut-on filter, filter. More detailed info on filters can be found in CEN/TS 16599:2014.
3.41
cut-on filter
optical device that only permits the transmission of radiation of wavelengths shorter than a specified
wavelength
Note 1 to entry: Although more rare than the cut-off filters, there are a few cut-on filters on the market.
Note 2 to entry: See also cut-off filter, filter. More detailed info on filters can be found in CEN/TS 16599:2014.
3.42
dark reaction
chemical reaction that does not require or depend on the presence of light
Note 1 to entry: Contrasts with a photochemical reaction, which is initiated by light absorption by one or more
of the reactants.
Note 2 to entry: A dark reaction is essentially a thermally activated reaction.
==
3.43
deactivation
< of catalysts> Reduced efficiency of the catalyst due to many possible reasons like poisoning, change of
the surface texture during reaction, chemical migration of species
3.44
decay time
time needed for the concentration of an entity to decrease to 1/e of its initial value when this entity
does not disappear by a first-order process
Note 1 to entry: Same as “apparent lifetime”. The use of the latter term is not recommended.
Note 2 to entry: Should the entity disappear by a first-order process, the term lifetime is preferred.
3.45
depth of penetration
inverse of the linear absorption coefficient
Note 1 to entry: SI unit is m; common unit is cm.
Note 2 to entry: in photochemistry the term applies to ultraviolet, visible, or infrared radiation.
Note 3 to entry: When the linear decadic absorption coefficient, a, is used, the depth of penetration (1/a) is the
P
distance at which the spectral radiant power, P , decreases to one-tenth of its incident value, i.e. to /10. When
λ
λ
the linear napierian absorption coefficient, α, is used, the depth of penetration (1/α = β in this case) is the distance
P
at which the spectral radiant power decreases to 1/e of its incident value, i.e. to /e.
λ
Note 4 to entry: See also absorbance, attenuance.
3.46
dichroic filter
filter that use the principle of thin-film interference
Note 1 to entry: see interference filter.
Note 2 to entry: The name dichroic arises from the fact that the filter appears one colour under illumination
with transmitted light and another with reflected light. They selectively pass light of a small range of colors while
reflecting other colors.
More detailed info on filters can be found in CEN/TS 16599:2014.
3.47
dichroic mirror
mirror used to reflect light selectively according to its wavelength
3.48
dichroism
dependence of absorbance of a sample on the type of polarization of the measuring beam
Note 1 to entry: This polarization can be linear, corresponding to linear dichroism (LD) in which the difference
in absorption for two perpendicularly linearly polarized beams is measured, ΔA = A – A , or circular
l Z Y
dichroism (CD) in which the difference in absorption for left minus right circularly polarized beams is measured,
ΔA = A – A .
C L R
3.49
dose
energy or amount of photons absorbed per volume (or per mass) by an irradiated object during a
particular exposure time
–3 –1 –3 –1
Note 1 to entry: SI units are J m or J g and mol m or mol g , respectively.
–3 –1
Note 2 to entry: Common units are einstein m or einstein g , respectively.
Note 3 to entry: In medicine and in some other research areas (e.g. photopolymerization and water purification
through irradiation) dose is used in the sense of exposure, i.e. the energy or amount of photons per surface area
(or per volume) impinging upon an irradiated object during a particular exposure time. This use is not
recommended. The terms photon exposure and radiant exposure are preferred. See also einstein, UV dose.
3.50
efficiency spectrum
plot of the efficiency of a step (η) against wavelength or photon energy
Note 1 to entry: Compare with spectral effectiveness; see also action spectrum, conversion spectrum.
3.51
einstein
mole of photons or an Avogadro number of photons
Note 1 to entry: Widely used, although it is not an SI unit.
Note 2 to entry: The energy of one einstein of photons of frequency ν is E = N hν, with h the Planck constant
A
and N the Avogadro constant.
A
3.52
electron back-transfer
back electron transfer
thermal inversion of excited-state electron transfer restoring the donor and acceptor in their original
oxidation state
Note 1 to entry: In using this term, one should also specify the resulting electronic state of the donor and
acceptor.
Note 2 to entry: The term electron back-transfer is preferred to back electron-transfer.
Note 3 to entry: It is recommended to use this term only for the process restoring the original electronic state of
donor and acceptor.
Note 4 to entry: Should the forward electron transfer lead to charge separation, electron back-transfer will result
in charge recombination.
3.53
electron transfer
transfer of an electron from one molecular entity to another or between two localized sites in the same
entity
Note 1 to entry: See also Marcus equation, outer-sphere electron transfer, inner-sphere electron transfer.
3.54
electronically excited state
state of an atom or molecular entity that has higher electronic energy than the ground state of the same
entity
Note 1 to entry: See also excited state.
3.55
emission of light
radiative deactivation of an excited state; transfer of energy from a molecular entity to an
electromagnetic field
Note 1 to entry: Same as luminescence.
Note 2 to entry: See also fluorescence, phosphorescence.
3.56
emission spectrum
plot of the emitted spectral radiant power or of the emitted spectral photon irradiance (spectral photon

exitance) against a quantity related to photon energy, such as frequency, ν, wavenumber, ν , or
wavelength, λ
Note 1 to entry: When corrected for wavelength-dependent variations in the equipment response, it is called a
corrected emission spectrum.
Note 2 to entry: When the emission is generated by a fluorescence process, it is indicated as fluorescence
spectrum.
3.57
emittance
emissivity
e
radiant exitance emitted by an object relative to that of a black body at the same temperature
Note 1 to entry: Emittance is dimensionless.
Note 2 to entry: Mathematical expression: M/M with M and M the radiant exitance of the object and of a
bb bb
black body, respectively.
3.58
energy transfer
excitation transfer
physical or chemical process originating from the excited state of another molecular entity
Note 1 to entry: In mechanistic photochemistry, the term has been reserved for the process in which an excited
state (produced by absorption of radiation) of one molecular entity (the donor) is deactivated to a lower-lying
state by transferring energy to a s
...