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SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014]
Measurement of photochemical or photobiological quantities and their corresponding units
SI Brochure, Appendix 3, Section 2

    The photometric quantities and photometric units which are used at present for vision are well established and have been widely used for a long time. They are not affected by the following rules. For all other photochemical and photobiological quantities the following rules shall be applied for defining the units to be used.

    A photochemical or photobiological quantity is defined in purely physical terms as the quantity derived from the corresponding radiant quantity by evaluating the radiation according to its action upon a selective receptor, the spectral sensitivity of which is defined by the actinic action spectrum of the photochemical or photobiological effect considered. The quantity is given by the integral over wavelength of the spectral distribution of the radiant quantity weighted by the appropriate actinic action spectrum. The use of integrals implicitly assumes a law of arithmetic additivity for actinic quantities, although such a law is not perfectly obeyed by actual actinic effects. The action spectrum is a relative quantity; it is dimensionless, with the SI unit one. The radiant quantity has the radiometric unit corresponding to that quantity. Thus, following the rule for obtaining the SI unit for a derived quantity, the unit of the photochemical or photobiological quantity is the radiometric unit of the corresponding radiant quantity. When giving a quantitative value, it is essential to specify whether a radiometric or actinic quantity is intended as the unit is the same. If an actinic effect exists in several action spectra, the action spectrum used for measurement has to be clearly specified.

    This method of defining the units to be used for photochemical or photobiological quantities has been recommended by the Consultative Committee for Photometry and Radiometry at its 9th meeting in 1977.

    As an example, the erythemal effective irradiance Eer from a source of ultraviolet radiation is obtained by weighting the spectral irradiance of the radiation at wavelength lambda by the effectiveness of radiation at this wavelength to cause an erythema, and summing over all wavelengths present in the source spectrum. This can be expressed mathematically as:

    Eer = integral ofElambda ser(lambda) dlambda

    where Elambda is the spectral irradiance at wavelength lambda (usually reported in the SI unit W m–2 nm–1), and ser(lambda) is the actinic spectrum normalized to 1 at its maximum spectral value. The erythemal irradiance Eer determined in this way is usually quoted in the SI unit W m–2.


We are pleased to present the updated (2014) 8th edition of the SI Brochure, which defines and presents the Système International d'Unités, the SI (known in English as the International System of Units).

Chapter 1: Introduction

Chapter 2: SI units

Chapter 3: Decimal multiples and submultiples of SI units

  • SI prefixes
  • Factor Name Symbol Factor Name Symbol
    101 deca da 10–1 deci d
    102 hecto h 10–2 centi c
    103 kilo k 10–3 milli m
    106 mega M 10–6 micro µ
    109 giga G 10–9 nano n
    1012 tera T 10–12 pico p
    1015 peta P 10–15 femto f
    1018 exa E 10–18 atto a
    1021 zetta Z 10–21 zepto z
    1024 yotta Y 10–24 yocto y
  • The kilogram

Chapter 4: Units outside the SI

Chapter 5: Writing unit symbols and names, and expressing the values of quantities

General principles for the writing of unit symbols and numbers were first given by the 9th CGPM (1948, Resolution 7). These were subsequently elaborated by ISO, IEC, and other international bodies. As a consequence, there now exists a general consensus on how unit symbols and names, including prefix symbols and names, as well as quantity symbols should be written and used, and how the values of quantities should be expressed. Compliance with these rules and style conventions, the most important of which are presented in this chapter, supports the readability of scientific and technical papers.

Appendix 1: Decisions of the CGPM and the CIPM

This appendix lists those decisions of the CGPM and the CIPM that bear directly upon definitions of the units of the SI, prefixes defined for use as part of the SI, and conventions for the writing of unit symbols and numbers. It is not a complete list of CGPM and CIPM decisions. For a complete list, reference must be made to the BIPM website, successive volumes of the Comptes Rendus des Séances de la Conférence Générale des Poids et Mesures (CR) and Procès-Verbaux des Séances du Comité International des Poids et Mesures (PV) or, for recent decisions, to Metrologia.

Since the SI is not a static convention, but evolves following developments in the science of measurement, some decisions have been abrogated or modified; others have been clarified by additions. In the SI Brochure, a number of notes have been added by the BIPM to make the text more understandable; they do not form part of the original text.

In the printed brochure, the decisions of the CGPM and CIPM are listed in strict chronological order in order to preserve the continuity with which they were taken. However in order to make it easy to locate decisions related to particular topics a table of contents is also provided, ordered by subject:

Appendix 2: Practical realization of the definitions of some important units

Appendix 3: Units for photochemical and photobiological quantities

Optical radiation is able to cause chemical changes in certain living or non-living materials: this property is called actinism, and radiation capable of causing such changes is referred to as actinic radiation. Actinic radiation has the fundamental characteristic that, at the molecular level, one photon interacts with one molecule to alter or break the molecule into new molecular species. It is therefore possible to define specific photochemical or photobiological quantities in terms of the result of optical radiation on the associated chemical or biological receptors.

In the field of metrology, the only photobiological quantity which has been formally defined for measurement in the SI is for the interaction of light with the human eye in vision. An SI base unit, the candela, has been defined for this important photobiological quantity. Several other photometric quantities with units derived from the candela have also been defined (such as the lumen and the lux, see Table 3 in Chapter 2).