Glossary
Many of the following UV definitions have been extracted from an article written by the UV Disinfection Committee of the International Union of Pure and Applied Chemistry. For further details on these definitions, contact Jim Bolton (jbolton@iuva.org).
The full technical paper from the UV Disinfection Committee is downloadable from the technical library, in the Information Centre menu.
Absorbance
The absorbance is a manifestation of the Beer-Lambert Law and is defined as the logarithm to the base 10 of the ratio of the incident spectral irradiance to the spectral irradiance of transmitted radiation when only absorption is considered.
Action Spectrum
The action spectrum is a quantitative indication of the relative biological or chemical photoresponse, per number of incident photons, as a function of wavelength or energy of radiation under the same incident fluence rate.
Many microorganisms have a disinfection action spectrum that is very similar to the absorption spectrum of DNA, while others have different action spectra. Ideally, one should use the action spectrum appropriate to the microorganism of interest. Unfort¬unately, in most cases the appropriate action spectrum is not known. In such cases, it is acceptable to assume that the action spectrum is proportional to that of the absorbance of DNA.
Biodosimetry
At present, the most accurate way to determine the biological effect of polychromatic fluence is to conduct bioassay experiments, known as biodosimetry.
Collimated Beam Devices
Collimated beam devices are often used to provide precise doses of radiation (fluence) to a sample. Their main objective is to provide a radiation field of uniform, measured fluence rate that can be used to irradiate a sample.
DNA
Deoxyribonucleic acid. DNA constitutes the molecules inside cells that carry genetic information and pass it from one generation to the next.
The molecule that encodes genetic information in the cells. It is made up of 4 bases (adenine, guanine, cytosine, and thymine) which are each bonded to a sugar (deoxyribose) and a phosphate to make nucleotides. These are repeated in various sequences along the molecule. Complementary bases pair to form a double helix structure. The sequence of nucleotides makes a gene which is used to make proteins.
Germicidal Wavelengths
The germicidal effectiveness of UV radiation depends on wavelengths that cause inactivation of microorganisms. For the purposes of UV disinfection, the adjective “UV” (e.g., UV irradiance, UV fluence rate, UV fluence, etc.) may be used to imply restriction to germicidal wavelengths. In practice, most waters absorb strongly below 220 nm and there is not much germicidal action above 300 nm. However, the germicidal wavelengths may be specific to each individual scenario. Thus a “standard” germicidal range cannot be absolutely defined.
Fluence (UV Dose)
Microorganisms are inactivated according to the total fluence incident on them as a result of the UV irradiation process.
Fluence (also called UV dose) (symbol H', units J m?2) is the total amount of radiant energy from all directions incident on an infinitesimally small sphere of cross-sectional area dA, divided by dA.
Irradiance (E) and Fluence Rate (E')
For purposes of UV disinfection work, a clear distinction needs to be made between “irradiance” and “fluence rate”. They are similar (same units) but conceptually quite different. Note that the definitions given here apply for any wavelength range.
Irradiance (symbol E; units W m?2) is defined as the total radiant power incident from all upward directions on an infinitesimal element of surface of area dA containing the point under consideration divided by dA.
Fluence rate (symbol E'; units W m?2) is defined as the total radiant power incident from all directions onto an infinitesimally small sphere of cross-sectional area dA, divided by dA.
The appropriate term for UV disinfection is “fluence rate” because a microorganism can receive UV radiation from any direction, especially when there is more than one UV lamp in the vicinity.
Photon Irradiance (Ep)
The photon irradiance (symbol Ep; units einstein s?1 m?2) is defined as the total number of einsteins (moles of photons) incident on an infinitesimal element of surface of area dA containing the point under consideration divided by dA.
Photon Fluence Rate (Ep')
The photon fluence rate (symbol Ep'; units einstein s?1 m?2) is defined as the total number of einsteins (moles of photons) incident from all directions onto an infinitesimally small sphere of cross-sectional area dA, divided by dA.
Photolysis
The cleavage of one or more covalent bonds in a molecular entity resulting from absorption of light , or a photochemical process in which such cleavage is an essential part. For example:
Cl2 -> 2 Cl.
Transmittance
As a beam of radiation passes through a solution, it will get attenuated by absorbing substances in the solution and by absorption, reflection, scattering, etc. from particles and the walls of the cell. (The internal transmittance refers to energy loss only by absorption, whereas the total transmittance considers that due to absorption, reflection, scatter, etc.)
Often in UV disinfection work, the absorption characteristics of the water are described by the “transmittance” of a beam passing through a solution in a cell with a defined path length of 10 (T10), 40 (T40) or 100 mm at a fixed wavelength (usually 254 nm).