Rethinking the Concepts of Fluence (UV Dose) and Fluence Rate: The Importance of Photon-based Units - A Systemic Review

• Published in: Photochemistry and photobiology Vol. 91; no. 6; pp. 1252 - 1262
• Main Authors: Bolton, James R., Mayor-Smith, Ian, Linden, Karl G.
• Format: Journal Article
• Language: English
• Published: United States Blackwell Publishing Ltd 01-11-2015
• Subjects: Atoms & subatomic particles; Dose-Response Relationship, Radiation; Mercury; Photobiology; Photochemicals; Photochemistry; Photons; Ultraviolet radiation; Ultraviolet Rays
• ISSN: 0031-8655; 1751-1097; 1751-1097
• DOI: 10.1111/php.12512

After a critical review of the fundamental equations describing photobiological and photochemical processes occurring in a medium exposed to a quasi‐collimated monochromatic UV light beam, the analysis in this review is extended to analogous processes driven by polychromatic UV light, such as that emitted by medium pressure mercury‐vapor arc lamps. The analysis is based on the Second Law of Photochemistry, namely that all photochemical events must be independent, and the rate of such events must be proportional to the rate of photon absorption. A consistent application of the Second Law of Photochemistry leads to a concept change; hence it is proposed herein to use photon fluence and photon fluence rate, rather than fluence (UV dose) and fluence rate, respectively, in the analysis and interpretation of photobiological and photochemical processes. As a consequence, many equations that have been used in the past must be revised, and some experimental information (e.g. action spectra) needs to be re‐analyzed.
This diagram depicts the reactor that could be used to study photochemical or photobiological reactions in a collimated beam apparatus. The reactor is configured so that photons from the UV lamp that pass through the hole in the mask impinge directly on the surface of the solution, but no photons impinge on the walls of the dish. This diagram illustrates the fundamental concept in the paper, namely that photochemical and photobiological reactions should use photon‐based terms, such as photon irradiance (einstein m−2 s−1), rather than power‐based terms, such as irradiance (W m−2).