Introduction

The validity of photobiological research depends strongly upon the accuracy and precision of the applied action spectra, measurement techniques and consideration of exposure geometry. It is therefore important to recognize the factors which influence the quality of the original photobiological research, the possible sources of error and the levels of uncertainty in applying laboratory action spectra to human health risk assessment. One need only study the variation in the reported action spectra published by different laboratories for the same biological effect to recognize that the derivation of sound action spectra is surely fraught with some problems. Both biological and physical factors influence the variation in published action spectra. Biological factors which may affect the actual biological effect of exposure to optical radiation include: variation in response among species, variation in response due to individual adaptation, influence of nutritional factors, the biological endpoint applied in each study, and the means and time of assessment of the endpoint in the plant, animal or human subjects. The physical factors which influence any reported action spectrum relate to the accuracy of radiometric and spectroradiometric measurements, the type of light source and the geometry and spectral bandwidth of each exposure used in the biological experiment.

To characterize photobiological phenomena, standardized terms and units are required. Without a uniform set of descriptors, much of the scientific value of publications can be lost. Attempting to achieve an international consensus for a common language has always been difficult, but now with truly international publications, it is all the more important. Since photobiology represents the fusion of several scientific disciplines, it is not surprising that the physical terms used to describe dosimetric concepts (e.g.., surface exposure dose, absorbed dose, dose rate, fluence, etc.) can vary from author to author. There are, however, international organizations that were established to minimize the confusion produced by poor or inconsistent technical terminology. Hopefully all scientific workers will apply the standardized terminology of the International Commission on Illumination, the CIE [1,2].

The International Commission on Illumination, the CIE, has provided international guidance in the science, terminology and measurement of light and optical radiation since 1913. The International Lighting Vocabulary (parts of which are also issued as an ISO or IEC standard) has been an international reference for photobiologists for many decades and the next revision is due to be published later this year. Despite its stature, many research scientists are unfamiliar with some of the subtle distinctions between the widely used dosimetric terms, such as irradiance, radiant exposure, fluence rate and fluence. For example, the standardized radiometric quantity, "radiant fluence," is defined as the radiant energy that passes through the circular cross-sectional area of an extremely small sphere. The photon fluence is the number of photons passing through this area. These quantities are most useful in quantifying a dose within a scattering medium, such as deep within tissue. Although radiant fluence has units of joules per unit area, that same unit applies to "radiant exposure." This latter radiometric quantity is what is frequently measured at the exposed surface of tissue and incorporates an added cosine dependence that is not present in the concept of fluence that captures energy arriving from all directions and entering the tiny dosimetric sphere. Similar distinctions apply to fluence-rate and irradiance. It is also of note that the same definitions apply in ionizing radiation dosimetry. The proper use of internationally defined quantities and units in scientific publications promotes the best international communication in the science of photobiology and photochemistry.

The CIE considers all aspects of the effects of optical radiation and lighting upon materials, plants, animals and humans other than vision. The Division prepares technical reports by technical committees (TCs) and by individual reporters, prepares standards, and promotes scientific meetings and seminars.

In this article the main definitions and very important procedure for determination of action spectra are described for practical use.

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