Radiometric quantities

Not surprisingly, S is in the direction of propagation. The magnitude of S, which we shall denote by the symbol I, is called the irradiance and its dimensions are energy per unit area and time. (The term intensity is often used to denote irradiance however, intensity is also used for other radiometric quantities, and we shall therefore tend to avoid this term because of possible confusion. E is now the recommended symbol for irradiance, but this hardly seems appropriate in a book where the electric field and irradiance often appear side by side.) As the wave traverses the medium, the irradiance is exponentially attenuated ... 

Since in industrial photochemistry mostly polychromatic light sources are used, photon quantities are relatively difficult to calculate and require knowledge of the spectral distribution of the radiometric quantity measured. Assuming on the other hand that the radiometric measurements do not need to be corrected for the spectral response of the probe, the photon irradiance at a given point within the reactor volume would then be given by Eqs. (39) and (40), respectively. 

In principle, there are two ways to achieve the radiometric calibration of an instrument measuring solar radiation. The first is by comparison to a standard radiation source of known output and the second by comparison to a prototype standard instrument that is capable in measuring the same radiometric quantity. The fist can be applied to broadband detectors only if their spectral response over the whole range of the radiation source is known with sufficient accuracy. The second method requires that the standard instrument has exactly the same spectral response, which is rather unlikely to occur. 

Irradiance — The irradiance E is the radiometric quantity that corresponds to the incident power per unit area at a given surface, given in W nT2. 

Radiometric quantities are important to describe and measure UV and VUV radiation. They are usually subdivided into energetic, spectral and photonic terms. Energetic terms (Tab. 3-9) are based on the energy of the radiation and they refer to all relevant wavelengths. Eor each of these terms a spectral derivative can be defined (Bolton, 1999) which is correlated with a specific wavelength X. Eor example, the extraterrestrial solar spectrum incident on the upper atmosphere is represented by the mean spectral irradiance Eq in W m nm over a defined wavelength interval AX in nm (CIE, 1989). Further, each of the spectral units can easily be transferred to photon-based units, which themselves are related to radio-metric units (cf Braun et al., 1991). 

Note The symbols for photometric quantities (see following table) are the same as those for the corresponding radiometric quantities (see above). When it is necessary to differentiate them the subscripts r and e respectively should be used. e.g.. Q, and Q,. 

In addition to these two radiometric assessments of the beam intensity, for beams of visible light there is a third possibility, which is to quantify the intensity of the beam by the intensity of visual perception by the human eye. Physical quantities connected with this physiological type of assessment are called photometric quantities, in contrast to the two radiometric quantities described above. In photometry, the intensity of the beam is called the luminous intensity 7y. The subscript v stands for visual . The luminous intensity 7v is an ISO recommended base quantity the corresponding SI base unit is the candela (cd). The luminous flux is determined as the product of the luminous intensity and the solid angle. Its dimensions therefore are luminous intensity times solid angle, so that the SI unit of the luminous flux < v turns out to be candela times steradian (cd sr). A derived unit, the lumen (Im), such that 1 Im = 1 cd sr, has been introduced for this product. 

The quantity Fg can be any of the above radiometric quantities photometric quantity is obtained by replacing the term radiant with the term luminous (e.g. luminous flux luminous intensity /, luminance L, illuminance E etc.). The crucial criterion for the assessment of a CMS is the final impact on the (human) driver. Therefore, the physical quantities to be considered are the photometric ones. However, there are significant differences between the luminosity functions for photopic vision (high intensity, daylight) and scotopic vision (low intensity, night scenes)—see Fig. 1. 

Radiometric quantity (physics) Photometric quantity (physiology)... 

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