Radiant quantities

Luminous and radiant quantities will be used interchangeably. Luminous quantities refer to measurements performed with a detector having a spectral response equivalent to the CIE standard photometric observer (1), whereas radiant quantities refer to spectrally resolved measurements. The measurement geometries used for luminous and radiant measurements are identical. 

The main parameters to describe light-related phenomena are classified as radiant quantities and luminous quantities the latter are psychophysical parameters. Table 2 shows their definitions and units. 

Laws of Photometry. The time rate at which energy is transported in a beam of radiant energy is denoted by the symbol To for the incident beam, and by P for the quantity remaining unabsorbed after passage through a sample or container. The ratio of radiant power transmitted by the sample to the radiant power incident on the sample is the transmittance T ... 

For environmental reasons, burning should be smokeless. Long-chain and unsaturated hydrocarbons crack in the flame producing soot. Steam injection helps to produce clean burning by eliminating carbon through the water gas reaction. The quantity of steam required can be as high as 0.05—0.3 kg steam per kg of gas burned. A multijet flare can also be used in which the gas bums from a number of small nozzles parallel to radiant refractory rods which provide a hot surface catalytic effect to aid combustion. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

The International Union of Pure and Applied Chemistry recommends that the definition should now be based on the ratio of the radiant power of incident radiation (Pq) to the radiant power of transmitted radiation (P). Thus, A = log(Po/P) = log T. In solution, Pq would refer to the radiant power of light transmitted through the reference sample. T is referred to as the transmittance. If natural logarithms are used, the quantity, symbolized by P, is referred to as the Napierian absorbance. Thus, B = ln(Po/P). The definition assumes that light reflection and light scattering are negligible. If not, the appropriate term for log(Po/P) is attenuance. See Beer-Lambert Law Absorption Coefficient Absorption Spectroscopy... 

A rate of transfer of entities, particles, fluids through a given point, surface, or pathway. For example, the different pathways for a particular enzyme-catalyzed reaction will have a different flux through each of those pathways. See also Chemical Flux. 2. A measure of the power associated with a particular quantity. See Radiant Energy Flux Radiant Power. 3. A measure of the strength of a particular field of force (eg., magnetic flux). 

A dimensionless quantity, symbolized by t or T, equal to the transmitted radiant power, Ptr, divided by the radiant power incident on the sample, Po thus, t = Ptr/ Pq. It is a measure of the ability of a body, solution, entity, eta, to transmit electromagnetic radiation. It is synonymous with transmission factor. See also Internal Transmittance Transmission Density Total Transmittance Beer-Lambert Law Absorption Spectroscopy... 

Irradiance is the energy per unit time per unit area in the light beam (watts per square meter. W/m2). The terms intensity or radiant power have been used for this same quantity. 

It is interesting to note that the fin efficiency reaches its maximum value for the trivial case of L = 0, or no fin at all. Therefore, we should not expect to be able to maximize fin performance with respect to fin length. It is possible, however, to maximize the efficiency with respect to the quantity of fin material (mass, volume, or cost), and such a maximization process has rather obvious economic significance. We have not discussed the subject of radiation heat transfer from fins. The radiant transfer is an important consideration in a number of applications, and the interested reader should consult Siegel and Howell IV1 for information on this subject. 

Planck constant — To describe the spectral distribution of energy of black body radiation -> Planck made the ad hoc assumption that the radiant energy could exist only in discrete quanta which were proportional to the frequency E = hu with h = 6.62 6 0 6 93(11) x 10 - 34 Js. Before 2003 the accepted value was 6.6260755(40) x 10-34 Js = 4.1356692(12) x 10-15 eV s. The quantity h later was referred to as Planck s constant. 

The symbols for the quantities radiant energy through irradiance are also used for the corresponding quantities concerning visible radiation, i.e. luminous quantities and photon quantities. Subscripts e for energetic, v for visible, and p for photon may be added whenever confusion between these quantities might otherwise occur. The units used for luminous quantities are derived from the base unit candela (cd), see chapter 3. 

Spectral quantities may also be defined with respect to frequency v, or wavelength 2 see spectral radiant energy density above. 

Fluence is used in photochemistry to specify the energy delivered in a given time interval (for instance by a laser pulse). This quantity may also be called radiant exposure. 

Gas Emissivities Radiant transfer in a gaseous medium is characterized by three quantities the gas emissivity, gas absorptivity, and gas transmissivity. Gas emissivity refers to radiation originating within a gas volume which is incident on some reference surface. Gas absorptivity and transmissivity, however, refer to the absorption and transmission of radiation from some external surface radiation source characterized by some radiation temperature 7. The sum of the gas absorptivity and transmissivity must, by definition, be unity. Gas absorptivity may be calculated from an appropriate gas emissivity. The gas emissivity is a function only of the gas temperature Tg while the absorptivity and transmissivity are functions ofboth Tg and Tt. 

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