See energy transfer, internal conversion, intersystem crossing. RADIATIONLESS DEACTIVATION ( Decay) - Loss of electronic excitation energy without photon emission or chemical change. In photochemistry Φ is reserved for quantum yield. Power emitted, transferred or received as radiation. RADIANT POWER ( P) - Same as radiant (energy) flux, Φ. The radiant power emitted in a given direction by a source or an element of the source in an infinitesimal cone containing the given direction divided by the solid angle of the cone (d P/dω, simplified expression: I = P/S when the radiant power is constant over the surface area considered). RADIANT INTENSITY ( I) - Radiant (energy) flux or radiant power, P, per unit solid angle, ω. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings fluence ( H 0) is an equivalent term. t when the irradiance is constant over the time considered).RADIANT EXPOSURE ( H) - The irradiance, E, integrated over the time of irradiation (∫ Ed t, simplified expression H = E See also photon exitance, spectral radiant exitance. It is the integration of the radiant power leaving a source over the solid angle and over the whole wavelength range. (d P/d S, simplified expression: M = P/S when the radiant power is constant over the surface area considered). RADIANT EXITANCE ( M) - The radiant power, P, emitted by an element of the surface containing the source point under consideration divided by the surface area ( S) of that element. See also photon flow, photon radiance, radiant energy, spectral radiant flux. ( P = Φ = d Q/d t, simplified expression: P = Φ = Q/t when the radiant energy, Q, is constant over the time considered). RADIANT (ENERGY) FLUX (Φ) - Although flux is generally used in the sense of the ‘rate of transfer of fluid, particles or energy across a given surface’, the radiant energy flux has been adopted by IUPAC as equivalent to radiant power, P. It is the product of radiant power, P, and time, t: Q = P t when the radiant power is constant over the time considered. ![]() RADIANT ENERGY ( Q) - The total energy emitted, transferred or received as radiation in a defined period of time ( Q = ∫ Q λdλ). ![]() RADIANT EMITTANCE - See radiant exitance. See also photon flow, photon radiance, spectral radiance, spherical radiance. For a divergent beam propagating in an elementary cone of the solid angle dω containing the given direction Ψ, the radiance is d 2P/(dω d S cos Ψ), with SI units W m −2 sr −1. RADIANCE ( L) - For a parallel beam it is the radiant power, P, leaving or passing through an infinitesimal element of surface in a given direction from the source divided by the orthogonally projected area of the element in a plane normal to the given direction of the beam, Ψ.
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