Reflectance spectroscopy is the investigation of the spectral composi tion of surface-reflected radiation with respect to its angularly dependent intensity and the composition of the incident primary radiation. Two limiting cases are important: The first concerns regular (specular) reflection from a smooth surface, and the second diffuse reflection from an ideal matte surface. All possible variations are found in practice between these two extremes. For the two extreme cases, two fundamentally different methods of reflectance spectroscopy are employed: The first of these consists in evaluating the optical constants n (refractive index) and x (absorption index) from the measured regular reflection by means of the Fresnel equations as a function of the wave A. This rather old and very troublesome procedure, which is length incapable of very accurate results, has recently been modified by Fahren fort by replacing the air-sample phase boundary by the phase boundary between a dielectric of higher refractive index (n ) and the sample (n ). 1 2 If the sample absorbs no radiation and the angle of incidence exceeds a certain definite value, total reflection occurs. On close optical contact between the two phases, a small amount of energy is transferred into the less dense phase because of diffraction phenomena at the edges of the incident beam. The energy flux in the two directions through the phase boundary caused by this is equal, however, so that 'total reflection takes place.
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Für Beruf und Forschung
Research
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ISBN-13
978-3-540-04587-8 (9783540045878)
DOI
10.1007/978-3-642-88071-1
Schweitzer Klassifikation
I. Introduction.- II. Regular and Diffuse Reflection.- a) Regular Reflection at Non-Absorbing Media.- b) Total Reflection.- c) Regular Reflectance at Strongly Absorbing Media.- d) Definition and Laws of Diffuse Reflection.- e) Experimental Investigation of Diffuse Reflection at Non-Absorbing Materials.- f) Diffuse Reflectance at Absorbing Materials.- g) Dependence of Remission Curves on Particle Size.- III. Single and Multiple Scattering.- a) Rayleigh Scattering.- b) Theory of Scattering at Large Isotropic Spherical Particles.- c) Multiple Scattering.- d) The Radiation-Transfer Equation.- IV. Phenomenological Theories of Absorption and Scattering of Tightly Packed Particles.- a) The Schuster Equation for Isotropic Scattering.- b) The Kubelka-Munk Exponential Solution.- c) The Hyperbolic Solution Obtained by Kubelka and Munk.- d) Use of Directed Instead of Diffuse Irradiation.- e) Consideration of Regular Reflection at Phase Boundaries.- f) Absolute and Relative Measurements.- g) Consideration of Self-Emission or Luminescence.- h) Attempts at a Rigorous Solution of the Radiation-Transfer Equation.- i) Discontinuum Theories.- V. Experimental Testing of the "Kubelka-Munk" Theory.- a) Optical Geometry of the Measuring Arrangement.- b) The Dilution Method.- c) Concentration Dependence of the "Kubelka-Munk" Function F(R?).- d) The Typical Color Curve.- e) Influence of Cover Glasses.- f) Scattering Coefficients and Absorption Coefficients.- g) Influence of Scattering Coefficients on the "Typical Color" Curve".- h) Particle-Size Dependence of the Kubelka-Munk Function.- VI. Experimental Techniques.- a) Test of the Lambert's Cosine Law.- b) The Integrating Sphere.- c) Measuring Apparatus.- d) Measurements with Linearly Polarized Radiation.- e) The Measurement of Fluorescent Samples.- f) Influence of Moisture on Reflectance Spectra.- g) Preparation of Samples for Measurement.- h) Adsorption from the Gas Phase and from Solution.- i) Measurements in the Infrared.- k) Discussion of Errors.- VII. Applications.- a) The Spectra of Slightly Soluble Substances, or Substances that are Altered by Dissolution.- b) Spectra of Adsorbed Substances.- Acid-Base Reactions between Adsorbed Substance and the Adsorbent.- Charge Transfer Complexes.- Redox Reactions.- Reversible Cleavage Reactions.- Surface Area Determination of Powders.- Establishment of Equilibria and Orientation at Surfaces.- Photochemical Reactions.- c) Kinetic Measurements.- d) Spectra of Crystalline Powders.- e) Dynamic Reflectance Spectroscopy.- f) Analytical Photometric Measurements.- g) Color Measurement and Color Matching.- VIII. Reflectance Spectra Obtained by Attenuated Total Reflection.- a) Determination of the Optical Constants n and ?.- b) Internal Reflection Spectroscopy.- c) Methods.- d) Applications.- Appendix: Tables of the Kubelka-Munk-Function.
