Diffuse reflectance measurements by infrared Fourier transform spectrometry

The prerequisites for applicability of the Kubelka-Munk theory of diffuse reflectance for obtaining characteristic color curves of powders are discussed, and some experimental support for the theory is given. Methods are described for eliminating surface reflection, which is always superimposed upon the diffuse reflection and which therefore distorts the spectrum. The effect of interactions with the adsorbent on the reflection spectra of adsorbed molecules is demonstrated by means of examples, and the special suitability of the method for investigating chemisorption and for following reactions at phase boundaries is pointed out. By analogy to the Beer-Lambert law, the Kubelka-Munk function can be used for quantitative photometric analysis. The reflectance curves of white standards are presented, and measurements relating to the conditions for “infinitely thick layers” are reported. The scattering coefficient of the Kubelka-Munk functions has been estimated for several samples of color-filters as a functions of grain-size and wavelength. The effect of moisture on diffuse reflectance spectra is discussed, and details of the measuring techniques are presented.

Principles and Techniques of Diffuse‐Reflectance Spectroscopy

Publisher Summary Radiation heat transfer involving molecular gases is the desirable effect in fossil-fuel or waste-fired steam generators for power production or process steam supply and in industrial heat-treating, melting, or enameling furnaces Heat radiation from gases may be utilized as a means of heat transfer and also of information transfer Information on the location, extent, composition, and temperature of a volume of molecular gases can be inferred from thermal radiation measurements The information obtained can be used by process control, fire and air pollution warning and monitoring systems, and by weapon and brush-fire-suppressant targeting or guidance systems This chapter examines how to make a nongray radiation heat transfer analysis for problem situations involving molecular gases with infrared vibration rotation bands when the gaseous mixture is in a state corresponding to thermodynamic equilibrium at the local thermodynamic state variables: pressure, temperature, and composition The special circumstances under which total emissivity and total absorptivity appear as useful quantities are derived, and approximations extending their utility are discussed The chapter examines how the more general and difficult problem of a nonisothermal gas is treated Some particular solutions for the parallel-plate duct are described, both to illustrate nonisothermal gas analysis and to obtain wall-layer transmission factors to extend the utility of engineering models employing an isothermal gas volume concept The chapter provides a brief description of the experimental knowledge of molecular gas behavior and a review of the fundamental ideas of radiation transfer The spectral band nature of gas radiation is briefly described and correlations for working calculations of gas radiation properties are presented

Molecular Gas Band Radiation

An integrating sphere for determining spectral reflectance and transmittance as a function of angle of incidence and wavelength in the 0.33- to 2.5-μ region is described. Geometrical arrangement of sample, entrance port, and detector as well as directional characteristics of detector and sphere wall coating permit absolute or relative measurements to be made for a sample with an arbitrary reflection-distribution function.

Integrating Sphere for Imperfectly Diffuse Samples

Die Voraussetzungen fur die Anwendbarkeit der Kubelka-Munk-Theorie der diffusen Reflexion zur Gewinnung typischer Farbkurven von Pulvern werden diskutiert und experimentelle Prufungen der Theorie angegeben. Es werden Methoden beschrieben, mit denen sich die der diffusen Reflexion stets uberlagerte Oberflachenreflexion, die die Spektren verfalscht, eliminieren last. Die Beeinflussung der Reflexionsspektren adsorbierter Molekeln durch Wechselwirkung mit dem Adsorbens wird an Beispielen gezeigt und auf die besondere Eignung der Methode zur Untersuchung der Chemisorption und zur Verfolgung von Reaktionen in der Phasengrenzflache hingewiesen. Die Kubelka-Munk-Funktion last sich analog dem Lambert-Beerschen Gesetz fur die quantitative photometrische Analyse anwenden. Es werden die Reflexionskurven von Weis-Standards angegeben und Messungen uber die Bedingung der „unendlichen Schichtdicke” mitgeteilt. Der Streukoeffizient der Kubelka-Munk-Funktion wird an einigen Farbfilter-Proben in Abhangigkeit von Korngrose und Wellenlange abgeschatzt. Der Einflus von Feuchtigkeit auf die diffusen Reflexionsspektren wird diskutiert, und Einzelheiten zur Mesmethodik werden angegeben.

Prinzip und Meßmethodik der diffusen Reflexionsspektroskopie

A device that can be used for the measurement of the absolute spectral reflectivity or reflectance of metals, paints, etc. is described. The unit is used in conjunction with a Perkin-Elmer Model 83 monochromator equipped with an NaCl prism for the wavelength range 1.0 to 15.0 microns. Preliminary results for MgO, polished copper, molybdenum, and electrolytic gold are presented.

Measurement of absolute spectral reflectivity from 1. 0 to 15 microns

Solar reflectance integrating sphere

Apparatus for the measurement of the band absorption of gases at pressures up to 10 atmos and temperatures up to 1400 K is described. An optical path of 15 in. is maintained with an accuracy of 1% by a nozzle seal system which allows measurements to be made without the use of windows in the high temperature zone. Use of KBr windows for the pressure seal permits measurements to 23 μ.

Joseph T. Gier

Papers

Measurement of absolute spectral reflectivity from 1. 0 to 15 microns

Solar reflectance integrating sphere

Apparatus for the determination of the band absorption of gases at elevated pressures and temperatures

Method of making electromagnetic wave reflector