Gustav Kortüm – författare

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.

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.

Unter Reflexionsspektroskopie versteht man die Untersuchung der von einer Oberflache reflektierten Strahlung in bezug auf ihre spektrale Zusammensetzung im Vergleich zu der Zusammensetzung der einfallen- den Primarstrahlung und auf die Winkelverteilung der Strahlungsleistung. Zwei Grenzfalle sind dabei wichtig: Entweder es handelt sich um regulare (Spiegel-) Reflexion von einer ideal ebenen Oberflache, oder es handelt sich um diffuse Reflexion von einer ideal matten Oberflache. Zwischen beiden Grenzfallen gibt es in Praxis alle moglichen Ubergange. Ent- sprechend diesen Grenzfallen gibt es zwei prinzipiell verschiedene Metho- den der Reflexionsspektroskopie: Die eine besteht darin, aus der gemessenen regularen Reflexion die optischen Konstanten n (Brechungsindex) und x (Absorptionsindex) des betreffenden Stoffes mit Hilfe der Fresnelschen Gleichungen in Ab- A. zu berechnen.Dieses altere und recht hangigkeit von der Wellenlange umstandliche Verfahren, das ausserdem keine sehr genauen Resultate liefert, ist neuerdings von Fahrenfort insofern modifiziert worden, als man fur die Reflexion nicht die Phasengrenzflache Luft/Probe benutzt, sondern die Phasengrenze zwischen einem Dielektrikum hoheren Bre- chungsvermogens (nd und der Probe (n ). Absorbiert die Probe nicht, so 2 beobachtet man oberhalb eines bestimmten Einfallswinkels Total- reflexion. Trotzdem tritt bei engem (optischen) Kontakt der beiden Phasen doch eine geringe Energie infolge von Beugungserscheinungen an den Randern des Bundels in die dunnere Phase uber, jedoch ist der Energiefluss in beiden Richtungen durch die Phasengrenze hindurch gleich gross, so dass man Totalreflexion findet. Absorbiert dagegen die Probe, so geht ein Teil der uberfuhrten Strahlungsenergie verloren, die Totalreflexion wird geschwacht.