Determination of optical constants from extinction measurements

Abstract

Traditional methods of determining the optical constants of particulate materials by means of transmission, absorption, and reflectance measurements are known to be inherently inaccurate. The use of the Lorenz–Mie formalism to derive the optical constants from extinction data overcomes the problems associated with the traditional methods; but, as currently practiced, this method has severe limitations. In this paper we report an entirely new approach to determining the optical constants of aerosols from extinction data. This is an iterative method that uses the Lorenz–Mie formalism in conjunction with the Kramers–Kronig dispersion relations in order to derive the optical constants of the aerosol material. The theory of the method is developed in detail and is applied successfully to find the optical constants of an o-phosphoric-acid aerosol in the 7–14-μm infrared. The numerical procedure is shown to introduce an error of less than 1% in the determination of the o-phosphoric-acid optical constants. Limits on n, k and the particle size distribution for which the method is valid are indicated.