A new absolute diffuse reflectance measurement in the near-IR region based on the modified double-sphere method

Abstract

A new method using integrating spheres for absolute reflectance measurements has been developed to establish a spectral diffuse reflectance scale in the near-IR (NIR) region. This method is a modified version of the double-sphere method developed in 1966, and the modification reduces the uncertainty due to the area measurement of the auxiliary sphere. In this new technique, the modified double-sphere method, a known absolute reflectance scale in the visible region derived from another absolute reflectance measurement is used to extend the absolute reflectance scale from the visible to the NIR region without requiring a strict area measurement. By using this modified double-sphere method, a spectral diffuse reflectance scale is established from 800 nm to 1600 nm with a relative expanded uncertainty (k = 2) of 0.48% to 0.54% depending on the wavelength. The same principle can be applied to determine a spectral diffuse reflectance standard in the ultraviolet region as well. In this paper, an outline of the modified double-sphere method is described with emphasis on its theoretical and instrumental features.