Outdoor experiments of LCTF spectropolarimeters for Earth observation

Abstract

The National Aerospace Laboratory (NAL) has developed a new type of imaging spectropolarimeter that uses a liquid crystal tunable filter (LCTF) which makes it possible to measure the optical properties of solar rays reflected from land and sea surfaces. The aim of this development is to pave the way for the establishment of polarimetric analysis of solar rays reflected from the Earth’s surface as a method of Earth environment observation. Two imaging LCTF spectropolarimeters that cover different wavelength bands have been developed: a visible light sensor for the 400-720 nm wavelength band, and a near-infrared sensor that covers the 650-1100 nm band. Efforts are now under way to apply these optical sensors to practical applications, for airborne and ultimately spaceborn Earth environment remote sensing. This paper first outlines the imaging optical sensors, including their operational principles and construction. Next, various spectral images acquired using the visible light optical sensor in outdoor field and flight evaluation experiments to measure spectral characteristics of solar rays reflected from land and water surfaces are shown. Then, the results of outdoor experiments conducted using the near-infrared optical sensor are shown, including the analyzed relative radiance of solar rays reflected from observed spots, and spectral images acquired at various wavelengths and polarization angles. These experimental results demonstrate clearly that solar rays reflected from targets with differing characteristics have different spectropolarimetric properties. Finally it is concluded that the way has been paved for determining surface conditions from the properties of the spectral images acquired by LCTF spectropolarimeters at wavelengths of 400-1100 nm.