Color ratios of the CE5 soil measured by the three-color goniometer (a)(b) and by the spectro-goniometer (c)(d). The color ratios between 633 and 473 nm with (a) absolute value and (b) normalized at the smallest phase angle (~3°). The color ratios between 500 and 800 nm with (c) i = 30° and (d) i = 45°. In each image plot, the radial distance from the center represents the zenith angle, and the azimuth angle measured from due north represents the relative azimuth angle. The incident zenith directions are indicated by orange dots. The backward and forward scattering directions are along the azimuth angles of 0° and 180° , respectively.