REFF measured by the three-color goniometer (a)(b) and the spectro-goniometer (c)(d), all in 633 nm. (a) Measurement made within the principal plane at four incident zenith angles: 0°, 30°, 45°, 60°; (b) REFF normalized at the smallest phase angle (~3°); (c) REFF measured by the spectro-goniometer in the upper hemisphere at i = 30° and (d) i = 45°. The image data shown in (c) and (d) were interpolated from ~100 discrete measurement points. 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.