|P_min|–albedo relation. The legends are identical to those in Fig. 3. The transparent glass powder samples are shown as a range, with the grain size (D) at the ends indicated because the albedo for each sample was unavailable. For lunar observations, P_min = P_r(α = 10.5°) is assumed (Shkuratov et al. 1992). For alumina samples, D/λ = 0.093 is enclosed in parentheses because their P_min values are extrapolated values (GG90).