Scattering

In the scattering measurements, the crystal was installed in front of the entrance slit of the monochromator and illuminated from the side, Fig. 7d. The optical chopper was installed between the lamp and the crystal. Therefore, the thermal radiation of the sample did not contribute to the scattering (or the reflection) spectrum. The long-pass color filter was used to block the visible light. In multiple measurements, the crystal was positioned at various angles relative to the lamp (θ _i, ϕ _i) and the monochromator’s slit (θ _j, ϕ _j) and was turned to them with its polished or unpolished sides. Correspondingly, in different particular measurements, the specular reflected light and the diffused light reaching the detector were mixed in different proportions and the photons experienced larger or smaller number of scattering events before they exited the samples.

In the control measurement, the crystal was replaced with the broadband white diffused reflector (from Lab Sphere). Experimentally, the two spectra were collected, one with the crystal and one with the diffused reflector. Then the former was divided by the latter, and the resultant scattering spectrum was scaled to reach unity in the spectral ranges where the crystals did not have any absorption (as it is explained above and in Section 2 of Supplementary Materials). This angular-dependent scattering spectrum σ(λ, θ _i, φ _i, θ _j, ϕ _j) was used to obtain the spectrum of the angular sample’s absorptance for given directions of illumination and data collection, α(λ, θ _i, φ _i, θ _j, ϕ _j) = 1 − σ(λ, θ _i, φ _i, θ _j, ϕ _j). (This procedure allowed one to deduce only the spectral shape of absorptance but not its magnitude)^60–63.