Fig. 2

A, spectral radiance of the two halves: as grey (neutral density) filter and polarizing filter have similar transmission spectra in the spectral interval allowed by the blue filter, the spectrum of the polarized half (dashed line) is similar to that of the non-polarized one (solid line). B, radiance measured along solar meridian of the bowl. Solid line, West and East parts are equalized when a polarized filter and a neutral density grey filter are both present on the East and West area over the bowl, respectively. A radiance gradient is present in the control experiments, when only the grey filter is present on the East part (dashed line). In both cases a blue filter is superimposed