FIG. 7.

Covariance maps of the transmitted spectra C_T ((a) and (b)), Eq. (12), and of the difference spectra $C_{\Delta }$ ((c) and (d)), Eq. (13), for an optically thin medium ((a) and (c)) and z = 1.25 mm ((b) and (d)) using an ensemble of 4000 spectra for the $N_p=N_d={10}^{10}$ photons.