Photospheric structure at 73.5 d after explosion for model a4. Top panel: temperature, the Rosseland-mean opacity, and the Rosseland-mean optical depth. The dashed lines help visualize the location of the photosphere (determined using the Rosseland-mean opacity). Bottom panel: evolution of the gray mean intensity and the gray flux (scaled by r², where r is the local radius, and subsequently normalized) computed by CMFGEN.