Apollo 8 Hasselblad image from film magazine 12/D - Lunar Orbit. Interior of Pasteur crater.
This photo was used in Figure 2-39 of the Analysis of Apollo 8 Photography and Visual Observations (SP-201), which has the following caption:

Apollo 8 frame 2147-D, showing densitometer trace, principal point, and zero-phase point.

The text of the Analysis refers to the figure as follows:

The principal point and zero-phase point were located on each frame, and densitometric traces (figs. 2-39 and 2-40) were extracted from frame edge to frame edge through these points. A second-generation master positive was used for all scanning. A mean brightness trace was fitted to each microdensitometer trace. The assumption was made that variations of the microdensitometer trace from the mean brightness trace were caused by local variations in albedo and slope, and that the mean trace corresponded to a uniform sphere and albedo. Eighty-two data points were extracted, and the phase angle g and luminance longitude a computed for each data point. Conversion of the microdensitometer data to exposures was performed through the use of sensitometric strip 4B (fig. 2-41) pre-exposed onto the head end of the flight film. The exposures were corrected for lens transmission characteristics, but have not been corrected for potential spacecraft-window-transmission effects. The values for each frame were then normalized to zero phase.