They used two photos per camera and changed the polarization between both Images. On one Image, the light from the lamps was polarized differently than the filters on the camera. This way no direct reflection is recorded. Then, they shot a "normal" Image. The difference between the two is the speculation, which they used for the normal map, probably after compensating for Fresnel based reflections.

The results of using Polarisation filters when photographing Skin are pretty drastic. I assume that Levius' latest photosets are made in a similar way, since they don't seem to contain much specularity. Sadly, there does not seem to be a "specular" Version as well, so creating spec maps isn't possible that way.

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.