Typewriter indentations in a letter by W. H. Auden - visualized with photometric stereo

<p>This figure shows a section of a letter from W. H. Auden to Stella Musulin.</p> <p>Top: conventional photograph.<br> Middle: raking light photograph. Indentations in the paper become visible, but hardly legible.<br> Bottom: a false-color visualization generated with photometric stereo. The simultaneous display of depth, albedo and curvature gradient allow the discrimination of four text layers.</p>