Samples are simply degreased and cleaned thoroughly. A laser beam is focused on a fine tip. As the tip scans across the surface of the metal, the reflection from the beam is detected and plotted by a computer. The reflections mirror the surface of the metal. The photodiode is the detector here.
Measurements reflect changes in the intensity of light at the photodiode.
The laser light moves over the surface. The light reflected from the surface is measured by the computer which calculates a 3-dimensional picture of the surface. The plots shown here are for the surface of a steel sheet which came straight from the mill without polishing. Valleys are dark, and humps are lighter in color. Note the instrument measures 0.5 micron in depth at these settings.