  The Siegert relation relates the electric field and intensity correlations of
light, under given assumptions. After a brief history of intensity
correlations, we give a derivation of the relation. Then we present an
experiment, which can be easily adapted for an undergraduate setup, and that
allows measuring both field and intensity correlations at the same time, thus
providing a direct test of the Siegert relation. As a conclusion, we discuss
typical situations where the relation fails.
