  We demonstrate the use of spatial emission patterns to measure the magnetic
fields used in a magneto-optical trap. The directional aspect of the Hanle
effect gives a direct, visual presentation of the magnetic fields, in which
brighter fluorescence indicates larger fields. It can be used to determine the
direction as well as the magnitude of the field. It is particularly well suited
for characterizing and aligning magneto-optical traps, requiring little
additional equipment or setup beyond what is ordinarily used in a
magneto-optical trap, and being most sensitive to fields of the size typically
present in a magneto-optical trap.
