FIG. O-3. Optically pumped magnetometer. Atoms in a vapor cell precess about the steady magnetic field that is to be measured, which is at an angle of 20–70° to the instrument axis. Monochromatic light, which is circularly polarized in the plane perpendicular to the instrument axis, has a component that can be absorbed by the precessing atoms. Once this absorption is complete, no further absorption can occur and then the cell becomes transparent, a condition that causes an increase in the light to a photocell. The polarized precessing atoms have a component along the axis of a transverse RF field that permits the atoms to undergo transitions and hence become available to absorb more light. The precessing atoms thus become a transfer mechanism between the light and the transverse RF field when the field is at the Larmor frequency. The light intensity is used to monitor the precession and automatically adjust the RF frequency, which can be measured to give the intensity of the steady magnetic field.