We develop a theoretical model for calibrating the absorption imaging of cold
atoms under high magnetic fields. Comparing to zero or low magnetic fields, the
efficiency of the absorption imaging becomes lower while it requires an
additional correction factor to obtain the absolute atom number under the
Beer-Lambert law. Our model is based on the rate equations and can account many
experimental imperfections such as Zeeman level crossing, failures of hyperfine
structures, off-resonant couplings, and low repumping efficiency, etc. Based on
this method, we can precisely calculate the correction factor for atom number
measurement without any empirical or fitting parameters. Meanwhile, we use a
cold-atom apparatus of rubidium-85 to experimentally verify our model. Besides
these, we find our work can also serve as a benchmark to measure the
polarization impurity of a circular-polarized laser beam with high
sensitivities. We believe this work will bring convenience for most of
cold-atom experiments using absorption imaging.Comment: 9 pages, 5 figure