Nearly all light sources are partially polarized, photomultiplier tube, photocell detector, and grating all have a polarization sensitive effect. When we use natural light to do some study, the light from the light source must be depolarized. In order to reduce the polarization sensitive effect of the detector, we need a fixed depolarizer in front of the detector. Since Lyot times, there has been considerable attention paid to the study of the polychromatic light depolarizer, but there are only a very limited number of ways to depolarize monochromatic light. In this article we introduce single element structure monochromatic light depolarizers made by using birefringent crystal. They include a single wedge depolarizer, a single element double wedge depolarizer, a right angle prism depolarizer, and a plano-convex lens depolarizer. We study their structure character, working principle, and use specification in detail and also point out some errors in the calcite right angle depolarizer.