A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method is called "polarized light microscopy" (PLM). Depending on the grade of observation required, petrological microscopes are derived from conventional brightfield microscopes of similar basic capabilities by:

• adding a Nicol prism polarizer filter to the light path beneath the sample slide
• replacing the normal stage with a circular rotating stage (typically graduated with vernier scales for reading orientations to better than 1 degree of arc)
• adding a second rotatable and removable Nicol prism filter, called the analyzer, to the light path between objective and eyepiece
• adding a Phase telescope, also known as a Betrand Lens, which allows the viewer to see conoscopic interference patterns
• adding a slot for insertion of wave plates