An instrument designer discusses the design, use, and limitations for optics based failure analysis tools for circuits, with added emphasis and new material on infrared microscopy. We begin with fundamental principles governing all microscopes and move quickly to microscopes specific to the FA lab including emission microscopes, laser-based microscopes (e.g: OBIRCH, XIVA), and various infrared microscopes. This year we discuss infrared detector design and consider how this impacts our FA microscopy.