What is TIRF? TIRF stands for Total Internal Reflection Fluorescence. TIRF microscopy is also called evanescent wave microscopy. The technique is useful for looking at fluorescent events that occur very close to a glass coverslip - less than about 100 nm from the glass. A laser shines up through a glass coverslip at a shallow angle, and is totally reflected back into the glass at the glass-water interface. But an evanescent wave probes the overlying fluid. This evanescent wave can excite fluorophores. In this example, secretory vesicles containing a fluorescent dye are excited. The depth of excitaion (z-axis resolution) can be nearly ten times better than with any other light microscope. Photodamage to living cells is also reduced.