Also known as Optical Coherence Tomography, this is a machine used to take a picture and “map” the surface contour of the retina. It works very similar to ultrasound, but instead of using sound, light waves are bounced off the internal eye structures. The scans produced by this machine look similar to that produced by a sonar depth finder in a boat … but instead of mapping the bottom of the ocean or river bed, we are measuring the surface of the retina (which is supposed to be flat and smooth like film in a camera). OCT is useful for detecting retinal distortions (like those caused by an epiretinal membrane) and for looking for macular edema in cases of wet macular degeneration or diabetic retinopathy. The OCT can also be used to map the optic nerve and is helpful for documenting and monitoring for glaucoma nerve damage (i.e., glaucomatous cupping).