A slit lamp is a microscope with a thin beam of light, used to examine the front of each eye, including your iris (colored portion), sclera (white area), eyelid, lens, and cornea, under magnification. The doctor may use special eye drops to dilate (expand) your pupils, then repeat the exam, to view the retina and back of the eye. The test checks for cataracts, macular degeneration, diabetes complications, cornea scratches or infections and chronic dry eye disease. A thorough retinal exam may detect many other diseases such as high blood pressure and cancer.