 The method involves a narrow slit of light from a slit lamp being projected onto the peripheral cornea at an angle of 60⁰ as near as possible to the limbus.The resulting image is a slit that is projected onto the surface of the cornea, the width is then used as a reference for the grading of the angle.The width of the angle is graded by the distance between the corneal slit image and the slit image on the iris.