A corneal topographer shines light onto the surface of the eye, then measures the reflected light to create a map of the cornea's curvature as well as any irregularities. The map is used for evaluations related to refractive surgery, contact lens fitting and corneal disease management. It is especially useful for measuring astigmatism. The colour map uses blue and green to represent flatter areas of the cornea, while red and orange represent steeper areas.