There are three main varieties of expanders , which have all been derived from Fresnel prisms : the Gottlieb Visual Field Awareness System , the Chadwick hemianopsia lens , and the EP horizontal lens .
There are three main varieties of expanders , which have all been derived from Fresnel prisms . 
 These include the Gottlieb Visual Field Awareness System , the Chadwick hemianopsia Lens , and the EP Horizontal Lens .