Patent ID: 6893126

Claim:
A plano-convex optical sheet having a positive power including: a substantially planar inner surface; opposed lateral edges, and a thickness s that is decreasing proceeding from a transversal central line passing through the geometric center of the optical sheet towards said opposed lateral edges along a portion of predetermined width w of the optical sheet, so as to define a convex outer surface having a curvature such that the optical sheet is capable of providing once bent a cylindrically shaped optically correct visor; wherein the thickness s of the sheet decreases from said transversal central line towards said opposed lateral edges in accordance with the following equation: s 2 =( x 2 −x 1 ) 2 +( y 2 −y 1 ) 2 d= (π−α) R 1 wherein: s is the thickness of the optical sheet at a point having a distance d from the transversal central line as measured along the inner surface of the optical sheet; d is the distance from the transversal central line as measured along the inner surface of the optical sheet at a point of thickness s; x 1 , y 1 are the Cartesian coordinates, in a Cartesian plane having its origin at the center of curvature C 2 of an outer surface of the visor to be obtained, of a point having a distance d as measured alone the inner surface of the visor to be obtained from the transversal central line; x 2 , y 2 are the Cartesian coordinates, in said Cartesian plane, of a point lying on the outer surface of the final visor to be obtained and on the same straight line 1 passing through the center of curvature C 1 and a point of coordinates x 1 , y 1 ; R 1 is the radius of curvature of the inner surface of the visor as defined by the following equation: 1 / R1 = 1 / R2 1 - [ s max · 1 / R2 · ( n - 1 ) / n ] wherein R 2 is the radius of curvature of the outer surface of the visor; S max is the maximum thickness of the optical sheet at the transversal central line; n is the refraction index of the optical sheet; α is the angle defined by the straight line 1 passing through the center of curvature C 1 and points x 1 , y 1 and x 2 , y 2 in the said Cartesian plane of coordinates.