Patent ID: 7310136

Claim:
A method comprising: illuminating a first surface and a second surface of a microstructured prism on a prism sheet with an incident light beam; wherein the first surface is illuminated by the incident light beam at an angle θ 1 with respect to a normal drawn to the first surface and wherein the second surface is illuminated by the incident light beam at an angle Φ 1 with respect to a normal drawn to the second surface; and wherein a third surface of the microstructured prism contacts a surface of a sheet substrate of the prism sheet; and further wherein the third surface is inclined at an angle of 90 degrees to the incident light beam; measuring a first distance “d 1 ” between a first image and a perpendicular to the surface of the substrate sheet on a measuring device that is disposed on an opposite side of the prism sheet from a light source used for the illuminating; wherein the perpendicular to the surface of the substrate sheet is taken at a point where the bisector of the apex angle of the prism meets the surface of the substrate sheet in contact with the third surface of the prism; and wherein the first image is obtained by refraction of the light beam from the first surface; measuring a second distance “d 2 ” between a second image and a perpendicular to the surface of the substrate sheet on the measuring device; wherein the second image is obtained by refraction of the light beam from the second surface; substituting values for d 1 and d 2 in the equation (1) and (2) below: sin(θ 1 −sin −1 ( n 1 /n 2 sin(θ 1 )))= n 1 /n 2 sin(tan −1 ( d 2 /d 3 )) (1) sin(Φ 1 −sin −1 ( n 1 /n 2 sin(Φ 1 )))= n 1 /n 2 sin(tan −1 ( d 1 /d 3 )) (2) wherein “d 3 ” is the distance between the prism sheet and the measuring device, n 1 is the refractive index of the media in which the measurement is made and n 2 is the refractive index of the prism sheet; and determining at least one value for either θ 1 , Φ 1 or n 2 .