Patent ID: 7929129

Claim:
Apparatus for inspecting a transparent glass sheet having a first surface and a second surface, the apparatus comprising: (A) a light source which illuminates a portion of the glass sheet; (B) a linescan camera which detects light scattered from defects on or in the glass sheet, the linescan camera comprising a plurality of pixels which form a light sensitive area having a length L and a width W; and (C) an optical system that transfers scattered light from defects to the linescan camera, the optical system having a numerical aperture NA and comprising: (i) a primary concave mirror having a first portion and a second portion, the mirror having a radius of curvature R; and (ii) a convex secondary mirror; wherein: (a) scattered light from a defect reaches the linescan camera by an optical path that includes reflection from the first portion of the concave primary mirror, reflection from the convex secondary mirror, and reflection from the second portion of the concave primary mirror; (b) the centers of curvature of the primary and secondary mirrors are substantially coincident and the radius of the secondary mirror is substantially equal to one-half of the radius of the primary mirror; and (c) L, W, and R satisfy the relationships: L/R≦ 0.25; and W/R≦ 0.14*(sqrt(1−( L/R ) 2 )−1)+0.046 for NA≧ 0.10; and W/R≦ 0.14*(sqrt(1−( L/R ) 2 )−1)+0.033 for NA≧ 0.12; and W/R≦ 0.14*(sqrt(1−( L/R ) 2 )−1)+0.020 for NA≧ 0.15.