Focussing error detecting apparatus

Claim 1

A focussing error detecting apparatus comprising: a composite lens (30) formed of a single lens (30 α) made of glass and a single lens (30 (ì ) made of plastics material; and a photo-detector (8) disposed behind said composite lens (30); wherein: said glass lens (30α) has a uniform focal length; said plastics lens (30 β) is formed of a pair of lens regions (30'a, 30'b) divided by a flat interface (T) including an optical axis (O) of light incident on said composite lens (30); said. pair of lens regions (30'a, 30'b) have back focal lengths (f' 1 , f' 2 ) different from each other and optical axes (0,, 0 2 ) spaced apart from each other; said photo-detector (8) is placed such that its photo-detecting face is positioned at a middle point between converged points of light rays emitted from said pair of lens regions (30'a, 30'b) on the optical axis of said incident light when said light incident on said composite lens (30) is in the focussed state; and said photo-detector (8) is formed of first and second photo-detecting sensors (8A, 8B) for receiving light rays passed through the far-side portion and near-side portion of said first lens region (30'a) of said pair of lens regions (30'a, 30'b) relative to said flat interface (T) and third and fourth photo-detecting sensors (8C, 8D) for receiving light rays passed through the near-side portion and far-side portion of said second lens region (30'b) of said pair of lens regions (30'a, 30'b) relative to said flat interface (T) so that a focussing error signal is produced from the difference between the sum (S A +S D ) of first and fourth photo-detected signals from said first and fourth photo-detecting sensors (8A, BD) and the sum (S B +S C ) of second and third photo-detected signals from said second and third photo-detecting sensors (8B, 8C).