Patent ID: 8085470

Claim:
A line head, comprising: a lens array that includes a first lens, a second lens and a light transmissive substrate on which the first lens and the second lens are arranged in a first direction; and a light emitting element substrate on which light emitting elements are arranged in the first direction, wherein the first lens is formed such that, in a cross section in the first direction including an optical axis of an imaging optical system comprised of the first lens, a curvature of the first lens at an outer peripheral portion has a sign opposite to that of a curvature of the first lens on the optical axis or has a smaller absolute value than the curvature of the first lens on the optical axis, and the first lens has a lens surface having a relation defined by either one of following formulas: ⅆ 2 ⁢ f ⁡ ( 0 , θ ) ⅆ r 2 ( 1 + ( ⅆ f ⁡ ( 0 , θ ) ⅆ r ) 2 ) 3 2 · ⅆ 2 ⁢ f ⁡ ( r , θ ) ⅆ r 2 ( 1 + ( ⅆ f ⁡ ( r , θ ) ⅆ r ) 2 ) 3 2 < 0 and  ⅆ 2 ⁢ f ⁡ ( 0 , θ ) ⅆ r 2 ( 1 + ( ⅆ f ⁡ ( 0 , θ ) ⅆ r ) 2 ) 3 2  >  ⅆ 2 ⁢ f ⁡ ( r , θ ) ⅆ r 2 ( 1 + ( ⅆ f ⁡ ( r , θ ) ⅆ r ) 2 ) 3 2  where r is a distance from the optical axis and θ is a rotation angle about the optical axis.