Patent ID: 7929008

Claim:
A line head comprising: a light emitting element substrate that has a first light emitting element; and a lens array that has a first glass substrate and a first lens, the first glass substrate having a greater width in a first direction than a width in a different direction from the first direction and being disposed apart from the light emitting element substrate, the first lens being formed from a resin and on the first glass substrate so as to image light emitting from the first light emitting element, wherein a cross section of the first lens taken in the first direction and including an optical axis of the first lens has a configuration wherein a curvature of the first lens at the optical axis has a larger absolute value than a curvature of an outer peripheral portion of the first lens, and the first lens has a relation:  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 of the first lens, θ is a rotation angle about the optical axis and f(r, θ) is a lens height in an optical axis direction in an r-θ coordinate system.