Patent ID: 7733364

Claim:
A line head, comprising: a substrate which is provided with a plurality of luminous element groups which respectively include a plurality of luminous elements in a first direction which emit light beams; a lens array which includes a plurality of imaging lenses which are provided corresponding to the plurality of luminous element groups; and a light shielding member which is disposed between the substrate and the lens array and includes a plurality of light guiding holes which correspond to the plurality of luminous element groups, wherein the lens array is away from the light shielding member, an inner diameter of each of the plurality of light guiding holes in the first direction is a first light guiding hole diameter (Ds), and a bore diameter of each of the plurality of imaging lenses in the first direction is a first lens diameter (D 1 ), the first light guiding hole diameter (Ds) is smaller than the first lens diameter (D 1 ), of the plurality of light guiding holes provided in the light shielding member, the light guiding hole located at one end in the first direction is a one-end light guiding hole, and the light guiding hole located at the other end in the first direction is an other-end light guiding hole, and the formula: D 1−( αs−αm )· L·T≧Ds is satisfied, where L is a distance in the first direction between an optical axis of the imaging lens which corresponds to the one-end light guiding hole and an optical axis of the imaging lens which corresponds to the other-end light guiding hole, αm is a linear expansion coefficient of the lens array in the first direction, αs is a linear expansion coefficient of the light shielding member in the first direction, T is a temperature range in use, Ds is the first light guiding hole diameter, and D 1 is the first lens diameter.