A lens array optical system including a plurality of lens portions is used for an imaging lens for a compound-eye imaging apparatus, a secondary image forming lens in an autofocus module of a single-lens reflex camera, an illumination lens in a semiconductor exposure apparatus, a condenser lens in a liquid crystal projector panel, and the like. The lens array optical system can be manufactured at low cost with optical plastic having high processability. However, the optical plastic not only has a high thermal expansion coefficient bus also is inferior in durability and light transparency, and accordingly is not suitable to be used stably on a high-temperature or high-humidity severe condition. Therefore, optical glass that has a low thermal expansion coefficient and is superior in durability under high temperature or high humidity is required to be used for the lens array optical system in order to enable stable use in various environments.
As a method for manufacturing a lens array optical system made of optical glass having multiple spherical or aspheric lenses, a method is proposed in which preforms made of glass with a larger curvature than a concave curvature of a forming die are placed, one by one, in transfer portions and are hot stamped (refer to Patent Literature 1). An excess of the preforms over the volumes of cavities for forming optical surfaces of the multiple lenses flows into junctions of the cavities and are integrally fused. However, there is a problem with the method of Patent Literature 1 that it takes much time and trouble to place the preforms on the optical transfer surfaces on the die. Moreover, air may remain in the junction (fused portion or joint) due to variations in the preforms to result in a reduction in the strength of the junction of the preforms. Moreover, there is also a problem that as the number of lens portions increases, the forming process becomes more difficult, and accordingly it becomes impossible to obtain a lens array having an intended optical surface shape.
Moreover, as another method for manufacturing a lens array optical system, a method is also proposed which does not use a die, supplies a liquid material to a substrate where a plurality of through-holes is formed, and forms a plurality of lenses on the substrate (refer to Patent Literature 2). However, there is a problem with the method of Patent Literature 2 that it is difficult to form an optical surface of the lens into an aspheric shape so that it is not possible to appropriately design the optical surface of the lens portion and optically correct various aberrations.
Moreover, as another method for manufacturing an optical glass lens array optical system, the drop method is known which forms a lens by dropping molten glass into a die. The drop method has few constraints on the shape of an optical surface since the forming process is performed in a state where the viscosity of the glass is relatively low. Moreover, a reduction in the occurrence of forming failure ascribable to the fluidity of a material can be expected. As the method for manufacturing a lens array by the drop method, Patent Literature 3 describes a method for manufacturing a lens array having 2×2 lens portions by dropping a glass drop on the center of one of dies having 2×2 optical transfer surfaces and pressing it with the other die. However, Patent Literature 3 does not describe the production of a lens array having lens portions more than 2×2, such as 4×4 or 5×5, in a lattice form. If an attempt is made to obtain a lens array having 4×4 lens portions by dropping a glass drop on a center portion of, for example, 4×4 optical transfer surfaces in accordance with the manufacturing method described in Patent Literature 3, an optical transfer surface that has been insufficiently filled with the glass may be produced as described below. Moreover, in Patent Literature 3, a projection is provided to change the flow of the glass drop to make adjustments such that the glass drop flows along the transfer surface of an optical surface near its edge side close to the projection, the edge side having a large inclination angle. Accordingly, an attempt is made to transfer the optical surface with high accuracy. However, there are problems that the manufacturing cost may increase if a glass projection is provided to the die as a preliminary process, and the die processing cost is high and the life of the die is influenced if the projection is formed upon die processing. If an attempt is made to drop a plurality of glass drops individually and respectively onto the optical transfer surfaces, forming failure may be invited, or gas entrainment may occur upon reaching the die, due to time differences in die arrival timings among the glass drops. Moreover, a problem also arises in which it becomes difficult to perform a forming process by pressing to a desired thickness since it becomes easy to become cold and harden as the result of a reduction in the volumes of the individual glass drops.