Technical Field
The present invention relates to an optical element for use in an optical system of such equipment as various camera devices, projectors, and the like, a mold for preparing the optical element, and an optical device employing the optical element.
Related Art
The optical element for use in an optical system of the optical device is in general subjected to reflection prevention or antireflection treatment to minimize ghosting and flare. Typically, an antireflection film is formed on the surface of the optical element.
However, the antireflection film must be formed as multiple layers for higher performance. Multiple layer formation of antireflection film requires a number of processes, preparing time, and processing cost. That is, it is difficult to design and manufacture a high-quality antireflection film.
Recently, various approaches have been tried to provide resinous optical devices at reduced cost. However, forming an antireflection film on the resinous material adversely affects heat and humidity resistance due to poor adhesion compared to formation of the antireflection film on glass.
Accordingly, a subwavelength structure (SWS) as a reflection prevention structure has been explored as an alternative to the antireflection film. The subwavelength structure has a microstructure with a pitch or width smaller than the wavelength of the light whose reflection is to be prevented. The optical effect of the subwavelength structure depends on the shape and material of the specific microstructure employed. The microstructure is formed by a dry process alone and is appropriate for a cost-effective production.
In order to produce an optical element having a minute concavo-convex structure, use of a nanoparticle etching method is conceivable. Considering the antireflection effect with regard to light with a wide wavelength area or light of infrared wavelength (i.e., longer wavelength than visible light), an interval of a lattice having a minute concavo-convex shape should be defined so that the diffraction light does not occur due to the microstructure up to a specific incident angle in the permeation reflection. Specifically, the interval of the microstructure should be preferably shorter than 200 nm. On the other hand, the height of the lattice of the microstructure should be defined so that the refractive index changes more smoothly for the higher performance. As a result, the height of the microstructure should preferably be more than 200 nm. Accordingly, an effective antireflection effect relative to light with a wide wavelength or the long wavelength longer than the visible light requires an aspect ratio greater than 1 and a height greater than 200 nm. Such a microstructure is difficult to produce with.