A diffuser that realizes a smooth intensity distribution profile of diffused light is demanded for a wide range of applications, such as general illumination including indoor light, light sources of optical sensors for industrial use, and screens for visual display.
A Gaussian diffuser that realizes a Gaussian intensity profile distribution of diffused light by refraction of incident lights is well known as a diffuser that realizes a smooth intensity distribution profile of diffused light. Gaussian diffusers include a rough surface having a perfectly random height distribution profile. As Gaussian diffusers, one that is produced by a method in which a base material such as glass is sanded to obtain a mold with a rough surface and the roughness of the mold is transferred to a plastic material, one that is produced by a method in which a base material is exposed with a so-called speckle pattern that is a random light intensity pattern produced by interference of light from coherent light sources to obtain a mold and the roughness of the mold is transferred to a plastic material and the like are known. Such Gaussian diffusers provide a natural and smooth intensity distribution profile of light. On the other hand, since the distribution profile does not substantially go out of Gaussian intensity profiles, the design flexibility is small, and the transmittance is reduced with a wide angle of light distribution. Further, with Gaussian diffusers, a surface tends to be recognized as a grainy texture, and a speckle pattern tends to be generated. Accordingly, Gaussian diffusers are not suited for such an application as screens in which a surface is required to have smooth appearance and feel.
For applications in which a higher transmittance or a distribution profile out of Gaussian intensity profiles is required, many devices using a microlens array instead of a Gaussian diffuser have been developed. With a microlens array, an intensity distribution profile of diffused light can be controlled by adjusting the shape of a microlens. Further, a higher transmittance can be obtained in comparison with a rough surface. However, in a microlens array in which microlenses are arranged at small intervals, wave fronts of rays from respective microlenses interfere with one another so that diffracted waves due to the periodic structure of the arrangement are produced, and thereby unevenness in light distribution is disadvantageously generated. Further, if a curvature radius of a microlens becomes small, it becomes disadvantageous that diffraction at the aperture itself of a microlens causes unevenness in light intensity distribution of diffused light.
Under the circumstances, microlens arrays in which arrangement, surface shapes or aperture shapes are made to vary to reduce unevenness of light intensity distribution due to interference and diffraction have been proposed. For example, a focal plane plate for camera focusing in which micro lenses are arranged on a random basis to reduce unevenness of diffused light intensity due to diffraction caused by the periodic structure of a microlens array has been developed (JPS63-221329A and JPH03-192232A). Further, a microlens array in which various features such as an arrangement, a surface shape and an aperture shape are provided with randomness has been developed (JP2006-500621A).
However, prior art documents including the patent documents described above fail to disclose, to a sufficient degree, how arrangement and shapes are made to vary to reduce unevenness of light intensity distribution. Further, in a microlens with a small coverture radius that is required to obtain a distribution with a great angle of divergence, unevenness of light intensity distribution due to diffraction at the aperture of a single microlens, not due to the periodic structure of the microlens array causes a problem. However, a microlens array in which arrangement and shapes are made to vary to reduce unevenness of light intensity distribution including that due to diffraction at the aperture of a single microlens, and an optical system including such a microlens have not been developed.
Accordingly, there is a need for a microlens array in which arrangement and shapes are made to vary to reduce unevenness of light intensity distribution including unevenness of light intensity distribution due to diffraction at the aperture of a single microlens, and an optical system including such a microlens.