Neutral density optical filter and image pickup apparatus

摘要

A spectral reflectance in the entire visible wavelength range may be decreased to decrease inconveniences in an image such as flare or ghost. A non-reflective periodic layer including a large number of fine uneven periodic structure bodies is formed on a substrate, and a neutral density ("ND") film is formed on the other surface of the substrate by vacuum deposition, to improve an anti-reflection function. Regarding a beam entering the ND filter, the MgF2 film of the formed ND film suppresses the spectral reflectance at the boundary of the atmosphere and the ND film and at the boundary of the ND film and the substrate. The non-reflective periodic layer decreases the reflectance at the boundary of the substrate and the atmosphere.