| 摘要 |
An ND filter of a thin film type is formed on a transparent substrate, such that a light-absorbing film and a dielectric film are laminated on the transparent substrate. The transparent substrate is formed of a resin film which can be fed at a constant speed. The light-absorbing film is deposited by sputtering a target material comprised of a metal or a mixture of a metal and a metal oxide onto the resin film traveling at the constant speed while using a mixture gas of oxygen, nitrogen, and argon as an ambient gas. Consequently, the light-absorbing film is composed of a mixture of the metal and metal compounds, and has an extinction coefficient of 0.3 or more in a visible wavelength range. Typically, the light-absorbing film is composed of 1% through 50% by weight of the metal, 30% or more by weight of saturated metal oxide, and residual components composed of the compounds of the metal containing lower oxide and nitride of the metal.
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