Neutral density filters (density filters for short) are filters that attenuate light intensity without changing the energy distribution of the spectrum. The fixed neutral density filter can attenuate the light intensity evenly in a wide spectrum area, and the surface is coated with a durable metal film layer. This type of filter can be used in natural light and laser fields. The fixed density filter is divided into absorption plus reflection type density film and low inversion type density film (ND mirror), which neutrally attenuates light in a large spectral range, and it is also particularly suitable for the calibration of optical systems. Density sheets with special sizes, densities, and wavelengths can be customized.