A neutral density (ND) filter is an optical thin film used to proportionally reduce light intensity without altering the spectral shape. It is commonly applied to control light flux, preventing overexposure or damage to optical components.

Based on design, ND filters can be categorized into absorptive and multilayer interference types. Absorptive types are typically single-layer metallic films, offering flat spectral response and simple fabrication. Multilayer interference types achieve high uniformity and low reflectivity through precisely stacked structures.

Common materials include metals such as Cr and Al, or a combination of dielectric materials like SiO₂ and TiO₂, with transmittance and spectral neutrality adjusted according to application requirements.

• Cameras and camcorders

• Microscope systems

• Laser processing platforms

• AR/VR devices

• Laser systems

• Optical measurement equipment

• Spectral analyzers

• Solar simulators

• Projection equipment

• Medical diagnostic instruments