Variable ND Selection Guide
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ND Filter Type	Round Step	Round Continuous	Cage Compatible	Rectangular Step	Rectangular Continuous

Features

• Provides Discretely Variable Attenuation
• OD Ranges from 0.1 - 4.0 or 0.3 - 2.0
• Two Size Options

Thorlabs' Rectangular Step Variable Metallic (i.e., reflective) Neutral Density Filters provide a discrete range of optical densities (OD) in a single optic. They are composed of a UV fused silica glass substrate and a metallic Inconel coating, ensuring a flat spectral response from the ultraviolet into the mid-infrared. These filters are commonly used as variable attenuators. Please refer to the Mounting tab for a suggested mounting method.

Our rectangular filters are offered in 10 mm x 50 mm and 25 mm x 100 mm sizes (H x W). Both sizes are available with OD ranges of either 0.3 - 2.0 or 0.1 - 4.0. In the versions that span OD 0.3 - 2.0, there are five discrete optical density steps, and in the versions that span OD 0.1 - 4.0, there are ten discrete optical density steps. Please refer to the Specs tab to determine the effective area provided for each step.

The optical density, OD, is defined by the equation:

Hence, a higher OD corresponds to lower transmission and greater reflection of the incident light, while a lower OD corresponds to higher transmission and less reflection. For example, an OD = 2 filter will attenuate the transmitted beam to 1% of the incident intensity.

The Inconel coating on these filters is a very hard metallic alloy that is resistant to aging under normal conditions, but will oxidize at elevated temperatures. It is therefore recommended that the neutral density filters be used at temperatures below 100° C.

These filters are not intended to serve as laser safety equipment. For lab safety, Thorlabs offers an extensive line of safety and blackout products that significantly reduces exposure to stray or reflected light.

*The optical density is specified at 633 nm.

Thorlabs' Step Variable Neutral Density Filters are commonly used as variable attenuators, where the filter must be translated to access its various optical densities. One way to mount this filter and achieve this flexibility is shown above. A CH1A mount holds the filter. This mount is then attached to our dovetail rail system, allowing it to quickly translate perpendicular to your beam path. To switch between optical densities, simply loosen the thumbscrew on the rail carrier and slide the filter along the rail.