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Round, Step-Variable, Metallic Neutral Density Filters
Ø100 mm ND Filter
Ø50 mm ND Filter
Mounted Ø100 mm ND Filter
NDC-50S-3 Variable ND Filter
Thorlabs' Round, Step-Variable, Reflective Neutral Density Filters provide 8 or 10 discrete optical density options, adjustable via rotation. They are composed of a UV fused silica glass substrate and a metallic Inconel coating through a full 270°, ensuring a flat spectral response from the ultraviolet to the mid-infrared. These filters have optical densities ranging from either 0.04 to 1.0, 0.04 to 3.0, or 0.04 to 4.0. Please refer to the Specs tab for more information.
The mounted versions of our step variable neutral density filters come with angular graduations and are mounted on a rotating axle. The mount has been carefully designed to minimize the effect on the clear aperture of the optic. Each mounted filter includes adapters for both 8-32 and M4 mounting holes, making the mounted filter compatible with Thorlabs' Ø1/2" TR Series Posts and accessories.
Optical density (OD) indicates the attenuation factor provided by an optical filter, i.e. how much it reduces the optical power of an incident beam. OD is related to the transmission, T, by the equation
where T is a value between 0 and 1. Choosing an ND filter with a higher optical density will translate to lower transmission and greater reflection of the incident light. For higher transmission and less reflection, a lower optical density would be appropriate. As an example, if a filter with an OD of 2 results in a transmission value of 0.01, this means the filter attenuates the beam to 1% of the incident power.
Inconel is a metallic alloy that ensures flat spectral response from the UV to the near IR. Unprotected metal coatings like this should only be cleaned by blown air, never touched, as contact may cause scratching to the unprotected surface. Although these are reflective ND filters, the Inconel coating does absorb some of the incident light, which limits the use of these filters to low-power applications. Inconel is resistant to aging under normal conditions; however, it will oxidize at elevated temperatures. To prevent oxidation, Thorlabs recommends using these ND filters at temperatures below 100°C. To achieve the best performance light should be incident on the side with the Inconel coating.
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.
Thorlabs offers unmounted, circular, step neutral density filters with angular graduations of increasing optical density. The filters have an optical density (OD) range of 0.04 - 1.0 (8 steps), 0.04 - 3.0 (8 steps), or 0.04 - 4.0 (10 steps) and are designed for use in the 240 - 1200 nm transmission range. Mounting of these filters onto a TR series post can be accomplished using the NDC-PM Post Mount Assembly shown below.
Our unmounted step neutral density filters offered above are also available in mounted versions that have angular graduations and a rotating axle. The mount has been carefully designed to minimize the effect on the clear aperture of the optic and is an ideal, compact replacement for a filter wheel. Adapters for both 8-32 and M4 mounting holes are included with each filter, enabling compatibility with Thorlabs' Ø1/2" TR Series Posts.
NDC-PM Post Mount Assembly Usage
Click to Enlarge
Step Filter Mounted on Post Using NDC-PM Assembly
The NDC-PM Post Mount Assembly allows you to mount any of our round, unmounted, step (or continuously variable) ND filters onto a rotating axle that can be threaded onto any Thorlabs Ø1/2" Post. The mount has a minimal effect on the clear aperture of the ND filter. Adapters for both 8-32 and M4 mounting holes are included with each mount.
To mount a filter, first tighten the setscrew on the side of the NDC-PM assembly using the included hex key to lock rotation. Using either a 5/64" or 2 mm hex driver, remove the silver screw, insert the filter between the two washers, and replace the wheel and screw assembly. Take care to replace the flat metal washer and spring washer in the original order (see video to the right), and tighten the bolt. The hex bolt threads into a 0.75" long Ø6 mm rod, the rotation of which may be locked using the included 0.050" hex key. If a longer distance is needed between the post and the filter wheel, the included 0.75" long rod can be swapped for a standard ER rod.