Thorlabs Mounted Continuously Variable ND Filter NDC-25C-4M

Linear, Adjustable Attenuation by Rotation Optical Density Range: 0.04 - 2.0, 0.04 - 4.0 UV Fused Silica Substrate Uncoated and AR-Coated Versions Circular, Continuously Variable, Reflective Neutral Density Filter Optical Density Range: 0.04 - 2.0 or 0.04 - 4.0 Ø25 mm, Ø50 mm, and ...Linear, Adjustable Attenuation by Rotation Optical Density Range: 0.04 - 2.0, 0.04 - 4.0 UV Fused Silica Substrate Uncoated and AR-Coated Versions Circular, Continuously Variable, Reflective Neutral Density Filter Optical Density Range: 0.04 - 2.0 or 0.04 - 4.0 Ø25 mm, Ø50 mm, and Ø100 mm Sizes Three Back-Side Coating Options: Uncoated (240 - 1200 nm) AR Coated for the 350 - 700 nm Range AR Coated for the 650 - 1050 nm Range Mounted and Unmounted Versions These circular, continuously variable reflective neutral density filters provide attenuation that can be adjusted linearly within the coated region 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 2.0 or 0.04 to 4.0.

The optical density (OD, see equations below) increases linearly from the start of the coating. The mounted versions of our continuously 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. Please note that it is typical to see as much as a 50% drop in power going from the uncoated region to the coated region for filters with a max OD of 4. 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. Click to Enlarge Click Link for Detailed Specifications on the Substrate Fields highlighted in green indicate specifications for unmounted filters. All filters are available with the back side uncoated or with an AR coating.

Filters with an AR coating have item numbers ending in -A (AR coating for 350 - 750 nm) or -B (AR coating for 650 - 1050 nm). The optical density is specified at 633 nm. The optical density is a linear function of the angle: OD = mθ where OD is the optical density and θ is the angle in degrees.

The value of m depends upon the optical density range of the particular ND filter: For filters with a OD range of 0.04 - 2.0, m = 0.00741. For filters with a OD range of 0.04 - 4.0, m = 0.0148. This formula assumes 0° at the point where the coating begins and 270o at the end of the coating (i.e., the point of highest optical density).

These curves illustrate the angular dependence of the AR coating that is on the back side of some of our ND filters. The specifications to the right are measured damage thresholds for our non-variable ND filters . Our continuously variable metallic ND filters will exhibit similar performance at the discrete values listed.

Damage threshold specifications are constant for a given optical density, regardless of the size of the filter. The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When