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Absorptive/Reflective Neutral Density Filters

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. Choosing an ND filter with a higher optical density will translate to lower transmission and greater absorption of the incident light. For higher transmission and less absorption, 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. There are basically two types of ND filiters: absorptive neutral density filters, reflective neutral density filters.

Our absorptive neutral density (ND) filters are available in different sizes with optical densities (OD) ranging from 0.1 to 8.0. Unlike their reflective, metallic counterparts, each ND filter is fabricated from a substrate of Schott glass that has been selected for its spectrally flat absorption coefficient in the visible region from 400 nm to 650 nm.

Reflective neutral density filters are available with a N-BK7 (CDGM H-K9L), UV Fused Silica (JGS 1), or Zinc Selenide substrate in different spectral ranges. N-BK7 (CDGM H-K9L) filters consist of an N-BK7 glass substrate with the metallic (Inconel) coating deposited on one side, Inconel is a metallic alloy that ensures flat spectral response from the UV to the near IR; UV fused silica filters consist of UVFS substrate with the nickel coating deposited on one side, which provide a flat spectral response; ZnSe neutral density filters consist of ZnSe substrate (optical densities ranging from 0.3 to 3.0) with the nickel coating on one side, which leads to a flat spectral response over the 2 to 16 µm wavelength range, please see the following graph for your references.

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Features:

Optical Densities:

Continuous or Step ND

Absorptive and Reflective Options:

Both Types of ND (Neutral Density) Filters Available

Shape Options:

Round or Squared

Version Options:

Unmounted or Mounted Available

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Common Specifications:

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Parameters

Ranges & Tolerances

  • Substrate Material

    Absorptive: Schott (Absorptive) Glass / Reflective: CDGM H-K9L or others

  • Type

    Absorptive/Reflective Neutral Density Filter

  • Dimension Tolerance

    +0.0/-0.2mm

  • Thickness

    ± 0.2 mm

  • Flatness

    < 2λ @ 632.8 nm

  • Parallelism

    < 5 arcmin

  • Chamfer

    Protective < 0.5 mm x 45°

  • OD Tolerance

    OD ± 10% @ design wavelength

  • Surface Quality (scratch-dig)

    80 - 50

  • Clear Aperture

    > 90%

  • Coating

    Absorptive: AR coated / Reflective: Metallic reflective coating

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Graphs

Transmission Curve for infrared reflective neutral density filters with optical densities ranging from 0.3 to 3.0 (blue curve: ND 0.3, green curve: 1.0, orange curve: ND 2.0, red curve: ND 3.0), these filters consist of ZnSe substrate with the nickel coating on one side over the 2 to 16 µm wavelength range. For more detailed information on other types of ND filters, please contact us.

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