Neutral Density Filters – Custom Specified

Product Description

Custom neutral density (ND) filters can be provided as either absorptive filters that are fabricated of colored glass, or as reflective-absorptive filters (also referred to as metallic) that are manufactured by applying optical coatings, typically to glass or fused silica substrates.

Custom Absorptive Neutral Density Filters

Absorptive ND filters are well suited for applications where the amount of heat energy impinging in the filter will be relatively low, where reflection is undesirable, and where the desired range of tightly toleranced neutrality is a relatively limited portion of the Visible (VIS) - Short-wave Infrared (SWIR) spectrum. These filters are manufactured by grinding and polishing a neutral density filter-glass blank that has been chosen in consideration of: (1) the desired finished optical density value at a nominal wavelength, (2) the spectral range of intended use, and (3) any thickness constraints. Thickness constraints are important to consider since the achieved optical density is a function of the finished thickness of the ground and polished raw glass. As a result of melt-to-melt variations in raw glass manufacturing, specifying both a tightly-toleranced optical density and a tightly-toleranced thickness can result in reduced manufacturability, particularly for OEM items that will remain in series production for long periods of time. It is important to note that the neutrality of absorptive neutral density filters is entirely dependent upon the raw glass. Most raw glass materials are fairly neutral in the range of 400 to 700 nm. Beyond 700 nm, the transmission tends to rise and in some cases, by a large percentage of the nominal optical density in the mid-visible. Transmission drops significantly at wavelengths lower than 400 nm.

Custom Reflective-Absorptive Neutral Density Filters

Absorptive-Reflective Neutral Density Filters are well suited for applications where a broader range of tightly-toleranced neutrality is desired, where reflectance in the optical system where the filter will be used will not cause problems, and where the amount of heat energy impinging in the filter will be elevated. As mentioned above, these filters are manufactured by vacuum-deposition of thin-film metallic coatings to transparent substrates, typically glass or fused silica. Through selection of metals to be deposited and optimized deposition methods, wider spectral ranges of neutrality can be achieved. Over the range of 250 nm to 1,200 nm, transmission can be fairly neutral. Using a nominal value in the mid-visible as a reference, a reduction in transmission from the nominal value will occur in the ultraviolet spectral band (250 to 400 nm) and an increase in transmission from the nominal value will occur in the near infrared to SWIR region of the spectrum. Unlike absorptive ND filters, thickness is not a constraint in the manufacture of reflective-absorptive ND filters. These filters accomplish light attenuation by a combination of absorption and reflection, with the larger component of the attenuation occurring by reflection.

Custom Neutral Density Filter Specification Checklist

The list of parameters that follows includes the most common parameters used to specify a neutral density filter. Many of the parameters are general enough to apply to any optical filter. Care should always be taken to avoid over-specification. For example, specifying some of the specifications in the optical category such as transmitted wavefront error can, depending on the chosen value, have a significant impact on cost while providing no increase in filter performance if the filter is used in a photometric detection application. When using this checklist, the best strategy is to provide what is known to be important and allow our technical team to recommend other specifications from the list that will insure the best performance for the application. After completing the checklist, please call 508-528-4411 to speak directly to one of our Technical Sales Engineers, or email us at to address your specific requirements.

Neutral Density Filter Specification Checklist

Type (Absorptive / Reflective-Absorptive)
Operating Conditions
Operating Temperature
Storage Temperature
Angle of Incidence
System Speed
Illumination Source
Polarization State
Length / Length tolerance
Width / Width tolerance
Thickness / Thickness tolerance (or max)
Active Area (min)
Edge treatment-Bevel
Edge treatment-Ring
Surface Quality
Flatness (S1)
Flatness (S2)
Wavefront Error
Range of Transmittance (Full range)
Range of Transmittance (Band 1)
Range of Transmittance (Band 2)
Range of Transmittance (Band 3)
Nominal Optical Density at Nominal Reference Wavelength (OD /nominal reference wavelength)
Desired Absolute % Transmittance Deviation from Nominal - Spectral Band 1
Desired Absolute % Transmittance Deviation from Nominal - Spectral Band 2
Desired Absolute % Transmittance Deviation from Nominal - Spectral Band 3