Neutral Density Optical Filters - Technical Information

Neutral Density Filters

Neutral density filters differ from other types of filters in that they equally reduce the intensity of light passing through the filter at all wavelengths in a specific wavelength range. As with the filters mentioned above, neutral density filters may be absorptive or reflective. Neutral density filters are given OD (optical density) numbers which provide the degree of opacity of the filter which determines its transmission. A lower OD number equates to a higher transmission and vice versa. The degree to which light is attenuated is dependent on the optical density of the filter and the thickness of the substrate. A filter with 100% transmittance has an optical density of 0.

Absorptive Glass Neutral Density Filters

Absorptive neutral density filters reduce the intensity of light over a wavelength range equally by a specific percentage as opposed to color glass filters which absorb some wavelengths or colors of light more than others. Neutral density filters are rated by an OD (optical density) number which defines the optical density and therefore the amount of light which is transmitted. OD is equal to the –log of transmission and is expressed as:

Nuetral densite Optical Filter transmission formula

In cases where there is more than one neutral density filter in use, the OD is additive. Available optical densities range in value between 0.1 and 6. In addition to being used in the visible spectrum, neutral density filters can also be used in the NIR. Another benefit of neutral density filters is that they are effective at absorbing heat in low power applications.

How to Order Neutral Density Optical Filters

Because Neutral Density Optical Filters are a very specialized service, a shopping cart purchase is not available. To request a quote or order optical filters, please fill out our Request a Quote form or call us directly at (262) 548-1155.

Advantages	Disadvantages
Low cost.	If the intensity of light (power) is too high they become heat sensitive.
Insensitivity to a wide range of angles of incoming light.	Not suited to high power applications.
Stackable to obtain a larger optical density without resulting in multiple reflections.	Effectiveness dependent on thickness and therefore may need additional polishing.
High durability, absorption not affected by minor scratches, easily cleaned.	May require an AR coating for use in the NIR which can add cost.
Effective in the NIR.

The Advantages of Polishing Neutral Density Filters

The advantage of polishing neutral density filters is they may not be commercially available in the proper thickness to provide the correct amount of absorption of light or in custom configurations which is where Advanced Optics can help. We polish all types of absorptive neutral density filters manufactured by Hoya and Schott to your exact specifications

Metallic Neutral Density Filters

Metallic neutral density filters work by a combination of reflection and absorption. They are produced by adding a thin layer of a metal alloy (typically Inconel) to the polished surface of a glass substrate. The optical density of the filter is obtained by careful selection of the metal alloy (optimal wavelength range of metal alloy) as well as by controlling the thickness of the coating. Coatings are applied to substrates such as fused silica, BOROFLOAT®33, N-BK7® and B270®. Substrate selection is based on how well a specific material transmits light in a specific wavelength region.

Advantages	Disadvantages
More spectrally neutral (uniform) when compared to absorptive neutral density filters.	Suitable only for low power applications such as low power lasers as part of the energy is absorbed and part is reflected.
Useful in the UV-VIS-IR spectra.	If stacked, these filters need to be slanted to avoid multiple reflections and any reduction in performance.
Substrate selection may be of a less expensive glass type (application dependent) as they are used in low power applications.	Must be used with reflective surface facing the illumination source.
May be used as beam splitters over a wide wavelength region.	More susceptible to scratches than absorptive filters and must be cleaned carefully.

Specifying the Correct Optical Filter

Optical filters are classified by their construction, which determines the way they filter light. There are many types of absorptive and reflective filters and careful selection should be based on the specific application in which they will be used. One should consider the following when selecting an optical filter for a specific application: wavelength range of interest as well as cut-on and cut-off properties, AOI of incoming light, energy of incoming light and operating environment.

Whether your application requires an absorptive filter, dichroic filter or neutral density filter, carefully considering your requirements is critical. Specific consideration should be given to the physical size, shape and tolerances of the substrate as tighter tolerances may require additional manufacturing which can add cost. The same applies to the surface quality of the filter which is expressed by a scratch and dig number. The lower the scratch and dig number, the more potential there is for a reduction in yield which drives up the cost of the filter.

Selecting the proper optical filter for your application is essential to saving manufacturing time, which translates to reduced costs. Advanced Optics can help you define your requirements.