Exposure Control

Neutral Density Filters are primarily used for exposure control, limiting an overwhelming amount of light that might otherwise inundate a subject. They are also used to retain image resolution and reduce depth of field.

Neutral Density (ND) Filters are generally gray in transmission, reduce light intensity equally over a specified wavelength range and result in no change to image color or contrast. MidOpt offers two types of Neutral Density Filters: “ND” series filters that use absorptive filter glass, and “Ni” series filters which partially reflect a portion of the light entering the lens. They are defined according to their optical density (OD). For example, “ND060” is an ND filter with an OD of 0.60. The higher the OD, the lower the transmission, as seen in the table below. All MidOpt mounted filters can be stacked together to achieve various optical densities. For example, an ND060 filter mounted together with an Ni120 filter will result in an OD of 1.80. (We suggest not combining two Ni Filters together in series, as multiple reflections can sometimes result.)

Absorptive (ND) versus Reflective (Ni)

Traditional ND filters aren’t perfect, since they don’t reduce the intensity of all wavelengths equally. This can create a color cast to an image, particularly with inexpensive filters. More importantly, most ND filters are only specified to perform over the visible region of the spectrum and don’t proportionately block all wavelengths of ultraviolet (UV) or infrared (IR) radiation. This can greatly affect contrast in the near-IR and can also be dangerous if using ND filters to view bright sources, such as the sun or molten metal or glass, which emit intense, non-visible radiation. The sensor, or even the eye, may be damaged, since the source may not visibly appear bright when viewed through the filter. In cases where imaging is done in the near-IR (with or without visible light), a MidOpt Ni series filter is recommended. Ni filters are designed to be spectrally flat through the near-IR and are enhanced to uniformly suppress reflected light from potentially interfering with application.

ND/Ni Filter Use

There are several practical uses for ND/Ni Filters in vision-related applications. In cases where the item being inspected is surrounded by an unmanageable amount of light, using an ND filter may be the only way to image the item. Applications that rely on very dark ND filters include imaging a welding process or looking directly into lamps, bulbs, light emitting diodes (LEDs) or other bright light sources. When a subtle reduction of luminous intensity is needed, a ND filter of a lower optical density may offer an appropriate solution.

ND/Ni filters allow for wider lens apertures, which naturally reduces depth of field. Shallow depth of field can isolate an object from its background and/or foreground. Portions of the image that could result in false readings or might be distracting to a viewer are now out of focus. With the current trend toward the more widespread use of shorter focal length lenses (cameras and lenses are generally required to be placed closer to the subject matter), this effect becomes even more pronounced.

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