Neutral Density Filters, or ND Filters, are optical filters designed to reduce the intensity of all light passing through the filter without changing the spectrum. In other words, an ND filter equally attenuates the wavelengths of light passing through it over a particular spectral band. They are used to reduce the power of transmitted light in high precision optical applications, but they are perhaps best understood in terms of photography.
A camera has two ways of adjusting the amount of light hitting the optical sensor—the size of the aperture opening (f-stop) and the shutter speed. However, there are times when the light intensity is so high that it’s challenging to find a combination of f-stop and shutter speed that doesn’t result in a blurry photo. In this case, a photographer may add a neutral density filter. The filter reduces the intensity of light without altering the color of the resulting image.
While bright light conditions are one reason photographers use ND filters, they are also helpful for enhancing contrast, dialing in the depth of field, and changing the photograph’s perceived motion. For example, using an ND filter enables photographers slow their shutter speed which (when done right) would transform the look a waterfall from individual droplets to a smooth, uniform veil. However, cameras operate discreetly—one frame at a time. They can compensate for light intensity with operational adjustments.
Another application of ND filters is in laser systems. Lasers are continuous light sources, and there are limited ways their output intensity can be minimized without altering the quality. In these applications, additional power reductions may be needed to preserve sensitive equipment. ND filters reduce the intensity of light passing through them without altering the laser light properties.
Types of Neutral Density Filters
There are two types of neutral density filters, absorptive and reflective. As their names indicate, absorptive filters absorb a percentage of the incident light while reflective filters reject a percentage of light. The classifications are not strictly divided along those lines as reflective filters also absorb a percentage of light, but their filtering capability is primarily due to reflection.
Absorptive ND Filters
Absorptive ND Filters typically rely on a glass substrate to absorb light. Since they rely on the glass itself for absorption, the thickness of the filter is critical. However, as the thickness of the glass increases, so does spectral distortion and light aberrations – read our previous blog post on Understanding Optical Aberrations. Absorptive filters also heat up as they absorb light energy limiting their ability to operate continuously in high-power applications.
Absorptive ND filters are primarily used in lower power applications where minimal reflection is desired. Camera filters are often absorptive, but they are also useful in medical imaging applications to protect measurement equipment.
Reflective ND Filters
Reflective ND filters utilize a thin layer of metallic film on the surface of the substrate to reflect light. The metallic film is deposited using chemical vapor deposition techniques. In practice, the reflective surface must face the incident light, and the system should be designed with that reflective light in mind, keeping sensitive components out of the path of reflection.
For visible spectrum applications, crown or float glass are typical substrates. UV-grade fused silica substrates offer UV protection, and germanium may be used in infrared applications.
Although various metallic alloys may be used for coating reflective ND filters, the austenitic nickel-chromium-based alloy Inconel is a popular choice. Inconel is a “superalloy” because of its high-strength properties, even at higher temperatures. However, in ND filter applications, Inconel is attractive for its optical properties. Inconel’s consistent filtering capabilities across a broad range of wavelengths make it ideal for ND filters. In addition, unlike other dielectric coatings with a reflection ratio typically 50/50, Inconel’s is 30/30 which helps dial in optical density (defined in the next section).
Since reflective ND filters absorb less energy, they are better suited for higher power applications such as laser systems. They are widely used for spectroscopy, machine vision, and thin-film inspection (ellipsometry). The reflection and transmission properties also allow them to be used as beam splitters.
Neutral density filters are primarily characterized by three physical properties—reflection, optical density, clear aperture.
Reflection (%): Describes the percentage of incident light reflected at the surface of the ND filter.
Optical Density, OD: The measure of transmittance through an ND filter. Optical density is logarithmically related to transmittance. It is important to note that since reflective filters exhibit some absorptive properties, reflection is not a direct relationship to transmittance.
OD = Optical Density
IO = Incident Power
IT = Transmitted Power
T = Transmittance
The OD is often given for a filter, so T = 10-D. The table below shows the transmittance for common ND filter optical densities.
Transmittance (%) for Common ND Filter Optical Densities
|Optical Density||Transmittance (%)|
ND filters can be stacked to achieve the desired transmittance. The optical density of filters in series is additive (e.g., two 1.0 OD filters in series are equivalent to one 2.0 OD filter). Just as it is crucial to install a reflective ND filter with the reflective surface facing the incident light, it is also important to combine filters in series with all reflective surfaces in the same direction.
Transmission Curves for EMF’s Neutral Density (ND) Filters are shown in Table 1 below. Notice the attenuation is nearly flat across the visible and near-infrared spectrums. This is characteristic of ND Filters.
Internal reflections between filters stacked in series are possible, especially with reflective filters. This may alter transmittance or increase distortion.
Clear Aperture (%): The clear aperture describes what portion of the filter meets the listed optical specifications. It arises dues to manufacturing limitations. It is often expressed in percentage (%) of diameter. So, for a 50mm diameter filter with an 80% clear aperture, 40mm meet specification.
In measurement and inspection, ND filters can equalize signals across multiple channels to improve processing and balance signals. Components such as photodetectors have input power limitations. In this case, ND filters can protect these components and increase their longevity. In addition to filtering, ND filters can be substituted for beam splitters. Overall, ND Filters are useful for various applications to reduce light intensity without sacrificing the light properties across the operating spectrum.