Neutral Density Filters Technical Discussion

An incident beam is attenuated by neutral density filters, without changing its spectral distribution. Oriel Instruments provides two types of neutral density filters, namely solid glass absorption type, and vacuum deposited metallic type.

Applications of Neutral Density Filters

Neutral density filters have found use in the following applications:

  • Equalize sensitivity of signal paths
  • Calibrate spectrophotometers and film
  • Monitor linear dynamic range
  • Split optical beams
  • Quantitatively decrease intensity for process characterization
  • Reduce intensity incident on photodetectors

Metallic Neutral Density Filters

Metallic neutral density filters involve the application of a thin inconel coating on a glass or fused silica substrate. The inconel material consists of diverse elements, such as chromium, nickel, iron, and cobalt. The alloy composition and vacuum deposition are carefully controlled, to form spectrally neutral coatings over a broad range of wavelengths. The Oriel metallic filters are typically more spectrally neutral when compared to the absorptive type of neutral density filter.

Filters with a fused silica substrate can be used in the UV region. The reflective side of these filters must face the radiation source during operation. Oriel Instruments provides two versions of fixed neutral density filters:

  • Vacuum deposited metallic coatings on fused silica, for the UV/VIS/NIR
  • Vacuum deposited metallic coatings on glass, for the VIS-NIR

Variable and step neutral density filters are also available from Oriel Instruments.

Metallic neutral density filters operate by a combination of absorbance and reflectance. The reflection to absorption ratio relies on the density value. The Oriel 0.5 density filters transmit roughly 32%, and reflect approximately 32%, while the coating absorbs the remaining 36%. This filter is typically employed as a beam splitter, to generate reflected and transmitted beams of the same intensity. Fixed and variable metallic neutral density filters are depicted in Figure 1.

Fixed and Variable Metallic Neutral Density Filters

Figure 1. Fixed and Variable Metallic Neutral Density Filters

Absorptive Type Neutral Density Filters

Absorptive type neutral density filters operate by absorption, which is ionic in nature. Hence, this type of density filter obeys the Beer and Bouguer's Laws:

      IT = I0 (1 - R)2 e-kct

Where IT = Transmitted intensity; I0 = Incident intensity; R = Surface reflectance (typically 0.04); e = 2.718; k = Constant; c = Ion concentration; and t = Filter thickness

Solid glass absorptive neutral density filters are somewhat neutral in the range of 400-700nm. They can be used from 350 to 2500, but with a transmittance varying from the value expected from the density value. Compared to the metallic type, these filters have fewer problems associated with multiple reflected beams, due to the absorption of most of the incident energy. However, solid glass absorptive neutral density filters are designed only for applications involving low power sources.

Selection Criteria

Compared to the absorptive type, the metallic type neutral density filters tolerate higher power and thermal shock. The Oriel metallic filters are spectrally neutral to beyond 2µm. This type is used for the Oriel low power sources with IR output, or for medium to high power sources in conjunction with a liquid filter. The fused silica metallic filters must be used for the ultraviolet.

Estimating Transmittance and Optical Density

Optical density is expressed as follows:

Where D = Optical Density; I0 = Incident power; and IT = Transmitted power

The transmittance, T, is , and is expressed as follows:

      T=10-D

Transmittance calculation of neutral density filters is illustrated in Figure 2.

Transmittance calculation of Neutral Density Filters.

Figure 2. Transmittance calculation of Neutral Density Filters.

Combining Neutral Density Filters

Neutral density filters can be serially stacked to yield higher densities. The total density is the summation of the individual densities. The total transmittance of a stack of neutral density filters is the product of the transmittance of each filter.

Using Neutral Density Filters Off Normal

The transmittance of the absorptive and the metallic filters relies on the angle of incidence. The transmittance turns out to be a function of angle and polarization when employed at angles beyond ~25°. The following equation can be used to calculate the angle dependence of the absorptive type at below 25°:

Where n = Index of refraction of the absorptive filter, ~1.51.

The increased density is because of the increased thickness, witnessed by a beam transmitting through the inclined filter. Since the metallic filters attenuate by a combination of absorption and reflection, their transmittance is less sensitive to angle, and their reflectance is not angle sensitive for angles less than 25°.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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