Circular Variable Filters Information

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Circular Variable FiltersCircular variable filters provide linear attenuation of incident light by rotating the filter. Circular variable filters are doughnut-shaped devices that allow narrow wavelengths from the electromagnetic spectrum to pass through circular substrates, known as segments, located on the units. They are one member of a family of variable filters that include relatives such as apodizing filters, comb filters, step filters and linear variable spectral filters. The filters are circle-shaped spectrometers. The location at which the radiation source travels through the filter determines the particular wavelength. The devices filter wavelengths that cover the full range of the visible spectrum and parts of the infrared spectrum. Circular variable filters are manufactured throughout a broad wavelength range, anywhere from 0.4μm on the spectrum’s visible region to 14.3 μm in the infrared region.


Joseph von Fraunhofer is considered the father of the spectrometer as a result of his early 19th century invention of the spectroscope. Spectroscopes are devices that separate all the wavelengths of visible light from a light source. Bernard Lyot is credited with the first use of filters as monochromators in the 1930s. Circular variable filters are monochromators that, due to their multi-segment design, can isolate any narrow range of wavelengths. Monochromators and other spectrometers were frequently used in laboratory experiments and other research and development efforts in the 1950s and 1960s, particularly government research. Monochromators have a range of modern applications. However, the use of the circular variable filter specifically, notably for ensuring food safety and quality, has seen a recent resurgence.



The underlying technology and design of the circular variable filter has remained consistent. However, the devices can be differentiated in two main respects: the range of the electromagnetic spectrum able to be filtered and whether they reflect or absorb the radiation source.


Types of filters include:


  • Circular Variable Filters (CVF)
  • Circular Variable Neutral Density filters (CVND)
  • Circular Variable Neutral Transmission filters (CVNT)
  • Linear Variable Neutral Density filters (LVND)
  • Linear Variable Neutral Transmission filters (LVNT)


Each circular substrate segment on a circular variable filter allows only a narrow band of wavelengths to filter through. When an input radiation source is focused on a portion of the filter, due to the circular design, users can rotate the filter to isolate the full spectrum of wavelengths present by observing the output radiation. The radiation beam traces a circumferential path around the filter as the device is rotated. The traced path causes the beam to pass through each segment. The varying film thickness, which defines the wavelength of peak transmittance, allows the segments of the circular variable filter to isolate particular wavelengths.


FeaturesCircular Variable Filters


The range of the visible and infrared spectrum the device filters is the central element of a circular variable filter. These filters provide high spectral resolution with a comparatively simple approach. The devices offer users the ability to switch quickly the wavelength being observed, which allows for increased efficiency and real-time inspection. They are well suited to acting as monochromators for the purpose of medium-resolution measurements involving spectral radiation. In addition, the filters are useful when information is needed for a variety of specific wavelengths of a spectral range.




Circular variable filters decreased in popularity when Optical Coating Laboratory Inc. stopped manufacturing them at the onset of the 21st century. However, demand for the filters has increased due to innovative new uses for the devices such as food safety applications. Several new manufacturers produce the product now. The use of the filters, whether in conjunction with in-line or handheld multispectral imagers, enables food processors to improve throughput and yield at their processing centers.

There is increased use of multispectral imaging in agriculture and livestock sectors. The technology assists in performing the in-line inspection of produce and livestock products. The technique offers a method of gauging the quality and safety of these products in a cost-effective manner.

A more traditional operating environment for the devices is in the laboratory, where they are used for a variety of purposes. The filters integrate into the condenser of a brightfield microscope to generate spectral images. Brightfield or fluorescent methods require the filter placement on. They are also placed on the microscope’s exit imaging port. This practice combined with a circular variable filter in microscopy (the wavelength-scan approach) is a simple yet effective means of gathering reliable measurements of live-cell specimens. In addition, it provides accurate measurements of samples that are stained or hybridized.


Other applications for circular variable filters include:


  • Quality applications in food safety, such as the inspection of poultry, fruits, vegetables and other crops
  • Industrial applications
  • Field testing applications
  • Laboratory applications
  • White light or laser wavelength filtering


SpecificationsCircular Variable Filters

Circular variable filters are designed with numerous capabilities, including:


  • Operating wavelength range
  • Spectral resolution
  • Diameter tolerance
  • Aperture clarity
  • Inner and outer diameter
  • Surface quality
  • Surface flatness
  • Parallelism error

Circular variable filters comprise of a glass substrate. For reflective models, the filters are additionally coated with a thin film optical coating optimized for specific wavelength ranges. When mounted, the filters are placed on rotating mounts made of metals such as anodized aluminum. Stainless steel is used in the construction of the filters’ posts that attach the mount.

Circular variable filters are ideal for applications where the user needs to switch easily between filtered wavelengths and conduct a real-time inspection. For such applications, the primary factor for choosing the right filter is the operating wavelength range (the range of the electromagnetic spectrum that the device can filter). In addition, the outer diameter and inner diameter of the filter is essential in determining which uses and in what setting the device can be operated.




Image Credits:


Newport Corporation

CI Systems