Linear Variable Filters – Types and Applications
Published: August 31, 2021
Linear variable filters (LVF), also known as continuously variable filters, or gradient filters, are special types of filters, where the filters’ spectral response changes continuously (or quasi-linearly), in a defined manner, along one dimension. In an example case a change of a linear variable filter position by 4mm will shift the filter center wavelength transmissivity e.g. from 500 to 520 nm, so by 20 nm (a slope of 5 nm on each 1 mm). The gradient may vary and is selected for a particular application.
In practice, LVFs enable to replace setups with multiple filters with a single filter, simplifying design and reducing costs of many measurement devices.
By design, LVFs are dielectric filters working on interferometric principles. They are manufactured by covering a filter substrate (for instance fused silica) with many thin film coating layers. The thicknesses of the thin film layers change along one filter direction (typically the length of the filter), creating the varying spectral response. Solaris Optics manufactures custom linear variable filters in its premises in Poland and in the below article we discuss the basics, types and main application fields of linear variable filters.
Conventional Wavelength Separation Techniques
A typical example of wavelength separation application is spectroscopy. In spectroscopy, wavelengths have been conventionally separated by diffraction gratings, prisms or a set of filters.
Diffraction gratings, for instance, have certain advantages for uses where light intensity is relevant, however as they are elements based on angle-based dispersion, they require more space and more complicated mechanical designs to work. Compared to diffraction gratings, linear variable filters require less space, yet allow smaller devices to be designed.
Another wavelength separation alternative – a set of filters, makes it necessary to ensure filter change mechanism, which can be realized e.g. by locating filters on a rotating wheel. This requires space, limits the possible pass bandwidths and rises overall cost.
In case of LVF the filter is held in a device so that a relative movement between the filter and the signal source is feasible. By moving the filter (or signal source) along the gradient axis, specific wavelengths are transmitted or blocked, according to the filter characteristics.
Main Characteristics of Continuously Variable Filters
LVFs are offered within UV to mid-IR range, e.g. from 230nm (then a filter range of e.g. 230-500nm) up to 5um (filter range 2.5-5 um). Apart from filter spectral range, they are typically characterized by bandwidth or cut-off wavelength, slope (change of center wavelength in nanometers per millimeter), pass bandwidth (usually specified as full width half maximum FWHM), transmission levels, filter blocking and physical dimensions. A manufacturer shall prepare also a chart showing the actual transmission wavelength versus filter physical position. In practice this chart shows actually a quasi linear relation between the transmissivity and position.
Some example specifications of commercially available continuously variable filters include:
- a slope of e.g. 5nm/mm, 20nm/mm, or any, e.g. 90nm/mm,
- pass bandwidth of less than 1% up to roughly 2.5% of the center wavelength,
- transmission of 50% to 90%,
- filter blocking of about OD3-4,
whereas a good level of linearity would be e.g. +- 1%.
The actual characteristics is largely affected and limited by the filter wavelength range, which for bandpass filters may be e.g. 230-500nm, 300-750nm, 700-1100nm, 1.3-2.6 or 2.5-5um. It is up to a specific application to design or select a suitable filter type.
Types of Linear Variable Filters
Linear Variable Filters can be created with various defined spectral properties, where the ratio between transmissivity and position may be also highly non-linear. However due to design convenience and requirements in many applications a linear characteristics is advantageous and in practice is the most sought. Variable filters are typically offered as long pass (Long Pass Variable Filters), short pass (Short Pass Variable Filters) and bandpass (Bandpass Variable Filters).
An interesting set of features can be achieved by using both Long Pass Variable Filters and Short Pass Variable Filters, so that they can be independently moved along the same axis. In such a setup it is possible to achieve a Linear Variable Bandpass Filter, which central wavelength (transmission spectrum) and bandwidth can be tuned continuously and adjusted as needed.
Applications of Linear Variable Filters
Continuously variable filters are applicable in the domain of measurement and detection. Hence they are used in many spectroscopic and analytical applications, in either laboratory as well as process applications. Uses such as gas analysis, material identification, wavelength sensors, hyperspectral imaging are some examples, where LVF are advantageous.
LVFs enable also more advanced designs. For instance when combined with detectors, such as line or square CCD/CMOS sensors the filters allow to build a compact and rugged detectors with large aperture and high transmission, which yields short measurement time, high suppression of straight light excellent signal to noise ratio.
As described above, linear variable filters offer a number of advantages compared to conventional wavelength separation techniques. Solaris Optics, as a throughout manufacturer of linear variable filters offers its customers support in filter design and selection. Please feel free to contact us!