Linear Variable Filter or Continuously Variable Filter?

Over the past six years, Delta Optical Thin Film A/S have been producing and further developing spatially variable filters. These types of filters have spectral characteristics that change spatially and were around for several years before we started to develop our own.

Traditionally, when people use the term linear variable filter they mean a filter whose spectral characteristic changes linearly across the filter, for example, the center wavelength for a bandpass filter.

The second meaning of linear is that the spectral change occurs along a straight line, giving the filter a rectangular shape, irrespective of if the relationship between spectral characteristic and position is linear, or for example exponential. Thus, the term ‘linear’ also describes the typical elongated rectangular shape of this type of variable filter.

Now, circular variable filters also exist. Using the same logic, in a circular variable filter the spectral characteristics change along a circle, regardless of the relationship between the spectral characteristic and angle. Circular variable filters are also circular in appearance.

Oliver suggests that another feature of variable filters can be highlighted, instead of the direction of the spectral characteristic change. A butcher block assembly of discrete filters can also be called a variable filter. Variable filters from Delta Optical Thin Film A/S  have a continuous relationship between the spectral characteristic and position, so why not call them ‘Continously Variable Filters’, which can include both rectangular and circular variable filters.

Continuously Variable Filters from Delta Optical Thin Film A/S

As mentioned above, a continuously variable filter has a continuous relationship between the spectral characteristics and the position. They are wedged filters and can be used to replace a number of fixed filters in an instrument. The center or edge wavelength can be adjusted by sliding the filter.

Delta Optical Thin Film has raised the standard of variable filters with our range of powerful continuously variable filters. Transmission levels and edge steepness is improved considerably, and these filters provide improved blocking over the complete reflection range.

These filters have a transmission that is higher than 92% and they work well with Supercontinuum Light Sources.

Our range of continuously variable filters are coated on single quartz substrates to reduce any auto-fluorescence and provide a high laser damage threshold. All of our filters are coated with ultra-hard surface coatings.

These filters are used in a wide selection of applications, from monochromators, hyperspectral imaging, and mini spectrometers.

Delta Optical Thin Film supply a large range of optical filters for different applications, if you would like any information about our range of products, including our Continuously Variable Filters, please contact us.