Continuously Variable Filters

Continuously Variable Filters (CVFs) – also known as Linear Variable Filters – from Delta Optical Thin Film offer unprecedented tunability over a large wavelength range, steep filter edges and high out of band blocking. These filters have enabled many applications that was previously not possible.

A CVF is a wedged filter, whose spectral properties vary continuously along one dimension of the filter. It is common to call these filters Linear Variable Filters (LVFs) but, we do however prefer to call our filters Continuously Variable Filters because the progression along the long side of the filter is never 100% linear and can sometimes be for example exponential.

A single CVF for example can replace a large number of fixed filters or a grating based monochromator in an instrument. It is possible to adjust the center or edge wavelength by sliding the filter or rotating it.

Our Continuously Variable Filters come with various functionality:
Continuously Variable Bandpass Filters (CVBP)
Continuously Variable Long Wave Pass Filters (CVLWP)
Continuously Variable Short Wave Pass Filters (CVSWP)
Continuously Variable Dichroic Beamsplitters (CVBS)
Continuously Variable Order Sorting Filters (CVOSF)

Product categories

We have divided our CVF’s into a number of relevant product categories:

Watch the video

Tune your own Bandpass Filter

This video show the benefit of combining a CVLWP with a CVLSP to form a Bandpass Filter where both center wavelength and bandwidth is tunable.

Further CVF tech talk

How linear is a LVF?
The term Linear Variable Filter seems to imply that the relation between edge or centre wavelength versus position along the filter is linear. However, in reality this is not strictly true.

Delta Optical Thin Film A/S used the term Linear Variable Filter in the beginning because it was coined several decades ago – long before Delta Optical Thin Film A/S began to produce this type of filters. The preferred term however is Continuously Variable Filter.

In reality, the relation is an s-shaped, monotonic function that deviates only slightly from a straight line:

For different purposes it is possible to design the function of edge or centre wavelength versus position along the filter deliberately non-linear, for example exponential, to compensate for angular effects or the change of bandwidth with center wavelength of variable bandpass filters.

In fact, there is a second type of non-linearity. This concerns the shape of lines of constant edge or center wavelength perpendicular to the wavelength gradient. Due to the production process the lines of constant edge or center wavelength are ring segments. Their curvature radius changes along the filter and is large compared to the filter’s dimensions:

Read more technical notes about the effect of beam shape and AOI with various light sources:

Edinburgh Instruments has written an article that shows the benefit of using Linear Variable Filters in grating-based spectrometers

Download article

Get in touch

Have a look at our existing products or get in touch regarding your needs – we are the optical filter company.

Get in touch