What are »continuously variable filters«?

A continuously variable filter (CVF)—also known as a linearly variable filter—is a wedge-shaped interference filter whose spectral properties vary continuously along one dimension. You determine the desired wavelength by shifting the filter (and, depending on the optics, also rotating it) instead of replacing the filter. Delta Optical Thin Film is a specialist in this field.

In many devices, a single CVF can replace multiple fixed filters or even a grating-based monochromator—with compact, robust mechanics.

Why is this exciting?

• Fewer parts, more flexibility: »Wavelength per position« instead of »wavelength per filter change«.
• Spectroscopy & fluorescence: CVFs are used in spectroscopy, among other things, and can serve as tunable filter monochromators in fluorescence microplate readers.
• Order sorting: Particularly elegant for diode array spectrometers, because variable edge profiles can smooth transitions.

The subtypes in brief

Linear Variable Bandpass Filters

A bandpass whose center wavelength shifts along the filter. Ideal when a more defined transmission window is required (e. g. 400 – 700 nm in several versions).

Linear Variable Edge Filters (LWP/SWP)

Variable longpass/shortpass edges; particularly universal because LWP + SWP can be used to build a tunable bandpass window. More information here.

Continuously variable dichroic beamsplitters (Linear/Exponentially Variable Beamsplitters)

Dichroics, mostly designed for AOI ≠ 0° (typically 45°). There are variants with nearly linear or exponential edge variation (LV/EV dichroics).

Variable filter sets (tunable sets, e. g., for EPI fluorescence)

Combinations of CVLWP + CVSWP (tunable bandpass) and optional variable dichroic; sample sets show defined tuning ranges and out-of-band blocking levels (OD specifications).

Continuously variable order sorting filters (CVOSF)

Specially designed to suppress higher orders in spectrometers with diffraction gratings – with a continuously running cut-on edge, resulting in fewer »undefined« spectral ranges.

»Linear« – truly linear?

Not necessarily. The characteristic curve »position ➙ wavelength« is typically monotonic, but can be S-shaped; in addition, lines of equal wavelength can be slightly curved across the filter surface (»ring segments«), which is taken into account in beam guidance.

Interested? Let us advise you!

Send us your key data (spectral range, AOI, beam diameter/spot, desired blocking (OD), design) and our team of experts will recommend the right CVF subtype and a specific variant for you.