Continuously variable edge filters (LV Edge Filters)

Linear variable edge filters are interference filters in which the edge (cut-on/cut-off) is not fixed but moves continuously along a filter axis. You select the desired edge wavelength by positioning the filter laterally in the beam (e. g., using a linear rail/scale). This means that a single filter can replace a whole series of fixed longpass or shortpass filters in many setups.

The principle

• Long Wave Pass (LWP): high transmission above the edge, blocking below.
• Short Wave Pass (SWP): high transmission below the edge, blocking above.
• »Variable« means that the edge (e. g., at 50 % transmission) increases along the length of the filter—you »tune« it via the position.

Practical note: The term »linear« is historical: the position-to-wavelength characteristic curve is often monotonic, but not perfectly linear (often slightly S-shaped) and, depending on the design, can also be deliberately nonlinear.

Typical applications

1. Tunable bandpass filters: Combining a variable LWP + a variable SWP creates a bandpass-type window whose center wavelength and bandwidth can be adjusted by the relative position of both filters.
2. Fluorescence setups / EPI fluorescence: Variable edge filters are often used in combination with a variable dichroic beam splitter to switch flexibly between dye/LED excitations.
3. Order sorting filters in mini spectrometers: A variable longpass can be particularly elegant as an »order sorting filter« because the edge »runs« along with the dispersion along the detector, thereby reducing transition/undefined areas.

What is important when it comes to design?

The following information is crucial for a quick and accurate selection:

• Wavelength range (min/max edge or tuning range)
• Angle of incidence (AOI) and polarization requirements (typically close to 0° for edge filters, often 45° for dichroics)
• Beam diameter/spot size (influences the effective bandwidth in setups with two edge filters, among other things; some data/examples use very small spots)
• Substrate/dimensions/thickness, mechanical constraints, environment (laser power, autofluorescence)
• Blocking requirement (OD): Standard blocking is often specified (e. g., OD3 over the relevant reflection range; higher if necessary by adding additional filters in series).

Standard types

The following variants are typical, »easy-to-use« entry-level types from our partner Delta Optical Thin Film. Other areas/geometries are possible.

Continuous Variable Short Wave Pass Filters

Continuous Variable Long Wave Pass Filters

Details and specific information on the individual variants (including data sheets) can be found on this manufacturer's website.

Extensions & Combinations

• More blocking / »deep blocking«: For demanding fluorescence measurements or stray light suppression, the blocking effect can be increased by using suitable blocking filters in series.
• Complete EPI filter sets: Combinations of variable dichroic + variable edge filters are available as coordinated sets (including high OD values for out-of-band blocking in set examples).

Interested? Let us advise you!

Send us your key data (wavelength range, AOI, beam diameter, desired OD blocking, dimensions). Our team of experts will be happy to help you find the optimal configuration for your specific application.