Continuously Variable Filters (a.k.a. Linear Variable Filters)

Overview

An extended set of performances within one single filter!

A Continuously Variable Filter (CVF) is a wedged filter, whose spectral properties vary continuously along one dimension of the filter.

A single CVF for example can replace a number of fixed filters in an instrument. It is possible to adjust the centre or edge wavelength by sliding the filter.

Delta Optical Thin Film has lifted the quality of variable filters to new levels by introducing a new powerful combination of continuously variable filters. Delta Optical Thin Film offers a Linear Variable Long Wave Pass filter (LVLWP), the corresponding Linear Variable Short Wave Pass filter (LVSWP) together with a Linear Variable Dichroic. Each of the filters can be used separately. Combining LVLWP and LVSWP enables the construction of band-pass filters that can be tuned continuously with center wavelengths from 320 nm to 850 nm, with the added benefit of tunable bandwidth. As LVF monochromator the filters are used in fluorescence microplate readers.

Besides setting new standards in transmission level and edge steepness, the filters offer blocking better than OD3 over the complete reflection range. It is possible to increase the blocking to beyond OD5 by placing another continuously variable filter in series.

The filters are coated on single quartz substrates for minimal auto-fluorescence and high laser damage threshold. All of Delta Optical Thin Film’s Continuously Variable Filters are coated with ultra-hard surface coatings (UHC) that are also used by Delta Optical Thin Film in traditional fluorescence filters.

Delta Optical Thin Film's Continuously Variable Filters are especially suited for applications in spectroscopy. Download the presentation on Continuously Variable Filters for Fluorescence and Hyperspectral Imaging Applications. Edinburgh Instruments has written an article that shows the benefit of using Linear Variable Filters in grating based spectrometers.

Models

Achieve full tunability in your system by using the following Continuously Variable Filters

  • 3G Short Wavelength Pass filter: Edge to be tuned from 320 nm – 850 nm, blocking up to 684 nm when edge at lowest wavelength. Deeper near-edge blocking, but lower UV transmittance than previous generations.
  • 3G Long Wavelength Pass filter: Edge to be tuned from 310 nm – 850 nm, blocking down to 190 nm when edge at highest wavelength, steeper edge. Steeper edge and broader blocking range than previous generations.
  • Different Order Sorting Filters for filtering higher orders of diffractive optics and suppression of background noise.
  • Dichroic: Edge to be tuned from 320 nm – 760 nm.
  • UV Bandpass filter: Center wavelength to be tuned from 300 nm – 330 nm.
  • UVVIS Bandpass filter: Center wavelength to be tuned from 320 nm – 560 nm.
  • VIS Bandpass filter: Center wavelength to be tuned from 400 nm – 700 nm.
  • Ultra-Narrow VIS Bandpass filter: Center wavelength to be tuned from 400 nm – 700 nm.
  • NIR Bandpass filter: Center wavelength to be tuned from 550 nm – 1000 nm.
  • Different Linear Variable Bandpass Filters for Hyperspectral imaging:
    • Centre wavelength range 450 nm to 880 nm, bandwidth approximately 2% of centre wavelength, transmission 60% to 90%, blocking range 200 nm to 1150 nm, blocking level OD4, sensor size 24 mm x 36 mm
    • Centre wavelength range 450 nm to 850 nm, bandwidth approximately 4% of centre wavelength, transmission 70% to 90%, blocking range 200 nm to 1100 nm, blocking level OD4, sensor size 25mm x 25 mm
    • Centre wavelength range 796 nm to 1084 nm, bandwidth approximately 1% of centre wavelength, transmission >85%, blocking range 200 nm to 1150 nm, blocking level OD4, for sensor size 32 mm x 18 mm

Features

The Continuously Variable Filters feature

  • Ultra-Hard-Coated filters.
  • Scratch/dig optical surface specifications 60/40.
  • Standard dimensions for LVLWP, LVSWP, LVUVBP are 60 mm x 15 mm x 3 mm.
  • Standard dimensions for LVOSF are 40 mm x 9 mm x 0.75 mm.
  • Standard dimensions for LVVISBP and LVNIRBP are 60 mm x 12 mm x 2.5 mm.
  • Standard dimensions for LVDichroic are 60 mm x 23 mm x 1 mm.
  • The wavelengths λ1, λ2 given in the specification are the wavelengths of the edge of the filter at the lowest spectral point (minimum thickness of the wedge) and the highest spectral point (maximum thickness of the wedge).
  • The data sheets show typical transmission curves for unpolarized light with an AOI=0° at different wedge positions.
  • If you need further information, please contact us. We can also develop your custom specific Continuously Variable Filter.

Specifications

Linear Variable Filters

Filter Part Number Edge/Center (nm) Transmission Details Price
3G LVSWP LF102474 320-850 T > 90% Data sheet 725 €
3G LVLWP LF102475 310-850 T > 92% Data sheet 700 €
LV Dichroic LF102227 320-760 T > 90% Data sheet 800 €
LVUVBP LF102190 300-330 T > 40% Data sheet 700 €
LVUVVISBP LF103566 320-560 20% < T < 85% Data sheet 850 €
LVVISBP LF102499 400-700 40% < T < 95% Data sheet 950 €
UNLVVISBP LF102773 400-700 25% < T < 80% Data sheet 950 €
LVNIRBP LF102637 550-1000 80% < T < 95% Data sheet 1250 €
LVVISNIRBP25 LF103252 450-850 70% < T < 90% Data sheet 2750 €
LVVISNIRBP36 LF103245 450-880 60% < T < 90% Data sheet 2750 €

What is the difference between 1G, 2G and 3G Linear Variable Filters? [1,2,3]G stands for different generations of our Linear Variable Filters – like with the iPhones 😉

The differences lie in broader blocking range and steeper edges for the latest generation. 3G Linear Variable Filters are our highest performance LVF.

How linear are LVF?

The term Linear Variable Filter seems to imply (read our blog post on LinkedIn) 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:

linearity

Typical function of edge wavelength versus position along the filter. The actual shape depends on filter type.

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:

LVF med cirkelslag

The black lines are ring segments that represent actual lines of constant edge or center wavelength.