The Continuously Variable Filter (CVF) (a.k.a. Linear Variable Filters) from Delta Optical Thin Film is a wedged filter possessing spectral properties that vary continuously along one dimension of the filter. The CVF offers an extended set of performances within a single filter.
A single CVF for instance can replace many fixed filters in an instrument. The center or edge wavelength can be adjusted by sliding the filter.
Delta Optical Thin Film has taken the quality of variable filters to a whole new level by introducing a new robust combination of CVFs. The company 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 individually. Combining LVSWP and LVLWP enables the creation of band-pass filters that can be tuned continuously with center wavelengths from 320 nm to 850 nm, with the extra advantage of tunable bandwidth. As LVF monochromator, the filters are employed in fluorescence microplate readers.
Apart from setting new standards in edge steepness and transmission level, the filters offer blocking better than OD3 over the full reflection range. The blocking can be increased to beyond OD5 by placing another CVF in series.
The filters are coated on single quartz substrates for high laser damage threshold and least auto-fluorescence. All of Delta Optical Thin Film’s CVFs 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 CVFs are particularly suited for applications in spectroscopy.
- Ultra-hard-coated filters
- Scratch/dig optical surface specifications 60/40
- Standard dimensions for LVOSF are 40 mm x 9 mm x 0.75 mm
- Standard dimensions for LVLWP, LVSWP, LVUVBP are 60 mm x 15 mm x 3 mm
- Standard dimensions for LVDichroic are 60 mm x 23 mm x 1 mm
- Standard dimensions for LVVISBP and LVNIRBP are 60 mm x 12 mm x 2.5 mm
- The wavelengths λ1, λ2 given in the specification are the wavelengths of the edge of the filter at the highest spectral point (maximum thickness of the wedge) and the lowest spectral point (minimum thickness of the wedge)
- The data sheets display typical transmission curves for unpolarized light with an AOI=0° at different wedge positions
- Delta Optical Thin Film can also develop custom specific Continuously Variable Filter
- 3G Long Wavelength Pass filter: Edge to be tuned from 310 nm – 850 nm, blocking down to 190 nm when edge at maximum wavelength, steeper edge. Steeper edge and broader blocking range than previous generations.
- 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.
- A variety of Order Sorting Filters for filtering higher orders of diffractive optics and suppression of background noise
- NIR Bandpass filter: Center wavelength to be tuned from 550 nm – 1000 nm
- UV Bandpass filter: Center wavelength to be tuned from 300 nm – 330 nm
- VIS Bandpass filter: Center wavelength to be tuned from 400 nm – 700 nm
- UV/VIS Bandpass filter: Center wavelength to be tuned from 320 nm – 560 nm
- Ultra-Narrow VIS Bandpass filter: Center wavelength to be tuned from 400 nm – 700 nm
- Dichroic: Edge to be tuned from 320 nm – 760 nm
- Linear Variable Bandpass Filters for Hyperspectral imaging:
- Center wavelength range 450 nm to 880 nm, bandwidth approximately 2% of center wavelength, transmission 60% to 90%, blocking range 200 nm to 1150 nm, blocking level OD4, sensor size 24 mm x 36 mm
- Center wavelength range 796 nm to 1084 nm, bandwidth approximately 1% of center wavelength, transmission >85%, blocking range 200 nm to 1150 nm, blocking level OD4, for sensor size 32 mm x 18 mm
- Center wavelength range 450 nm to 850 nm, bandwidth approximately 4% of center wavelength, transmission 70% to 90%, blocking range 200 nm to 1100 nm, blocking level OD4, sensor size 25mm x 25 mm
How Linear Are LVFs?
The term Linear Variable Filter (LVF) suggests that the relation between center or edge wavelength versus position along the filter is linear. However, in fact this is not exactly true.
Delta Optical Thin Film A/S used the term LVF because it was coined many years ago – long before the company started to produce this type of filters. The ideal term however is CVF.
In fact, the relation is an s-shaped, monotonic function that deviates just slightly from a straight line:
Typical function of edge wavelength versus position along the filter. The actual shape depends on filter type.
For various purposes, it is possible to design the function of edge or center wavelength versus position along the filter purposely non-linear, for instance exponential to compensate for angular effects or the alteration of bandwidth with center wavelength of variable bandpass filters.
In fact, 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 alters along the filter and is large compared to the filter's dimensions.
The black lines are ring segments that represent actual lines of constant edge or center wavelength.