Linear Variable Filters (LVFs)

LVFs for Biophotonics, Spectroscopy and Hyperspectral Imaging

Some applications may benefit from using Linear Variable Filters (LVFs) instead of conventional optical filters, which exhibit distinct transmission and blocking bands uniformly across their whole surface. In contrast, LVFs provide continuously variable transmission characteristics across their surface. For instance, such a filter might transmit the whole visible spectrum on one end while selectively allowing only the red spectrum to pass through on the other end (Fig. 1). Users can adjust their required transmission characteristic simply by altering the position of illumination on the filter, winning more flexibility. LVFs especially excel in applications requir-ing precise wavelength control, such as spectroscopy and hyperspectral imaging. They should also be considered in sophisticated optical setups requiring multiple wavelength bands, such as in fluorescence applications.

Benefits

High versatility
Wide range of variable filter types
Deep blocking by reflectance, low absorption
High transmittance in the pass range
Long-term shift-free spectral performance
High environmental stability
Customized dispersion
Customized filter sizes

Applications

Linear variable filters (LVFs) are utilized in numerous optical fields such as spectroscopy, hyperspectral imaging (HSI), and fluorescence microscopy. LVFs are advantageous in applications that require lightweight and compact instruments, such as HSI devices installed on unmanned aerial vehicles (UAVs), which are increasingly utilized in agriculture or for environmental observations. LVFs are also applied as wavelength selectors, order sorting filters in grating-based systems, or in purely filter-based spectrometers.

Technical Data

Filter type Longpass, Shortpass, Bandpass
Further types available on request

Wavelength range as per customer request, e.g.
from 400 nm to 800 nm
from 380 nm to 1100 nm
from 1100 nm to 1700 nm

Transmittance T > 90 – 97%
(depending on wavelength range)

Blocking up to OD8
(according to requirements)

Reflectance R > 90 – 99%

Angle of Incidence standard 0°
(different AOI on request)

Substrate Fused silica or BK7 or equivalent

Dimensions as per customer request

Spatial dispersion up to 100 nm / mm

Parallelism < 3 arcmin

Surface Defects 5 / 3 × 0.1

Environmental Stability Temperature – 100 ... + 150°C
Humidity up to 99%

Fig.1: Linear variable longpass filter in the VIS, measured on different positions. The average transmission is > 97%.

Fig. 2: blocking performance measured on different positions on the filter. The absolute blocking is better than OD6 while the edge steepness 10%-90% is less than 3nm. The theoretical blocking of OD8 cannot be measured due to noise floor limitations.

Fig. 3:Linear variable shortpass filter for the NIR, measured on different positions. The average transmission is > 97%.

Fig. 4:blocking curve measured on different positions on the filter. The absolute blocking is better than OD6 while the edge steepness 10%-90% is less than 5nm.

Fig. 5:NIR-shortpass measured at various lateral positions. It‘s noteworthy that the dependence of the cutoff wavelength on the measurement position follows a linear trend, with a coefficient of determination of R²=1.

Fig.6: Linear variable bandpass for Si-sensors. The passband shifts from 360nm to 1000nm while blocking the full range 300nm – 1100nm with OD5avg.

LVF based on the combination of LP-filter and SP-filter

Bandpass filter adjustment for tuning of different filter characteristics

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