Continuously variable bandpass filters offer a good middle ground for building hyperspectral imaging solutions, says Delta Optical Thin Film’s Oliver Pust
Hyperspectral imaging has been used for a couple of decades in applications such as satellite imaging, air reconnaissance and other not overly price-sensitive markets. Classical hyperspectral cameras with gratings and prisms achieve the highest spectral resolution and are well suited for demanding applications in research.
An alternative approach comprises sensors that are coated at wafer-level with fixed wavelength bandpass filters. These sensors provide compact cameras, offer total flexibility with respect to filter pattern, and are readily suited for snapshot imaging.
A third approach is cameras based on continuously variable bandpass filters (CVBPFs), which combine high light efficiency, high signal-to-noise ratio and high spectral resolution with compactness and robustness.
Delta Optical Thin Film manufactures custom CVBPFs for mid-size and full-frame CCD and CMOS sensors. These filters offer very high transmission levels and are fully blocked in the light-sensitive wavelength range of silicon-based detectors (200nm to 1,150nm or higher).
The first filter from Delta that was used for hyperspectral imaging was a linear variable bandpass filter with a centre wavelength range from 400nm to 700nm. It was actually designed for a non-imaging application in an absorbance reader. Although it was not fully blocked in the silicon range and was rather large, customers saw its potential for HSI and used it to build prototypes and products. A second LVBPF extending the wavelength range to 1,000nm was developed and produced on customer request.
As a result of further customer projects, standard filters are available covering 450nm to 880nm and 796nm to 1,084nm, transmission is up to 90 per cent in some models, a blocking level of OD4, and sensor sizes range from 19 x 8mm to 32 x 18mm.
Delta’s CVBPFs are thin film filters that are coated with silicon dioxide and metal oxides on a single fused silica substrate without the use of glue, colour glasses or thin metal layers. The resulting filters are very robust against environmental conditions like temperature and humidity, and are spectrally and mechanically stable. The filters are ideally suited for long-term use in airborne or space applications without any degradation.
The filters can be mounted directly on top of, or close to, the sensor. Options include gluing onto the sensor surface, replacing the cover glass with the filter or a mechanical holder. The optical design does not require the use of a slit and light is collected through the full aperture of the lens. The deep broad-band blocking of the filter ensures a high signal-to-noise ratio and eliminates spectral crosstalk.
Without a slit, every acquired image shows the complete scene. This makes it possible to arbitrarily image the scene from different positions without the need for precise synchronisation of lateral movement and image acquisition, such as with the push broom technique. In addition, it is possible to construct a hyperspectral data cube using image pattern recognition techniques.
Filters that cover a larger wavelength range over a shorter length are desirable. Currently, filter designs are under development that, for example, cover 450nm to 950nm over 36mm with an exponential dispersion function.
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Midwest Optical Systems has designed two new triple bandpass filters for imaging vegetation from the air. The TB550/660/850 filter adds green to traditional red and NIR measurements.
Green, red and NIR wavelengths are used for applications where chlorophyll vegetation index (CVI), normalised green (NG) and other vegetative index monitoring is needed.
The TB475/550/850 gives blue, green and NIR, which is recognised as the enhanced normalised difference vegetation index (ENDVI), a technique that can provide more detailed information. ENDVI may better isolate plant health indicators and can generate results and false colour mapping to indicate values at the pixel level.
MidOpt triple bandpass filters are offered in various standard threaded mounts and custom mounts that are cut to fit any lens or camera size. Standard material thicknesses include 0.5mm, 1.1mm and 2mm.
Edmund Optics has introduced flexible longpass filters. These versatile cut-on filters offer deep blocking and excellent transmission over a large spectral range from 400-1,600nm to accommodate a wide variety of applications. The filters are constructed using ultra-thin flexible polymers and dyes. They are scratch insensitive and provide equivalent durability to most industry hard-oxide coatings, making them extremely robust.
The filters are made of hundreds to thousands of polymeric and dyed sub-layers. Using these stacked materials allows them to be ultra-thin, measuring only 200-500µm thick. The filters are manufactured using a process combining features from plastic extrusion and fibre drawing processes. Able to conform to both flat and curved surfaces, the filters can be cut into complex shapes and small dimensions down to 0.5mm by the customer using scissors, blades, or lasers.