Innovative Filter Solutions for Hyperspectral Imaging

Abstract

Hyperspectral imaging (HSI) has been used for a couple of decades in applications such as satellite imaging, air reconnaissance and other not overly price sensitive markets. Still, there is no clear definition of hyperspectral imaging. Sometimes techniques that produce 2D images with more than the typical three RGB colors or spectral channels -for example by inclusion of a near-infrared channel -are already called hyperspectral. Mostly though, this is not considered sufficient. Typically, even ten spectral channels are still to be called multi-spectral rather than hyperspectral.In the following, we require that certain criteria are fulfilled for an imaging technique to be hyperspectral: ■ ■ For every pixel in the image, we measure the spectrum of the incident light or radiation.■ ■ The measured spectrum is continuous and not discretized to a limited number of channels.■ ■ The spectrum covers more than one sub-wavelength range, for example UVA, visible and near-infrared or NIR and SWIR. The advent of alternative approaches makes HSI attractive for volume markets or even consumer products, for example cancer detection, precision farming with UAV or directly at the plant, food testing in supermarkets and many more.Alternative approaches comprise sensors that are coated at wafer-level with fixed wavelength bandpass filters, e.g. Pixelteq or imec. Common are also thin film coatings on glass substrates that can be patterned during deposition (in situ), or by using a photolithographic process over the coating to block the addition or subtraction of materials deposited on the substrate surface, e.g. Materion. These micro-patterning techniques allow either filters that have a staircase of different center wavelengths in one direction, suited for the so-called push broom technique, or 2D mosaics, suited for the so-called snapshot technique.However, according to the above criteria, strictly speaking these solutions do not provide hyperspectral capability but are inherently multispectral due to their discrete changes in center wavelength. Typically, between 10 and 100 different wavelengths or channels are offered. A truly hyperspectral sensor offers a continuous change in center wavelength and as such a virtually unlimited number of channels.Delta Optical Thin Film follows a different approach to filters for hyperspectral imaging. Based on our extensive experience with linear variable filters, we develop and manufacture custom linear variable bandpass filters (LVBPF) for mid-size and full-frame CCD/CMOS sensors (e.g. 25 mm × 25 mm or 24 mm × 36 mm). These filters offer very high transmission levels and are fully blocked in the light-sensitive wavelength range of silicon-based detectors (200 nm to 1150 nm or higher). The combination of LVBPFs with silicon detectors allows the design of very compact, robust and affordable HSI detectors that offer sev- Delta Optical Thin Film A/S is the leading supplier of advanced, high performance linear variable filters commonly used in a variety of biomedical imaging applications including...

Miniaturization of optical spectrometers

“…However, this is relatively slow and necessitates extra moving parts. Combining a linear variable filter with a detector array for parallel measurement offers an ideal solution (55). Microspectrometers based on this configuration have already been demonstrated, as shown in Fig.…”

A Review of Current and New Optical Techniques for Coral Monitoring

“…A new type of hyperspectral imaging technology that utilises linear variable filter(s) (LVF) has recently emerged. A LVF is an optical filter whose bandpass windows varies continuously across its surface [74]. LVFs allow for lower cost imagers to be produced [74].…”

“…A LVF is an optical filter whose bandpass windows varies continuously across its surface [74]. LVFs allow for lower cost imagers to be produced [74]. For example, LVFs have been integrated with consumer grade digital cameras to convert them into hyperspectral imagers [74][75][76].…”

Single-Point Detection Architecture via Liquid Crystal Modulation for Hyperspectral Imaging Systems

“…This technique is complicated, relative to our use of LC shutters, given that it makes use of broadband laser emission. Filter array techniques can also be noted [29]- [33], Innovative Filter Solutions for Hyperspectral Imaging, (2016), Optical Components. Here, a mosaic of filters are arranged consecutively before a photometer.…”