Hyperspectral imaging (HSI) is finally gaining the momentum that it deserves as a new vision technology for industrial on-line quality control, inspection and process monitoring. Applications cover wide range of fields from food sorting to printing, recycling and vegetation to name a few. Compared to more familiar black and white or RGB -imaging it provides more detailed chemical, physical and biological information for automation and decision making.

Technological power comes from measuring the information above visible wavelength range of light to extract relevant information. Ambitious machine vision companies that want to stay at the technological forefront should not bypass current opportunity to adopt HSI as an important competition asset.

General considerations

When selecting instrument for hyperspectral application there are many functional and non-functional quality attributes to consider. Typically, spectral range, resolution, speed and cost (return of investment) set by application are the first ones to think of.

Main technological differences are related to how areal and spectral measurement is carried out in practice. It is not possible to measure two dimensional spatial image and spectrum from each position simultaneously. This dilemma divides current instruments as follows.

Push-broom based instruments

Push-broom instrument is based on matrix camera and spectrograph to form a line imaging device where one axis of the detector registers spatial position from a line and the other axis spectral information in each spatial position. This assures that resulting spectrum is perfectly co-registered so that all spectral bands are measured simultaneously and from the exactly same sample position. Illumination is needed only for this narrow line and result can be processed immediately after each line measurement.

Other HSI technologies

Current main competing technologies are tunable filter and variable filter instruments. In tunable filter only one wavelength band is measured from two-dimensional area at each time. In linear variable filter all wavelength bands are measured simultaneously but each band from different position on the target area. Both suffer from difficulty in getting co-registered spectrum and wasting most of the illumination energy due to operation principle. Co-registration problems lead to difficulties in data processing, unreliable spectral signatures and delayed processing results.

Quality attributes of different hyperspectral imaging technologies while used in industrial on-line applications.

Push-broom for industrial on-line applications

It is reasonable to conclude that only line imaging push-broom HSI is suitable to all industrial on-line applications where moving and changing targets are measured and spectral co-registration is required. This property is necessary to assure spectral purity and for getting reliable classification results.

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