Development of a New Line Illumination for Industrial Hyperspectral Imaging Systems

Abstract

Hyperspectral line scan systems are used in industrial sorting solutions to differentiate materials by subtle spectral features (spectral fingerprint) which are not resolvable by the human eye. Prominent applications are the detection of contaminants in food or the separation of different types of plastic. Hyperspectral line scan systems are suitable for many high throughput applications. A line across the sample, perpendicular to the direction of the relative movement, is projected into an imaging spectrograph. The spectral information for each pixel along this line is projected along the second axis of the two-dimensional detector chip. Thus, it is only necessary to illuminate a narrow line shaped area across the sample. This is usually done with halogen spots which deliver an elliptical illumination spot at the sample. By using several spots in a row and superimposing the illumination spots the area of interest is covered. However, this leads to a non-homogeneous intensity distribution along the scan line and an illumination of a much larger area than necessary. In our approach we use readily available automotive H7 halogen headlight bulbs in combination with an elliptical reflector. We compare different approaches based on mirrors as well as lens arrays to compress the elliptical illumination spot into a line shaped illumination. We compare the different approaches by their intensity distribution of the light along the scanline and the footprint of the light source. Optical simulations and practical measurements of the different light sources are presented.