Combined 3D model acquisition and autofocus tracking system for hyperspectral line-scanning devices

Obtaining a sharp and focused image is essential to fully utilize the advantages of hyperspectral imaging. For line scanning hyperspectral devices, focusing is a challenge in applications where the height and/or position of the imaged object might vary during a scan. Initial focusing is in addition a tedious process that has to be repeated for each sample or measurement. In this paper, a new continuous autofocus tracking system for hyperspectral line-scanning cameras is reported. The presented system is able to automatically and objectively find the correct distance between the camera and the imaged object for proper focus, and retain this focus distance during scan using a laser triangulation system. Concurrent with the focusing, a 3D model of the object is constructed. The system was tested and found to perform well for NIR-SWIR imaging of human hands and test objects with sharp changes in height and contrast. The method is easily adaptable to other spectral ranges and applications, such as industrial conveyor belt applications. The method significantly eases the acquisition of hyperspectral images by ensuring optimal image quality in every scan and eliminating the need for manual refocusing between individual samples.