ARS | Publication request: Optical design and system calibration for three-band spectral imaging system with interchangeable filters

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: February 19, 2008
Publication Date: June 30, 2008
Citation: Kise, M., Park, B., Lawrence, K.C., Windham, W.R. 2008. Optical design and system calibration for three-band spectral imaging system with interchangeable filters. ASABE Annual International Meeting. Paper No. 084685.

Interpretive Summary: This paper reports the optical design of multispectral camera system, the calibration algorithm that compensates optical misalignment, and an evaluation of the imaging system. Spectral imaging is to acquire the image of specific spectral bands for measuring spectral characteristics of the object in a non-destructive manner. It is a phenomenon to characterize the pattern of materials in terms of absorbance and reflectance at different. Typical applications for visible and near-infrared (Vis/NIR) region, with emphasis on agriculture, can be found in remote sensing, food safety and quality inspection, fruit mutuality detection, and mineral inclusion analysis. Multispectral imaging is not able to measure as many spectral bands as hyperspectral imaging that acquires several hundred spectrally contiguous bands. However, most of multispectral imaging systems can acquire defined band images simultaneously, which makes multispectral imaging system more suitable for real-time applications. The objective of this study is to develop a compact, cost effective, and spectrally flexible multispectral camera system that can measure two visible and one near-infrared images simultaneously. This system can be retrofitted for a variety of applications, such as food safety and quality inspection as well as airborne remote sensing.

Technical Abstract: The design and calibration of a three-band image acquisition system was reported. The prototype system developed in this research was a three-band spectral imaging system that acquired two visible (510 and 568 nm) images and a near-infrared (NIR) (800 nm) image simultaneously. The system was prototyped with a three-port imaging system that consisted of three identical monochrome cameras, optical assembly, and three interchangeable optical filters. Spectral reflectance from an object was collimated by a front lens, and split into three ways by a cold mirror and beam-splitter: A cold mirror reflects 90% visible light and transmits 80% NIR light. The visible light was again split identically into two directions by a beam-splitter. Focusing lenses then projected each image on its corresponding sensor. By incorporating an interchangeable filter design, the imaging system is able to measure any two visible spectral bands that range between 400 nm and 700 nm, and one NIR band that ranges between 700 nm and 1000 nm without additional complicated manufacturing process. In order to co-register the three-band images, a system-specific calibration algorithm was developed that compensates lens-sensor geometric misalignment.