Abstract

Multi-spectral laser imaging is a technique that can offer a combination of the laser capability of accurate spectral sensing with the desirable features of passive multispectral imaging. The technique can be used for detection, discrimination, and identification of objects by their spectral signature. This article describes and reviews the development and evaluation of semiconductor multi-spectral laser imaging systems. Although the method is certainly not specific to any laser technology, the use of semiconductor lasers is significant with respect to practicality and affordability. More relevantly, semiconductor lasers have their own characteristics; they offer excellent wavelength diversity but usually with modest power. Thus, system design and engineering issues are analyzed for approaches and trade-offs that can make the best use of semiconductor laser capabilities in multispectral imaging. A few systems were developed and the technique was tested and evaluated on a variety of natural and man-made objects. It was shown capable of high spectral resolution imaging which, unlike non-imaging point sensing, allows detecting and discriminating objects of interest even without a priori spectroscopic knowledge of the targets. Examples include material and chemical discrimination. It was also shown capable of dealing with the complexity of interpreting diffuse scattered spectral images and produced results that could otherwise be ambiguous with conventional imaging. Examples with glucose and spectral imaging of drug pills were discussed. Lastly, the technique was shown with conventional laser spectroscopy such as wavelength modulation spectroscopy to image a gas (CO). These results suggest the versatility and power of multi-spectral laser imaging, which can be practical with the use of semiconductor lasers.

Highlights

  • Optical spectroscopic imaging and the related multi/hyperspectral imaging are highly useful techniques for a wide and diverse range of applications, ranging from microscopic chemical/biological imaging to stand-off mapping of chemical distribution and long-range remote sensing [1,2,3]

  • This paper describes some recent studies [22,23,24,25,26,27] in laser multi-spectral sensing and imaging with semiconductor lasers ranging from near-IR (NIR) to midwave- and longwave-IR (M/LWIR), showing the technique capability and potential for spectroscopic discrimination of objects

  • A challenge in multispectral imaging is to distinguish this type of variation from that associated with the material dielectric property

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Summary

Introduction

Optical spectroscopic imaging and the related multi/hyperspectral imaging are highly useful techniques for a wide and diverse range of applications, ranging from microscopic chemical/biological imaging to stand-off mapping of chemical distribution and long-range remote sensing [1,2,3]. As far as the measurement approach is concerned, the trend has been to use passive multi-/hyperspectral imaging, which employs detectors coupled with wavelength filters/multiplexers to measure the emission or scattered radiation from targets in the natural environment. Broad-band non-laser light sources are used when illumination is needed. Lasers uniquely offer radiometric and spectroscopic accuracy and resolution, and multispectral imaging technology can be greatly expanded with the laser. There are applications in which the laser multispectral capability provides invaluable performance; some examples are in the field of LIDAR [4]. For the last few decades since late 1970s to early 1980s, the value of multispectral LIDAR has been well demonstrated as numerous work developed multi-wavelength or tunable/frequency agile LIDARs for applications that range from chemical agent detection [5,6] to atmospheric sensing [4]

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