## 2 Hyperspectral Imaging Configurations
### Instrumentation  
Despite various image acquisition configurations, most hyperspectral imaging systems share three major components, i.e., light source, wavelength dispersive device and area array detector. Light sources generate light to illuminate the sample, which basically fall into three types, i.e., halogen lamps, light emitting diodes (LEDs) and lasers. A halogen lamp, also known as quartz tungsten halogen (QTH) lamp, provides a smooth broadband spectrum covering the region from VIS to NIR, which is commonly used in hyperspectral imaging for food detection. However, QTHs have a relatively short lifetime, high heat generation, spectral shift due to temperature changes and unstable output due to operating voltage fluctuations. LEDs have evolved rapidly in recent years and become increasingly popular in machine vision applications. They are able to generate both narrowband and broadband light, and support flexible illumination arrangements such as spot, line, and ring lights, depending on applications. Compared to QTHs, LEDs have numerous advantages including small size, fast response, low cost, long lifetime, low heat generation, and low energy consumption. Lasers are powerful monochromatic light sources, distinguished from QTH and LED by the properties of monochromaticity, directionality, and coherence. Lasers are frequently used as excitation light sources in hyperspectral fluorescence and Raman imaging measurements for food quality and safety detection. A wavelength dispersive device is to disperse the incident broadband light into different wavelengths and project the dispersed light to the detector, which is essential for a hyperspectral imaging system. The common wavelength dispersive devices are imaging spectrographs and electronically tunable filters (ETFs). Imaging spectrographs use diffraction gratings for wavelength dispersion based on the fact that the direction of light propagation is wavelength dependent. There are two types of imaging spectrographs, as illustrated in Figure 2, according to whether a reflection or transmission grating is used for wavelength dispersion, both of which have been used in line-scan hyperspectral imaging systems. Two major types of ETFs are liquid-crystal tunable filter (LCTF) and acousto-optic tunable filter (AOTF). The LCTF is a birefringent-based filter that uses the phase retardation between the ordinary and extraordinary rays passing through a liquid crystal to create constructive and destructive interferences, thus dispersing an incident broadband light into different wavelengths. The AOTF, on the other hand, is a solid-state device that isolates a single wavelength based on light-sound interactions in an anisotropic crystal that behaves like a transmission diffraction grating, and the tuned wavelength can be controlled by varying the frequency of an acoustic wave propagating inside the crystal. Unlike imaging spectrographs, both LCTF and AOTF are generally used in area-scanning hyperspectral imaging systems. Figure 1: Four approaches to acquiring a three-dimensional hyperspectral image cube: (**a**) point scanning, (**b**) line scanning, (**c**) area scanning and (**d**) single shot, where \(x\) and \(y\) represent the spatial dimensions, while \(\lambda\) is the wavelength. An area array detector, or also commonly called a digital camera, is to measure the intensity of the collected light in two spatial dimensions. Charge-coupled devices (CCDs) are the mainstream detectors used in hyperspectral imaging systems. The CCD sensor consists of many small photodiodes, known as pixels, which are made of photosensitive materials such as silicon. Different sensing materials have different spectral responses of the CCD sensor, which are quantified by quantum efficiency (QE). Regular silicon-based CCD cameras have a bell-shaped QE curve over the visible and short-wave near-infrared regions of 400-1000 nm, and they usually have lower QEs.