## 2 Line-Scan Hyperspectral Measurement Techniques
### Line-Scan Measurement Methods  
Line-scan hyperspectral imaging can be arranged in different configurations to satisfy different detection requirements. Reflection and transmission measurements (Figure 1a) are two basic modes used in line-scan hyperspectral imaging techniques. In a reflection mode, a source and a detector are placed on the same side of the sample. The detector collects signals reflected from the sample, such as specular and diffuse reflectance, fluorescence, and backscattering Raman, which are commonly used for sample surface inspection. In a transmission mode, a source and a detector are arranged on opposite sides of the sample. The detector acquires transmittance signals passing through the sample, which can be used to assess bulk composition of the sample, especially for small individual items with an internal condition of weak absorption. For either reflection or transmission configurations, the line-scan hyperspectral imaging method (i.e., the pushbroom method) simultaneously acquires a slit of spatial information as well as full spectral information for each spatial point in the linear field of view (FOV) illuminated or excited by line or area light sources. A special 2-D image (y, \(\lambda\)), with one spatial dimension (y) and one spectral dimension (\(\lambda\)), is collected from each scan (Figure 1a). A complete hypercube (x, y, \(\lambda\)) is built line by line as the scan is conducted along the direction of motion (x) to cover the whole surface of the sample.  
Hyperspectral transmittance imaging is able to obtain overall internal sample information. However, it generally cannot differentiate the information from individual sample layers due to lack of variations for the transmittance signals. Spatially resolved spectroscopy is a technique that can obtain subsurface information from a layered sample. The basic principle is to separate a point light source and a detector to allow light to travel through a deeper area of the sample (Figure 1b). Spatially resolved spectra are collected from a series of surface positions laterally offset from the point source, allowing compositions of the individual layers to be evaluated. The concept has been adopted for different spectroscopy techniques, such as spatially resolved diffuse reflectance [8], spatially resolved fluorescence spectroscopy [9], and spatially offset Raman spectroscopy [10]. Measurement methods using optical fiber probes are either slow (single fiber probe with mechanical movement) or restricted in offset range and interval selection (fiber probe array) for acquiring the spatially resolved spectra. Line-scan hyperspectral imaging techniques provide a flexible and efficient method for spatially resolved spectroscopy measurement [11; 12]. The method collects a series of spectra all together in a broad offset range with a narrow spatial interval using one CCD exposure (Figure 1b). The scattering image (\(y,\lambda\)) acquired from a single scan is a complete set of the spatially resolved spectra with sufficient spatial and spectral information for subsurface evaluation of heterogeneous or layered food samples.  
Figure 1: Line-scan hyperspectral measurement methods for (**a**) reflectance and transmittance imaging and (**b**) spatially resolved spectroscopy.