## 3 Spectral Scattering Imaging
### Spectral Scattering Imaging (SSI) Configurations  
As shown in Figure 4, a general SSI system mainly consists of an illumination unit, an imaging unit and a computer. The illumination unit consists of a light controller, a light source, an optic fiber and a focusing lens. A QTH lamp is commonly used as a light source for providing broadband illumination over a wavelength range of about 400-1000 nm in both multispectral and hyperspectral measurement. The imaging unit includes a CCD camera and a wavelength dispersive device attached with a zoom lens. Light passes through an optic fiber and a focusing lens to form a sharp, highly focused beam (usually between 1-2 mm in size), which is incident on the fruit surface. As the light propagates into the fruit tissue, photons are either absorbed or scattered. The photons, which are backscattered and then exit from the fruit in an area (scattering area), illuminate a portion of the fruit tissue surrounding the incident area, as shown in Figure 5. In designing an illumination unit, attention should be paid to the size of light beam for generating good scattering images [24]. A larger beam gives a higher light throughput and a larger scattering area, but it may not be desirable for quantifying light scattering characteristics. A smaller beam, on the other hand, albeit desirable for scattering measurement, results in lower intensities of images acquired and a lower signal-to-noise ratio (SNR) as well. A beam size of 1.6 mm in diameter is often used for acquiring scattering images from apple fruit with the maximum scattering area of about 25 mm in diameter [43; 48]. The wavelength dispersive device is used to disperse the incoming broadband light into separate wavelengths, and it can be a line-scan imaging spectrograph using a prism-grating-prism configuration for hyperspectral scattering imaging [43; 48], or a common aperture multi-channel imaging spectrograph for multispectral measurement [25; 49]. In addition, multispectral measurement can also be achieved using a LCTF [50; 51; 52], or a mechanical rotating filter wheel containing four to six bandpass filters [24]. Hence there are basically four sensing configurations, as shown in Figure 6, for measuring scattering images. Figure 4: Schematic of a spectral imaging system for acquiring scattering images of fruit. Figure 5: Schematic of light scattering within fruit resulting from a sharp, focused incident beam. The line-scan spectrograph based configuration (Figure 6a) allows simultaneous recording of a line of spatial information with a full spectral range for each spatial pixel of the scanned line, thus creating a special 2-D image with one axis representing the spatial dimension and the other spectral. For acquiring a complete 3-D hypercube, a series of scans need to be captured sequentially, which is time consuming and generates a large volume of data as well. To expedite the acquisition of hyperspectral scattering images, a simplified scanning approach can be used, since light scattering in the fruit tissue could be assumed to be symmetric with respect to the incident point. Thus, the complete scattering information of the fruit may be well represented by one single line scan that passes through the center of the incident area. However, in practical implementation, the scanning line often is slightly away from the incident center (1 to 2 mm) to avoid signal saturation [43; 48]. This simplified approach to acquiring hyperspectral scattering images is suitable for real-time, online implementation. Likewise, the common-aperture multispectral imaging configuration (Figure 6b), which records scattering images at multiple wavelengths simultaneously, also is suitable for fast, real-time online applications, but it suffers from reduced spatial resolution and slightly deteriorated image quality, i.e., lower SNR and image distortion.