## 3 Hyperspectral Acquisition System  
In order to capture spectral signature curves from fish, a customized hyperspectral imaging (HSI) acquisition system was employed. Image acquisition was performed with a push-broom hyperspectral  
Figure 2: Scale-space significance map for the comparison between the hyperspectral image of a skin mole and an image obtained by manually distorting it. The original and distorted spectral signatures are shown in Figure 1. For the tests, the Hannig-Marron global rejection threshold was used both for the range and the mean.  
camera with a spectral range of 410-1000 nm (see, for example, [3]) and spatial resolution of 0.3 mm across-track by 0.6 mm along-track (Norsk Elektro Optikk, model VNIR-1024). The camera was fitted with a lens focused at 1000 mm, mounted 1020 mm above a conveyor belt. Samples were illuminated using two custom made fiber optic line lights (Fiberoptics Technology inc., Pomfret, CT, USA), fitted with custom made collimating lenses yielding light lines approximately 5 mm wide (Optec S.P.A., Milano, Italy). Each line light was 400 mm wide, with six bundles of optical fibers. The light from 12 focused 150 W halogen lamps with aluminium reflectors (International Light Technologies, Peabody, MA, USA, model L1090) was fed into the fiberoptic bundles. The imaging and illumination setup is seen in Figure 3a. The optical power actually hitting the sample is approximately 0.16-0.79 Watt/(nm\(\cdot\)sr\(\cdot\)m\({}^{2}\)).  
The illumination system is composed of a controller unit which allows controlling the brightness and the light source. This system permits us to regulate the light intensity according to the sample characteristics, such as color, size or other parameters dependent on light. The acquisition technique employed by this camera is the so-called push-broom method, which consists of an optical system capturing an image from a line in a plane as depicted in Figure 3b. The camera collects images as seen in Figure 3b.  
To capture a hyperspectral image, either the camera or the sample must be moved synchronously with the shoot of the camera. In this case, the sample is moved using a linear actuator by a stepper motor along a line. The light used has been tested to emit in the whole spectral range. Before starting the capturing process, the camera must be focused and calibrated with a dark reference and a white reference. In this process, a tile with 99% of reflectance was chosen for the white reference.  
The spectral signatures of the same frozen fish are taken on day 0, day 2, day 4, day 7 and day 10. On each day, we captured 912,082 signature curves in each of the four acquisitions made. The four acquisitions from day 0 were then compared to the other acquisitions in order to find significant differences as described in Section 4.  
After image acquisition, the data from the reference images were used to perform a radiometric calibration of the raw spectral signature of each pixel of the HSI cube as suggested in [13].  
\[CI=\frac{RI-DI}{WI-DI}, \tag{5}\]  
where \(CI\) is the calibrated image, \(RI\) is the raw image and \(WI\) and \(DI\) are the white and dark reference images, respectively.