## 4 Integrated Reflectance and Transmittance Imaging
### Instrumentation  
A laboratory online hyperspectral imaging prototype was developed originally for quality evaluation of pickling cucumbers. It can operate in three different imaging modes, including single reflectance (400-1000 nm) and transmittance (400-1000 nm) modes and the integrated reflectance (400-675 nm) and transmittance (675-1000 nm) mode. As illustrated schematically in Figure 12, the system consists of three basic units, i.e., illumination, imaging and conveying [82].  
The hyperspectral imaging unit is composed of a high-performance 12-bit CCD camera, a line-scan imaging spectrograph attached to the camera and a focusing lens connected to the front side of the spectrograph. The integrated reflectance and transmittance imaging mode is realized through the specially designed illumination and conveying units, plus an additional in-line light correction unit. The illumination unit uses two light sources. One 150-W QTH lamp light source with a DC-regulated power supply connected to a dual fiber optic line light is used for reflectance measurement. When the system operates in the integrated mode, a low-pass blocking filter with the cutting wavelength of 675 nm is placed between the reflectance lamp and optic fibers (Figure 12b). The other light source is a 410-W QHT lamp used for transmittance measurement (a long-pass filter is used for blocking the light below a specified wavelength or 675 nm), and the lamp housed in a custom-made enclosure is placed beneath the convey belts (Figure 12). It is noted that a blocking filter at other wavelengths can be also used to lend measuring flexibility to the system. The transmittance light is focused onto the bottom part of the sample between the two belts, and the optical axis of the transmittance light is slightly angled with respect to the vertical axis (Figure 12a), to avoid direct light from entering the camera when no sample is present in the scene.  
The conveying unit consists of two lanes, operating with the same imaging unit, and each of the lane is constructed with two round belts driven by pulleys. The distance between the two belts can be adjusted to accommodate different sizes of commodities being detected, and the two-belt configuration enables the transmittance measurement.  
The prototype has a built-in unit for continuous in-line light correction. During imaging, the output of the light sources can fluctuate due to the lamp aging and the variation of environmental factors. Correction for the output change is thus needed to ensure robust, consistent performance of the imaging prototype. As shown in Figure 13, three reference discs, two of them outside the lanes for correction of the transmittance image and the other placed between the lanes for reflectance image correction, are mounted onto the frame of the conveying unit, aligned with the scanning line  
Figure 12: Schematic of a hyperspectral imaging prototype integrating reflectance and transmittance sensing modes.  
of the imaging spectrograph. The reflectance reference, made of PVC, is illuminated by the optic fiber line lights above; while the two transmittance references, made of Teflon, are illuminated by the corresponding transmittance light source below (Figure 12). The three references are imaged at the same time with the sample for each scan line. Flat-field corrections are then performed to eliminate the effect of light source fluctuation, which will be further discussed in the following section.