Accurate spectral calibration is crucial when measuring with a hyperspectral camera. The position of the spectral features needs to be as precise as possible. A peak’s center position must be accurately estimated within the spectral sampling. However, as introduced here, even though the sampling is the same between cameras, the position of the sampled bands may differ between devices. To ease the exchange of data and the transferability of models between devices, unified spectral calibration (i.e., the position of the spectral bands in nm) between cameras of the same type is the most significant help. This feature is characteristic of Specim FX cameras.

VNIR = VISIBLE NEAR INFRARED (400 – 1000 nm)
AIE = AUTOMATIC IMAGE ENHANCEMENT

Typically, every spectral camera working in a push-broom fashion has a different spectral calibration. This is due to the way these devices are assembled. A spectrograph disperses light passing through an entrance slit and onto a 2D detector (see Fig.1 below). The detector has two axes: a spatial one (horizontal) and a spectral one (vertical). This 2D matrix digitizes the analog signal coming from the spectrograph.

Figure 1: Schematic principle of a hyperspectral camera working in a push-broom fashion.

Spectral calibration assigns a wavelength (in nm) to each detector row. The spectral distance between the center of two rows corresponds to the spectral sampling. When such cameras are produced, the spectrograph’s mechanical position concerning the detector chip’s position varies between devices because of manufacturing mechanical tolerance. Therefore, each camera of the same type has different spectral calibration. In addition, the numbering of the first row of interest (if the VNIR spectrograph is utilized, this first row corresponds to ca. 400 nm) may have a different position between devices. We speak here of a spectral offset, an essential parameter of spectral calibration.

Each FX camera is provided with two spectral calibration packs:
– One with the camera-specific spectral calibration
– One with the unified spectral calibration (with FX in the naming)

This gives users the freedom to choose which calibration pack to use. With the camera-specific one, each device would have a different calibration, as explained above. However, unified spectral calibration includes relevant parameters for running the AIE algorithm developed by Specim, correcting, e.g., the smile and the keystone (see TN2022_7), and unifying the spectral calibration. This means that once the AIE is enabled, data streamed from all FX10, FX17, and FX50 will have the same spectral calibration. For GX17, a similar feature is available when used with SpecimONE.

Fig.2 illustrates the benefit of this unified calibration. An HgAr spectral lamp generated peaks at 546.07 nm, 750.39 nm, and 912.30 nm. Those peaks were measured at the center of FOV with 100 FX10, with and without the unified spectral calibration enabled. It appears that with the unified spectral calibration, peaks are measured precisely at the same position (i.e., band) at 546.24 nm (band 114), between 750.24 nm and 751.61 nm (bands 265 and 266), and at 911.92 nm (band 382).

Without unified calibration, the center position of the peaks varies within the sampling accuracy of the camera (ca. 1.34 nm for FX10 without binning), and there can be a difference of +-1 band. Notice that here, the spectral offset of the calibration is taken into account.

Three spectral peaks measured by 100 individual FX10 units, with and without the unified spectral calibration enabled.

Figure 2. Three spectral peaks measured by 100 individual FX10 units, with and without the unified spectral calibration enabled.

Conclusion

This unified spectral calibration is of great help when comparing data acquired with different cameras. Also, since spectral features will be similar between devices, sorting models based on data collected with FX cameras are easily transferable. Not only it is cost-efficient when building OEM machines, but also facilitates collaborative research by reducing variability historically among HSI cameras. This reduction in variability enhances transferability across FX cameras, as highlighted in TN2021_1.

Disclaimer

This technical note is prepared by Specim, Spectral Imaging Ltd. and is for general guidance only. We keep all the rights to modify the content.