With a working slit spectrometer, we are now one step from being able to make a raw datacube.
An hyperspectral datacube is an image acquired on an hyperspectral camera made of two spatial dimensions (x,y) and one spectral dimension (lambda). The slit spectrometer delivers a full slit spectrum (x,λ) and the full datacube is acquired through a line-scan along the direction normal to the slit (y).
We do this by employing a motorized rotation stage placed under the camera.
The rotation speed of the motorized stage is optimized to get a square pixel. The side of the pixel along the slit (x direction) should indeed have the same side size along the scanning direction (y direction).
While the rotation stage moves, each spectrum is then recorded and the camera image looks like this :
On the image above, the vertical axis corresponds to the spatial direction along the slit (x axis), and the horizontal axis represents the spectral direction along the slit position (λ). The system is a “spatial scanning” device, and the scan produced by the rotation stage could be replaced in the future by the scanning motion of an airborne platform.
There are 3 obvious dark vertical bands which correspond to the 3 absorption bands of water, with the large one on the right centred on 1.4 micron.
Once the scan is completed, the datacube is sliced in the (x,y) direction and the image is reconstructed as shown on the following image:
This image is taken at the wavelength offering the best contrast and the best atmospheric transmission at around 1 micron. Each spectra is accessible by post processing for every pixel within the image using a python GUI.
The next stage will be to perform the system spectral calibration. We will remove the effect of the camera optics and sensor from the measured image by acquiring a dark and white frame which will subsequently be used to correct the datacube. The dark frame calibration will remove the sensor response produced by the camera electronics (which depends on temperature or exposure time). The white reference frame will allow us to remove the illumination spectral signature from the scene. With this calibration, it will be possible to ultimately derive the scene reflectance, which is the absolute spectral reflectivity revealing the physical or chemical nature of the sample.