AirSpex 2007

4. Calibration


The calibration of the spectrograph was made in the Optical Lab at UNIS. The instrument is both intensity and wavelength calibrated.


Wavelength calibration


For the wavelength calibration a Hg-lamp was used. The lamp was placed in front of the spectrograph and we sampled data for 30 seconds on the ARCHOS VIDEO AV320 recorder. The following lines were detected in the spectra: 5770/5791 , 4358 and 4047 . The first doublet is resolved which means that the bandpass has a maximum value of 10 . From the calibration data we found that the spectral range of the instrument is 37007000 . The calibration is done by identifying and then fitting the spectral lines recorded by the spectrograph to the theoretical Hg lines.


Intensity calibration


We used a 1000 W lamp called ORIEL 68835 which is coupled to a Light Intensity Controller (LIC). The lamp runs at 8.2 Ampere. We placed it at a distance z = 0.738 m from a diffuse reflective Lambertian surface, to evenly illuminate the spectrographs field of view. The spectrograph is placed in such a way that the screen covers its full field of view, pointing toward the middle of the screen. The setup is shown in figure 4.1 and 4.2. We collected data for 60 seconds on the video recorder. The intensity of the screen is given by






Where is the known intensity of the lamp in mW/nm m2, z0 is the distance used to get B0 , z0 = 0.5m, z is the distance from the lamp to the screen (see figure 1), ρ = 0.98, which is the diffuse re-emitting coefficient of the screen and α is the tilt angle, in this case 0.



From this, the calibration curve is obtained through





where C is the instrumental response to the intensity of the screen in relative units [2]. The calibration curve gives the factor at each wavelength, by which the measured intensities need to be multiplied to get the physical values. This curve is saved in a calibration file, and loaded into the yaPlaySpecX software before extracting images from the spectral movie.
















Figure 4.1: Calibration setup. The 1000 W lamp (2) is illuminating the screen (1) from the distance z. To protect the spectrograph (4) from light contamination a baffle (3) is placed between the lamp and the detector.
























Firgure 4.2: The Calibration room at UNIS: (1) 18 x 18 inch 2 Lambertian surface, (2.) rails, (3) adjustable mobile table, (4) entrance fiber to spectrograph, (5) door with baffle, (6) room lights, (7) tungsten lamp, (8) power cable to lamp filament. [1]





[1] Sensitivity calibration of narrow field of view optical instruments, F.Sigernes et al., In press.

[2] Airborne Hyperspectral Imaging, F. Sigernes, 2005.