A Hyperspectral Camera in the UVA Band

This HSI camera has been extended to work in the UVA band (315–400 nm). The limit at 315 nm is due to the limited transmittance of the glass substrate of the semireflective mirrors of the F–P interferometer. The responsivity of the camera goes to zero at about 300 nm. The objective is made from quartz lenses coated with MgF2 and is coated for applications in the UV. The resolution is related to the maximal optical path delay and in this application is 12.6 THz in frequency (corresponding to about 5 nm at 315 nm). The hypercube contains about 400×400 pixels, for a total of 160 000 spectra.

Text extracted from "“A hyperspectral camera in the UVA band”, M.Zucco, V.Caricato, A.Egidi, M.Pisani, Special Issue of IEEE Transactions on Instrumentation and Measurement, Volume:PP, Issue: 99, DOI: 10.1109/TIM.2014.2381751 (January 2015)"

Fig.1, Picture of the setup for HSI with F–P interferometer in the UV. The CCD camera, the F–P interferometer, and the objective to form the image on the camera. The beam splitter, the diffuser, and the laser to calibrate the retardation.

Result:: Characterization of the mirror dispersion

To extend the application of the HSI to UV bands, we have to characterize the dispersion and the reflectivity of the mirrors to know the correction to be put into the algorithm based on the Fourier transform used to calculate the spectra. The phase has been measured with the technique explained in detail in [11], where a broad-spectrum lamp with UV components, like a xenon lamp, shines the F–P interferometer and the transmitted spectra are recorded by a spectrometer (Ocean Optics HR4000) while the distance between the mirrors is changed. From the analysis of the interferograms extracted from the succession of the spectra, it is possible to fit the Airy function and extract the reflectivity R(λ) and the dispersion phase O(λ) as a function of the wavelength and an estimation of the penetration depth of the mirrors. Phase correction in reflection of the single metallic mirror inserted in the F–P interferometer. In the picture, the black line is the measured phase correction as a function of wavelength for metallic mirrors obtained by fitting the interferograms (measured with a spectrophotometer) with the Airy functions. The seven red squares are the phase corrections measured using the HSI with the setup in Fig. 1 with two lasers (532 and 633 nm) and five LEDs (at maximal wavelengths of 345, 355, 375, 396, and 397 nm).

Result:: Hyperspectra of the Sky in the UVA band

HSI has been used to measure the spectra of the sky. The picture of the setup with the portion of the sky measured with the HSI is presented in Fig. 8. The effective angle of view of 40° of the optical system is imposed by the UV objective and by the region of contact of the F–P interferometer and is reported in Fig. 9.

The spectrum of the sky enclosed in the white area is reported in Fig. 10. The Fraunhofer lines of the solar spectrum due to the absorption of the elements present in the outer layers of the sun are clearly visible in the spectrum of the sky.

Result:: Hyperspectra of LEDs in the UVA band

With the set-up in Fig 1 we acquired the hypercube of a scene with five LEDs (at maximal wavelengths of 345, 355, 375, 396, and 397 nm). From the hypercube we obtained the spectra presented in FIg 3 and the picure in false colors in FIg 5.

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