A device for hyperspectral imaging in the UV V. Caricato*, A. Egidi*, M. Pisani*, M. Zucco* and M. Zangirolami† *
INRIM, Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce, 91, 10135, Torino (Italy) † Fandis Lab srl, via per Castelletto, 69, 28040, Borgo Ticino, Novara (Italy)
[email protected]
Abstract — We present an optical device for hyperspectral imaging designed to work in the 300-400 nm range. The system is basically based on a scanning Fabry-Pérot interferometer with reflective mirrors in the range of interest. The instrument allows capturing the UV spectrum of each pixel of an image generating a 3D matrix called “hyperspectral cube” with spatial and spectral information. This system can be integrated with a digital camera provided with a UV sensitive sensor and a UV fisheye optic for the measure of the UV sky radiance. Index Terms — Fabry-Pérot, Fourier transforms, Hyperspectral imaging, measurements techniques, optical device, spectral analysis, ultraviolet sources.
I. INTRODUCTION In a Hyperspectral Imaging device (in the following HSI) a spectrophotometer is integrated in an imaging device (a digital camera). This system allows to generate the spectral content of each pixel of a 2D scene in the so called “hyperspectral cube”, i.e. a 3D matrix where the third component is the spectral distribution. The advantage of this technique is the wide range of the fields of application, from colorimetric analysis [1] to cultural heritage [2], from thermal imaging [3] to fluorescence microscopy [4], from food analysis [5] to Earth survey [6]. Two HSI devices have been realized and validated at INRIM based on a low finesse scanning Fabry-Pérot interferometer. The first one has metallic mirrors, with a reflectivity of about 20% and it has been tested in the visible [1] and short wave IR [3] range; the second one has dielectric mirrors and it has been also validated in the visible range with the advantages of lower losses at lower costs and the availability off-the-shelf [7]. Here we present the design of a HSI device optimized to operate in the 300 nm - 400 nm range for measurements of spectra of ultraviolet sources. A possible application of the new device is the integration with a digital camera with a UV sensitive sensor and a fisheye optic for the spectral irradiance mapping of the sky. II. THEORY
The Fabry---Pérot (F-P) interferometer is a moving cavity with two partial reflecting mirrors at a distance d. When the cavity is lighted by a monochromatic radiation, the irradiance transmitted (the interferogram) I(δ), during the scan of the distance, is mathematically described by the Airy function (1) 1 I (δ ) = S (ν ) . 4R 2 ⎛ 2πνd ⎞ 1+
(1 − R)2
sin ⎜ ⎟ ⎝ c ⎠
978-1-4799-2479-0/14/$31.00 ©2014 IEEE
where R is the reflectivity of the mirrors, v is the frequency and δ is the optical path delay. When the two mirrors have a R smaller than 30% it is possible to approximate the Airy fringes in (1) with a cosine function giving the new equation ⎛ πνd ⎞ ≈ S (ν )(1 − 2R ) + S (ν )2 R cos⎜ ⎟. ⎝ c ⎠ 2 ⎛ 2πνd ⎞ sin ⎜ ⎟ ⎝ c ⎠
1
I (δ ) = S (ν ) 1+
4R
(1 − R )2
(2)
Using this approximation it is possible to apply on the interferogram an algorithm based on the Fast Fourier Transform to calculate the spectrum. The same algorithm works for a broad band source as well, if two conditions are satisfied: firstly, the source must have a spectrum smaller than an octave in order to discriminate the fundamental component from the second harmonic generated by the Fourier transform; secondly, the number of sampled points per fringe has to be sufficiently high to reduce as much as possible the contribution of the aliases in the spectrum. Due to nonnegligible penetration depth of the mirrors, the interferogram lacks the central points, near the zero retardation, and the Fourier Transform cannot be applied straightforwardly. To overcome this limit the algorithm is able to perform a mathematical reconstruction of the missing points [8]. III. THE NEW F-P The new F-P cavity has been realized based on the experience gained from the realization and validation of two mentioned above HSI devices. The main tasks addressed in the new design are: a motor driven position control of the mirrors and a control of the curvature of the mirrors. A. Mirror movements In this F-P cavity the two mirrors move from the contact position to the maximum distance which sets the obtainable spectral resolution. The movement must be parallel, controlled at the nanometer level and possibly repeatable. In the F-P cavity with metallic mirrors the coarse position and the parallelism are controlled by three screws and a reduction mechanism implemented with a system based on leverages and elastic hinges, while the fine position is controlled by three piezo actuators. When the temperature is stable (laboratory conditions), the system works sufficiently well. In the F-P cavity with dielectric mirrors the coarse and the fine movements are controlled by a variable leak valve connected with a peristaltic pump [7]. Also this device is
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sufficiently stable when the temperature doesn’t change significantly. B. Mirror curvature Sticking effects, due to intermolecular forces between the two mirrors in contact, can be reduced with a slightly convex shape of the mirrors. The curvature is limited to few optical fringes over 50 mm diameter. So we have designed a mechanism to impose a desired bending to the mirrors. C. The motorized system The new F-P is based on motorized system both for coarse position and for fine movements. The core of the mechanism is a set of three extra fine thread screws driven by three small step motors through a set of reduction gears to achieve the required nanometric resolution. The micromotor has a step resolution of 18°/step and integrates an epicyclic gear reducer with a 1/256 ratio. The screws have a 100 µm pitch. The screw is driven by the motor through a home-made worm gear reductor with a 1/30 gear ratio. The overall calculated resolution of the mechanism is 0.65 nm per step. Fig.1 shows a picture of the motorized F-P.
The light source is a xenon lamp with a strong spectrum component in the range of interest and the spectrum has been collected with the two mirrors in contact. V. FUTURE WORK AND CONCLUSION We are going to characterize the F-P cavity for ultraviolet sources from the spectral point of view in order to compare our device with other reference instruments (classical image spectrometers). The characterization will be performed with different laser and LED sources in the range of interest. We have presented the design and the characterization of the HSI device for hyperspectral imaging in the range 300-400 nm. This device can find application in the measure of spectra of ultraviolet sources. Moreover this HSI can be implemented with a UV camera for the realization of a hyperspectral camera for the observation of the diffused UV radiation of the sky. ACKNOWLEDGEMENT This work was partly funded by the Project “Traceability for surface spectral solar ultraviolet radiation” EMRP ENV03. REFERENCES
Fig. 1.
A picture of the F-P for the 300-400 nm range.
D. Transmittance in the UV range We have measured the transmittance of the UV mirrors of the F-P with a reference spectrophotometer (Ocean Optics USB4000). Preliminary results are in Fig. 2.
Fig. 2. Transmittance of the UV mirrors of the new F-P from 290 nm to 410 nm when in the contact position. The transmittance shows a number of interference fringes due to the penetration depth of the coating and multiple reflection of light.
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[1] M. Pisani and M. Zucco, “Compact imaging spectrometer combining Fourier transform spectroscopy with a Fabry-Perot interferometer,” Opt. Express , vol. 17, n. 10, pp. 8319-8331, May 2009. [2] M. E. Klein, B. J. Aalderink, R. Padoan, G. De Bruin, and T. A. Steemers, “Quantitative hyperspectral reflectance imaging,” Sensors, vol. 8, no. 9, pp. 5576–5618, September 2008. [3] M. Pisani, P. Bianco, M. Zucco,“Hyperspectral imaging for thermal analysis and remote gas sensing,” Appl. Phys. B, vol.108, no. 1, pp. 231–236, April 2012. [4] M. Pisani, M., Zucco, M., Caricato, V. and Egidi A., “Hyperspectral imaging: a tool for biological measuremets ,” 16th International Congress of Metrology, p. 14007, October 2013. [5] M. Kamruzzaman, G. ElMasry, D. W. Sun, and P. Allen, “Application of NIR hyperspectral imaging for discrimination of lamb muscles,” J. Food Eng., vol. 104, no. 3, pp. 332–340, June 2011. [6] M. J. Barnsley, J. J. Settle, M. A. Cutter, D. R. Lobb, and F. Teston, “ The PROBA/CHRIS mission: A low-cost smallsat for hyperspectral multiangle observations of the earth surface and atmosphere,” IEEE Trans. Geosci. Remote Sens., vol.42, no.7, pp. 1512–1520, July 2004. [7] M. Zucco, M. Pisani, V. Caricato, and A. Egidi, “A hyperspectral imager based on a Fabry-Perot interferometer with dielectric mirrors,” Opt. Express, vol. 22, n. 2, pp. 1824-1834, January 2014. [8] M. Pisani and M. Zucco, Fourier Transforms - Approach to Scientific Principles, InTech, 2011.
