Paper Details

The techniques for pre-specified modification of the imaging characteristics of an optical system receivedconsiderable attention since the early days of systematic investigation of optical imagery [1 -2]. It was found that thepupil function of any optical system plays an important role in this regard. The desired modification of the imagingcharacteristics is usually implemented by using a mask on the pupil of the optical system. Extensive studies have beencarried out in this direction and the use of amplitude masks and phase masks have been reported [3-7]. Usually eachmask is dedicated to a specific purpose and on-line modification of such masks is not possible. At the same time,synthesis of the pupil function for yielding a specific imaging characteristic is indeed a formidable task and remainspractically insolvable except for some limitingly terminal and simple cases.rnIt has been recognized that the polarization properties of light, if ingeniously used, offers additional flexibilities to theoptical designers that are unachievable by scalar wave properties alone [8-16]. The advantage of these polarizationbased optical systems is that their imaging properties can be continuously altered in situ. Polarization is used as aparametric variable to introduce a variation in the complex amplitude of the pupil. Obviously, two degrees of freedomdue to two orthogonal states of variation are available to continually change the imaging properties of these systems byvarying the relative contributions of the two orthogonal components. However, fabrication and accurate alignment ofsuch masks are quite troublesome and a considerable loss of light due to absorption, scattering etc. takes place in themasking element. Thus, a system, easily realizable, equally effective as well as free from the previously stated problems,had long been a requirement. Optical systems fabricated with birefringent materials have this potential and thismotivated many researchers to investigate the behaviour of a birefringent lens made of a uniaxial crystal [17-26]. Auniaxial birefringent lens sandwiched between two linear polarizers with its optic axis perpendicular to the lens axisbehaves like an ordinary lens with a radially varying complex filter at its pupil plane [17 -18]. Since the filter isgenerated because of the interference phenomenon, the problem associated with the alignment is removed. The imagingcharacteristics of the said system can be altered continuously just by changing the orientation of the any of the twopolarizers. This makes the proposed system more versatile.The imaging characteristics of the proposed system under diffraction-limited condition were studied [17-21]. Thesame system may be adapted either for enhanced resolution or for apodization just by rotating any polarizers included inthe system. This system behaves as a double focus lens in general and by varying the birefringent lens parameters it ispossible to change the separation between the two foci. The proposed system may be designed to obtain noticeably highdepth-of-focus compared to an identical ordinary lens [19]. The imaging characteristics of the proposed system inpresence of pre-specified on-axis and off-axis aberrations were studied [22-24]. It shows higher tolerance to theaberrations than an identical conventional lens. The effect of polychromatic visible light on the performance of thesystem was also investigated [25]. The study revealed that the focusing characteristics of the proposed system do notchange appreciably under polychromatic illumination from that under strictly monochromatic illumination. The need forappropriate infrared imaging devices and components has been on the rise [27 -34]. The applications of proposed systemunder monochromatic or polychromatic beam illumination in the infrared region have not beenexplored till date. We study the viability of the proposed system considering crystal quartz as the birefringent lensmaterial in the infrared region. In this connection it may be mentioned that the coefficient of absorption for the crystalquartz is less than 0.030 cm-1in the region approximately from 190 nm to 2900 nm [35]. The infrared band is oftensubdivided into smaller sections though the divisions are different for different applications [36]. We consider the rangeof wavelengths for which infrared photography (700 nm to 900 nm) and short-wave infrared (SWIR) defence applications(900 nm to 1700 nm) usually take place. The wavelength of around 1550 nm is used for mode rn fiber-opticcommunication [37]. Thus the proposed system may find applications in multiple domains, such as SWIR defence andsurveillance purposes, infrared photography, medicine, archaeology, modern fiber-optic communication systems andmany other infrared imaging devices. The axial irradiance distribution function and the intensity point spread functionare considered as the image assessment parameters. The results show that all the above properties of the said system areretained even under infrared illumination having large bandwidth.

Pranati Mandal, Sandeep Kumar, “NOVEL IMAGE PROCESSING APPLICATIONS OF A BIREFRINGENT LENS IN THE INFRARED REGION”, International Journal of Advance Engineering and Research Development (IJAERD), Vol. 2, Issue 6, pp. 517-533, June 2015.