1 Scratch Detection in Optical Elements Using Digital Holography Sonia Verma1, 2, Subhra S Sarma1, 3 and Rajkumar1* 1CSIR- Central Scientific Instruments Organization, Chandigarh-160030, India 2 Department of Applied Physics, Guru Jambheshwar University of Science and Technology, Hisar-125001, Haryana, India 3Assam Don Bosco University, Guwahati-781017, Assam, India *Email: raj_csio@yahoo.com Abstract— There are various applications including space, biomedical etc. which put stringent conditions on precision optical components. Thus, it becomes important to develop efficient techniques for testing of these components in a non-contact manner. Optical testing methods fulfill these requirements. Here we report use of digital holography for detection of scratches in optical elements. A hologram of the wavefront generated by the test element having a crack is recorded digitally. Subsequent numerical reconstruction of recorded hologram allows detection of the scratch. Further image processing is performed on reconstructed image to improve visibility of the scratches. Keywords—digital holography, scratch detection, image processing. 1. INTRODUCTION In imaging the surfaces of optics used should be preciously made, finished and handled. Even a minute scratch or crack in the optics will generate noise signal by scattering the incident light and thereby may severely affect the results. The scattered light can generate troublesome ghost interference patterns. Thus, it becomes imperative to keep the optics, scratch and dust free, for obtaining accurate results. Defects could be a minute scratch, dust particles, irregularity in periodic pattern of semiconductor wafer etc. Various methods were employed to detect these defects and are basically divided into two categories i.e. optical techniques and digital techniques [1, 2]. In digital techniques the pixel information is compared with one another in image plane but in this one cannot do parallel processing while in case of optical techniques, the subject information is manipulated using lens system. Over the time, various methods have also been developed for optical inspection [2, 3]. Although, optical techniques process whole subject at same time but these are complex and expensive. To overcome these disadvantages, optical and digital methods have been combined in single setup [4]. The digital holography is one of the important methods for non-destructively testing/inspecting the large area in parallel processing. Some of the previously reported holographic techniques for this purpose are conventional and use photographic plate to capture the information of subject and thus are time consuming [5, 6]. Digital holography does not require chemical processing after recording of hologram thereby allows faster reconstruction of recorded holograms [7, 8]. Further use of numerically generated reference wave for reconstructing hologram avoids errors which may be present in optically generated reference beam. Considering these advantages here, we report use of digital holography for detection of scratches in optical elements. 2. THEORY Dennis Gabor first introduced holographic method in 1948 to capture an object along with its depth of focus. He used in-line arrangement for recording the hologram. But that arrangement was not useful because the two images produced during reconstruction overlap. Then E. Leith and J. Upatnieks introduced off-axis holography in 1962 in which there is some angle between reference wave and object wave. This off- axis holography has advantage of not having overlapped reconstructed images. These methods initially done by the use of photographic plate to record the interference pattern of reference wave and object wave. This conventional method is time consuming, require perfect dark room, and expensive. Numerical hologram reconstruction was initiated by Yaroslavskii in 1970s in which the digitized ‘conventional’ holograms were reconstructed numerically. After that the devel