Large-area full-field thickness measurement of glass plates by an optical interferometric system Po-Chi Sung a , Wei-Chung Wang a,n , Chi-Hung Hwang b , Meng-Hsiu Li a a Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013, ROC b Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan 30076, ROC article info Article history: Received 10 May 2014 Received in revised form 26 July 2014 Accepted 29 July 2014 Available online 11 August 2014 Keywords: Flat panel display (FPD) Glass plate Thickness Full-field measurement Interferometry abstract Thickness uniformity has been a crucial issue for glass plates used in the flat panel display (FPD). However, point-by-point measurement and/or phase-shifting technique must be employed in the current optical interferometric techniques. Therefore, instantaneous full-field thickness measurement cannot be implemented. Besides, the measurement area is limited by the dimensions of optical components employed in the current optical interferometric techniques. In this paper, an optical interferometric system named angular incidence interferometry (AII) was proposed so that large-area full-field thickness measurement can be achieved. By using AII, the full-field continuous phase difference can be determined by using only one interference image. When the thickness at one point of the specimen is known, the full-field thickness distribution can be obtained immediately. Moreover, with the use of only a basic point-expanded laser light and an image acquisition system, no other special optical components are needed in AII. The applicability and feasibility of AII on the measurement of thickness were investigated by a typical commercially available glass plate of 0.7 mm nominal thickness. & 2014 Elsevier B.V. All rights reserved. 1. Introduction With the development of the display technology, the flat panel display (FPD) gradually becomes indispensable equipment in daily life. In the FPD, the glass plate is one of the important elements. Glass plate has been used as the substrate in both the thin film transistor-liquid crystal display (TFT-LCD) and the organic light emitting diode (OLED). To provide airtight seal protection to prevent moisture and oxygen from permeating the OLEDs and environmentally sensitive electronics, glass plate has been used as encapsulation glass for OLED [1]. Even in the curved display and flexible display, glass plates are also used. The thickness unifor- mity is crucial to glass plates used in the FPD since the non- uniform thickness may induce the change of the viewing angle and the Mura phenomenon. Mura is the display defect related to the non-uniform brightness exhibited in the panel, and it has been one of the most challenging issues in quality control. Besides, as thinner and larger size FPDs have become the market trend, stringent requirement of thickness uniformity of thinner and larger size glass plates has been urgently requested. Therefore, an on-line real-time full-field thickness inspection system is urgently needed in the manufacturing process of glass plates. Because of full-field, non-destructive, highly accurate and real-time advantages, various optical interferometric techniques [2–13] have been developed and widely used on the thickness measurement of transparent plates. In order to perform the full-field thickness mea- surement, interferometric systems [2–11] based on the point test have to implement the point-by-point measurement. The interferometric system proposed by Hibino et al. [12] is able to measure the full-field thickness; however, the phase-shifting technique has to be adopted. Hence, the instantaneous full-field thickness measurement cannot be essentially performed by the above-mentioned interferometric sys- tems. The phase shift can be immediately performed by the sampling Moiré system proposed by Ri and Muramatsu [13], the measurement accuracy of the proposed system is able to compete with that of the laser interferometry, however, the employed grating may restrict the measurement area. In this paper, an optical interferometric system named angular incidence interferometry (AII) was proposed so that real-time full-field thickness measurement is possible for a large area. In implementing the thin film interferometry [14], the incident angle at different positions of the test specimen is kept approximately the same. Unlike the thin film interferometry, when the light impinges on the specimen by an inclined angle, AII employs the variation of the incident angle at different positions of the speci- men produced by the point-expanded laser light. Due to the variation of the incident angle, the information of the spatially Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2014.07.086 0030-4018/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Optics Communications 333 (2014) 243–252