White light interferometry for surface profiling with a colour CCD U. Paul Kumar a , Wang Haifeng a , N. Krishna Mohan b,n , M.P. Kothiyal b a Department of Physics, National University of Singapore, Singapore 119260, Singapore b Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India article info Available online 25 February 2012 Keywords: Fringe analysis White light interferometry Phase shifting Surface profiling Single-chip colour CCD camera abstract In laser based interferometry, the unambiguous measurement range is limited to half a wavelength. Multiple wavelength or white light interferometer is used to overcome this difficulty. In this paper a white light interferometer with a colour CCD camera is discussed. We access interference intensity information from the three channels of the colour CCD simulating three-wavelength measurement. This makes the data acquisition as simple as in single wavelength interferometry. The unambiguous measurement range however gets limited by the coherence length of the CCD. The usefulness of the proposed method is demonstrated on a micro-sample. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Laser interferometry using phase shifting technique is a widely used tool for precision surface metrology [1,2]. However, the unambiguous measurement range for the laser is limited to half a wavelength. The unambiguous range can be increased using the more wavelengths [3–8]. Lasers have been used for this purpose. White light interferometry (WLI) is also a state-of-the-art technique for measuring discontinuous surface profiles [9,10]. WLI combining with advanced CCD cameras, computers, image processing cards, and software packages has given an extremely powerful measure- ment tools. Extremely short coherence length of the white light source results in high contrast fringe occurs only when the optical path difference (OPD) is close to zero. The 3-D plot of the axial positions of the zero OPD along the optical axis represents the surface profile of the object under test. Compared to single wave- length phase shifting interferometry, the white light interferometry is rather slow, because the number of frames to be recorded and evaluated is rather large. The spectrally resolved white light inter- ferometry (SRWLI) [11,12], on the other hand gives only a line profile of the object, although the requirement on number of frames is similar to the single wavelength phase shifting interferometry. In this paper, we describe a single shot white light microscopic interferometer with a colour CCD camera. For recording of inter- ferograms a three-chip or a single-chip colour CCD cameras can be used. Maximum resolution will be achieved using 3-chip colour CCD. In this investigation, however, a single chip colour CCD camera is used and the data from the three channels is used as three wavelengths source. We believe that such experiment has not been done before. Typically five to six frames are required to use phase shifting algorithm for phase calculation. Thus the data acquisition is as simple as in single wavelength case. The stored phase shifted interference data from RGB (red–green–blue chan- nels) is then separated into its components. The phase at individual wavelength is calculated using phase shifting algorithms. The combination of white light interferometry with colour CCD camera makes the measurement faster, simple, and cost effective. There is a practical issue in implementing this approach with a phase- shifting technique. The single chip-CCD has three separate spectral bands, R, G, B centred at Red (l 1 ¼ 620 nm), Green (l 2 ¼ 540 nm), and Blue (l 3 ¼ 460 nm) wavelengths, respectively. The phase shifts are usually produced by using a piezoelectric transducer (PZT) to change the length of one of the optical paths. The phase step produced at single wavelength can be set precisely at 901, but the same motion of the PZT introduces a phase-step miscalibration at the other wavelengths. This problem can be overcome in various ways [8]. In the current work, first the phase shifts (a i ) at individual wavelengths are calculated and then used in the 5-step phase equation to calculate the error free phase [13] as explained in Section 3. For comparison we have also used the higher order (8-step) phase shifting algorithm, which has a higher tolerance of 720% for phase-shift miscalibration error and is well suited for RGB wavelength analysis [8,14]. Use of this approach combining white light interferometer with colour CCD camera increases the unambiguous range for surface profile measurement. We have also applied the two weavelength procedure (equivalent wavelength) to get unambiguous result, which is limited to half the equivalent length. Experimental results on a micro-specimen are presented. 2. Optical interferometry The schematic of the microscopic white light interferomeric arrangement as shown in Fig. 1 is used for the measurements. The white light beam is collimated using a collimating lens (CL). The Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2012.02.002 n Corresponding author. E-mail address: nkmohan@iitm.ac.in (N.K. Mohan). Optics and Lasers in Engineering 50 (2012) 1084–1088