SICE Trans. on Industrial Application Vol. 8, No. 14, 113/116 (2009) 113 Crosstalk Compensation for Three-Wavelength Interferometry Katsuichi KITAGAWA * A crosstalk compensation technique is proposed for three-wavelength single-shot interferometry. This technique is essential when a commercially available RGB LED illuminator and a color camera are used for the imaging system. Based on a linear model, the crosstalk compensation algorithm is derived. The crosstalk coefficients are obtained from the linear regression analysis of the R,G,B signals in three images captured with a different LED illumination. The effectiveness of the proposed technique has been confirmed by several methods. Finally, it has been applied to the single-shot interferometric measurement of 1 m step height. The algorithms used and experimental results are presented. Key words: crosstalk compensation, three-wavelength, interferometry, surface profiler, single-shot 1. Introduction In recent years, in various industrial fields such as semiconductors and LCD panels, there is an increasing demand for accurate measurement of surface profiles on the order of nanometers. The surface profile measurement method using optical interference is the most promising measurement method from the viewpoint of speed, measurement accuracy, and maintainability. In the phase shifting method 1) , which is a typical optical interferometry method, multiple interference images are taken while changing the relative distance between the measurement surface and the reference surface of the interferometer, and the surface profile is estimated from that information. With this method, it is necessary to capture multiple images, so there is a problem that the accuracy is greatly reduced in an environment with disturbance such as vibration. As a solution to this problem, a single-shot measurement method for obtaining the surface profile from a single image has been proposed. A typical method is to generate carrier fringes by tilting the reference plane, which is called spatial carrier fringe method 2)-8) (Fig. 1). From one interference fringe image (Fig. 2) obtained by this method, the surface profile can be obtained by Fourier transform method 3) , spatial phase synchronization method 4)-6) , or local model fitting method (LMF method) 7)8) . However, since the fringe order cannot be determined from one fringe image, there is a problem that correct phase unwrapping cannot be performed when there is a step difference of 1/4 or more of the light source wavelength between adjacent pixels. In order to solve this problem, the author realized a dual-wavelength simultaneous imaging system using a color camera and two blue and red color LEDs, and succeeded in measuring a step of 350 nm by two-wavelength single- shot interferometry. 9)10) In this method, only the B and R signals of the color camera are used in order to avoid the crosstalk between the signals of two wavelengths. However, crosstalk compensation is indispensable in order to realize a three-wavelength single-shot measurement using G signals for the purpose of further expanding the measurement range. Crosstalk here means that the R, G, B components of a color camera do not completely correspond to the illumination light of three colors because the spectral sensitivity distributions of the R, G, B components of the camera overlap. It means that, for example, the signal from the B illumination is also mixed in the G component of the camera. By the way, Pfortner et al. 11) proposed an interferometric method using a multi-wavelength simultaneous illumination light source and a three-chip color CCD camera as in this report. There, three lasers are used as a light source. The three-wavelength phase shifting method is realized. However, details of the optical system have not been reported, and crosstalk compensation is not performed as unnecessary. On the other hand, in the field of three-dimensional measurement using the color projection method, there have been some reports regarding crosstalk compensation. 12-16) For example, Sato et al. 14) performed crosstalk compensation between RGB channels of a LCD projector and a color camera in a range finder study based on the color pattern projection method. The author applied the concept of compensation in the color projection method to single-shot interferometry, and obtained good results. In addition, a method of confirming the effectiveness of crosstalk compensation was examined by utilizing the fact that the light source for interferometric measurement is approximately monochromatic. In this paper, we report the algorithm of crosstalk compensation, how to obtain the crosstalk coefficients, how to confirm the compensation effectiveness, and some experimental results. 2. Imaging system The imaging system consists of a commercially available 3-color LED lighting device (CCS, model HLV-3M-RGB-3W) and a color Fig. 2 Interferogram with carrier fringes. Fig. 1 Optics of spatial carrier method.