Color correction for calorimetric color reproduction

of input system are equivalent to the color matching function of the display primaries [ 41, the spectral sensitivities of the real system do not agree with the color matching functions. Thus, we employed a simple 3 x 3 matrix to perform color correction of the input system. It should be realized that this color correction method is not new, having been applied for televisions [ 11. Optimizing the correction matrix of the electronic endoscope for clinical use is, however, a novel application. Such optimization necessitates having the statistical distribution of spectral reflectance of the real mucous membrane, and fortunately, since this data was in-hand, having been previously measured by a specially made endoscopic system for spectral photometry [ 5 1, we utilized it to optimize the correction matrix. In the following sections, we describe the theory of the optimization method and then present computer simulation results that confirm the effectiveness of the proposed method.
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Fig. 1. Diagram of a typical electronic endoscope. 0030-4018/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SsDrOO30-4018(94)00531-l
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and diagnosis possible. In fact, if numerous endostopic images are collected to build a database, this significantly enhances such analysis and diagnosis. Towards this end, however, a crucial problem exists, i.e., the reliability and stability of color reproduction for the generated endoscopic images must be improved. The associated transmission path for color information is quite complex. Namely, the spectral information from the object is initially reduced to three values by weighting and integrating with the spectral sensitivities of the input system for each color. If necessary, these values are gain-controlled at the video processor, and then excite the CRT’s red, green, and blue phosphors. At this time the light intensity is not linear to the input signals because of the gamma characteristics of the CRT. Consequently, the reproduced color has a strong dependence on the following: (i) the spectral characteristics of the white light source, (ii) the spectral transmittance of the color filters, (iii) the gain at the video controller, (iv) the spectral distribution of the three phosphors, and (v) the gamma characteristics of the CRT. Obviously the reproduced colors in the computeraided image analysis and diagnosis should be close to the true color of the object and should not depend on the image acquisition site or time. The electronic endoscope, though, has an inherent limitation in that it is impossible to retrieve the spectral information of the object. To mitigate this drawback, a next-best solution for obtaining accurate color reproduction involves using a calorimetric color reproduction in which tristimulus values of the object are employed [ 11. In conventional endoscopic systems, however, such color reproduction is not guaranteed. This problem led to the present paper which describes a new color correction method for calorimetric color reproduction of electronic endoscope images. The proposed method is comprised of a two-step calibration procedure, where an input system calibration is first done to obtain the ideal R, G, B tristimulus values, which are determined by the chromaticities of the display primaries. Next, the CRT’s gamma characteristics are calibrated to reproduce these tristimulus values. The latter calibration is accomplished using either tabulated values or corresponding analytic functions [ 2,3 1. Here, we focus on the former calibration. Although the ideal tristimulus values can be obtained when the spectral sensitivities
15 January 1995
OPTICS COMMUNICATIONS
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Optics Communications 114 (1995) 57-63
Color correction for calorimetric color reproduction in an electronic endoscope
1. Introduction The utilization of electronic endoscopes vice conventional optical fiber scopes has recently gained much popularity in the medical profession. The typical electronic endoscope (Fig. 1) incorporates an integrated CCD area sensor at the head of the scope which can image gastric or intestinal surfaces. The light source part consists of a white light source (xe-
Received 5 April 1994; revised manuscript received 23 August 1994
Abstract A color correction method is presented for application in an electronic endoscope. This method performs color correction in the endoscope’s input system using a simple 3 x 3 matrix to reproduce a calorimetrically correct color. The correction matrix is optimally designed using about 300 actual spectral reflectances of the gastric mucous membrane. The effectiveness of the proposed correction method is demonstrated both numerically and visually through computer simulation.