Optimum Design of Spectral Characteristic of Light Source for an Optical Endoscope

The design of spectral characteristic of the light source in an optical endoscope is discussed. First, two criteria are introduced for the evaluation of the light source and the evaluation result for two CIE standard illuminants and 15 light sources commercially available is presented. Next, an optimization method using POCS (projection onto convex set) is proposed for the design of the light source. It is shown that the optimum light source achieves much better score for the introduced criteria than the 17 light sources. Simulation of image reproduction of mucous membrane under an arbitrary light source is also described. Introduction In endoscopic diagnosis, an optical endoscope (OE) with fiber bundle is widely used as well as an electronic endoscope (EE) with CCD area sensor. The OE has some advantages; it is less expensive than EE and the observed color is more stable because it is composed of optical elements only. As a shortcoming, the OE available now can hardly enhance the color variation of mucous membrane which is important in diagnosis, while the EE can easily do that by changing some parameters of video circuits. However, we considered that the OE can potentially have a similar effect of color enhancement by optimizing its light source. In this paper, we first formulate the color observed in an optical endoscope. Then we present two criteria for evaluation of the light source and actually evaluate 17 kinds of light sources. Next, we propose a technique to optimize the spectral characteristic of the light source of OE. In this optimization, three conditions corresponding to two criteria introduced and a physical constraint are expressed mathematically and a solution holding those conditions simultaneously is found by POCS algorithm. The effectiveness of the optimized light source is confirmed through the criteria introduced. Finally, we briefly introduce a simulation of image reproduction under an arbitrary light source. Formulation Fig. 1 shows a schematic illustration of an optical endoscope. The light emitted from the light source passes through the light guide fiber and illuminates the object surface. The intensity distribution of the light on the object 43 surface is transmitted through the image guide fiber and observed by a physician. Letting the spectral radiance of the light source be e(λ), the overall spectral transmittance of transmissive optical elements t(λ), spectral reflectance of mucous membrane o(λ), the observed light is given by the spectral product of these characteristics, e(λ)t(λ)o(λ). The tristimulus values of this light are expressed as, (1) where ) ( ), ( ), ( λ λ λ z y x are the color matching functions. Figure 1: Schematic illustration of an optical endoscope. For the mathematical expression used in later sections, we express each spectral characteristic discretely by a vector or a matrix as follows. Reflectance of mucous membrane: o(λ) => o = [o1,..., on] Radiance of xenon lamp (conventional lamp): ex(λ) => ex = [ex1,..., exn] Radiance of test light source: e (λ) => e = [e1,..., en] Overall transmittance of optical elements: t(λ) => T = diag[t1,..., tn] ∫ ∫ ∫