Spectrum imaging detection techniques currently face the new challenge that it's difficult to detect similar foreign materials for their same appearance as the background. To resolve the problem of detection of similar foreign materials in the complex background, the objective of the present research was to develop a transmission/reflection spectrum image detection method based on metamerism breakdown characteristic. In the discrete spectrum wavelength region from 370 to 1 100 nm, this method is based on the principle that different materials have different spectral transmission and reflectance characteristic. When submitted to a source of illumination at different wavelength, foreign materials present different transmission and reflectance values in comparison to those from background. In the present paper, for simultaneously discriminating several types of foreign materials from background, the transmission and reflectance characteristic discrimination between foreign materials and background in the metamerism breakdown process was analyzed, then the optimal wavelength evaluation function for describing the background/foreign materials transmission and reflectance discrimination was set up to seek an optimal transmission/reflection illumination wavelength. Through the spectrum imaging experiment, the optimal operating condition including optimal detection wavelength and optimal illumination energy for these detected foreign materials was determined and accordingly an optimal wavelength imaging system was developed. In the actual application to the detection of foreign materials in cotton, the foreign material image features were acquired using the optimal wavelength transmission/reflection imaging system, also the image features were enhanced using image decision band fusion, then the targets of foreign materials were extracted effectively from image using binary processing. The experiment result indicated that the image features of similar foreign materials acquired by imaging system were obvious, and the detection result consisted with the fact in the double wavelength image detection system, which include 390 nm reflection wavelength and 580 nm transmission wavelength. This method provided an effective way for the detection of white fiber foreign materials in cotton and a novel practical means to solve the current problem of foreign material detection.