The Research on Reconstruction of Spectral Reflectance in LCTF Imaging System Based on Comparative Measurement

The spectral camera based on the liquid crystal tunable filter (LCTF) can be used to different platforms due to its advantage of small volume, low power consumption and convenient integration. In addition, LCTF spectral camera identifies the color of the target in the scene according to the spectral reflectance characteristic of the target, which can address the problem of metamerism very well. Thus, LCTF spectral camera is also applicable to areas of camouflage recognition and estimation of production of fine agricultural products. The comparative measurement method, which can effectively discount effects of illuminant and background in the scene, has been widely used to reconstruct the spectral reflectance of the target. The comparative measurement method mainly adopts a standard white surface as a reference. However, in some scenes, because of the limitation of the space, it is very difficult to put in a standard white surface. In order to address this question, in this paper, a comparative measurement by referring to the bright targets in the scene obtained by LCTF spectral camera was proposed. In the experiment, five different surfaces in the Gretg Macbeth Color Checker (MCC) were respectively used as references to construct the spectral reflectance of red, green and blue surfaces in MCC. The reconstructed spectral reflectance of red, green and blue surfaces was compared with that measured by a spectrophotometer (X-Rite Color Eye 7000A). Results showed that reflectance curves reconstructed were basically similar with those measured by the spectrophotometer in the shape; root mean square errors for red, green and blue surfaces were all less than 0.05. For blue surface, the reconstructed reflectance was the most accurate by referring to blue-green surface whose reflectance was close to that of blue surface; for red and green surfaces, the reconstructed reflectance was the most accurate by referring to yellow surface which has the second-largest luminance after the white surface; the above reconstructed reflectance was more accurate than that by referring to the standard white surface. Results indicated that the proposed method by referring to the bright surfaces can be used to construct the spectral reflectance of the target very well in LCTF spectral camera. The proposed method is useful in the expansion of application fields of LCTF spectral camera.