Mars is one of the focal points of human deep space exploration in the 21st century, and also the first choice of Chinese research objects. As an important payload for Mars exploration, a multi-spectral camera is of great significance for studying the morphology and texture of Mars's surface.
True color images of Mars are important for mineral analysis. The working light source of the Mars Exploration multi-spectral camera is different from the laboratory calibration light source, which makes the common RGB calibration method have a large color difference when it is in orbit image. After analyzing the influence of light source brightness and relative spectral distribution on camera output value, a new normalized RGB color model based on the multi-spectral camera calibration method was proposed by a research team leading by Prof. XUE Bin from Xi'an Institute of Optics and Precision Mechanics (XIOPM) of Chinese Academy of Sciences (CAS). The study was published in the journal of Physical Achievement.
Outdoor imaging test and difference of color correction (Image by XIOPM)
At present, pixel-level pseudo-color image fusion is the easiest to achieve in multi-spectral image color recovery, and the fusion results are easily distinguishable by human eyes. But the fly in the ointment is that false color fusion does not reproduce the target's true color. To solve the problem of chromatic aberration, the R, G and B values of the camera output were normalized and converted into chromaticity values, and the outdoor images were corrected. The experiment showed that the increment of chromatic aberration caused by the change of light source was significantly reduced. The validity of this method in the image correction of Mars in orbit is verified, which provides data support for subsequent engineering applications.
The color value of the camera and the true value of the standard color map was obtained by using a multispectral camera and a spectral radiometer under the D65 light source of the standard light box. CMFs method, RGB method, and the new method proposed in this paper are used to calibrate the color of multispectral cameras. Before calibration, the mean chroma difference of the CMFS method was 19.21. After calibration with the CMFS method, RGB method, and the method proposed in this paper, the mean chroma difference was 8.89, 5.31 and 4.35. After color correction using the method presented in this paper, the average color difference is reduced to 5.02, and the color error of calibration is reduced by 0.96 compared with the traditional method. When correcting outdoor images, the corrected chromatic aberration increment caused by the change of light source decreases by 0.54. This method promotes the practical application of multi-spectral true color image in image display and rock mineral analysis.
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