FIG. 5. White reference image obtained with a 10x MS-PLAN objective lens at 438±10 m. The original image appears homogeneous to the human eye as it displays grey-level values between 180 and 206 (left). After stretching of the histogram (right), attenuations due to dust, interference patterns and optical aberrations are clearly perceptible which demonstrates the importance of a background correction for proper imaging of mineral spectra.