Spectral light reflectance is a measure of the relative amount of light reflected by a material under incident white light. It is expressed within an absolute range from 0 (minimum) to 255 (maximum) and specified as average value for the red (600-700 nm), green (500-600 nm) and blue (400-500 nm) color band (RGB system). The reflectance properties of sediments relate to their chemistry and structure and are dominated by pigmented trace constituents, typically brownish or greenish Fe-bearing minerals (clays, oxides, sulfides), grayish silicates and darker organics. Reflectance logs provide high-resolution records of terrigenous content (total reflectance) and redox state (red/blue ratio). Scanned at high spatial resolution, reflectance images provide sharp, undistorted, true-color core photographs scarcely affected by undesirable artefacts known from classical core photography (shadows, reflections etc.).
The digital imaging module of the GEOTEK MSCL consists of a camera containing three separate 3*1024 pixel CCD detectors mounted in the focal planes of split light beams ~40 cm above the surface of the sediment and equipped with red, green and blue dichroic filters. This camera captures consecutive, strictly orthogonal line images of the bypassing split core surface. The sediment is illuminated from above by two white fluorescent tubes. Freshly cut archive halves were carefully leveled to prevent shadows from residual surface roughness.
All cores were scanned at an axial resolution setting of 100, corresponding to 1 row of pixels for every 100 μm in core depth. The resolution achieved across the core is nearly equivalent. The brightest part of each core was selected to determine the lens aperture value which allows the entire core to be measured on the same setting without saturating any of the color channels. Each reflectance value is calibrated against the range defined by a white tile (white calibration) and a closed lens cap (black calibration). A color test card was measured before and after each core to determine and linearly correct drift effects of the CCD sensors.
A specialized post-processing software was used on board to perform all necessary image corrections and calculations. The processing starts by cropping end-cap and cavity sections and by removing spurious color stripes caused by a non-uniform response of individual color channels. This task is efficiently solved by normalizing the means within each down-core column of data to the same mean-core value. The segment images are then merged into a full core image and numerically compressed in various ways. The median value of each data row was chosen as representative reflectance value in the depth series of red, green and blue reflectance, total reflectance (mean value of R, G and B) and for the red/blue ratio.
