[en] The development of a unique, bulk density, photoelectric cross section logging tool and its features is chronicled from conceptual infancy to initial field trials. A program for developing the Spectral Litho-Density (SLD) began in earnest with the construction of a four part test fixture. Studies, utilizing this fixture, were conducted to evaluate various source to detector spacings along with source and detector collimation angles. A broad range of formation densities, (1.688 to 3.107 grams/cc) and lithologies, (P/sub e/'s of 0.764 to 5.06 barns/electron), were assembled for these studies and for final tool characterization. Test fixture results reveal an optimum source to long space detector spacing with respect to formation density measurements. As well, it is shown the source collimator and its geometry play an important role in controlling the SLD density spine response. Further tests included the insertion of intermediate mudcakes, of varying thickness and density, between the tool pad face and borehole wall in order to appraise assorted tool designs. Upon completing the test fixture studies a set of prototype tools were assembled incorporating state-of-the-art design features. These features include beryllium encapsulated detectors for maximum P/sub e/ sensitivity, a beryllium windowed dewar flask (housing the detectors and photomultiplier tubes) for protection from hostile thermal logging conditions, and digitized long and short space spectra (256 channels/detector) for uphole gain stabilization and post processing benefits