Modern third generation storage rings, require state-of-the- art grazing incidence x-ray optics, in order monochromate the Synchrotron Radiation (SR) source photons, and focus them into the experimental stations. Slope error tolerances in the order of 0.5 (mu) Rad RMS, and surface roughness well below 5 angstrom RMS, are frequently specified for mirrors and gratings exceeding 300 mm in length. Non-contact scanning instruments were developed, in order to characterize SR optical surfaces, of spherical and aspherical shape. Among these, the Long Trace Profiler (LTP), a double pencil slope measuring interferometer, has proved to be particularly reliable, and was adopted by several SR optics metrology laboratories. The ELETTRA soft x-rays and optics metrology laboratory, has operated an LTP since 1992. We review the basic operating principles of this instrument, and some major instrumental and environmental improvements, that were developed in order to detect slope errors lower than 1 (mu) Rad RMS on optical surfaces up to one meter in length. A comparison among measurements made on the same reference flat, by different interferometers can give some helpful indications in order to optimize the quality of measurement.