Describing the length of physical standards in terms of wavelength of a primary length standard represents the highest level of traceability for length; the basic goal of this research is to provide this level of traceability in Croatian National laboratory for length. Extensive modifications were made on a Zeiss gauge block interferometer to achieve this goal; a new optical system was made to allow use of laser light sources and establish traceability, and numerous additional modifications were made to upgrade measurement possibilities and to reduce overall uncertainty for gauge block calibration. These modifications span through the entire measurement system, from improved environmental conditions and digital sensors for their measurement, to improved digital acquisition and analysis of interferograms. Phase shifting interferometry was also implemented in Zeiss interferometer, increasing its resolution below nanometre level. Performance of these modifications was verified through EURAMET Key Comparison with several European institutes. After this phase of research was complete, development of a new gauge block interferometer was started. As a result, a Twyman-Green interferometer was built, enabling implementation of Synthetic Wavelength Phase Shifting Interferometry. This interferometer was thoroughly tested, and results indicate that it can match the performance of commercial gauge block interferometers. Finally, as direct result of research described above, two new measurement methods have been proposed. A novel method for direct measurement of wringing layer was tested, with very good results. This method eliminates the variability of wringing layer thickness, without the need for additional measurement equipment, and therefore provides a universal tool for reduction of measurement uncertainty for absolute gauge block calibration. Combination of this method with a modified stack method resulted in a new method for determination of phase correction, which greatly reduces the uncertainty of phase correction and enables its determination with just a fraction of previously required measurements.