New parallax-free methods of interferometric length measurements, developed recently at INMETRO, give the possibility to exclude, practically completely, the uncertainties related to the wringing procedure of a gauge block to a reference plate. It is shown experimentally that the uncertainty of the length measurement by optical interferometry can be reduced to the value of about 1 angstrom. The temperature measurements of a material artifact are demonstrated with the uncertainty of less than 0.1 milli-Kelvin. There are no basic restrictions for the improvement of the practical realization of the SI length unit, based on the measurements of material length standards by optical interferometry to the level of approximately 0.001 ppm. Two new, independent length specifying parameter fora gauge block are introduced, i.e. the mechanical and optical lengths of a block, which correspond to the length measurements with the uncertainty level indicated above. Parallax-free measurements of the optical length of the block has been performed for a 'free' block in unperturbed state. Basically new concept for length metrology, i.e. the concept of a free, unperturbed artifact, has been introduced into the length measurements.