Primary level gauge block interferometers for realization of the SI length unit

The main features of the small Carl Zeiss comparator (for the gauge blocks up to 100 mm), and a large Kosters interferometer (for gauge blocks up to 1000mm) are discussed in some detail. Special emphasis is made on the problem, requirements and specific feature of the differntial optical measurements, which become feasible with these new devices. Very high resolution of the instruments, resulting in resolving the oblique incidence effects for "sad" and "smile" type of interferograms, which correspond to different directions in the measurement of the fringe fraction values, imposes some restrictions on the measurement procedure, realized with their use. For example, the the optimum parameters of the interferogram, which are required for the most accurate and reliable operation of the fringe-pattern analyzing comparators, the measurement can be performed for only one type of interferogram ("sad" in our case), and for the measurements of the other type ("smile") a new optical tuning of the interferometer is required. The problems of temperature measurements with a sub-mK accuracy level are formulated, and the ways to overcome them are outlined.