At NMISA the SI unit for length is realised by an iodine stabilised He-Ne laser, an
optical measurement. In industry most measurements are performed by mechanical probing.
Gauge blocks are the link between the optical measurements and the mechanical
measurements, and are therefore critical in disseminating traceability. The gauge block
length is currently determined by wringing the gauge block onto a platen. This is
laborious, requires skill and causes contact errors. Since around 1943, there have been
efforts to build a double ended interferometer where gauge block length can be determined
without wringing it onto a platen. While there has been a lot of progress in building
DEIs, to date, there is still no commercially available DEI. We present a collaboration
project between the National Metrology Institute of South Africa (NMISA), the National
Institute of Standards (NIS Egypt) and Stellenbosch University (SUN) in South Africa to
build a double ended interferometer for use at the respective National Metrology
Institutes. We investigate improvements to the interferometric calibration of gauge blocks
and recent developments. The different systems currently in use in the national metrology
laboratories are described and various designs are investigated. Lastly the expected
outcome of the project will be discussed.