Gauge blocks and length bars provide industry with a convenient form of secondary length standard for checking the accuracy of mechanical components. To ensure that the length of these gauges is traceable to international standards they are measured in instruments which use calibrated sources of light as the measurement scale. The design of a new generation of instruments for the automatic measurement of length standards has been made possible by recent developments in frequency stabilized lasers and in advances in instrumentation and computing techniques. NPL has developed an automatic gauge block interferometer which can provide the absolute measurement of length of gauge blocks from 0.1 mm to 300 mm with an accuracy of ±20nm and ±90nm respectively. The instrument uses red and green HeNe lasers to provide the measurement wavelengths and television/computer techniques to analyze the data. A very recent development has been to add a white light and fringe counting interferometer to this instrument. This enables gauges to be measured in one instrument, without the normal requirement of determining the approximate gauge length by another technique. NPL is also developing an interferometer for the automatic measurement of length bars from 100 mm to 1500 mm. This will use a phase stepping interferometer with very careful control of the environment in the measurement area.
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