The natural standard for the metre is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second.

Optical Flats Optical flats are used to test the flatness of fine-lapped surfaces such as the measuring faces on gauge blocks, measuring inserts, measuring tables, micrometers etc., which have to meet extremely high demands on accuracy. Interference fringes appear when the optical flat is laid on the surface to be tested. Their number shape and configuration make it possible to identify geometric deviations in form on the surface. The interference fringes are caused by the

BS 969 : 2008 This standard specifies the tolerances and limits for simple forms of limit gauges, such as: It is intended for reference by component producers and gauge makers when gauge limits have not been stipulated. It specifies gauge tolerances for work piece tolerances in ranges between 0.009mm and 3.200mm and how gauge limits are related to the work piece limits. It also specifies types of gauge according to size and minimum work piece