Optical flats are polished surfaces used to measure the flatness of machined surfaces and are a vital tool to enable precision engineering and manufacturing. This flat was used for more than 40 years by an important Sydney based engineering firm, Ross Simpson Engineering Pty Ltd. This company generously reconditioned parts for locomotives 3830 and 3265 at no cost to the Museum.

This flat may have been used to determine flatness to approximately 25 nanometres (one millionth of an inch). One or both sides of the flat have been polished to enable flatness to be determined to the specified resolution. To use, the flat is placed on the unknown surface and illuminated from above the flat. Optical interference fringes (bands of light and dark) form at the contact surface and will be seen through the top of the flat. For higher resolution and easier measurement the flat is illuminated with monochromatic (one colour) light. Straight parallel fringes indicate the flatness to the specification of the flat and the wavelength of the light. Curved or irregularly spaced fringes indicate convex, concave, spherical or cylindrical departures from flatness.

The flat is calibrated on a regular basis by a registered testing laboratory to determine changes in its flatness over time. This flat was calibrated in 1995 by the National Measurement Laboratory (of CSIRO). In 2004 this lab became part of the newly formed National Measurement Institute which brought together the National Measurement Laboratory from CSIRO, the National Standards Commission and the Australian Government Analytical Laboratories.

Jesse Shore
12 February 2008

This optical flat is made of glass, perhaps a clear optical fused silica. This material combines durability, resistance to scratching with low thermal expansion.

The maker of the flat is unknown; possibly it is 'Lensmaster'. It was made in Illinois, United States of America, c. 1960.

This flat was used by Ross Simpson Engineering Pty Ltd, Sydney, New South Wales, with an optical comparator to measure the flatness of surfaces. The donor said the flat was used from 1963 to December 2007.

Optical flat, with case, glass / leather / velvet / metal, possibly made by Lensmaster, Illinois, United States of America, c. 1960, used by Ross Simpson Engineering, Sydney, New South Wales, Australia, 1963-2007

The optical flat is a thick clear glass disc with polished flat top and bottom and frosted cylindrical side. Three black arrows, all pointing the same direction, are lightly etched into the side of the disc. The flat is stored in a square grey leather covered case with black velvet lining. The flat fits into a cylindrical recess in the case. The case has two hinges at the back and two latches at the front which are opened by horizontally sliding outward spring loaded buttons. Marks on lid.

Made: Illinois, USA; 1955 - 1965

The cover of the case is imprinted with the manufacturer's name or brand but this is not clearly legible. It might read 'Lensmaster'.
Underneath the name/brand is an illegible word which might be the company name followed by "Company". Next line is the city (illegible - looks like "Gilove"), "Illinois".

There is a silver sticker on the upper left of the top outside of the case from the "National Measurement Laboratory" with the CSIRO logo and "CSIRO" under the logo. "Date of Calibration" is printed and "June 1995" is handwritten. "Report No:" is printed and "RS25575" is handwritten.

Above the CSIRO sticker is a black Dymo label imprinted "M21407".

2008/128/1

1955 - 1965