Reference Standards on UKAS calibration Certificates

The purpose of this section is to assist you with the discussions you may have with assessors from certification bodies about the content of calibration certificates.

Difficulties sometimes occur because assessors wish to see details of the reference standards used for calibration included on UKAS calibration certificates.

Any calibration laboratory that holds UKAS accreditation has to operate in accordance with UKAS requirements at all times. These requirements apply not only to the calibrations for which the laboratory offers a service, but also to all subsidiary measurements (for example, of environmental conditions) whose accuracy may significantly affect the accuracy or validity of such calibrations.

The laboratory has to have procedures for carrying out the calibrations & for the management & calibration of its reference standards of measurement & other measuring equipment. These procedures have to meet the requirements of the the current UKAS accreditation standard.

To satisfy the UKAS requirement, laboratories must hold all of the appropriate reference standards of measurement that they need, and maintain them in an appropriate state of calibration at all times.

UKAS checks that the above requirements are being satisfied. There is therefore no need to provide details of the equipment used on calibration certificates since UKAS accreditation provides all the assurances that the user needs.

This text is based upon a technical policy statement issued by the United Kingdom Accreditation Service.

Flatness - Metrology Measurement

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 nature of light waves. They appear when a thin wedge of air is formed between the glass surface and the surface to be tested. incidental light is reflected and split into two partial beams, which become superimposed due to the different frequencies and thus produce interference fringes. When daylight is used (i.e. light with various wave lengths), the fringes are coloured. With monochromatic lights (i.e. light with only one wave length), alternate light and dark fringes appear. The distance between the fringes is determined by the width of the air wedge.


optical flat

If the surface tested is flat, the fringes are straight and parallel to the corner edge. An uneven surface shows curved fringes, e.g. circular if the surface tested is concave or convex. The deviation in size of the distance between two fringes of the same colour corresponds to an error in flatness of hale a wavelength of the colour used. This value is approx 0.3 um for daylight.


Surface plates / Surface tables

Surface plates (BS 817:2008) are used as a datum surface on which to make measurements. There are 2 types of surface table:

  1. Granite
  2. Cast iron

Both of these are available in 4 grades:

  • Grade 0 (highest accuracy)
  • Grade 1
  • Grade 2
  • Grade 3 (lowest accuracy)

Example 1: (using BS817:2008 table 1)

1000mm x 630mm table:

Grade Variation Overall Flatness
0 0.003mm 0.0055mm
1 0.006mm 0.011mm
2 0.012mm 0.022mm
3 0.024mm 0.044mm


The surface plate is measured using a sensitive electronic level following either a union jack method or a grid method:

Union Jack Method:

Lines are drawn on the surface table that mimic those of the union jack flag. The electronic level is then moved down each line at predefined spacing recording the angular changes at each position.  Flatness calculations are then made electronically using this information.

Grid method:

Lines are drawn on the surface table in a grid pattern. The electronic level is then moved down each line at predefined spacing recording the angular changes at each position.  Flatness calculations are then made electronically using this information

In a addition to both these method, local variations in flatness are also checked using a variation gauge.