Walton and Beckett Eyepiece Graticules
Pyser-SGI have developed a range of eyepiece graticules and stage micrometers for use in compound microscopes, both phase contrast (for analysis and counting), polarization (for identifying) and stereo microscopes (for initial examination) of Asbestos fibres.
When counting asbestos samples, it has been found that limiting the area of valuation to that defined by the grid on an eyepiece graticule can give significantly higher operator concentration values than when the full field of view of the microscope is used. The published work by S T Beckett et al in 1976 recommended that the graticule grid method of counting be adopted for
asbestos analysis and that steps be taken to reach national or, preferably, international agreement on a standard form of graticule. The Walton and Beckett graticule was designed specifically for the evaluation of fibrous dust and was adopted worldwide.
The Walton and Beckett graticules are used for counting fibrous dust and are particularly useful where the majority of fibres to be counted are shorter than 5 microns. The circle is divided into four by two diametrical lines scaled in units of 5 and 3 microns. 3 and 5 microns are the critical measurements of fibre lengths and diameter used in fibre counting. Unlike the usual globes of other particle graticules the Walton and Beckett has a series of shapes to compare objects with.
These shapes have been designed for comparison with fibres, especially since they incorporate aspect ratios of 3:1 (G22) or 5:1 (G24) essential for such analysis.
The circle on the Walton and Beckett graticule must represent 100 microns at the stage of the microscope when used with a 40x objective lens. The microscope needs to be calibrated to ensure that the Walton and Beckett graticule will give true measurements when it is fitted.
Based on the G22, the G25 is produced to a new design by the Institute of Occupational Health in 1996.
Details of the Pyser-SGI Walton and Beckett Eyepiece Graticules can be found in the Pyser-SGI Asbestos Microscopy catalogue.