Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
with eye relief (i.e. distance from eye lens to eye point) of 20 to 30 mm and these should be
used by observers wearing spectacles.
Figure 7: The Huygens eyepiece.
Figure 8: The Ramsden eyepiece.
Eyepieces produced by the same manufacturer are usually mounted so that they are
parfocal. For microscopes with binocular heads eyepieces are normally provided in pairs,
each pair being chosen for accurate concentricity and equal magnification.
Although eyepiece markings have still to be standardised, the magnification is always
marked; additional information may include the field of view in mm and the type of
eyepiece, for example, Hy (Huygens), Comp (Compensating) and WF (Wide Field).
2.6 INTERCHANGEABILITY OF EYEPIECES AND OBJECTIVES
Due to the standard dimensions involved, the physical interchanging of objectives and
eyepieces from one microscope to another presents no difficulties, even if these are of
different makes. Thus, the objective mount has a standard thread (of Whitworth form)
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