Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
2.2.4 Curvature of field
Field curvature in an objective causes a plane object to be imaged on a curved surface.
Thus the whole field of view of the objective will not be sharply defined at one focus
setting; however, provided the objective is free from other off-axis aberrations, the whole
field can be critically examined by making slight changes in the focus setting.
Image distortion i.e. a gradual change in magnification from the centre to edge of the
image field causes a square object to be imaged with the characteristic 'barrel' or
'pincushion' shape. Microscope objectives commonly exhibit some distortion (typically of
the order of one percent), but this may not be apparent in the observation of many types of
object. However, if dimensional measurements are to be made directly from the
microscope image and over a large field of view, the performance of the objective and
eyepiece system should be checked in this respect using a stage micrometer or other type
of graticule.
2.3 OPTICAL COMPONENTS OF THE MICROSCOPE
In very basic terms, the optical system of a microscope consists of an objective forming the
primary image of the object and an eyepiece producing a secondary virtual image, which
is then viewed by the eye (see Figure 6). For relaxed viewing, the focusing may be
adjusted so that the virtual image is effectively at infinity. The condenser illuminates the
object, with the objective itself acting as condenser in most reflected light microscopy.
Microscope manufacturers produce several different types of each of these components
and it is therefore useful to summarise their respective properties.
2.4 THE MICROSCOPE OBJECTIVE
The microscope objective is the most critical component of the optical system. Ideally this
lens should produce a well-defined image with any residual spherical aberration, coma
and astigmatism minimised to ensure that it is capable of resolving the theoretical limit
discussed in section 2.1. In addition, the image should be substantially free from curvature
of field and distortion and with excellent colour correction to allow a wide illumination
bandwidth to be used. Microscope objectives are generally classified according to their
colour correction with an additional reference if they are of the flat field variety. The
complexity of the objective i.e. the number of component lenses it contains increases with
its numerical aperture and with the overall state of correction required; a high power dry
objective may contain ten or more component lenses in order to achieve a high degree of
optical correction over a large field.
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