Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
Figure 6: Image formation in the microscope
2.4.1 Achromatic objective
The achromatic objective is the type most commonly used in microscopy and is capable of
giving good image quality at relatively low cost. It is designed so that light of two
specified wavelengths, one in the red and the other in the blue region of the spectrum,
comes to a common focus, there being some residual chromatic aberration (usually known
as secondary spectrum). Image performance of an achromat is likely to be best in green
light and appropriate filtration of the illumination is therefore desirable. The simplest
types of achromatic objective produce an image with some curvature of field.
2.4.2 Apochromatic objective
In this type of objective, which is of more complex design and uses a wider range of
optical materials than the achromat, light of three different wavelengths is brought to a
common focus. The colour correction of the apochromat is significantly superior to that of
the achromat and indeed, over the centre of its field the overall standard of performance is
extremely high. The cost of an apochromat is usually several times that of an achromat of
comparable numerical aperture.
As a compromise between these two objective types, objectives known as 'fluorites' or
'semi-apochromats' are available from most manufacturers. These contain fewer optical
components than the true apochromat and are correspondingly cheaper; their
performance is somewhat superior to that of the achromat.
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