Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
condenser. The latter produces a sharp image of the field stop and thus minimises stray
light, which might otherwise reduce image contrast.
Condensers are conventionally designed with sufficient working distance for use with a
standard microscope slide 1 mm in thickness. Glass photomask substrates are typically
1.5 mm or 2.3 mm in thickness and it is important to ensure that condensers used for
illuminating these masks have adequate working distance.
In reflected light microscopy (apart from very low power work) the microscope objective
itself acts as condenser. A beam splitter located above the objective directs light to the
specimen and then after reflection allows the light to pass through to the eyepiece. Under
these conditions of illumination, image contrast can be seriously reduced by back
reflections from the lens surfaces of the objective since spurious light is able to reach the
eyepiece without taking part in the true image forming process in the microscope.
This effect can be reduced by means of antireflection coatings on the lens surfaces and
these are now almost universally applied to microscope optics. With a single layer coating
the reflectance at an air/glass interface is about 1.5 per cent, compared to a figure of 4 per
cent for an uncoated surface. However, by the use of a multi-layer coating, this reflectance
can be reduced to well below 0.5 per cent and multicoated objectives are therefore
particularly recommended for reflected light microscopy.
2.8 COHERENCE
To study image formation in rather more detail than in section 2.1, it is necessary to
introduce the concept of coherence and to consider the difference between temporal and
spatial coherence.
Assuming light to be propagated as a transverse wave motion, a 'perfectly'
monochromatic beam of light can be represented as consisting of a continuous train of
waves, effectively of infinite length. Light from a helium-neon laser possesses an
extremely narrow bandwidth and can be considered to conform reasonably closely to this
model. In contrast, light emitted by more conventional sources, such as discharge and
tungsten lamps, has a much broader bandwidth, even when interference filters are
employed to isolate a narrow spectral range. The emitted wave trains can then be
regarded as having a finite rather than infinite length, with no fixed phase relation existing
between successive wave trains. The length of the wave trains will decrease with
increasing illumination bandwidth.
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