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10. ASSESSMENT OF IMAGE QUALITY
The need to be able to measure optical quality was recognised many centuries
ago, but no really adequate concept was put forward until the 19th century,
when several scientists progressively refined the concept of resolving power -the
limit of ability to detect that two adjacent objects were present as the
separation between them was reduced. This 19th century work culminated in
the classical papers of Lord Rayleigh, which defined the limit of resolution for a
perfect optical system to be the angular separation of two point objects when
the first dark ring of the Airy disc produced by one object coincided with the
peak of the Airy disc formed by the other. This concept was later extended by
Rayleigh’s followers to the case of two line objects, and later again became a
general concept for specification of performance of most optical systems. Useful
discussions of the concepts of resolution and resolving power are to be found in
Ronchi, Brock and Perrin.
Since the 1930’s there has been an increasing awareness that this simple
concept was other than satisfactory, because of the dependence of its value on
numerous factors. As a result of this, other more adequate and universal
measures have been sought. Some 35 years ago a concept already widely used in
the fields of electronics and control systems was ‘borrowed’ by optical
scientists - that of frequency of response. From this grew the concept of
Optical Transfer Function (OTF) which, in theory, was a complete measure of
the degradation in sharpness due to an optical system, Many methods have
been developed for the instrumental measurement of the .OTF of optical systems
(see Section 10.2.2). If used in conjunction with some measure of gross change
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