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of scene contrast (e.g. veiling glare - see Section 9.1.6) such a measure should be
capable of completely specifying an optical image. However, it soon became
apparent that there was no obvious, direct relationship between OTF and visual
performance in normal viewing, partly due to the unknown effects of coupling
between the optical component and the eye, and partly due to an incomplete
understanding of the relationships between visual performance when looking at
periodic functions and when looking at isolated objects.
In recent years there has thus been a considerable effort made to find a figure
of merit related to OTF which can define visual performance. Several possible
empirically derived figures of merit have been investigated. These will be
discussed later in this chapter (Section 10.3). Finally, from the modelling
described in Chapter 7 it has been possible to propose a physically-based figure
of merit (visual efficiency) which may be used as a starting point for the
prediction of visual performance for many simple and definable viewing
situations. This will also be discussed later in the chapter (Section 10.3.5).
Whilst progress was being made in the development of methods of measuring
frequency response, attention was also being given to the possible forms of
veiling glare. This has resulted in the definition of a Veiling Glare Index (VGI) as
a standard for specifying general loss of contrast, and other glare functions
applicable to ghost images and local glare. These will be discussed in
Section 10.4.
10.1 Limitations of resolution
10.2 Spatial frequency response (The Optical Transfer Function)
10.3 Other quality measures
10.4 Veiling glare
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