The Furnival Press, (Geo. Barber & Son, Ltd.), Furnival Street, London, E.C.4 (7144)
15 years has thrown light on many aspects of image quality
and has advantages in design, but this approach has not
produced any simple test to replace resolving power for
general use, and from its nature is most unlikely to do so.
Moreover, some of the conclusions to which it leads, such as
the unsuitability of resolving-power for predicting perform
ance on isolated details, can be deduced without using
Fourier ideas. 4 It is conceivable that emphasis may shift
away from sine-wave analysis during the next few years, as
apparently happened several years ago in television.
The single bar test lacks the fundamental qualities and
some of the advantages of sine-wave analysis, but being in the
space domain, has the advantage of presenting its results in
an immediately comprehensible form and thus avoids
confusion. At the same time it can act as a bridge to the
frequency domain. Its correlation to subjective quality
remains to be proved, but the same is true of the transfer
function.
The resolving power test will probably continue by virtue
of its simplicity and familiarity. If properly understood it
can still be of value. Even a high contrast test, though
utterly remote from the conditions of aerial photography, can
be satisfactory for controlling the production of lenses where
the variation of transfer function shape is small. If all
transfer functions had the same shape it would also be
satisfactory for ranking systems, but since they do not it can
be highly misleading in certain circumstances.
The low contrast resolving power test is not, strictly
speaking, valuable because it matches the conditions of aerial
photography, but rather because it discriminates against
those systems which have a wide bandwidth but poor modula
tion transfer. For most practical purposes this test will rank
systems in their correct order and for that purpose the author
has yet to see it proved wrong or surpassed by any other test.
It will not predict what size of isolated detail can be seen,
but at present there is no simple and proven test which will
do this.
In conclusion, studies during the past few years have not
made it easier to obtain a full evaluation of a system or the
quality of an image. They have greatly improved our
understanding of the elements of image quality, so that we
know what questions a given evaluation technique can or
cannot answer.
Acknowledgment
Acknowledgment is made of Itek Corporation’s per
mission to publish this paper. The opinions expressed are
the sole responsibility of the author, but their development
during the past few years has inevitably been influenced by
numerous discussions with colleagues at Itek and (formerly)
at the Royal Aircraft Establishment, and with Professor
Erik Ingelstam ; also by research programmes sponsored by
Itek Corporation and by the United States Air Force under
Contract No. AF 33 (647)-9158. To all of these grateful
tribute is paid.
REFERENCES
1. Photogrammetria, XVI, 1959-1960 : 3. Special Congress Number B.
2. An excellent comprehensive account is given by Perrin. Perrin, F. H.
Journal of the Society of Motion Picture Engineers, 69, 151, 239 (1960).
3. Lewis, N. W., and Hauser, T. V. Journal of Photographic Science, 10,
No. 5. September/October, 1960.
4. Contract No. AF 33 (657)-9158. Fourth Engineering Report. Itek
Corporation. Performed for United States Air Force.