26 THE PHOTOGRAPHIC IMAGE, BROCK
ments of image quality by inspection of the negatives, especially if exposed over a
standard test area of country, the lack of any quantitative scale of goodness is felt to
be a serious deficiency. Resolution targets have of course been used, but they are ex-
pensive to construct and maintain in permanent form, and are always liable to be moved
due to requirements of building expansion, etc. Portable targets need additional people
to transport them to the test site and lay them out. If high contrast is used, the results
tend to be unrealistic, while low contrast targets need constant attention to maintain a
chosen contrast level against the changes due to wear, fading of paints, etc. Due to all
these limitations, outdoor targets have not been used by the writers organisation for
many years, and the final testing of lenses, cameras, and emulsions has been done by
photographing representative areas of ground from appropriate altitudes, eliminating
as far as possible such factors as vibration which introduce extra unknowns.
There might be some hope of improving this situation by applying contrast trans-
mission techniques. A white line on dark background could serve as a *slit" source, and
micro-densitometry plus the necessary analysis on the image would give a CT function
from which the performance on low contrast detail could be deduced. Except for tests on
very large scales, however, (say 1 : 5000 or larger) the size of the test-object remains a
serious problem. Given sufficient finance, the problem could be solved, e.g. by construc-
ting a line of fluorescent lamps supported on masts over a pine-wood. This is perhaps a
situation where international co-operation might lead to fruitful results. The weather
difficulty would remain — any such test-range erected in Northern Europe could be used
on perhaps ten days in the year, while if erected in some less cloudy part of the world
the expense and inconvenience of travel to it would also limit its usefulness. Nevertheless
this seems a promising line which could strengthen what is at present the weakest link
in testing aircraft cameras.
6. Present status in cameras, lenses and sensitive emulsions.
6.1. General.
In this section brief notice is taken of the present position and significant advances
in the equipment and materials used for air photography, so far as they affect the quality
of the negative image.
No mention is made of several important things such as windows, filters anti-vibra-
tion, mountings, etc., because the writer knows of no significant advances which have
come into general use. He suspects that the excellent image quality now possible may
often be degraded by vibration due to inefficient mountings, but has no positive evidence
relating to photogrammetric work.
There have been no remarkable advances in lens quality during the past four years,
though the improved wide angle lenses are now very generally used. At least two firms
now make cameras with high speed shutters, which should contribute towards a general
improvement of image quality.
New emulsion types have become available commercially, and some comments are
made on these.
6.2. Negative emulsions.
6.2.1. Available types.
The characteristics of emulsions available on the British and North American mar-
kets are summarised in Table 3.
These characteristics refer to full development in a developer of the D19 type and
have been slightly rounded up.
Compared with four years ago the chief difference is the greater variety now com-
mercially available. The high speed emulsions of reference 2, which have been the stand-
ard for many years, remain substantially unchanged. Technical advances have made it
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