Proceq SRT 5800 Manual page 21

quick and easy to operate. The unit described in this Appendix fulfils
these requirements and, although some of the high-quality finish of the
photographs produced by the more elaborate apparatus is lost, the
reproduction obtained is quite satisfactory for record purposes.
Description of apparatus
The camera unit, which is shown in Plate 4, consists of a camera with
a 120 roll-film back mounted vertically with its focusing plate 55 cm
from the road surface, and with a synchronized flash mounted within
the same housing. The camera used had a 7-6-cm, f3-5 lens,
permitting a coverage of 26 cmX30 cm on the 120-size negative. To
illuminate this area the flash is located midway along one of the longer
sides of the box at a distance of 43 cm and 55° from the centre, as
indicated in Fig. 3. This position was found by experiment to be the
best, having regard to the essential requirement that the overall
dimensions of the unit should be as small as possible; the direction of
the flash is sufficiently oblique to show up details of surface texture,
and the source of the flash sufficiently distant to give reasonably
uniform illumination. (Ideally the direction of the incident light should
be more oblique, but this requires a much more distant source.6)
With this arrangement, the flash was used in combination with a slow
panchromatic film, the camera lens being set at an aperture of fll with
a fixed focus. With this setting the depth of focus is about 4 cm, which
is sufficient to accommodate unevenness and roughness in the road
surface. The flash was mounted below a hinged door to facilitate
replacement of the batteries. Each photograph can be identified by
marking with crayon a traffelyte rule, which is also engraved with a
scale in cm; this rule is placed directly on the road surface.
In operation the camera is set on the road surface, the identification
rule set in position, the flash set, the door closed and the trigger fired.
Thus the time taken to obtain a permanent record of the surface,
showing all the details of texture and general appearance required,
occupies only a few seconds at each site. Standard conditions of
illumination ensure a fair comparison between different surfaces.
Typical photographs showing the range in texture (taken on Pan X 120
film) are those shown in Plate 2. Experience has shown that in wet
weather the road may be dried with a sponge and rag and a
reasonable photograph still obtained.