| we should use the
nance range of the
printing medium's
s, however, a com-
seem to differ con-
wn the paper char-
verall slope at that
iner details because
racteristic too steep
trong separation of
lespread acceptance
jecessity. Moreover,
just criticised, since
ty in choosing and
ry valuable in hazy
gatives in clear air
difficult cases, some
ble loss of speed is
r, but this is often
le to control gamma
a luxury fundamen-
"radient, as distinct
us limitations.
characteristic curve
out with increasing
roblems in matching
auch a characteristic
with electronic cam-
eased * noise"). The
lge if they could be
tice in that the limit
710 mu. This has no
because of a fortui-
ibsorption band and
(Fig. 17). There has
, say to 750 mu, to
, as often suggested
ld in principle have
n with an orange or
. It has been pointed
infra-red if not ac-
correction easier by
THE PHOTOGRAPHIC IMAGE, BROCK 29
virtue of the smaller disper- Moe qe UTER
sion. All of these factors fa- |
vour a shift of effective sen- |
sitivity from the present | |
typical area towards infra- |
red.
No notable developments
in the manufacture or use of
infra-red emulsions appear
to have taken place.
N
n
bs S ul
| SENSITIVITY
Ui
o
6.25. Colour of base.
So far all film used for
aerial photogrammetry has
been coated on colourless base.
Motion picture and minia-
ture camera negative emul-
sions are always coated on
grey (or other anti-halo) 400 500 600 700 800
base, to avoid halation and
preserve maximum high-light
detail. Preservation of maxi-
mum detail is vital for photo-
grammetry also; why then is
grey base not used for aerial cameras? In the past it could have been argued that aerial
lenses were so much inferior to short-focus and miniature lenses that the good influence
of the grey base would have been swamped. This is no longer true; the best survey lenses
have very fine image quality but colourless base persists.
Experimental coatings of high speed and low speed emulsions on grey and colourless
base have been made by both Ilford Ltd. and Kodak Ltd. and resolution tests show a
small but significant advantage for the grey base. This is apparent as an extension of the
exposure range over which a given resolving power can be obtained.
An interesting side effect is a reduction of apparent speed which must be connected
with the suppression of back reflection by the grey base, though the exact mechanism is
not obvious. The speed for a given resolution, however, is not reduced.
Although the advantage of grey base can be shown in laboratory tests, repeated
trials by the writer’s organisation have failed to show any significant effect in actual
air photography. It was not practicable to make the air tests with a dual installation of
exactly matched cameras, and the comparison was done by substitution of magazines in
successive runs over the same target. No consistent difference could be found in the
amount of detail shown by the grey and colourless bases, even when using a short-focus
(37) lens of very good quality. The tests did not cover the condition of heavy over-
exposure, but it must be concluded that the small advantage given at correct exposure is
swamped by the other factors which in practice reduce image quality to something less
than its optimum. The case for its use to enhance image quality has yet to be proved.
It is noted that Plus X Aerecon is now being issued with a disappearing black anti-
halo backing.
Grey base is being tried as a means of reducing the print-through of the pressure
plate pattern when photographing very low contrast subjects with certain cameras using
vacuum film flattening. Insofar as the effect is associated with differential reflection of
light from the plate and the air channels, grey base should eliminate it. So far the ex-
perimental evidence is incomplete.
RELATIVE REFLECTANCE
RELATIVE: SENSITIVITY
o
i
| |
| |
WAVELENGTH (mu) |
Fig.17. Generalised spectral reflectance curve of green
foliage compared with approximate spectral sensitivity
of Plus X Aerecon.
