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Transactions of the Symposium on Photo Interpretation

Tone reproduction. Reproduction of macro details in terms of
photographic densities
Of the many aspects of aerial photography, tone reproduction has probably
received the greatest attention from photo interpreters. Tone reproduction is
the presentation of the various brightnesses of larger object patches - such as
object details much larger than the limit of recognition or resolution - in the
form of various photographic densities in the image. This tone reproduction
depends on:
- the sun’s intensity and colour temperature of illumination - position and lati
tude on earth - time of the day and year, sun’s elevation - spectral diffusion
and absorbtion characteristics of haze and its spatial distribution - spectral
remission of the object under the prevailing conditions, - non-spectral remis
sion determined by shape and texture of the object - shadow contents of the
scene and of the details - wind strength and direction - state of humidity of the
object - various soil conditions at the moment of photography - flight altitude
- filter characteristics - lens characteristics - exposure level - negative emul
sion choice - H & D characteristic curves of the emulsion chosen - position of
the various object brightnesses on this characteristic curve - developer choice -
development time and agitation - evaluation instrument used.
When positive copies are made the tone reproduction further depends on:
- the copying instrument - positive emulsion gradation - exposure level of the
positive copy - positive development, etc. to name only some of the most
important factors.
It sometimes strikes us that from this abundance of influences only the spec
tral “objectff filter + emulsion” conditions have received the greatest attention
from photo interpreters: in practically all cases of applied aerial photography
most of the above mentioned factors were not taken into consideration at all.
This, in my opinion, accounts for the fact that until now it has been impossible
to plan the desired tone reproduction in advance. It can be shown that the
small photographic density differences caused by significant colour differences
in vegetation are practically always outweighed by the many other effects.
Here, again, consideration of only one influence - i.e. the spectral response -
will never allow a reliable planning of aerial photography.
Spectral differentiation by means of film-filter combinations can be succesful
if a pronounced spectral difference is indicative of certain features, as is the
infrared remission for the presence of chlorophyll. This well known fact is
applied in the use of IR emulsions for the recording of general vegetation
(differentiation between coniferous and broadleaf species, identification of
areas where chlorophyll has been destroyed by disease or by insect damage),
but even this pronounced IR remission in chlorophyll is useful only if the re
cording of the IR contents does not take place at the cost of good rendering of
texture details. Experience shows that any loss of sharpness in IR photography
makes this type of photography nearly useless and therefore the photo scales
used should not be smaller than 1 : 10,000 or as a minimum 1 : 15,000. The