SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
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