Full text: Proceedings of the Symposium on Global and Environmental Monitoring (Pt. 1)

important subtle differences 
within each ofthe categories. 
How these differences affect 
the resultant image information 
is of key importance to the end 
user. The major objective of 
this paper is to delineate the 
differences in mapping grade 
films and identify the primary 
benefits and/or drawbacks in 
their application. 
In 1982, the Canadian 
Interdepartmental Committee on 
Aerial Surveys (ICAS) revised 
their photographic acquisition 
specification regarding tonal 
quality. Emphasis was placed on 
acquiring imagery with a proper 
contrast (density range) [5]. 
The aerial photography firms 
using the techniques to expose 
for, and modify the density 
range [6], developed a greater 
awareness of the possibilities 
and limitations of emulsions to 
satisfy the conditions of ICAS 
quality criteria. This 
awareness has translated to 
more efficient photographer- 
laboratory interfaces and, 
ultimately, to users obtaining 
a better quality product. This 
modification of the acquisition 
procedure has had an impact to 
the user as significant as the 
introduction of newer films; a 
review of this procedure 
defines a secondary objective 
of this paper. 
BLACK AND WHITE FILMS 
Black and white (BW) aerial 
films, as noted, have typically 
been categorized into two basic 
categories: infrared and 
panchromatic. 
Infrared 
The infrared film, Kodak 
Infrared Aerographic 2424, is 
regarded as a near infrared 
sensitive emulsion. Its 
sensitivity spans from the 
ultraviolet spectral region to 
approximately 930 nanometers 
[7]. A minus blue filter is 
used during exposure to absorb 
the wavelenghts shorter than 
525 nanometers to compensate 
for the film's high sensitivity 
in the blue spectral region. 
This convention places into 
question the recording of 
shadow detail by the film since 
the spectral quality of the 
light in these regions tends to 
be scattered blue light. 
Evidence shows that image 
contrast significantly 
deteriorates and no additional 
information is gained by 
removing the minus blue 
filtration [8]. 
The exposure latitude of this 
film is relatively narrow, 
making it prone to errors in 
exposure (Figure 1). The 
problem is further aggravated 
by the unreliability of 
conventional exposure meters in 
sensing infrared radiation. An 
effect commonly noted is the 
lack of highlight detail in 
deciduous tree canopies. The 
infrared reflection is high 
therefore producing high 
densities on the negative film. 
These high density values are 
thus recorded on the shoulder 
region of the characteristic 
curve where the brightness 
variations of the deciduous 
tree cover type may not be 
adequately portrayed by the 
emulsion (Figure 1). 
Regardless of the film's 
operational drawbacks it 
remains the film of choice in 
many natural resource 
applications. The two most 
prominent applications being 
vegetation data analyses and 
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