Photographic Density Slicing 
  
Photographic density slicing is a technique by which very small 
differences in film density (shade of gray) may be separated 
and displayed individually or together in various colors. A 
normal human eye can separate only about 200 shades of gray, 
but can distinguish about 20,000 hues, values, and chromas of 
color (Estes and Simonett, 1975, p. 931). Electronic instruments 
in normal use can detect density differences as small as 0.05, 
and delta density of 0.01 has been achieved. When a particular 
feature of interest has a specific spectral characteristic either 
on multiband or panchromatic imagery and this spectral characteristic 
or response differs even slightly from those of surrounding features, 
density slicing can be used to bring out this feature wherever it 
may exist in the image, at the same time suppressing the other 
features. New applications and techniques are discussed by Ross (1976). 
Color Additive Viewing 
  
The successful operation of Landsat-1 and Landsat-2 has encouraged 
widespread use of color additive viewing as an interpretation technique. 
Color additive viewing may be done using either analog or digital 
methods. In analog instruments, two or more transparencies are 
registered and light of different color (controlled by filters) and 
intensity is transmitted through the image and projected onto a 
viewing screen or other display device. In digital devices, computer- 
compatible tapes or discs are used to modulate a color television-type 
cathode ray tube. In both the analog and digital systems, the inter- 
preter "interacts" with the data to select the appropriate colors and 
intensities for individual bands. The digital systems are more flexible 
than optical instruments, and permit additional manipulation and extrac- 
tion of statistical information from the data. 
The theory behind optical color additive viewing is treated by Wenderoth 
and Yost (1972, ch. 9) who also provide examples of the applications 
(ch. 12). Many interactive digital systems that serve as color addi- 
tive viewers also permit the analyst to use various enhancement and 
level-slicing techniques to assist in image analysis. 
Digital Image Processing and Analysis 
  
Remote sensor data suitable for computer manipulation are obtained by 
digitizing the analog electrical output of an optical-mechanical 
scanner, television system, radar receiver, microwave radiometer, 
or microdensitometer or other device that converts the densities of 
photographs to an analog electrical signal. 
Computer image processing is summarized by Billingsley, Gillespie, 
and Goetz, (1975). According to them, digital image processing 
techniques basically consit of rectification, cosmetic, and analysis 
-10- 
(