images on the viewing screen is easily accomplished. This also facil- 
itates precise processing of the negatives to obtain a known relation- 
ship between image density and exposure in each spectral band so that 
the colors on the final additive image can be more precisely related to 
the reflectance of objects in the scene. In other systems, any images 
of similar scale can be used in the additive process. 
Advantages of optical-additive viewers in relation to the alter- 
natives are: (1) they provide greater resolution than electronic coun- 
terpart systems, (2) precise, independent processing of data in each 
spectral band is possible. This cannot be done using color films (color 
photographs are, of course, color composite images). (3) Combination 
of multidata information is possible. Again, this is not possible 
using color films. Disadvantages arise from the difficulties in further 
computer processing of data (which is possible with electronic systems) 
and the cost of equipment and the relatively time consuming combination 
process, both of which are not encountered with color films. Also, in 
optical-additive systems, the image is projected on a viewing screen, 
which must be photographed if multiple or permanent copies are desired. 
Full size (20.3cm x 25.4cm) color negative films placed in lieu of the 
viewing screen provide the highest resolution output product. 
Recently the use of diazo products, which produce a continuous tone 
mono-color contact-size copy of each original black and white image, 
have been used extensively as a low-cost alternative to color additive 
viewers. The color images are registered and taped together to form a 
color composite image. While the diazo process does provide a quick 
hard-copy product, precise calibration of colors is not possible. 
Another low-cost alternative to additive viewers is a multiple exposure 
of multiband black and white images, each through a different colored 
filter, registered onto a single piece of color film. This simplifies 
the problem of reproduction of additional copies, and has the advantage 
over the diazo process of only requiring careful registration once, at 
the time the master color photo is made. 
Density-slicing equipment 
While density-slicing systems vary in the techniques employed, they 
all are designed to provide an image in which each specific density or 
signal strength range is presented in a unique tone or color. This may 
be accomplished either through photographic processing techniques or 
electronically, and the final image may be in black and white or color. 
The greatest operational application of density slicing has been 
in water resources work to enhance differences in turbidity, water color, 
or water temperature (using thermal infrared imagery). Density slicing 
does find a use in these applications because the analysis is, in general, 
dependent on single-band density rather than on geometric pattern, and 
the target of interest, i.e., water, is essentially free of significant 
texture. 
The most successful and widely accepted method for photographic den- 
sity slicing is that used by Agfacontour film (Nielson, 1974). Through 
careful exposure and processing, a specific density range on the input 
film is isolated and rendered as a low density on the Agfacontour copy 
negative, while all other densities on the original are recorded as a 
high density. By far the most useful electronic systems for density