Nikon LS-3500 - LS-3510 - Manual (Page 65)

Scanning for Reproduction 7-1
Software Reference for Scanners
Scanning for Reproduction
Two areas of great concern in image input and output are density and color
balance. What follows is a description of the problems the imaging system may
have to deal with and how we manipulate the controls to achieve the best
corrections.
Gamma
There are eight bits available in the scanner output LUT for assigning tonal
values or shades. (in the 12-bit version of the LS-3510AF, there are 12 in and 8 out
of the 12x8 LUT, or 12 raw bits out). In our binary system, twelve bits results in
2
12 , or 4096 shades and eight bits results in 2 8 or 256 separate and distinct shades
that can be read and encoded. This means that if we begin to reduce the number of
bits used, (when we significantly alter the endpoints or highlights and shadows of
our table) then we have fewer displayable shades with which to imitate the tonal
range of the original. For example, if we reduced the range from 0-255 to 0-128
(or 2
7 ), then we effectively would only have 128 shades with which to represent
every shade from the original image. This can lead to tone wedging, sometimes
called “posterization” or “tone breaks” that make continuously shaded surfaces
look “banded” where certain in-between shades are missing.
If we need to make extreme changes to image densities or color balance, the
Analog Controls internal to the scanner allow a greater degree of manipulation
without relying on a set number of bits for continuous shading, as the image
processing application might do. Since, in a non-digital electrical circuit, there are
no discrete steps imposed by the number of bits to encode and decode a certain
shade, there will be no gaps between points, no aliasing or stair-stepping, on an
analog gamma curve. All non-digital electrical circuits can vary their power
output, whether they are controlled by current or voltage devices (transistors or
tubes), in a smooth and continuous way, and are limited only by the precision of
the electrical components in the system. This precision limitation is what causes us
to use the scanner or computer’s digital LUT’s for more accurate, if not
continuous manipulations.
The essential concept here is that you must make use of both types of controls
in any digital input system. The Nikon scanner’s auto modes make use of this, and
you can control it manually as well.
Color Balancing
As an example of color balancing using digital LUT precision, when a photo
shows a green color cast in the shadows and a magenta color cast in the highlights
we have a common occurrence in traditional imaging called a 'cross-curve'. The
color cast cannot be corrected using Analog RGB controls, since globally adding
magenta or subtracting green to correct the overly green shadows, would make the
highlights even more magenta, and adding green or subtracting magenta to clean