[See part 1 for bibliography]

****Who Invented Unsharp Masking?****

[Hunt, p263]
"Another useful feature often used in masking was first suggested
by Yule (Yule, 1944). Exact registration of the mask when bound
up with the original is obviously difficult, and if not perfectly
achieved results in halos appearing around any well-defined
edges. Yule suggested that the masks should be made deliberately
*unsharp* by printing them with a thin spacer between the
transparency and the mask material. This not only helps to
obscure slight lack of registration of the masks, but also
improves the reproduction of fine detail. A negative mask reduces
contrast, but fine detail is seen more clearly if reproduced at
high contrast; by having the mask unsharp the fine detail is not
resolved by the mask and hence, when it is bound up with the
original transparency, it does not reduce the contrast of fine
detail, but only of large areas."

[Yule, p74]
"Color-correcting masks should be made somewhat unsharp; for
example by the use of a diffusion sheet (Spiegler and Juris, 1931
and 1933; Yule, 1944). This makes the registration of the masks
less critical, and it improves the sharpness of the detail in the
reproduction."

****Moire and Overlapping Absorption Bands****

[Yule, p343]
"It is often said that the yellow printer does not usually cause
noticable patterns because its color is so light. Although there
is some truth in this, Pollak (1958, 1959) has shown that it is
more a question of overlapping absorption bands. Each wavelength
of light produces moire patterns independantly, and what we see
is the sum of all of these. If only one halftone print absorbed
light at each wavelength, there would be no pattern. A pattern is
produced because of the overlapping absorption bands of the inks.
The absorption band of the black ink, which covers the whole
spectrum, is bound to overlap with all three of the other inks.
At wavelengths where the magenta and cyan both absorb light
(mainly in the green region of the spectrum), a moire pattern
will therefore be produced between the magenta, cyan, and black
if the 30 or 60 degree angles are incorrect. This pattern will
appear most pronounced if observed though a green filter."

[Yule, p343-4]
"When the yellow ink is at 45 degrees to the black, these two
images will not form a noticeable moire pattern. The yellow is
then at an angle of 15 degrees to the other two colors, and it
will normally form a small moire pattern with either one if the
absorption bands overlap, and a somewhat larger second-order
pattern with both of them together if all three absorption bands
overlap. The yellow ink absorbs light only in the blue region of
the spectrum. With a reddish magenta and a greenish cyan, both of
which absorb blue light, a more noticeable pattern will be
produced by interaction with the yellow ink than when bluish
magenta and cyan inks are used. With a clean bluish cyan, in
fact, the moire pattern formed between the cyan and yellow may be
so weak that they can both be printed at the same angle without
intorducing much variation from sheet to sheet on account of
register changes."

****Moire and Color Shifts****

[Yule, p335]
"It might be thought that the best way to avoid moire patterns
would be to print all the halftones at exactly the same angle.
This would not be satisfactory because the color produced would
depend on whether the dots fell on top of each other or side by
side. With the dots on top of each other, a much lighter tone
would be produced, because there would be a much larger area of
unprinted white paper. The color produced would therefore be
extremely sensitive to minute variations in register."

[Yule, p339]
[Describing screens not quite 60 degrees apart]
"In some parts, rosettes of dots are formed with a single dot in
the center (composed of three superimposed dots). Elsewhere,
double rings of dots with a clear center are produced. In the
latter case, the centers of the clear openings are superimposed.
Between these two extremes, irregular patterns of dots are
formed."

"If the screen angles in Fig. 13.07B had been exactly 60 degrees,
the moire pattern would have become infinitely large. The dot
arrangement would be similar over the whole sheet. It might be
dot-centered or clear-centered, or it might have the irregular
intermediate pattern. A shift in register of only half a screen
period would shift it from one to the other. With the
clear-centered pattern, there would be no point at which three
dots were exactly superimposed; and with the dot-centered
pattern, the centers of the clear openings would never coincide.
The relative areas of the eight colored components of the pattern
would change with this shift in register, and consequently the
color would change slightly. This always occurs in four-color
printing, especially when there is much black in the middletones,
but it is usually not distinguished from other causes of color
balance shifts. It becomes very noticable when an error in screen
angle brings the different patterns close to one another in the
form of moire."

[Yule, p341-2]
"It is, however, if practical importance that shifting one of the
screens by a distance of only half a screen period (usually about
0.004 inch) will convert the pattern from a dot-centered one to a
clear-centered one, which produces a color shift if the
absorption bands of all three inks overlap. The differences in
color are quite small (corresponding to a difference in
green-filter density of about 0.04 in the middletones for typical
good quality magenta, cyan, and black inks), but they are very
noticable when they occur in a moire pattern."

[Yule, p342]
"One interesting consequence of this is that printing a black
halftone over a neutral three-color gray (composed of cyan,
magenta, and yellow) can make it slightly greenish or pinkish
depending on the register of the dots. Another consequence is
that gray-balance measurements (with yellow, magenta, and cyan
inks) will be unreliable if 30 and 60 degree angles are used in
the test object. In this case, the blue-filter density is the one
which is variable, resulting in slight blue-yellow color shifts."

"The dot-centered and clear-centered patterns are complimentary
to each other in that dot-centered highlights will be accompanied
by a configuration in the shadows like the clear-centered
pattern. The small openings in each of the halftones are seen as
dots in the shadows, and these are arranged in a ring as in the
clear-centered pattern."

"The formula for the period of the moire pattern (Eq 13.01) shows
that with a 150-line-per-inch screen, an error of half a degree
will produce a pattern with a period of 0.75 inch, which is very
noticable. The angles should be accurate within +/- 0.1 degree,
which will give a pattern with a period of nearly 4 inches. This
will not be visible except in large uniform areas. The pattern is
most noticable in a medium gray with a dot area of about 50% in
the cyan, magenta, and black printers."

"Even if the angles are exactly correct, this pattern is still
troublesome, since a slight variation in registration of the
three colors will cause a change on color from sheet to sheet
(Pollak, 1955a). This must occur consistently in four-color
printing, but it appears unavoidable and is usually unnoticed. It
provides an additional argument against carrying black in the
middletones of the picture. With a skeleton black plate, the
tones containing black are so dark that the pattern is not
noticable."

[Yule, p344]
"In conventional gravure printing, there is very little halftone
pattern and all colors may be printed at the same angle. This has
been suggested even for the halftone gravure process, such as the
Dultgen process, which have a much more pronounced dot pattern.
It should be remembered, however, that when all the angles are
the same, the moire pattern is of infinite size, and what occurs
is not a visible moire pattern but a change of color from sheet
to sheet as the register changes. Large, irregular blotches of
varying hue, due to local distortions, are also seen. To
determine how serious this is, a proof should be made with two of
the halftones at slightly incorrect angles. The intensity of the
moire pattern thus produced will be a measure of the
sheet-to-sheet variation to be expected."