|
  |
| August 2004 issue of PRINTWEAR |
|
 |
|
|
What
the "PI" Terms Mean…Alone and Collectively
by Douglas Grigar, Master
Screen Printer
|
(A version of this article originally appeared
in the August 2004 issue of PRINTWEAR.)
|
Halftone line count, screen mesh, and printer
output - that is, LPI, TPI, and DPI. What exactly is their relationship?
Before we can even begin to choose what mesh to use for any given art
requirement, we must first understand that basics of this relationship
and the terms used to identify the participants - specifically, the
three “PI” terms. There is often confusion about not only
what the terms mean, but more importantly, what they mean to each other.
“PI” Definitions
Let us look first at “dots per inch,”
or DPI. Most positives are produced from computer-generated files on
electronic devices of one sort or another. “Dots per inch”
has taken on the specific meaning of how many blocks of contrasting
areas a given device can print in a linear inch. Just as children’s
building blocks are limited in the shapes they can produce using just
a handful of blocks, so is a printing device able to print more accurate
and even shapes with a larger number of smaller blocks printed within
a given area (see Figure 1).
|
[Figure 1: Output devices with higher
DPI capacity can form more even dot shapes.]
|
|
Next comes “lines per inch,”
“line count,” or LPI. This refers to the number of rows
of dots that will print in a linear inch. LPI only dictates the number
of dots formed into rows along the inch, but not the specific proportional
size of each dot. The proportional size of each dot is variable (see
Figure 2).
|
[Figure 2: “LPI” indicates
how many rows or lines of dots are in a linear inch, rather than the
exact size of each dot.]
|
|
Third is “threads per
inch,” “thread count,” or TPI. This term is specific
to screen printing and refers to the number of threads in a linear inch
of mesh. TPI only refers to the count of threads, not the diameter or
the threads or the type of weave (see Figure 3).
|
[Figure 3: “TPI” indicates
how many threads are in an inch of mesh, but not the size of the threads
or the type of weave.]
|
|
|
The Three Dot Shapes
Every professional-level graphics
program with a printing interface will allow users to choose from at least
three types of dot shapes. The three most common dot shapes are round,
elliptical, and square (see Figure 4).
|
[Figure 4: The common dot shapes from
left to right are round, elliptical, and square.]
|
|
|
The most popular dot shape is the simple
round dot. The round dot works well in nearly all printing methods from
single-color halftone work to process-color separations. The round dot,
due to its compact, even shape, does not aggravate moiré problems
as more exotic-shaped dots can. The round dot also forms the least distracting
“rosette” pattern when used in process-color printing.
Elliptical (oval) dots can help with
line counts that could be a bit too high for the screen mesh desired.
Elliptical dots have a better chance of crossing several mesh openings
to form a more printable dot, but also have what some consider a disturbing
“string of pearls” effect when some ends of the oval dots
connect into long lines that cross the image (examples of this can be
seen in Figures 4 and 5).
|
[Figure 5: Both round and elliptical
dots form “rosettes,” but note the rosette on the left is
slightly less distracting to the eye.]
|
|
| Square dots produce a harder,
more mechanical look that could be useful in creating an effect to match
a given subject matter, but is generally considered too “harsh”
for normal use. Square dots often end up with rounded corners as a full
exposure tends to erode the small corner points.
Image Banding at Lower DPI
Most printers will have a limit to the smallest
line count they can print without visible banding appearing in the printout
(see Figure 6). Banding is most distracting in large areas of subtle tone
change, such as a deep-blue sky fading in color as it nears the horizon.
Dividing your printer’s DPI by 12 to 18 (depending on the device)
will provide the general maximum line count printable without banding.
|
|
[Figure 6: Printers have a maximum
line count that can be printed without visible banding breaking up the
desired smooth tonal transition.]
|
Line Count Choice
for Thread Counts
Years ago it was common to see printers struggle
with line counts much too high for garment screen printing, often on meshes
of 305-TPI and lower. The macho (and masochistic) desire to print the
highest of line counts on garments has thankfully been replaced with the
search for lower line counts that provide good visual resolution, while
reducing the tonal compression (see below) encountered on absorbent textile
substrates.
Also years ago, factors of four to five were
used to find a corresponding mesh, while the current trend is to multiply
the line count by five or six, or more. For example, using 5.5 as a factor,
we find that a 55-TPI positive would need a mesh of 302.5-TPI; the closest
standard choice available would be 305-TPI. The 5.5 factor is just a guide,
and lowering the line count is often the better action. A firm and exact
factor is never going to work across the entire spectrum of your printing
demands. Individual plant testing is the only realistic way to approach
finding the best match.
Visual Gauge of Dots on Mesh
One of the easiest means of gauging dots
to mesh is to use a loupe, microscope, or magnifying glass to view the
positive over the mesh threads. Place the positive in direct contact with
an uncoated screen. Make sure the smallest dots you wish to print cover
at least two mesh openings, and two threads span the width of the dot.
This two-and-two ratio will work for a positive dot, but if you want a
small negative dot to form, you will need a minimum of three threads and
three openings to overcome the ink gain into the open area (see Figure
7). Rough substrates such as canvas and fleece will generally require
even more threads and openings to print clearly.
|
[Figure 7: The red areas denote the
dark areas of a positive that create the minimum open area needed to
print a clear dot.]
|
|
| Tonal Compression
Tonal compression in screen printing is aggravated
by screen resolution and dot gain. When both ends of the scale fail to
form correctly, combined with dot gain, it causes a flat, muddy image
with lowered tonal contrast. On the light end of the scale (1 to 15 percent)
the dots start to “drop off.” Dots in this light range become
too small for the mesh and emulsion to form open areas to let ink pass
to the substrate. Dot gain compounds the problems in the lightest range,
as even the smallest dots gain in size. The light dots will begin to drop
off - causing a ragged, uneven edge as threads cross the desired opening
- or simply close in when exposed.
On the dark end of the scale (85 to 99 percent),
dot gain from the absorbent substrate and ink movement close in on the
space forming around the negative dot. The smallest negative dots will
fall into mesh open areas and drop off opposite the way the positive dots
fail to form. Dots just cannot form if blocked by mesh threads or if they
fall into open areas where they have insufficient mesh with which to bond.
Line Count to the Rescue
The answer to tonal compression is to decrease
the line count, causing the subsequent dots to enlarge and, by this size
increase, to create tones with greater contrast. The red rings in Figure
8 represent typical dot gain and show how tonal compression is higher
with smaller dots that are closer together. It seems strange at first
that lower line counts help alleviate some of the tonal compression, but
when taken to the simplest extreme, it is clear that larger dots, farther
apart, can create greater contrast from dot to dot.
|
[Figure 8: Two line counts, on the
left a higher count and on the right a lower count.]
|
|
| |
|
|
|
|
|
|
|
|
---- --- - --- - 
----- - --- - 
---
-
---- --- -
---- --- -
---- --- -
