print color reproduction is primarily dependent on the following
factors: · ink film thickness · dot size ·
registration · ink trapping. Most of these factors can be
measured with a densitometer. With the use of a densitometer, a
press operator will have greater control of the color throughout
a press run. In order to achieve improved color print reproduction
it is helpful to understand how a densitometer works (Figure 1).
The eye is very sensitive to color. It can perceive density and
shade variations and compare them to a given calibration standard
that identifies specific color standards. The eye cannot however,
assign precise numerical values to perceived variations. A densitometer,
on the other hand, can assign numbers to the density variations
the eye perceives by quantifying the amount of light that is reflected
from the surface of a printed sheet (Figure 2). The densitometer
cannot be used to measure color differences. It is strictly a device
to measure the optical density of the process colors of a printed
Densitometers are used for quality control in printing. Measurements
are primarily concerned with the process colors of cyan, magenta,
yellow, and black. The light emitted by the densitometer is white
light. White light consists of the three primary colors of light:
red, green, and blue. When the proportions of these three primary
colors of light are approximately equal, the eye will perceive this
light as white light.
A densitometer, in order to measure a printed sample, will produce
light from a stabilized source (1) (See Figure 2). The light passes
through a lens (2), where it is focused on the printed surface.
Depending on the film thickness and the pigmentation of the ink
involved (4), part of the light is absorbed. The non-absorbed content
of the light is reflected by the surface of the printed stock. A
lens system (5) now captures the reflected light returning from
the ink film at an angle of 45 degrees to the light source, and
focuses them into a receiver (Photo Diode) (6).
The quantity of light received by the photo diode is converted into
electricity. The electronics of the densitometer (7) now compares
this current with a reference value (white). The difference obtained
is the basis for calculating the absorption characteristics of the
ink film being measured. The results of this show the value an operator
will see on the densitometer display. Color filters (3) in the light
path restrict the light to the range of wavelengths in question.
Different densitometers utilize different filters to measure their
responses. In the U.S.A, it has been the custom to use the wide
band or Status T filters. Several different types of filters are
available. The filter set being used must be clearly stated when
communicating any densitometer values, or there will be differences
in the numerical values.
The printing ink to be measured (cyan for example) affects the light
similar to a color filter. Color filters possess the property of
allowing their own color to pass through and absorbing or blocking
the light of the other colors. The mixture of arriving light colors
of blue and green will produce cyan. These blue and green light
contents are able to pass through the ink film unimpeded and reach
the white surface of the paper before being almost completely reflected.
The red light content, on the other hand, is absorbed by the cyan
ink film to a greater or lesser degree. Consequently, depending
on the pigmentation and the ink film thickness, only a relatively
small proportion of the red light content is reflected. The eye
perceives this reflected light as cyan, which consist of mainly
blue and green components.
For measuring the ink density, however, only the smaller, red content
of the light, which is strongly influenced by the ink film thickness,
is significant. For this reason, a filter (see Figure 3) is inserted
in the path of the light, which holds back the blue and green light
contents. This allows only the red light content relevant for measuring
the cyan color to reach the photo diode of the receiver. It is important
to understand that the densitometer does not see color only light.
Therefore, the other colors of light must be filtered out to achieve
an accurate measurement on a sample. Depending on the type of instrument
involved, the color filters are placed in the path of the light
either before or after the measuring specimen.
The ink density values that are shown on the instrument display
are always expressed as logarithmic numbers. As the logarithmic
density values increases, the amount of available light decreases.
For example, a density of 0.00 indicates that 100% of the light
falling on the sample is being reflected. A density of 1.00 indicates
that only 10% of the incident light is being reflected. A density
of 2.00 indicates only that 1% of the light has been reflected.
This is shown in graphical form in figure 4.
The densitometer is designed to adapt the density measurement to
the peculiarities of the human sensory perception. The human eyes
and ears evaluate optical and acoustic stimuli on a logarithmic
scale. This means that the uniformly rising intensities are not
perceived as uniformly rising. For example, if an observer is looking
at a light table, where the glass top is being illuminated by a
fluorescent bulb, he then perceives a light of certain intensity.
If a second fluorescent bulb of equal brightness is now switched
on, then although twice the amount of light energy is striking the
glass top of the light table, the observer will not perceive the
new energy level as double the first. Further doubling of the energy
would be perceived to even a lesser degree. The more often the light
energy is increased, the less the increase is perceived.
These logarithmic differences are extremely important to understand
when setting color on press. The logarithmic increase in visual
perception relates to the amount of ink on press to achieve this
level of perception. What this means is the level of ink will also
have to be increased on a logarithmic scale to achieve higher press
To Illustrate this point in the printing world, an excellent example
would be to look at how rub off is affected by the print density.
The amount of rub-off of a newspaper print is highly dependent on
the print density or the ink film thickness. As can be seen in Figure
5, this varies exponentially with film thickness and becomes particularly
bad when the density exceeds 1.10.
Figure 6 lists these values as a percentage increase in terms of
rub off from a print density of 1.0. As you can see, if you raise
the print density from 1.0 to a density of 1.25 ( an increase of
25%) the rub off of the printed sheet increases approximately 45%.
The drying or setting process for newsprint is by absorption, and
as the volume of ink is increased, the newsprint can only absorb
a fixed amount. Therefore more ink will remain on the surface to
In addition to the adverse effect on rub-off, higher print density
has other consequences leading to poorer ink mileage. This relationship
is illustrated in Figure 7, where it can be seen that very little
increase in density is achieved when excessive amounts of ink are
printed. For example, an increase in print density of 20% (from
1.0 to 1.2) requires an ink film weight increase of 58%. This film
weight percentage increase is over double the percentage of the
print density change.
A densitometer, like any other instrument, will not work properly
unless it is calibrated. If the instrument is not using traceable
calibration references, each instrument is its own system. One unit
will not relate to another. The values obtained with a non-calibrated
densitometer will not accurately reflect the variability of the
process being monitored. Calibration of a densitometer is simple.
In calibrating the instrument, a value for the "zero point",
or low end response, is established first. Then a "high end"
point is set. In setting these two points a slope between them is
established, thus allowing for accurate measurements. Besides the
calibration plaques offered by the densitometer manufactures, a
"T-Ref?" is available from the Graphics Communications
Association (GCA), which is a printed reference used to verify that
a densitometer is truly a Status T response unit.
In today's times of rising costs it is easy to see the functionality
of the densitometer in the press room. A densitometer will not have
"eye fatigue" problems where the human eye becomes "used
to" looking at a subject. When the human eye does this, it
becomes less sensitive to changes. The densitometer is easily one
of the most important instruments in the pressroom environment.
Without it, problems beyond the rub off mentioned above, such as
set off, pipe roller build up, tracking or marking, and color balance
would be difficult to control. Much of the printing done today without
this tool would be severely hampered, and production