SpectroDens Basic comprises all functions related to density measurement.
A very practical feature is:
The Automatic Density mode. By a push of the green measurement
button the relevant measurement values are displayed. Depending on the type
of measurement patch this can be:
• Solid density D • Dot area in % • Gray balance
In this example the measurement has been taken on a Cyan patch. SpectroDens recognizes
the type of process color and displays the solid density value of Cyan, which is
displayed with the “C” symbol. Values for Cyan (C), Magenta (M), Yellow (Y) and Black
(K = key color) can be shown.
The density value is a value without a unit. It corresponds to the amount of ink applied on the
paper surface. The higher the density value the higher the thickness of the ink layer.
The correct density value is dependent on the printing parameters (paper, ink, printing
process). Usually, density values are between 1 and 2.5.
The second measurement has been taken on a Magenta patch. The former Cyan measurement
still gets displayed. Measurements of a specific process color will be overwritten as
soon as a new measurement of this specific color will be made.
For the display of
dot area (or dot percentage) in %, two measurements have to be
made: First the solid density patch and then
the %-patch, which is in this example a 80 %-
patch. The resulting dot percentage is in this
case 92 %; the referring dot gain is 12 %:
92 % - 80 % = 12 %
Should a solid density value be displayed instead of the desired dot percentage value,
then the density measured first was significantly higher than the second one. In this
case we recommend to delete the displayed value by pressing the red Escape button and
to measure again.
When making a measurement on a
gray balance patch, the referring density values
for all four process colors are shown in a bar graph. The values for Cyan, Magenta and
Yellow should be in close range to each other to ensure a neutral gray balance without a
color hue.
It is important that the gray balance patch, on which the measurement is taken, comprises
the right %-values to achieve a neutral gray
when printed correctly. The process standard
ISO 12647 for offset print defines the values as:
C = 50 %,
M = 40 % and
Y = 40 %.
The value for Black indicates how dark the gray color is. The higher the value for Black
(= K) the darker the sample.
In DELTA Mode
(= comparison mode)
the measured values are compared to reference values. The mode is activated
by navigating to the “soft key” with the triangle symbol and selecting it.
In the example on the left side a Yellow patch was measured.
The measured solid density is:
Y = 1,37
This is 0,08 less then the set reference
(= target) density of Y = 1,45.
The measured value is outside the limits of
the set tolerance of ± 0,06. Therefore it is
marked with a cross symbol in the round icon.
A checkmark symbol would be shown in case of a measurement within the tolerance limits.
The setting of
reference values and tolerances is done in the menu under the item
measurement conditions.
In the
Density Reference Setup up to five data sets (REF 1 – 5) for reference (= target)
densities can be edited.
As a preference the reference densit
ies
recommended by the German Printing and
Media Association (bvdm) for the five defined paper classes are already pre-set.
The tolerances define the lower and upper limits around the reference values. In this
example for Yellow:
1,45 + 0,06 = 1,51
1,45 - 0,06 = 1,39
In the measurement function
Densities CMYK all four partial densities for Cyan,
Magenta, Yellow and Black are displayed.
In this example, a solid Magenta patch was measured. The solid density is:
M = 1,42.
But why are densities for the other colors shown as well ? Shouldn’t they have a value
of 0 ? No. These are the so-called
partial densities. No printing ink is 100 % spectrally
pure. There will always be partial colors besides the actual main color.
In this measurement function measurements can be compared to reference values as well.
In this example the reference value was not taken from the reference memory but was
measured just before by putting the “soft key” to
Reference:
1. First select via “soft key”
Reference and make a measurement on a patch which will
be the future reference patch.
2. Now activate via “soft key“
Sample and make a measurement on a sample patch. A
triangle symbol in front of the measurement
value indicates that delta values (difference between reference and sample) are shown.
Future sample measurements will be compared to the once measured reference values
as well.
3. Now activate the
DELTA Mode by selecting the triangle symbol.
The function
Densities CMYK is especially suitable for the measurement of multicolored
composed CMYK colors, e.g. in logos, to find out the reason for possible color shifts.
Dot Area
For the function Dot Area (= dot percentage) two measurements have to be carried
out. First a measurement on a solid patch and then a measurement on a %-patch
(= screen patch). The dot area value is calculated with the Murray-Davies formula.
1. First place the measurement head above a solid patch and start the measurement by
pressing the green button. The solid density value will be displayed.
2. Now make a second measurement on a %-patch and the dot area in % will be
shown. In this case a measurement has been taken on a 40 %-patch. The resulting dot gain can be
easily calculated:
53,6 % - 40 % = 13,6 %
The dot gain value gives the information to what degree a point is enlarged (or decreased)
during each process step of the reproduction process chain.
The double arrow on the left side of the device display indicates which measurement
has to be done next.
Should several dot area values refer to the same solid density, this value has to be
measured only once and can be saved for consecutive %-measurements. After a solid
density measurement, just keep the Enter button pushed for approximately 3 seconds
until the density value is framed. Now an unlimited number of %-measurements can
be made. Deactivating is done by keeping the Enter button pressed again.
Dot Gain / Contrast
The function Dot Gain / Contrast is closely related to the previously described
function of dot area. Here also, first a solid density measurement is carried out and
then %-measurements follow.
In the lower part of the device display there is shown which patch has to be measured
next.
In this case there just has been measured a solid density patch with a density of 1.74.
The double arrow points to the “80” meaning an 80 %-patch has to be measured next.
The round Escape button will always lead back to the previous step in case a wrong
measurement has been made.
After the last %-measurement has been done (in this case the 80 %-patch) additionally to
the Dot Gain value the Printing Contrast will be calculated and displayed.
The setting for the %-patches to be measured is done in Dot Gain Setup.
Up to three percentage values can be edited.
The TECHKON TCS print control strips include two different %-patches; 40 % and 80 %.
Trapping
With the feature Trapping there can be checked, how good two layers of ink which
are printed over each other represent the resulting mixed color. In the Basic version
of SpectroDens the trapping calculation is carried out according to the formula of
Preucil.
There are no standard values for the trapping measurement. The values which can be
obtained depend on the printing process and particularly on the type of ink and paper
used. Typical values for standardized print with Paper Class 1 (glossy, coated) are
C+M > 60 %, M+Y > 72 % and C+Y > 85 %. The SpectroDens models of the Advancedand
Premium-class offer extended trapping calculation according to Professor Ritz
and Felix Brunner, which ensure a better comparison between C+M, M+Y and C+Y
measurements. An alternative determination how well two separate colors overprint
can be checked with a colorimetric function, such as the L*C*h*-feature, where the
L*- and C*-values give the information of the printable gamut.
Printing Curve
The measurement function Printing Curve displays a complete transfer curve,
which describes how screen dots (and tonal values) are transferred during the several
process steps of the reproduction process.
First, make the required settings in the Printing Curve Setup:
1. Tolerance range can be set according to standard values (in this case the ISO Paper
Class 1/2 was selected)
2. Dot (screen) frequency (only relevant for the display of tolerance values)
3. Increments of the wedge to be measured, e.g. in 10 % steps
In the first step a 100 %-(solid) patch is measured.
SpectroDens recognizes automatically the color and displays the solid density. In this
case D = 1.47 for Cyan.
The double arrow in the upper left corner indicates that the next patch to be measured
will be a 90 %-patch.
Now all other pre-set %-patches can be measured on the measurement wedge.
Now the corresponding current dot area is indicated in the lower right section of the
display. In this case: 94.1 % dot area at 90 % dot percentage results in a dot gain of 4.1 %.
The shaded area close to the curve shows the tolerance range. The curve should be
within this area.
A wrong measurement can be easily deleted by the red Escape button and the device will
be ready again to read the patch correctly.
With the Arrow buttons the vertical, dotted cursor line can be navigated to display previous
measurements.
Printing Plate
Beside energy and focus of the laser, dot gain is the most important information
which has to be checked in plate exposure. Special plate measuring instruments
working with video analysis have been developed for this application; for example
TECHKON SpectroPlate. Compared to densitometers and colorimeters they offer
big advantages.
Nevertheless it is basically possible to make measurements with a densitometer on a
printing plate. In doing so one should regard the following: Variations in the coating of
the printing plates, may result in strong influences on the measurements especially in
the crucial range under 10 %. Variations of 0.02 D can cause errors up to 6 %.
The values measured with a densitometer do not correspond with the geometric dot
area, due to the fact that the densitometric measurement process is subject to optical
influences, which increase the measurement value. Therefore densitometers measure
always a higher value than video analysis devices. But the measurement value of the
densitometer can be taken as a reference value for the tonal value adjustment.
For the measurement of the zero point (1.), the solid patch (2.) and the screen patch
(3.) you should select areas on the printing plate which are located as close as possible
to each other.
The measurement feature Printing Plate measurement allows reading values directly
on printing plates. The process is comparable to reading on printed paper.
Before taking a measurement it is important, that the rubber coating is washed off
the printing plate, because otherwise light gets lost and the measurement result will
be distorted.
In addition a correction factor, the so-called Yule-Nielsen factor, can be set which is
dependent on the material characteristics of the printing plate. Usually the plate manufacturer
will furnish the information about the factor for the different plate types.
The setting of the Yule-Nielsen factor is done in the settings window of the measurement
conditions. If there is no information about the factor available from the documentation
furnished with the plates, we recommend to keep the value in a neutral range at n = 1.
The measurement is performed as known from measuring on printed paper:
1. First, calibrate on a light, not imaged area of the printing plate, as it would be a paper
white calibration.
2. Now measure a dark, 100 %-imaged patch. SpectroDens will select one of the four
CMYK channels where the contrast signal is highest depending on the color shade of the
plate. In this case the Cyan channel gave the highest signal, because the plate is blue/violet.
3. Now the measurement on a %-patch is carried out and displayed. Additional
%-measurements can be easily made. It is not necessary to measure the solid density again,
because it is already saved, which is indicated by the frame around the value.
Density graph
The function Density graph enables the exact display of density values for any spot
color. Usually the standards define density filter characteristics only for the process
colors Cyan, Magenta, Yellow and Black. Thanks to the spectral measurement of
SpectroDens, a virtual spectral density characteristic for any spot color can be obtained.
The great advantage is that the process control of spot colors can now be
as easily achieved as known from the four process colors.
In the following example a measurement has been made on a blue spot color.
The density graph is derived from the remission spectrum and represents the
density spectrum for wavelengths from 400 to 700 nm. The peak of the curve
displays where the spectral density value has to be taken.
In this case the spectral density for the blue color is D = 1,37 at a peak wavelength of
580 nm.
Only spectral densities relating to the same peak wavelength (Dmax) can be compared.
The Advanced- and Premium-models display the spectral density automatically as well in
the Densities CMYK measurement mode.
The density values for CMYK show to what extent the spot color can as well be registered
by the CMYK-color related standard density filters.
Here in this case, the blue color is close to a Cyan, therefore the density value in the Cyan
channel is quite high.
CIE L*a*b*
The previously described measurement functions all were based on densitometry.
Thanks to the spectral performance of SpectroDens it is possible to calculate and
display colorimetric values as well. Measurements in colorimetry modes have the
advantage of an absolute description of color based on characteristic values.
The thematic separation between densitometric and colorimetric functions is displayed
by a dotted line within the measurement function window. CIE stands for the international
color institute Commission Internationale de l’Eclairage which publishes the
standards for colorimetry.
The most popular color system in the printing industry is the CIE L*a*b* color space.
Every color is precisely described by three values: The L*-value stands for the luminance
and can have values between 0 (a theoretical,absolute Black) and 100 (a theoretical, ideal
White).
The a*-value describes the color value on the Green/Red-axis (- a*: Green, + a*: Red)
and the b*-value the color value on the Blue/Yellow-axis (- b*: Blue, + b*: Yellow).
Example: Colorimetric L*a*b*-measurement on a blue sample. The blue color has a luminance
of L = 41,5 and a low portion of Red (a < 1).
In colorimetry the white reference is absolute in contrary to densitometry where the
white is usually referenced to paper white. Before making colorimetric measurements,
an absolute white calibration has to be carried out by placing SpectroDens on the
charging console and calibrating on the round, white ceramic standard.
Colorimetric measurements can only be compared when the measurement conditions
are the same:
• Polfilter: off
• White reference: Absolute
• Illuminant: D50
• Observer: 2°
When selecting “Auto” in the polarizing filter setting, there will be automatically the polarizing
filter inactivated when making colorimetric measurements.
The color distance ΔE* describes how close two colors match. A value of 0 means
that they are identical. ΔE*-color differences in the range of 5 can be seen by the
human eye when reference and sample are close together and observed at the same
time.
For measurements of the color distance ΔE* of two different colors, activate the DELTA
Mode first by selecting the “soft key” with the triangle icon. Now two measurements
are made:
1. Reference measurement
2. Sample measurement
The color distance ΔE* is displayed with two digits. Additionally the components ΔL*, Δa*
and Δb* can be seen.
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