LABORATORY 3: PUBLICATION GRAVURE INK

GOALS: Prepare press ready Yellow, Magenta, Cyan, blacK inks with/without extender. Observe how extender affects print density. Adjust ink to target density on LWC paper grade. Print on gravure K-proofer with yellow, magenta, cyan and black ink on LWC, SCB and newsprint grade. Determine reflective density, gloss, mottle, and color using CIE  L*a*b* color coordinates. Discuss how the paper quality affects the print.



Adjustment of Reflective Density for Gravure Ink

Take small amount of virgin ink, dilute with toluene to efflux time of 22 s (Shell #2 cup). Print using K-proofer. Measure density of solid areas.
Take the same amount of virgin black ink; add extender (about 1/3 of original ink volume). Dilute with toluene to efflux time of 22 s (Shell #2 cup). Print by K-proofer. Measure density of solid areas.


Reflective Density

Instrument: X-Rite 408 Reflection Densitometer
Samples: Measure magenta and black solids only
Take 10 measurements of density at every sample. Determine Average Reflection Density [%] and Standard Deviation of Reflection Density [%].

Principle: Densitometers measure difference between light projected onto (through) sample and amount of light reflected back (or transmitted by sample).
Reflection and Transmission Densitometers.
Reflection densitometers- measure reflected light.
Transmission densitometers- transmitted light – for transparent film measurement.
Reflection densitometer: red, green, blue filters to represent different thirds of visible light spectrum. Complementary filters:
Blue filter for yellow ink
Red filter for Cyan (blue) ink
Green filter for Magenta (red) ink
When complementary colored filters placed over the inks, they appear Black (or shade of gray). Densitometers “see” in black and white !
Color values are not referenced to a model of human vision.
Densitometric measurement of color can be calculated by colorimeters and spectrophotometers, not vice versa!
Tabular data or spectral curves
Reflective density data from 0 to 2.0
Reflection densitometer uses incident light source (tungsten filament lamp) to illuminate the print either at 45 or 90 degrees
Light reflected from the print recorded by photocell and numerically displayed – digital display (analog).

Assessment of color- not linear, but nearly logarithmic relationship
Density related to reflectance as follows:

Density   D =  log 10 1/R

Where reflectance   R  = R1/Rw
R1 = Intensity of light reflected from print
Rw = Intensity of light reflected from white paper

And density  D =  log 10 Rw /R1
Densitometers equipped with computer chips – calculate density, dot gain (tone value increase), ink trap, print contrast. Densitometry – used for controlling color at press.



Specular Gloss

Take 10 measurements of ink film gloss and 10 measurements of paper gloss.
Delta gloss is calculated as: Ink gloss – Paper gloss [%].
Instrument: Gardener Gloss Meter. Gloss reading: [%] of standard
Gardener Gloss Meter consist of: Light source, Collimating lens (parallel beams), Slot (limit the parallel beam). Method of directing the beam against the paper specimen at fixed angle of incidence
Method for measuring the amount of light reflected from the paper (angle of incidence equals angle of  reflection).
Working standard: black polished glass (100% gloss).

Gloss is qualitative property which cannot be expressed readily in fundamental therms. Related to:
Luster - sudden selective reflection of light (pleasing effect)
Glare  - undesirable reflection of excessively bright light (unpleasant, blinding effect)
Gloss: Characteristic of paper or printed surfaces which causes to reflect the light at given angle of reflection in excess of the diffuse reflection at that angle.
Gloss: Degree to which the surface simulates a perfect mirror in its capacity to reflect incident light
Specular reflection refers to the portion of the incident light which is reflected from the surface of an object with an angle of reflection being equal to the angle of incidence.
The amount of specular reflection is determined by:
The index of refraction of the object,
The wavelength of the incident light,
The angle of incidence.
Parallel beams of light striking optically flat surface at the same angle of incidence will be reflected as parallel beams. Diffuse scattering- occurs at non- flat (rough) surfaces
Leveling off  (Calendering), filling the valleys by coating
Print gloss measurements: 60 o from the normal to sheet (Best compromise for all prints)
The amount of specularly reflected light increases with increasing angle of incidence.
Gloss can be measured at 20 o, 45 o, 60  o, and 75 o.



Ink Mottle

Mottle is defined as the unevenness in print density or print gloss. Mottle = Substrate ability to produce an uniform flat printed tint.
Mottle - affected by non uniform wetting and penetration of ink into substrate.
Mottle - affected by surface tension properties of substrate.
Mottle caused by poor paper formation (coated papers and board).
Calendering and wet pressing affect variation in base sheet structure.
One result of calendering- reduction of non-uniformity in z-direction (Roughness).
Smoother sheets believed to be more receptive to ink – more uniform ink coverage.
Negative effect – random densification of surface layers of paper
Mottle: easily to recognize, difficult to measure
Equipment for mottle determination: Tobias Mottle Tester: MTI measures the mottle by measuring the variation in reflective density of a sample at controlled ink film thickness.
The variation of density over the area of the sample = indication of sample mottle. Mottle index. Regular densitometers – accuracy of 0.01. To measure mottle – accuracy of 0.001 is needed.
Tobias Mottle Tester:
4 main components:
A probe (measurement head- gloss head or density head)
Rotating drum that carries the sample
A microprocessor performing all control and analysis functions
A video display monitor
The probe performs the actual density measurements.
Probe: illumination source, an optical measurement system, and associated electronics.
Illumination source: produces small area of light of controlled intensity, which defines the area to be measured.
Optical measurement system: detects the amount of light that is reflected by the sample from the illuminated spot.
Electronics convert the output from the measurement system to a signal that can be analyzed by the microprocessor.
The sample is mounted on the measuring drum. The drum rotates under the probe for scanning.
Electronics-microcomputer controls: Scanning, recording, analyzing, and displaying the data.
The MTI uses standard densitometric technique of illuminating a 3 mm diameter circular area with a beam of light that is perpendicular to the sample surface and measuring the light that is reflected from the surface at the angle of 45 degrees.
Mottle tester measures mottle by analyzing variation in the reflective density with accuracy of 0.001 units. System is calibrated to give output that is 1000 times the conventional density reading. (Density of 0.8 produces a reading of 800). Adjacent sample areas 0.15 mm, continuous reading).
Density readings- at high speed, multiple reading of each sample area. Readings are then averaged to produce single data point for each area. Mottle tester filters out the variations in density occurring gradually over large distances, and measures those that occur over small distances.
MTI = average deviation of density per processed data points
MTI takes up to 20 scans. Each scan can be up to 500 data points. Each data point is the average of a number of measurements (Typically 64) of single sample area.
Sample can cover whole drum, ½ or 1/3 of the drum.
The MTI uses standard densitometric technique of illuminating.


CIE L*a*b* Color Coordinates

Equipment: Datacolor Datamaster Spectrophotometer
Calibrate the instrument according to instructions at computer screen. Measure yellow, magenta, cyan, and black  color, two times each. Record the color coordinates at D65 light source only.
Measurement of L * a* b * values of color, so called color coordinates.  L* refers to lightness of color L=100 white, L=0 black; a* denotes degree of redness or greenness, b* indicates the degree of yellowness or blueness of the color.