Densitometer

Densitometry / Chung 1

Densitometry

Bob Chung, R • I • T

A densitometer measures light absorption due to the presenceof colorant, such as ink and toner. It mainly measures colorantamount, and not color.

(Rev., 062)

Topics How a densitometer works and what it

measures—Reflection and transmission densitometers—Wide-band and narrow-band densitometers—Hand-held and scanning densitometers

Density-derived values—Dot gain / Print contrast—Ink trapping—Hue error / Grayness


Densitometer Fundamentals A densitometer is an electronic instrument

consisting of—Light source—Photocell—Associated logic and circuitry—To measure the degree of light absorption of a

sample Two types of densitometer

—Reflection and transmission

Reflection Densitometer For press sheet analysis

—CMYK inks or colorants are printed.

1.36

light source

optics

sample

color filter

detector electronics display


Transmission Densitometer For film density and dot area measurement

Reflection Density Density is defined as the logarithm (base 10)

of the reciprocal of the reflectance of a sample.— Absolute density is relative to the calibration white point.— Relative density is the absolute density minus the paper

density.

1.28


Reflection Density Correlate with the ability of a sample to

absorb light—Density ranges from 0.0 to 2.0 (typical)

1.28

Transmission Density Transmission density is log of the

reciprocal of the transmittance (T) of thesample—Transmission densities range from 0.0 to 5.0

Zero density is calibrated with no sample (or air).

3.00


% Film Dot Area Halftones are measured in % dot area.

% dot area = (1 - T) x 100 = (1 - 10-D) x 100

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

0 20 40 60 80 100

% Film dot area

Tra

ns.

D

Density

D Trans.

= log [ 1

100 ! % FDA

100

]

3.00

% Film Dot Area Halftone positive vs. halftone negative

% dot area (pos) = 100 - % dot area (neg)

3.00


Densitometer Calibration Adjust the instrument to known values

—Zero (white point) adjust—Upper scale (black point) adjust

1.28

Transmittance of Filters Applicable to reflection densitometers

—Red Wratten #25

—Green Wratten #58

—Blue Wratten #47

—Amber Wratten #106

For color separation

To convert photocell to the luminosityresponse of the eye

1.28


Transmittance of Filters Spectral transmittance curves of RGB filters

—Wide band filters transmit about 100 nm orone-third of the visible spectrum.

1.28

Reflectance of Process InksCyan ink

Magenta ink

Yellow ink


Filter Densities Cyan ink absorbs long wavelength light

—Red filter density implies cyan ink amount

1.28

Density and Visual Sensation Equal density-incremented scale is not

visually uniform.—Density difference in light region (0.4 - 0.3)

has greater visual difference than the samedensity difference in the shadow region (1.4 -1.3). Density differentiates dark shades.

— Effective process control in pressroom


Wide-band Densitometer Reflection densitometry practices in the

U.S. are wide band ANSI Status T.—CGATS.4

0/45 geometry Black opaque backing The source-filter-detector combination define its spectral responses.

Narrow-band Densitometer The densitomertry practices in Europe is

narrow-band. Advantages

—Less variations due to lamp & photocell—Higher sensitivity to ink film variation—Better inter-instrument agreement

Disadvantages—Not Status T


Polarizing Filter Use of polarizing filters

—Better agreement between wet ink and dry inkdensity readings Minimizing the difference in light scattering

between gloss and matte surfaces—Measurements are different from non-

polarizing densitometers Practices in the United State are non-polarizing

Densitometer Limitations Only measure partial print quality

—Other attributes that affect print quality include Color fidelity Tone reproduction Sharpness Gloss, texture, visual demerits, etc.

May not agree with visual judgment—Visual judgment of the okay press sheet should

precede the density measurement.


Densitometer Limitations A densitometer measures light absorption.

—It does not measure color. It may be termed as “anomalous trichromat”

600400 500 700

wavelength ( )

0

50

100 ! "#

$

ρ is the red (L) cone sensitivityγ is the green (M) cone sensitivityβ is the blue (S) cone sensitivity

Densitometer Usefulness Consistent over time

—No "eye fatigue" problems Not influenced by surrounds

—Eyes are often fooled by surrounds.


Densitometer Usefulness Characterize the input-output relationship

from film, to platemaking, and to printing—% digital dot vs. density of print.—Plate/press curve or amplitude response curve

Provide quantitative data for process control

020 40 60 80 1000

0.5

1.0

1.5

2.0

Drepro

% Dot Area (film pos)

Ds

50%tints

Solid

D50

Density-derived Values Ink-paper-press characteristics

—Dot gain (use relative density values)—Print contrast (use absolute density values)—Trapping (use relative density values)

Process ink characteristics—Hue error / Grayness / Efficiency

Use relative density values


Dot Gain The increase in halftone dot size from the

film stage to the reproduction stagefilm

plate

paper

Dot Gain% Dot Gain = % Paper Dot - % Film Dot

% film dot is measured by a transmission densitometeror defined by digital values.

% paper dot is calculated by the Murray-Davies formula.


Murray-Davies Formula Complementary filter densities of the solid

(Ds) and its tint (Dt) are used—The default dot size on film is 50%

% Paper Dot = 1- 10-Dt

1- 10-Ds ! 100

020 40 60 80 1000

0.5

1.0

1.5

2.0

Drepro


Ds

50%tints

Solid

D50

Print Contrast A measure of the shadow contrast rendered

by the printing process.—Using absolute density values

% Print Contrast = Ds - D75

Ds

! 100

020 40 60 80 1000

0.5

1.0

1.5

2.0

Drepro


Ds

75%tints

Solid

D75


Ink Trap A measure of how well the second ink is

transferred (or trapped) on top of the first ink.—Influence the hue and strength of two-color

overlaps, i.e., red, green, and blue

Ink Trap Formula The most commonly used ink trap formula

was developed by Frank Preucil (GATF).

—All three densities are measured with thecomplementary filter of the second ink layer.


Standard-based Printing GRACoL 6 has been a density-based

printing standard.—Density and % dot gain depend on paper grade.

Higher paper grade can produce higher SID withlower dot gain.

Paper Paper Paper

Name Description (Db) K C M Y K C M Y

Premium gloss/Grade 1 & 2 dull coated

matte coated 0.09 1.60 1.30 1.40 1.00 24 22 22 20#3/#5 coated SWOP 0.14 1.60 1.30 1.40 1.00 +/- 0.10 26 22 22 20 +/- 3%Supercal SC 0.17 1.40 1.10 1.15 0.95 28 26 26 24Uncoated offset 0.17 1.25 1.00 1.12 0.95 28 26 26 24Newsprint SNAP 0.28 1.05 0.90 0.90 0.85 32 30 30 28

* Status T, Absolute, Dry density

Solid Ink Density (aim)*

1.70 1.40 1.50 1.05

Solid Ink

Density

0.07

% Dot Gain (aim) % Dot

Gain tol.

22 20 20 18

Process Ink Evaluation Process inks are evaluated against the ideal

inks.—An ideal process ink absorbs 1/3 of the visible

spectrum completely (high wanted density)and reflects the other 2/3 (zero unwanteddensity).

Solid ink patch

Filter Yellow Magenta Cyan

Red 0 0 High

Green 0 High 0

Blue High 0 0


Typical Process Inks Wanted densities are not high enough.

—Indicated by density of solid tone Unwanted densities are not zero.

—Indicated by medium and low densities

Solid ink patch

Filter Yellow Magenta Cyan

Red low low High

Green medium High medium

Blue High medium low

Process Ink Evaluation Based on the high, medium, and low values

of the three filter densities.

% Hue Error = Dmed - DlowDhigh - Dlow

x 100

% Grayness = DlowDhigh

x 100

% Efficiency = 1 - Dmed + Dlow

2 x Dhigh x 100


Features and Benefits Hand-held densitometers are for lab use.

—Specify printing requirements for legacy jobs, suchas reprint

—Ensure that quality of color printing conforms tocustomer’s requirements.

Features and Benefits Scanning densitometers are for production use.

—One-dimensional scanning densitometer scanscolor control bar.

—Two-dimensional scanning densitometer scanscolor control bar and printed image spots.

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Densitometer