printing industries- density

© Grapho Metronic - Anwendungstechnik

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Color + Quality

Densitometry and colorimetry for standardised, high-qualityprinting

Part II 08/2003


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Part 1: Description of color- Light is color- Light sources- Standard light- Additive und subtractive color mixing- Complementary colors- Reflection and absorption

Part 2: Densitometry and colorimetry- Measuring principle of a densitometer- Densitometric readings- Ink film thickness and density- Densitometry in printing- Colorimetry in printing

Part 3: Color systems

Part 4: Observation conditions

Agenda


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Densitometry and colorimetry

Densitometry has proven to be the ideal measuring technique forregulating color in printing, because there`s a direkt relationshipbetween ink film thickness and density.

but...

Density as a measured value does not represent the opticalappearance of the color on the printed sheet. The densitymeasurement does not give any definite informationon color impression.

Final assessment of the color isonly possible using a colormetricmeasurement, which simulates thesensitivity of the human eye.


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halogen bulb

polarization filter 1

polarization filter 2

ring mirror

beam splitter

spectral filter for Yellow

spectral filterfor Magenta

spectral filter for Cyan

silicium sensor

Reference channels

Measuring principle of a densitometer


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e.g. HKS 66 K (green)

1.66

0.280.93

At each measurement, simultaneous individual measurements are carriedout in the 3 measuring channels and 2 reference channels.

The individual measurements in the reference channel are forcomparison. Alterations in the measured values which result fromfluctuations at the light source can thus be recognized and eliminated.

It provides ring-type illumination of theprinting control strip at an angle of 45°.This compensates for predominantdirections on the surface of theprinting material.

The three light beams are eachpassed through one of theinterference filters and theamount of light which passesthrough is measured by asilicon detector.

Measuring principle of a densitometer


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Spectrum without a filter Seen through a green filter

Filter effects

Color filters in the ray path of a densitometer restrict the light to thewavelengths relevant for the printed color in question. As well as thebasic rules of filter effects, a filter lightens its own color and darkens thecomplementary colors. The densitometer is used to calculate themeasured ink film.


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Polarization filter

Polarization filters suppress the surface gloss.

With polarization filters a greater linearity is also achieved betweenink film thickness and measured density, i.e. equal alteration offilm thickness always produces equal alteration in density value.


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Densitometric readings

Density = - lg 0.9 = 0.05Reflectance 90%

The optical density is derivedfrom the remission

Calculating the density:D = - lg (β)

The less the reflectance,the greater is the ink filmthickness and the (logarithmic)density.

Density = - lg 0.5 = 0.30 Reflectance 50%




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Ink film thickness and density

In the range of normal film thicknesses for process colors (approx.1/1000 mm) there is an almost linear relationship between densityand film thickness.

This means there is also a directrelationship to the ink slide opening.

Therefore, the densities arehighly suitable for recording andcontrolling the film thicknesseson the printed sheet.


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Densitometry in printing

C+Y

C+M

M+Y

colorspace

Cyan

Magenta

Yellow

Density measurement is optimised for four-color superimposed printing.

The process colors cyan, yellow and magenta are defined, standardcolors. For printing, it is assumedthat their color is correct.

This means that only the filmthickness needs to be checked.

This is done using a densitometer.

The ink density measuring systemsFDM 17 and FDM 20 (CCI + CCI 2D)are suitable for this.


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Colorimetry in printing

Magenta

Yellow

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Cyan

In order to make an objective assessment of the visual impression of acolor, one needs a measurement and evaluation system that reproduceswhat the human eye sees. And this is what colorimetry provides.

The color tone in the black circle can havethe same density as the process colormagenta. However, it is obviously notthe same color.

The task of colormetrics is toprecisely define color tones.

Colormetrics can be used toprecisely compare different colors.


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Quality control and regulation of specialcolors

Colormetrics is required to obtain the correct target density valuefor density control of special colors. In order to do this, the colordeviation from a required set color is measured on full surfaces.

Densitometry cannot be used for this.

Consequence:

The combination of densitometryand colorimetry provides animportant prerequisite forstandardised, high-qualityprinting.

Measuring principle FM 19(ColorPilot)


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ColorPilot - Further developmentof the CCI technology

The proven densitometric measurement and regulation of CCI hasbeen supplemented by colorimetric quality evaluation.

The printing machine is still regulated in accordance withdensitometrically measured values.

The densitometric and colorimetric valuesin the print control strip are measuredsimultaneously.

Measuring speed and regulatingquality are the same as CCI.

Measuring head FM 19(ColorPilot)

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