Measurement of print

The colour parameters XYZ or L*a*b* are not related in a simple way to the characteristics of the half-tone image.  Densitometry refers to the measurement process used to describe the print “density” characteristics of images produced by the four-colour printing process.

Reflection density is widely used for quality and process control within the graphic arts industry.  Once the half-tone characteristics of a visually acceptable print is established, densitometry offers a way of assessing the properties of a print using parameters that are directly related to the halftone process.  The information from the instrument can be used to monitor the production process so that corrections and adjustments of the press can be made in order to maintain acceptable reproduction.

The methods and terms used here follow the practice of the Committee for Graphic Arts Technologies Standards (CGATS).

Printed colour

It is helpful to understand the way which the dot-areas of a half-tone pattern are related to the amounts of light in the red wavelength band (R), in the green wavelength band (G) and in the blue wavelength band (B) that are reflected by each area of the print.

Colour mosaic

In the case of half-tone printing, only eight different coloured dots are produced.  Typical colour separation values (amounts of light in the red, green and blue wavelength bands) for each type of dot relative to those of the white substrate are shown in Table 1.

All the colours available in the printed image are formed by changing the area occupied by of each type of dot in the mosaic of dots in a localised area of the print.  The area of a single dot is so small that there will be several hundred even in the smallest part of the image that the eye can resolve.  The colour seen by the eye in each part of the image arises from the blending of the light reflected by the mosaic of dots.  Each type of dot will contribute a proportion of light from the red, green and blue bands of the spectrum and the net effect is determined by the sum of these contributions.

Densitometer: c y m k

Light reflected by the printed surface is collected at an angle of 45° to the surface, as defined by a pair of apertures.  The spectral character of the 45° light is analysed using optical filters to determine the amounts of light in four bands of wavelengths; red, green, blue and visual.  National and International Standards establish the characteristics of the filters and of the weighting functions.

The densitometry standards allow the positions of illumination and collection to be reversed to provide an alternative design for the instrument.  In the 45/0 arrangement light is shone onto the print surface at 45° to the surface and the reflected light is collected normal to the surface.

R: Reflectance factor

In all cases the reflectance factor is defined as the ratio of the amount of filtered (or weighted) light detected from the sample relative to the amount of filtered (or weighted) light detected from a perfect white diffuser measured under the same conditions.  In normal use the term “reflectance” is used rather than “reflectance factor”.

D: Optical density

C or red-filter density: DR

Y value or blue-filter density DB

M value or green-filter density DG

K value or visual-filter density: DV

Typical values

The density values will naturally vary slightly during a print run, a typical range of variation when operating to a close tolerance is ±0.05D units

Analysis filter characteristics

Densitometers use optical filters with a transmission characteristic similar to the filters used in the colour separation process of printing plate production.  There are two major types of response in common use for reflection densitometry, Status T (USA) and Status E (Europe).  It is important that the Status response of the densitometer is included in any specification or report of density values.

Status E

Status E is generally accepted in the UK and Europe as the Wide Band Graphic arts filter response characteristics, see Figure 3.  It has the advantage over the USA based Status T response in that the c y m density values of prints from a balanced set of process inks are similar in value.  A balanced set of inks will print a neutral, achromatic colour for overprints of cyan magenta and yellow.

Status T

Polarisation filter

It is generally agreed that polarisation filters can give less difference between the density values of a wet and a dried printed sheet.  However the use of polarisation filters in a densitometer is controversial since the effect is not always consistent, there is the possibility of distortion of the spectral response and there are no published standards for the use of polarisation filters.

Print control strips

It is difficult to isolate areas on a printed image that are suitable for use in printing process control, as a result a test strip is often printed in an area that is normally hidden or discarded in the final product.  The control stripe has a series of test areas whose density values can be used in the control of various aspects of the printing process.  The size of the test areas should be larger than the aperture of the densitometer.

A typical general-purpose process control strip or colour bar may include the following elements.

Greyness and hue error

Definition

Greyness and hue error and are measures of the non-ideal nature of the ink-substrate combination, the values are used as quality control tests and are applied to incoming ink or paper/board and during a production run.

If the values vary during a production run, then it is an indication of contamination of the printing plates or the ink.

Explanation

The density values of a solid process-ink patch can be thought of as the sum of three contributions.

(greyness)+ (hue error) + (ideal process-ink)

The greyness contribution adds equally to each density value.  The hue error contribution adds to just one of the density values.

Consider a yellow solid patch with the values:

This is equivalent to the sum of the following contributions:

Method

The measurements are made on the solid patch of the cyan, yellow or magenta ink.  The solid ink patch is measured and the c, y and m density values units of the patch are noted.  The values are examined and sorted into the highest (H), the middle (M) and the lowest (L).

The values obtained refer to the properties of the printed patch, so it is not necessary to take into account the density of the unprinted substrate.

Example: The yellow solid patch is measured and the values:

are obtained,

Table 4 shows Example values that are expected for different types of print process.

It should be remembered that lower values of hue error and greyness represent a process-ink with properties closer to the ideal.

Tone value, apparent dot area (A%),

The apparent tone value is on a printed sheet is often different from that intended in the image file or on the plate.  There are two reasons for the change in apparent area.

Definition

The tone values or dot area of a print is the percentage of the surface that appears to be covered by dots from single process-ink, ignoring light scattering in the print substrate and other optical phenomena.

Method

The apparent area is determined using the Murray-Davies equation.  Three density values are needed.

The appropriate c or y or m density values are used in the equation according to the colour of the ink under test.

Tone value increase, dot gain (TVI %)

Definition

The tone value increase is the difference in tone value between any two steps in the printing process.  Modern definitions include electronic data and measured values on any media.

A statement of the tone value increase should include a statement of steps in the printing process that are being compared.  For example:

Explanation

The TVI or dot gain value gives a measure of the likely tonal quality of the print.  The tonal balance of the print depends on the dot gains of the inks being similar, which is critical for grey balance, and for maintaining critical overprint colours such as flesh tones, green grass and blue sky.

The screen areas of the test patches are picked to correspond to three characteristic tonal regions of an image, highlight (25%), midtone (50%) and shadow(75%).  Excessive dot gain in highlights make pastels nearly impossible to reproduce.  Excessive dot gain in the shadows will cause loss of detail in these areas of the image.

Print contrast

Definition

The print contrast is the ratio of the difference in density between the solid area and the shadow tint area to the density of the solid, expressed as a percentage.  This indicates the printing system's capability to hold image detail in the shadow tone region.

Explanation

The shadow detail carries important information in many images, subjective evaluation such as “flat” or “jumps off the page” indicate a low or a high print contrast value.  As a result, the % print contrast is determined using the shadow (80% or 75%) screen dot-area patch.

Method

The appropriate c or y or m density values are used in the equation according to the colour of the ink under test.  Since the values refer to the quality of a print, no correction is made for the density of the unprinted substrate.

Two measurements are needed

The print contrast is determined from

Example

Ink trapping

The four process-inks are printed in sequence, with black printed either first or last, then cyan, followed by magenta and then yellow.  The inks are formulated to with a graduation in tack according to the sequence of application by the press.  This ensures that an ink will adhere to the previous ink layer that was printed rather than lifting it from the paper.

An ink printed on top of another ink in a multi-colour press may not achieve the same density as when printed directly onto bare paper or board.  One explanation for this is that the second-down ink does not transfer as efficiently to ink on paper as it does to paper alone.  Factors influencing trap include ink film thickness, ink tack and viscosity, printing sequence and the mechanical adjustments on the press such as rollers and impression settings.

Poor trapping will result in a hue shift in overprint reds (magenta and yellow), greens

(cyan and yellow), and blues (cyan and magenta).  Both overtrapping and undertrapping occur.

Definition

Ink trapping is the relative transfer of an ink onto previously deposited ink(s) as compared to the transfer of the same ink onto the bare paper/substrate.

Method

Ink trapping compares the density of the overprinted ink layers with that of the same ink layer printed directly onto the substrate.  The appropriate c or y or m density values are used in the equation according to the top colour of the overprint.

Identify the top colour, or second ink laid down, of the two-colour overprint target to be measured.  When taking measurements for this patch, use the filter for the top colour.  For example, for a green overprint made by printing yellow over cyan, a yellow density (blue optical) filter is used.

Perform the following measurements using the filter for the top colour:

The apparent trapping of the top colour ink on the ink first laid down is given by the Preucil quation:

Example