Posts Tagged ‘Print control’

CIE Lab & LCH

This is a very basic introduction to two related colour models which are becoming increasingly important in the world of colour reproduction. A colour model is merely a way of describing colour. These are among the tristimulus (three-dimensional) colour models (‘spaces’) developed by the C.I.E.

What is the CIE? C.I.E. is short for ‘Commission Internationale de l’Eclairage’, which in English is the ‘International Commission on Illumination’.  A professional scientific organisation founded over 90 years ago to exchange information on ‘all matters relating to the science and art of lighting’. The standards for colour spaces representing the visible spectrum were established in 1931, but have been revised more recently.
For those of us involved in creating colour which will be reproduced on a printed page, it is easy to forget that there are other industries which need to accurately describe colour! RGB or CMYK descriptions won’t be of any use to paint or textile manufacturers! Terms such as ‘maroon’ or ‘navy blue’ won’t be precise enough.
There are many CIE colour spaces, more correctly known as models, which serve different purposes. They are all device independent, unlike RGB or CMYK colour spaces which are related to a specific device (camera, scanner, or press, etc.) and/or material type (paper, ink set, film emulsion or lighting, etc.). These RGB and CMYK spaces usually do not cover the entire visible colour spectrum or gamut. The CIE also specify lighting conditions.

The CIE LCH Colour Space or Colour Model.

This is possibly a little easier to comprehend than the Lab colour space, with which it shares several features. It is more correctly known as  L*C*H*.  Essentially it is in the form of a sphere. There are three axes; L* , C* and H°.

The L* axis represents Lightness. This is vertical; from 0, which has no lightness (i.e. absolute black), at the bottom; through 50 in the middle, to 100 which is maximum lightness (i.e. absolute white) at the top.

The C* axis represents Chroma or ‘saturation’. This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or ‘colour purity’.

If we take a horizontal slice through the centre, we see a coloured circle. Around the edge of the circle we see every possible saturated colour, or Hue. This circular axis is known as H° for Hue. The units are in the form of degrees° (or angles), ranging from 0° (red) through 90° (yellow), 180° (green), 270° (blue) and back to  0°.

The LCH colour model is very useful for retouching images in a colour managed workflow, using high-end editing applications. LCH is device-independent. A similar colour model is HSB or HSL, for Hue, Saturation and Brightness(Lightness), which can be used in Adobe Photoshop and other applications. Technically this is ‘device-dependent’, however it is particularly useful for editing RGB images. For example to edit a green:  Adjust the Hue angle by increasing it to make it ‘bluish’ or by reducing it to make it ‘yellowish’; Increase the Saturation (Chroma) to make it ‘cleaner’;  increase the Brightness or Lightness to make it lighter. Go on give it a try!

The CIE Lab Colour Space or Colour Model

CIE Lab colour space used in ICC Colour ManagementThis is more correctly known as  L*a*b*.
Just as in LCH, the vertical L* axis represents Lightness, ranging from 0-100.  The other (horizontal) axes are now represented by a* and b*.  These are at right angles to each other and cross each other in the centre, which is neutral (grey, black or white). They are based on the principal that a colour cannot be both red and green, or blue and yellow.
The a* axis is green at one extremity (represented by -a), and red at the other (+a).
The b* axis has blue at one end (-b), and yellow (+b) at the other.
The centre of each axis is 0. A value of 0 or very low numbers of both a* and b* will describe a neutral or near neutral. In theory there are no maximum values of a* and b*, but in practice they are usually numbered from -128 to +127 (256 levels).

CIE Lab is extensively used in many industries apart from printing and photography. It’s uses include providing exact colour specifications for paint (including automotive, household, etc.), dyes (including textiles, plastics, etc.), printing ink and paper. Nowadays it is becoming of increasing importance in specifying printing standards such as in ISO-12647, where it is usually used instead of densitometry.
For example Paper Type 1 (115gsm gloss coated white, wood-free) has ‘Paper Shade’ described as ‘L* 95, a* 0, b* -2’. So the L*95 is very light, the a*0 neutral, and the b*-2 very slightly ‘blueish’.
Paper Type 5 (115gsm uncoated yellowish offset) is described as ‘L* 90, a* 0, b* 9’. So it is a darker, more ‘yellow’ paper. If you compare the different Lab values for Type 1 & 5 you will understand the descriptions.

Lab measurements can be used to control printing, typically by monitoring a 3-colour neutral grey mid-tone patch. It is also very useful for specifying a spot colour, perhaps an important “house” or “corporate” colour such as “Coca-Cola Red”. The same colour definition could be used for printed matter, vehicles, clothing, buildings, and of course tin cans.

To obtain CIE Lab measurements from an RGB image in Photoshop etc., you will need to have assigned the correct ICC profile to that image.

In ICC Colour Management  CIE Lab is often used as the Profile Connection Space (PCS) where it provides a link between two colour profiles, such as Input RGB (scanner or camera) and Output (CMYK or RGB press or inkjet). All ICC profiles contain a PCS. In an input  profile the tables will convert the scanner’s or camera’s RGB space to the PCS (Lab). An output profile will convert the  PCS (Lab) to the digital printer or printing press colour space (CMYK). The other PCS colour space is CIE XYZ, which is often also used by spectrophotometers to report colour, see the nextarticle.

Delta E Differences and Tolerances.

The difference between two colour samples is often expressed as Delta E, also called  DE, or ΔE. ‘Δ’ is the Greek letter for ‘D’. This can be used in quality control to show whether a printed sample, such as a colour swatch or proof, is in tolerance with a reference sample or industry standard. The difference between the L*, a* and b* values of the reference and sample will be shown as Delta E (ΔE). The resulting Delta E number will show how far apart visually the two samples are in the colour ‘sphere’.

Customers may specify that their contract proofs must have tolerances within ΔE 2.0 for example. Different tolerances may be specified for greys and primary colours. A value of less than 2 is common for greys, and less than 5 for primary CMYK and overprints. This is somewhat contentious however. Proofing RIPs sometimes have verification software to check a proof against  a standard scale, such as a Ugra/Fogra Media Wedge, using a spectrophotometer. Various software applications are available to check colour swatches and spot colours, proofs, and printed sheets.

Delta E displays the difference as a single value for colour and lightness. ΔE values of  4  and over will normally be visible to the average person, while those of 2 and over may be visible to an experienced observer. Values for neutrals are lower. Note that there are several subtly different variations of Delta E: CIE 1976, 1994, 2000, cmc delta e. When no particular ‘flavour’ of DE is mentioned, it is usually DE76 (as in 1976).  The later DE2000 is more perceptually uniform, and becoming more common.

Information source:

http://www.colourphil.co.uk/lab_lch_colour_space.html

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Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.

2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).

Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK
imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

Under Color Removal & Grey component Replacement (UCR & GCR)

The principle of RGB to CMYK separation:
In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or “patchy.” Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet.

Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray
tones), and and better printability.
The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally
outputting the result as a 4 channel CMYK image.

Monitor Calibration Test Chart:

Want to check whether your monitor is calibrated?  Please find above the attached test file to check the same.

The desktop background images are used to check and evaluate the monitor calibration. The test elements are used for a visual check of highlight and shadow, the tonal response curve (gamma) and a neutral continous gradient on the monitor.
Additional test images permit to control the uniformity of the monitor

ColorSolutions desktop images v3.zip
(6,5 MB)
 (Click above to download)

Monitor calibration test image

If sheets are pulled freshly printed from the delivery and measured, the ink is still wet and has a shiny surface. While drying, the ink penetrates the paper (absorption) and loses its gloss. This not only changes the color tone, but also the density. It is only possible to a limited extent for the press operator to use densitometry to compare wet sheets with the reference values, which also refer to dry ink.

Why Polarizing filters?

To get round this problem, two linear polarizing filters at right angles to one another are placed in the path of the densitometer. Polarizing filters only permit light waves oscillating in a certain direction to pass. Part of the resultant aligned beam of light is reflected by the surface of the ink, but its direction of oscillation remains unchanged. The second polarizing filter is rotated 90° in relation to the first, which means that these reflected light waves are blocked.

Fig1. Polarizing filter

However, if the light is only reflected after it penetrates the film of ink, either by the ink or the paper, it loses its uniform direction of oscillation (polarization). Consequently, part of it passes through the second polarizing filter and can be measured.

Filtering out the light reflected by the glossy surface of the wet ink thus has the effect of making the densitometric measurement values for wet and dry ink roughly equivalent.

Brilliant colors true to the original on a premium surface – that ̓s what distinguishes a high-quality print product. Many elements in prepress and printing impact on a product ̓s color fidelity. The various input and output units in a print shop ̓s production process, consumables and other factors can cause deviations in color. A consistently employed color management system eliminates this problem.

In this series of blog tips we ̓ll show you the most common sources of error and give you tips on color management. We ̓re pleased to have caught your interest.

What is Color Management?

The color management process enables an identical color impression to be produced error-free on various output units, such as monitors, proofers and printing presses.

a. World of Color Management

With a consistently employed color management system, a template made on any input unit can be reproduced virtually identically at any output unit. Color management systems can also harmonize a range of devices, such as scanners, digital cameras, monitors, printers, filmsetters and platesetters. The color is then shown according to the print conditions, for example.

The print result is at the heart of the color management process, because the possibility of making corrections directly at the printing press is limited.

A host of factors affect the quality of your printed products: in addition to the machine itself, there are the printing stock and inks, as well as external factors such as atmospheric humidity, and room temperature. Often, when problems arise in the pressroom, the root cause is not immediately apparent. Frequently, it takes time to filter out the underlying cause from among the many possible variables.

This series of blog with Professional Tips offers advice on tips in the most common problem cases. This handy guide should not be missing from any pressroom.

Horizontal Stripes

The problem in this case involves disturbances in the printed image, which are manifested as bright or dark stripes running horizontally to the direction of printing.

Horizontal stripes arise for a variety of reasons. Besides adjustment errors, errors in screen vignettes (prepress errors) may also play a role. Additionally, the formation of stripes may also be influenced by contamination from the build up of powder and coating, from damage to the printing blanket or the printing plate, or from the ink/dampening solution feeds, as well as from the quality of the ink. The machines themselves may also be causing the stripes, for example from an incorrect alignment of the ink and dampening units, or from rolling errors (too much pressure). Tone stripes can be caused by the relative lateral movement of the rollers on the printing plate.

remedy for stripe formation conditioned by machines

• When maintaining the rollers, only use appropriate cleansing agents; the weekly application of a wash paste removes lime deposits, and will regenerate the rollers.

• Adjust the rollers in accordance with the instruction manual; inspect the adjustment regularly.

• Change used rollers: the rubber surface of older rollers becomes glossy and over-smooth. At the same time, as their hardness increases, the edges bulge out in the shape of a trumpet. More pronounced abrasion becomes evident.

• Adjust reciprocation of the ink form rollers

• The dampening distributing cylinder must be kept clean and receptive to water.

• The roll bearings must be in fine working condition (no play, no sticking).

• Lubricate the roll bearings

• By setting the ink form rollers as gently as possible on the printing plate, impacts stemming from the run-on and run-off edges are reduced.

• The bearer ring should be kept clean and grease-free.

• The calibrated under packing should be clean, cut in a format to fit, and correctly inserted.

• Clamping between the plate and the blanket should range between 0.1 mm and a max. 0.13 mm. (0.004 in and 0.051 in).

• Clamping between the blanket and the counter pressure should be set in consideration of the surface of the printing stock; avoid clamping that is set too high.