The colour sequence to be followed during printing plays an important role in the reproduction of the original. In conventional printing whether Offset or Letterpress, four different coloured inks are printed on top of each other in a specific sequence to obtain the intended tonal and colour effects of the original under reproduction. The ultimate colour effects to match the original on print is derived by printing with Cyan, Magenta, Yellow and Black printing inks in that sequence which wherever needed turns into secondary and tertiary shades to match with the colours on the original being reproduced. Some times extra shades, which again are within the shades of the three primary colours and black are also printed as overprint to enhance the reproduction. This is rare.
Similarly in the Security Printing process, where the design elements are of intricate design elements likes various thickness of lines, wavy patterns, Guillosche designs etc for example the currencies, cheques, stamp papers, and labels with hidden security features, certain sequence of colour printing is followed with separate plates after the proof is approved with the same sequence. The colour sequence will not be however Cyan, Magenta, Yellow and Black as in conventional printing process, but each plate will have a rainbow of colours of entirely different hues and shades in different band widths. Since the primary colours are not used, the question of forming secondary and tertiary colours also does not arise as the shades will be entirely different in nature and the design elements criss-cross over the other to give visual shade. The colour separated plates does not have halftone dots in specific angle and not printed on top of each other with Cyan, Magenta, Yellow and Black in order. This is the fundamental difference between the conventional printing process and the Security Printing process.
Against this background one need to understand what governs the principle of the sequence of printing the colours. How important is to follow the sequence of colours in both the processes of printing and what will happen if they are changed ? It may so happen at times that in order to save idle hours whenever some technical problem or the other lead to the temporary stoppage of the machines either for change of a specific colour plate, or get modified ink for smooth flow on the machine, all of which may take quite some time to arrive, some alternate actions may be taken by the printers to keep the printing continue.
In situation when technical problem lead to the temporary stoppage of the machines either for change of a specific colour plate, or get modified ink for smooth flow on the machine, the printer will be left with two options. Carry on the printing with the next colour printing plate and another shade of ink both of which are readily available so that the machine idle time could be avoided. The other option would be to wait for the supply of fresh plate and inks against the discarded plate and troubled ink.
This is easier on the machine which are single colour press- printing one plate at a time. But on a multi plate printing machine this is not possible and one may have to wait till the new plates and the modified ink is received.
It is not necessary that every time the plate may be required to be changed or modified ink required leading to the temporary stoppage of the machine. It may also happen that the image on a particular plate may not be printing satisfactory and leave dark image at some spot due to some technical problem such as insignificant jerk happening due to worn out gears of the impression or blanket cylinders or even from the ink rollers at that spot. It may require replacement of the gear on machine or long period of maintenance may become imperative to readjust the settings and the loss of such a huge time may not be acceptable due to urgency of work. In such a scenario some times by interchanging the plates on the plate cylinder segment printing with darker shade having more design elements with lower shade of colour with less design elements and vice versa the printer may notice that the problem disappear from the print. But is it a practical solution and what will be the impact in initiating this action? See the illustration below to understand the effect of interchanging the plates between segments.
In normal circumstances the customer may agree to the moderate change in appearance of the reproduction subject to the condition that the security features are not compromised. However in the case of important documents like Currencies, Stamps, Licenses and many such vital documents, the changes may not be accepted as they may cause confusion as the visual appearance is the first step in authenticating and accepting the documents.
Universally certain standards have been laid out for reproducing the multi colour originals in certain colour sequence since the sequence of printing plays a vital role in determining the quality (appearance) of printed color reproductions even though few other factors too influence the quality of reproduction such as the transparency and the tack of the ink.
Theoretically speaking, colour image is reproduced by two types of Colour models namely additive and subtractive. They are technically termed as RGB meaning additive colour with Red, Green and Blue lights while the second one is called CMYK, which is subtractive color with Cyan, Magenta, Yellow and Black using printing inks.
In the case of RGB sequence red, green and blue light are added together in various combinations to reproduce a wide spectrum of colours for the display of images in digital photography and web designs on computers. In respect of printed reproductions on paper or any other surface using coloured inks, the accepted norm for the printing sequence is Cyan, Magenta, Yellow and then Black, shortly termed in technical language as CMYK sequence if it is four colour process of printing, or CMY if three colour process of printing.
CMYK is a subtractive color model, used in multi color printing on
printing machines using semi solid wet inks. The printed ink reduces the
light that would otherwise be reflected. That’s why this model is
called subtractive because inks subtract the brightness reflected from a
white background from the colours printed. Some presses even resort to printing in YCM order.
order to understand the implications and the importance of printing in a
specific sequence in both these processes, and the possible impacts of interchanging the sequence of colour printing, we need to understand the basics involved in the processes of both in brief.
Conventional printing process:-
In conventional printing process the first stage is to convert the multi
tonal originals into several several dots on printable plates or blocks to reproduce
multi colour original effect on print. Once separate plates are prepared with the design elements broken into several thousand dots of varying sizes, they are printed over the surface one after the other with primary colours. At least four basic colour
separated plates/ blocks are required to print colors like cyan, magenta, yellow and black colours to match the
coloured original may comprise of many shades and the shades too in
different hues and tones. For example the shades may be bluish green,
darkish green, pinkish, pure blue, pure yellow or pure Red while several
other shades have merged with them. Let them be any shade or colour,
always remember that they fall within the range of Rainbow spectrum of
colours only. Thus the effect of any shade is as closely as possible
archived on print by printing them with three to four
primary colour inks. This means the primary colours produce secondary
and tertiary colours by different tonal values of the primary colours.
Say yellow plus blue gives green. Blue plus Red gives Violet. Red plus
yellow gives orange and yellow plus Red plus Blue may give black etc.
The plates for printing are prepared with a set of four colour separated
halftone negatives/positives using colour filters and
halftone screens to print yellow, cyan, magenta and black colours on
paper to render the true colour image as per original copy. Once the colour separated halftone negatives/positives are prepared, so
many plates and blocks are prepared and each plate or block is printed
with four basic coloured inks such as cyan, magenta, yellow and black,
known as CMYK. While registering the images on the substrate on which
they are printed the dots in each plate sit in an angle of 15 to 30
degree from each which in technical term is called juxtaposition of
dots and gives the desired colour effect as close as possible on print.
Unless halftone screens are used, the continuous tone shades in the
photos cannot be converted into printable form. And unless the screens
are positioned in a particular screen angle with a difference of 30° for
each colour separated negative or positive, the juxtaposition of dots
i.e. printed side by side cannot be achieved and if all dots fall one
upon other, it will then become a single colour dark image.
This is the basic structure of processing in the conventional printing process, whether it is Letterpress, Offset or Gravure. What will happen if the sequence of printing is changed from the accepted norms of CMYK into CYMK or YCMK ?
Not much difference in the appearance of the ultimate print will result in. Even if there is small change, it can still be brought to the original shade by slight adjustment of the ink shades since the dots fall in juxtaposition leaving the other elements fall in their own position. See the following illustrations to understand the effect of following and deviating from the sequence of printing.
Security printing process:-
The fundamental deviation is in the basic processing compared to the conventional printing. Security printing
technique is devoid of colour separated, halftone screened negatives
or positives to reproduce the original colour print. Further the primary
colour inks like Yellow, Magenta and Cyan are also not used to print
juxtaposition of dots to reproduce the coloured original. No screens or
colour filters are used for colour separation. In short, basically the
Security printing is reproduction of the coloured original in true form
incorporating lines and other patterns of varying sizes and varying thicknesses
on the plates or blocks sans juxtaposition of dots on print.
photographs or portraits are first converted into elements consisting
of minute dots and lines of varying thicknesses, wavy patterns,
Guilloche designs and several types of straight lines and with micro
letters in between.
The artist converts the continuous tone photographic image or portraits
into various tonal values with lines, dots and other patterns by hand
engraving them on metal blocks which is used as base for further
processing. The separately engraved blocks with shades are finally
joined together to form the printable image as close as possible to the
How are the colour effects achieved on the reproduced prints then ?
The various colour effects are not achieved by super imposing the images
one on top of the other using conventional shades like Yellow, Blue and
Red. The relief images from Dry Offset plates or images from plane
plates of Wet Offset are printed in subdued colour combinations through a
process called Rainbow colour printing in which the colours merge in a
varying, uneven manner creating a spectrum of colour effect.
Security printing different colour shades appear wherever the
design elements cross over in Rainbow printing or the design elements in single colour in the same plate have been merged in different angles to show different hue. The colours used are
also subdued to ensure that they are not easily copyable a scanner or in colour copiers.
is the basic structure of processing in the security printing
process. What will
happen if the sequence of printing is changed from the approved proofs already taken in a particular sequence of printing similar to the accepted norms of
printing CMYK into CYMK or YCMK in conventional process?
Certainly the appearance of the ultimate
print will show a different result and will not match with the approved proof. Reason ? There are no fixed tertiary or secondary colours emerging from the combination to cause different hues within CMYK. The colour effects will be much different as the inks are of semi translucent in nature.
The most important aspect is that the different hues appearing all over the body of the print are all independent shades or colours, some of which are so subdued that there is no scope to show a third colour glaringly. The wavy lines and straight lines which are positioned in different angles in the design itself gives different hues to naked eye even though the shades may be same. The lines are not in juxtaposition and instead fall on one another crisscrossing in rainbow of independent shades. The shades of the lines that fall on top of the base design elements dominate in visual appearance. Thus altering the sequence of printing after the proof is approved with certain sequence of rainbow colours matching to the image will show different hue on the print. the See the following illustrations to understand the position.
Print with a approved specific
sequence of Rainbow colour
Appearance of Print by interchanging the
sequence of Rainbow colour
Appearance of Print by
sequence of Rainbow colour
Finally there is another important aspect in these two processes of printing. In single colour press in conventional printing the sheets will be printed first in one colour and then after completing printing the entire lot, it will be printed again with the second colour. However on the multi colour printing machines, the sheets will be printed with all the colours, one after the other falling on top of each in succession without waiting for the entire sheets to be printed for taking the print of other colours. The inks being semi wet in that stage and state of body, the two or three coloured inks which fall one on top of the other in juxtaposition are able to partially get mixed with each other to show a distant second or third colour. However in the case of security print designs, the colours being independent, dull and light shaded ones, more in no instead of four standard colours, even if the design elements fall on each other they do not merge and cause a third or fourth colour element. You can still take practical trial and see the difference in both the processes.
All the images and design elements reproduced
above are only illustrative models to explain
the concept, and as generated in computer
by the author himself and are not the actual
prints from any press.