FOUR COLOUR PRINTING PROCESS

Title: FOUR COLOUR PRINTING PROCESS

The present invention refers to printing proc sses in general, and more specifically to a colour print ing .process particularly suited for four colour printing of. images.

As is well known in the art, the conventional four colour process for printing a colour image consists of the superimposing of three subtractive primary colour ed inks (yellow, magenta and.cyan) with a fourth ink (black) utilized as a compensation to provide the out¬ lines of the shapes of the image with greater definition. This printing process by superimposing inks, however, suffers from a number of serious disadvantages which will now be discussed.

A first disadvantage of this conventional print ing process is related to the quantity of ink that has to be used. Since full quantities of inks having all the subtractive primary colours are applied one on top of the other, the amount of ink used is much larger than is really necessary.

A second disadvantage is a consequence of the first. Theoretically, the separation of the primary co- lours for printing is done in such a manner that when the primary colours are combined and printed, the result should be a true reproduction of the original. In prac¬ tice, however, the impurities contained in the inks cause slurring, that is to say, they dull the colours and com¬ promise the fidelity of the reproduction.

A further disadvantage is related to the print¬ ing speed and the fact that each layer of ink of a given colour requires a drying time so as to avoid it from be¬ ing set off, that is to say, transferred to the reverse side of the next sheet of paper. Obviously, the greater the drying time for each layer of ink, the slower the printing speed which means an increase in the time nece¬ ssary to print a series of images.

A large number of solutions have been proposed in the art to overcome the above problems, amongst which the reduction of the amount of ink of each colour. This reduction, however, results in the image having a "washy" look which is not normally well accepted.

Another previously proposed attempt is the so- •called UCR (Under Colour Removal) printing process in which the quantities of the three inks having the sub¬ tractive primary colours are reduced in the neutral and shaded areas, any intensity lost being made up by an in¬ crease in the intensity of the compensating colour (black) . Obviously, in accordance with such a process, the quan¬ tity of ink deposited on the paper is reduced which per¬ mits quicker drying and a consequential reduction in prin ting time .

However, in spite of the above process partial¬ ly solving the problem related to the limitation of prin ting speed, it does not eliminate slurring of the colours due to the accumulation of the inks of the three primary colours together with black in each printed area.

A solution to this problem of slurring would be the total removal of one of the subtractive primary col¬ ours, thus eliminating the acromatic value which defines the greys.

UK Patent N9 1.513.421 discloses a process for forming colour separation records for printing in which, apart from mentioning such removal, it is stated that it cannot be carried out in the lighter areas of the image to be reproduced, and it is suggested to increase the a- mount of primary inks to obtain better saturation.

The purpose of the present invention is to pro¬ vide a printing process which overcomes the problems of the prior art.

A first object of the invention is to provide a four colour printing process which provides substantial economy of the primary colour inks which are more expen¬ sive than black ink.

Another object of the present invention is to provide a four colour printing process in which the pro¬ blem of slurring is eliminated.

A further object of the present invention is to provide a four colour printing process which permits a greater printing speed than is normally obtained.

Yet another object of the present invention is to provide a four colour printing process which provides sharper reproduction apart from greater fidelity in the reproduction of the image.

According to the present invention, these ob¬ jects are obtained by means of a four colour printing process using three subtractive primary colours and black, characterized by the total removal of one of the subtra£ tive primary colours in the colour separation phase, the proportional reduction of the remaining subtractive pri¬ mary colours and the cαπpensation of the primary colours removed by black, whereby no printed area contains more than two primary colours together with black.

The present invention will now be described in greater detail, by way of a non-limiting example, with re¬ ference to the attached drawings in which:

Figure 1 is a graph illustrating the approximate percentages of the primary colours and black in a conventional four colour printing process; Figure 2 is a graph similar to that of figure 1 showing the approximate percentages of the pri¬ mary colours and black in a UCR printing proc¬ ess; and

Figure 3 is a graph similar to that of figures 1 and 2, showing the approximate percentages of the primary colours and black in a printing pro_ cess according to the present invention. Referring now more particularly to the drawings, and as can be seen from Figure 1, in a four colour print ing process the quantities of primary coloured inks used in a given printing area are extremely high when compar¬ ed with the quantity of black used in the same printing area. This is due to the fact that, in the conventional four colour printing process, the subtractive primary c£ lour printing inks are used in maximum amounts with black being used only as a "skeleton", that is to say, only to complement the three full primary colours in the shaded areas.

Referring now to Figure 2, it will be seen that the so-called UCR colour separation four colour process which involves the partial removal of the subtractive pri mary colours and their compensation by black, results in a reduction in the quantities of each subtractive primary colour used, the quantities used of such colour ink a- pproaching that of the black ink. However, this process also uses the basic principle of the conventional four colour printing process, that is to say, the use of black ink only as a skeleton.

In the process of the present invention, print¬ ing is effected "the other way round", that is to say, a full amount of black is used throughout the complete i- mage (light and dark tones) to form the outlining and the shading of the image, whereas the three subtractive pri¬ mary colours appear only in skeleton, that is to say, they enter only to provide the desired colour. Thus, since there is no superimposing of four reticules, the forma¬ tion of the contours of the image may be more precise and sharper, obtaining moreover greater quality and fidelity when compared with the conventional process due to the greater saturation and purity of the colours obtained by the smaller amount of ink and the smaller number of lay¬ ers of ink which results in more reflection of incident light.

As can be seen from Figure 3, there is clearly a substantial reduction in the quantity of each subtract trive primary colour necessary for four colour printing. Apart from eliminating slurring of the inks, this reduc¬ tion enables one to obtain greater printing speeds since the less ink used, the quicker the drying time.

In order for these results to be obtained in a- ccordance with the present invention, during the colour separation phase for printing, effected by electronic se lection in a scanner (or by a photo-mechanical method) , the subtractive primary colour which appears with least intensity in each given printing area (including the light areas) is totally removed, the remaining primary colours being reduced in a proportional amount and black being increased, also proportionally, so as to compensate loss of intensity due to the colour which is removed. As a result of such removal, no printing area contains more than two subtractive primary colours and black.

The process of the present invention is comple¬ mented by a specific colour chart with three colour scales. each representing a different combination of the primary colours (magenta, yellow and cyan) together with black. These scales prove that it is possible during printing to produce all the desired colour matrices with the red zone consisting- of black,, magenta and yellow, the green zone consisting of black, yellow and cyan, and the blue zone consisting of black, cyan and magenta.

Having now described the invention, it should be understood that it may undergo modifications with res¬ pect to the manner of carrying it out provided that such modifications be within the scope of the following claims.