1991-09-12
1993-04-13
Evans, Arthur G.
395112, G06K 1500
Patent
active
052029590
DESCRIPTION:
BRIEF SUMMARY
In printing, colours can be produced in two ways:
(1) By using pigmented inks in a range only limited by the availability of pigments, to produce so called flat or solid colours by a single mix.
(2) By printing a smaller set of these colours using for example only the pigmented colours of cyan, magenta and yellow together with black. This method produces colours by using solid and/or screened values defined in percentage terms of the four inks superimposed one on the other. These are so called process colours.
Conventionally colours can be measured by means of one of various types of colour difference equations to give colorimetric values by a standard code such as that known as CIELAB numbers. This particular classification incorporates a combination of three factors which create a particular visual effect of a colour namely, (A) its hue, (B) pigment saturation level or degree of separation and (L) the degree of lightness which is a function of the ink thickness or density. The CIELAB standard for any particular colour can be measured by a number of known existing scientific methods. CIELAB numbers can be calculated from colours produced by either of the above methods. While such a code, if identical for two colours produced by both methods, would indicate that the colours are the same, there is no simple method of defining both the ink mix for the flat colour and the percentage values for the process colour which would be necessary to achieve these equivalent results.
Moreover it can be shown that although it is possible to start with a process colour and duplicate this as a flat colour by finding a given ink mix to achieve the same CIELAB numbers, the reverse is not true. There is no known method by which it is possible to start from a flat colour produced by the first method and reproduce this exactly as a process colour by any combination of the four colours using screened percentages of any value.
It is the object of this invention to provide a means whereby an adequate reference system of printed process colours can be duplicated by single ink flat colours to the same visual appearance and substantially identical colour difference equation values. Such a system then provides a simple means of producing a range of colours by either method without further calculation. This would enable a printer to create a flat colour which is equivalent to a process colour being used (and vice versa), where colour matching is necessary when printed material is being produced by differing methods.
Accordingly this invention provides a colour referencing system whereby a process colour is first defined in terms of percentage levels of a number of base colours to create a standard reference, an ink mixture is experimentally defined for a flat colour which corresponds in appearance to the process colour and the colour difference equation value is determined by conventional means for that matched colour.
Such a system is only possible if the process colour reference standard can be repeated without error. The illustration of the process colours in relation to their standard reference will ideally be by the system as is defined in our existing European Pat. No. 0119836.
In determining the equivalent flat colour for a particular process colour a spectrophotometer could be used to give an approximate measurement of the process colour and a possible flat colour mix could be calculated by a suitably programmed computer. This might give a number of possible choices of mixes of pigment to make a flat colour. These can be prepared and then compared by eye with the process colour. A skilled operator can then make the necessary small adjustments to the mixes to bring the flat colour as close in appearance to the process colour as it is possible to judge by eye.
An advantage of this method is that the number of pigments required for ink mixing purposes for the process colour method is largely confined to three or four colours. A most important advantage is that both sets of colours can be precisely defined in colorimetric values by
REFERENCES:
patent: 4881182 (1984-11-01), Hank et al.
Journal of the Opt. Soc. of Amer, vol. 68 No. 8 Aug. 1978 "Preparation of the USA.," pp. 1141-1142.
Din 16534, Oct. 1971: "Europaische Farbskala fur den Offset Druck" Deutsche, Normen.
Evans Arthur G.
Gordon Phillips Limited
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