Technical Field

The present invention relates to an inkjet ink set that provides reduced metamerism, a method of printing using the inkjet ink set, and an inkjet printer configured to carry out the invention. Background and Prior Art

In inkjet printing tiny drops of ink fluid are projected directly onto an ink receiver surface without physical contact between the printing device and the ink receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops onto the ink receiver. Printing can be accomplished by moving a print head across the ink receiver or vice versa.

The jetting of the ink droplets can be performed in several different ways. In a first type of process called continuous inkjet printing, the ink stream jetted from an orifice of the print head is broken up, by applying a pressure wave pattern to this orifice, into ink droplets of uniform size and spacing, which can be electrostatically charged or not as desired.

According to a second process the ink droplets can be created by a“drop on demand” method. A drop-on-demand device ejects ink droplets only when they are needed for imaging on the ink receiver, thereby avoiding the complexity of drop charging, deflection hardware and ink collection. In drop-on-demand inkjet printing the ink droplet can be formed by means of a pressure wave created by a mechanical motion of a piezoelectric transducer, or by means of discrete thermal pushes. Colour gamut is an important feature of colour inkjet printing, since it is a measure of the range of colours that can be reproduced using a given combination of colourants. It is desirable for the colour gamut to be as wide as possible so that as many colours as possible can be reproduced by a given set of inks. The colour gamut is controlled primarily by the absorbance characteristics of the set of colourants used to produce the image. Subtractive imaging systems typically employ at least cyan (C), magenta (M) and yellow (Y). It is common for such systems to include an achromatic (neutral density) colorant such as black (K). In many cases these primary colours are supplemented with additional spot colours (e.g. red, green, blue, orange etc) to increase colour gamut and create wider possibilities when choosing colour designs. Thus, inkjet printers are manufactured to accept four or more ink cartridges.

The colour perception of a printed colour image, being a result of interpretation in the human brain, is not necessarily constant and can vary under different lighting or viewing conditions. This phenomenon, known as metamerism, is crucial for an industry where colour is an important aspect of the final product.

There are four types of metamerism: illuminant metamerism, observer metamerism, field size metamerism and geometric metamerism. However all of the different types relate to the same phenomenon that the coloured object does not have colour constancy under all conditions. It is also known that the human eye is more sensitive to metamerism in the

550nm wavelength region, and less sensitive in the 400 and 700nm wavelength regions.

The present invention provides improvements in this area. Detailed Description of the Invention

It has been found that for some inkjet printing applications a narrower but deeper gamut can provide improved coverage of all the ranges of colours required for that application. For example, when printing wood effect colours, it has been found that such colours are well described by reds and yellows, and there is little or no need for cyan or magenta.

It has also been found by the inventors, that metamerism can be reduced when fewer colours are involved in the ink set, as this is believed to reduce the possible sources of colour variance. Furthermore, by printing with only three inks in an ink set the digital inkjet printing process is made less complex and therefore at reduced cost.

Thus, in a first aspect, the invention relates to inkjet ink set consisting of two inks selected from the list (1) consisting of: a cyan ink, a magenta ink, a yellow ink and an achromatic ink; and one ink selected from the list (2) consisting of: a red ink, a green ink and a blue ink, wherein one of the inks from list (1) is an achromatic ink. By“consisting of” is meant that no other inkjet inks are present in the set and the ink set has only three inks.

Thus, provided that the application does not require a wide variety of colours, i.e. a wide gamut, the present invention provides a number of significant advantages in the field of digital inkjet printing.

As is well known in the art, inkjet inks produce colour in the printed image by a so-called subtractive method. This means that the inks absorb a portion of incident light (e.g. a white daylight illuminant) and reflect the incident light with a portion subtracted

(absorbed), to provide the printed image (i.e. reflected light) with a perceivable colour.

A cyan ink absorbs red light, i.e. light in the range of from 620 to 750nm. A magenta ink absorbs green light, i.e. light in the range of from 495 to 570nm. A yellow ink absorbs blue light, i.e. light in the range of from 450 to 495nm.

A red ink absorbs both green and blue light, i.e. light in the range of from 450 to 495nm and from 495 to 570nm. A green ink absorbs both blue and red light, i.e. light in the range of from 450 to 495nm and from 620 to 750nm. A blue ink absorbs both green and red light, i.e. light in the range of from 495 to 570nm and from 620 to 750nm.

An achromatic ink absorbs light from across the entire visible spectrum of from 400 to 700nm. The degree to which the light is absorbed determines whether the achromatic ink is a white-grey (with very little light absorbed) or a black-grey (with a large amount of light absorbed).

Although not wishing to be bound by any particular theory, it is believed that the two inks from list (1) essentially provide the width of the gamut of colours and the ink from list (2) enhances the depth of that gamut.

One of the inks from list (1) is achromatic, preferably black or light black (i.e. grey).

In a preferred embodiment the ink set consists of a yellow ink, an achromatic ink and a red ink. This has been found to provide a gamut of colours which can reproduce a wide variety of wood effects and the red ink provides a depth to the gamut that a CMYK ink set could not provide. As is well known in the technical field, inkjet inks comprise a colourant and a means of delivery.

The colourant may be a pigment or a dye. The choice of this will depend on the circumstances and does not have a large effect on the way in which the invention works. However if the colourant is a dye this has been shown to provide reduced metamerism as compared to a pigment.

If the colourant is a dye the dye can be any of acid, metal complex, reactive, solvent, sulphur, direct, cationic, disperse and vat dyes. If the colourant is a pigment then it may be an organic or inorganic pigment.

To provide the means of delivery the inks can be aqueous based, oil based, solvent based, hot melt or phase change or UV curable, or a combination thereof. It is not believed that that present invention is sensitive to the choice of delivery method.

In a second aspect, the invention relates to a method of inkjet printing, the method comprising printing an image onto a substrate using a digital inkjet printer, wherein the method is carried out by printing ink from only three inks from an ink set as described herein installed in the printer.

The inkjet printer may be based on a drop-on-demand or continuous principle.

As discussed above, the present invention is particularly useful for printing wood effect images with reduced metamerism. The invention can also easily be envisaged to provide the same benefits to other printing applications such as brick images which share a similar gamut to woodgrains.

The present invention is particularly advantageous in printing wall coverings e.g.

wallpaper.

In a third aspect, the invention relates to a digital inkjet printer configured to carry out a method according to the present invention and having an inkjet ink set installed therein, the ink set being as described herein.

The invention will now be illustrated, by way of example, and with reference to the following figures, in which: Figure 1 is a chart showing a comparison between the gamuts of the 5-ink and 3-ink ink sets. Figure 2 is a chart showing a comparison between the gamut of the 3-ink ink set and the range of colours represented in the woodgrain types

Figure 3 shows the difference in perceived colour DE of the woodgrain colour chart as between the illuminants D65 and TL84.

Figure 4 shows the difference in perceived colour DE of the woodgrain colour chart as between the illuminants D65 and A10.

Figure 5 shows the difference in perceived colour DE of the woodgrain colour chart as between the illuminants D65 and CWF.

Examples

In order to determine a representation of the colour gamut of the range of wood effects available, a selection of commercially available wood laminate products were selected and their colours measured. The sample selected represents a wide variety of woodgrain designs (dark oak-North American, European, South American and Asian) comprising white ash, brushed teak, light maple, white oak, butter birch, Adam’s apple, natural varnished French oak, natural varnished oak planks, jungle plum, homage oak grey oiled planks, chocolate oak, grey varnished French oak and wenge (tropical timber).

These were used to prepare a woodgrain colour chart, providing a set of thirteen colours (one for each wood type mentioned above) defined by respective a* and b* values. Ink Sets

Two ink sets were prepared based on aqueous pigment inks. The pigments were carbon black Cl PK 7 (black ink), pigment yellow bisacetoacetarylide Cl PY155 (yellow ink), magenta pigment Quinacridone Cl PR122 (Magenta ink), pigment blue copper phthalocyanine Cl PB 15:4 (cyan ink), and red pigment anthraquinone Cl PR 177 (red ink). The five inks were used to prepare a 5-ink ink set in the traditional CMYK representing the prior art and a 3-ink ink set for the YKR set according to the present invention. Printing

An Epson Stylus™ colour 7700 was used to print the thirteen colours on the woodgrain colour chart. Printing was carried out onto a 100 g/mm2 decor paper.

After printing, the colour batches were measured using colour spectrophotometer i7 equipped with Colour Control professional software. Aperture size of 6 mm was used. Observer degree set at 10°. UV filter is set off. Colour data measured with specular included. Colour data of each patch measured was collected under different illuminants as follows:

1. Daylight D65:“Artificial Daylight" Fluorescent lamps conforming to the

international Standard llluminant D6500 within the tolerances prescribed in BS 950: Part 1.

2. Cool white fluorescent CWF

3. A10- Tungsten Filament lighting required by BS 950: Part 1 as a test for

metamerism. (CIE llluminant 'A').

4. TL84: Philips Triphosphor fluorescent lamps often chosen as a "Point of Sale" lluminant.

This provided a range of a* and b* values for the woodgrain types under the different illuminants for both the 5-ink and the 3-ink ink sets.

Figure 1 shows a comparison between the a* and b* values (representing their gamuts) of the 5-ink and 3-ink ink sets. It can be clearly seen that, as expected, the gamut of the 5-ink ink set covers a wide range of a* and b* values representing hues covering the whole visible spectrum. It can also be seen that the width of the gamut of the 3-ink ink set is narrower as compared to the 5-ink ink set. However the width of the gamut is still approximately one third of that of the 5-ink ink set and therefore still provides a usefully wide gamut. It can also be seen that the gamut for the 3-ink inkset is more focussed in that it can provide a deeper range of colours by extending further outwards from the origin. Thus the 3-ink inkset provides a narrower but more focussed gamut. Figure 2 shows a comparison between the gamut of the 3-ink ink set and the range of colours represented in the woodgrain types. It can be seen that although the gamut of the 3-ink ink set is smaller than a conventional 5-ink ink set the full range of wood colours is covered.

Thus it can be concluded that the full range of wood grain types can be easily

represented by the reduced width gamut of the 3-ink ink set. Moreover a higher quality of reproduction of the woodgrain types may be achieved as shown by the deeper range of colours possible in figure 1. As a still further advantage, the 3-ink ink set provides greatly reduced metamerism as will now be shown.

Metamerism measurement

Metamerism in the printed images was assessed by comparing the measured colour of woodgrain colour chart printed images as measured under artificial daylight D65 as compared to each of the illuminants 2 to 4 mentioned above.

This provided a set of DE values for the difference in perceived colour. A histogram showing the values of DE measured is shown in figures 3 to 5. Figure 3 is for a comparison between D65 and TL84, Figure 4 is a comparison between D65 and A10 and Figure 5 is a comparison between D65 and CWF.

As is known to the skilled person, the human eye can perceive a DE of 1 or more, depending on the colour. Across all colours, it is usual for a DE of greater than 3 to be used as a control limit.

It can be clearly seen that the DE values for the 3-ink ink set are consistently statistically less than those for the 5-ink ink set. This indicates that metamerism in the 3-ink printed wood colours is noticeably less than those printed with the 5-ink ink set. Moreover, the DE values under some illuminants pairings are reduced to values of at most 3 or 4, indicating that metamerism has not only been reduced but that it has been essentially removed entirely.