GB2281249A - Printing on black background. - Google Patents

Abstract

A method of printing a colour image onto a black background comprising the steps of first printing a white, positive, followed by printing three other colour separations (eg yellow, cyan and magenta) individually and separately on top of the white thereby to provide a colour image wherein the black background shows through to provide definition in the image. The white positive is produced from positives of the three colours, black not being used. The process is suitable for replicating the effect of a chalked blackboard using screen printing. <IMAGE>

Description

PRINTING PROCESS The invention relates to a method of printing onto dark backgrounds such as blackboards.

It is known to print images using for example silk screening or lithography where generally these methods are used for printing on white background. However, it is also known to print an image onto a black background but such printing processes involve a large number of steps and are therefore relatively expensive. Additionally, these known processes provide a finished result which does not accurately reproduce the original image for example when the original image was drawn on a blackboard using coloured chalks.

In the known printing process where an original drawing in coloured chalks is made on a black background the following steps are used to print the image onto a blackboard. The image is created on a black flexible card which emulates the blackboard. This is scanned to obtain a four colour separation in a known manner, for example, using a Plessey ITEK four colour scanner. In this way, the four so-called colours of the four colour separation of the image are obtained, these being namely cyan, magenta, yellow and black.

The four colour separation of the known method provides negatives of the image in each of the four colours. By contact printing each of the four negatives, a positive version of each is obtained, i.e. in yellow, cyan, magenta and black. The positives are generally provided on translucent film of some description.

The black image is obtained from the deepest shadow (i.e.

most contrasting) regions of the image and is used in the normal printing process, where the original is printed on a white surface, as the final stage to provide definition to the printed image. This is also true of the known methods of printing onto dark backgrounds such as blackboards. However, since the colour printing inks are translucent they do not appear to have any colour when printed directly onto a black background hence an additional stage needs to be carried out before the image can be printed onto black compared to the more usual printing process.

Where the method of printing is screen printing, then the positives are laid against a prepared screen such as a stretched nylon screen (or mesh) which is treated with certain chemicals sensitive to ultra-violet radiation. By placing each of the positives against separate, prepared screens and exposing the screen to ultra-violet radiation projected through the positive, the exposed sensitized chemical is transformed into a relatively hard non-soluble form. When the screen is washed in water (or other suitable solvents), the original chemical applied to those areas of the screen not exposed to ultra-violet radiation (i.e. due to the shadow of the image on the positive) is washed away leaving a silk screen having regions through which inks can pass. This is known as a stencil.

After preparing all four screens for each of the four colour separations, these can then be applied separately to a white surface in the more usual process wherein cyan, magenta and yellow are printed, followed by the black.

However, when printing onto a black background using the known process, a fifth stencil is used which is produced from a positive of the entire original image including the black. This image is printed as a white background onto the black printing material as a preliminary stage to printing the four colour process just described. Thus none of the black from the background is seen through the printed image in the known art.

That this process is perceived as necessary is due to the fact that each of the three colour inks (i.e. cyan, yellow and magenta) used in usual screen printing methods are transparent and rely on a white background in order to be visible in the image. If they are printed directly onto a black background then they will appear to be substantially black themselves. Therefore, in the known technique, a white background is printed onto the blackboard against which the other four colours are printed. Thus, it is known to provide a five colour printing process in order to print onto blackboards.

The known art therefore provides a lengthy process which is both time consuming and costly. Additionally, the known art does not provide effective reproduction of coloured chalk images drawn on blackboards.

The present invention seeks to avoid or at least mitigate the problems of the prior art. Accordingly, there is provided a method of printing onto a black background such as a blackboard comprising the steps of printing a first white image composed of a positive produced from a contact print of three colour separation negatives, that is, cyan, yellow and magenta colour separations but not the black, followed by printing individually each of the three colour separations onto the first laid white image, thereby allowing black from the background to show through that part of the image onto which the white layer was not printed thereby to provide definition in the image from the blackboard itself.

Other aspects of the invention are set out in the claims.

Beneficially, the invention enables realistic reproduction of chalk images to be made on blackboards in permanent inks such that they do not rub off. Ideally the printed image has the loose feel and texture of a grainy chalk image which is produced by allowing black from the background to show through the image for example.

A preferred method of printing according to the invention will now be described, by way of example only, with reference to the accompanying drawings which are illustrative of the product of the process, in which: FIGURE 1 shows part of a coloured design on a blackboard, reproduced here in black and white only, where the image has been printed on a blackboard using the colour printing process according to the invention to reproduce effectively the original chalk colour image; FIGURE 2 shows an enlarged portion of part of Figure 1; and FIGURE 3 shows an enlargement of another part of the image shown in Figure 1.

In a preferred method according to the invention an image is drawn onto a dark background such as black card using the preferred drawing materials such as chalks for example.

In the case of chalks, the image applied to the medium could be erasable simply by rubbing. However, in order to provide a permanent image on a blackboard, for example, the invention provides a process of applying relatively permanent inks to the black surface which are not erasable simply by rubbing. In this way, blackboards can be permanently decorated for example around their boarders as shown in Figure 1.

The image can be applied to black card which is flexible so that it can be applied to the drum of a colour separation device such as Plessey's ITEK four colour separator. The image is rotated in the known manner at about 1250rpm and scanned thereby to provide electronically stored colour separations of the image. Of the usual four colour separations, only the three colours yellow, cyan, and magenta are used in this process to produce colour negatives or positives of each of these three colours and a white, which can be applied to transparent film using for example an ITEK converter and image setter.

The transparency produced comprises black dots or lines as is known in the art which have a certain size depending on the nature of printing. For example, a dot size of about 50 to 1 square inch (50 to 650 micro m2) can be used here.

The actual density of dots on the positive transparency represents the intensity of a certain colour in the final print. The combination of the three colours provides a complete range of colours.

Whilst in the usual print process pure white in the original is ignored and only cyan, magenta, yellow and black transparencies are produced, here the black is ignored and a white transparency is produced together with the three other colours. The white transparency can be formed for example by a method of contact printing each of the three colour separations (cyan, magenta and yellow) onto one transparency. By using negatives of the three colour separation transparencies (with the black background marked) a white positive can be produced. The white positive can be modified to suit the actual image or design by the print setters. A process of spreading, known in the art, can be used to enhance white regions of the original image. By then reversing the three colour negatives to positives, four positives are provided for the printing stage.

A contact print of each of three colour separation negatives provides a white positive providing dots on the positive which will underlay the portions of each of the cyan, magenta and yellow positive in the final print, thereby enabling reproduction of these colours at the printing stage which is described later. Each of the colours is thus printed at least in part on white dots with which they register at the printing stage and the translucent inks are visible.

The process of spreading to provide the white base for printing the three colours and for providing the white of the original image, can be done at various stages of the process as would be apparent to the skilled man. Firstly, it can be done at the stage of preparing a white positive transparency whether by contact printing or by adjustment of the electronic image handled by the colour scanner and setter. Secondly, it could be done at the stencil preparation stage or thirdly at the inking stage.

When printing by screen printing, which is the preferred form, in the usual way each of the positives can be mounted against a separate chemically treated nylon screen for example. Each screen is exposed to ultraviolet light which passes through the positives thereby transforming the chemically treated areas exposed into a relatively hard non-soluble region. The dots on the positive prevent ultra-violet light being exposed to specific regions of the screen. When the screen is washed using known solvents such as water for example after the ultra-violet light exposure, the unexposed regions become clear since the chemical treatment remains soluble. Thus the nylon mesh is exposed in places and ink is therefore able to pass through the mesh at these regions.Thus for screen printing a stencil is prepared for the white positive and for each of the three colour separations namely cyan, magenta and yellow.

The final part of the process is actual inking at the print stage. First white colour ink is printed on a black substrate such as a blackboard, followed then by printing each of the colours, cyan, magenta and yellow individually in a given sequence.

Thus, there is provided an image in which a certain percentage of black shows through from the background to provide definition to the image analogous to the appearance of chalk on blackboards for example. The regions surrounding each white dot appears substantially black including regions to which colour (i.e. cyan, magenta and yellow) is applied which do not register with a white dot.

Also the black region included in the original image show through from the background. Of course, where a region of high intensity colour is printed the black background is virtually masked to the naked eye but is clearly visible using a magnifying glass in a similar manner to the white background of a normal print. However, where for example in the original the end of a chalk stroke fades away leaving a somewhat grainy and light effect the black background shows through in both the original and in the final printed version according to the invention.

This is in complete contrast to the usual process of printing onto white wherein a fourth black colour separation is added to provided definition in the dark regions of the image. Furthermore, the present process is in contrast with the known method of printing onto black backgrounds wherein a white layer is added over the entire image followed by conducting the normal four colour process including adding a black to provide contrast as described earlier.

Of course, with the latest scanning machinery, it is possible to extract a large range of different colour separations from white through magenta, cyan, yellow to black and to produce either negative or positive transparencies direct from electronic colour scanners and separators such as Plessey's ITEK four colour scanner convertor and setter. Thus, the technique described above can be varied somewhat in the production of the positive as will be apparent to the skilled man.

Claims (5)

1. A method of printing a colour image onto a black background comprising the step of producing a white positive to be used to provide a first white layer in the printing process which white positive comprises a positive of each of the colour separations to be used wherein none of the colour separations are black or white (i.e. yellow, cyan and magenta in a four colour process) and a white positive based on the actual white content of the image, for example, provided by a negative, a black separation of the image, the printing process comprising the steps of first printing the white positive followed by printing the three other colour separations individually and separately on top of the white thereby to provide a colour image wherein the black background shows through to provide definition in the image.

2. A method of printing onto a black background such as a blackboard comprising the steps of printing a first white image composed of a positive produced from a contact print of three colour separation negatives, that is, cyan, yellow and magenta colour separations but not the black, followed by printing individually each of the three colour separations onto the first laid white image, thereby allowing black from the background to show through that part of the image onto which the white layer was not printed thereby to provide definition in the image from the blackboard itself.

3. A method of printing a coloured image onto a black background comprising the steps of first printing a white layer representative of all regions of the image comprising any colour, including white but excluding regions of substantial blackness, printing in sequence three colour layers, such as cyan, magenta and yellow, to form the coloured image wherein regions of the black background provide contast within the image.

4. A blackboard having a coloured image printed thereon using the process according to any preceding claim.

5. Use of the process of claim 1, 2 or 3 to print coloured images on blackboards.