Several design aspects can be incorporated in the bank notes, which cannot be easily copied. A number of options are available to the Designers. Now Computer Graphics are capable to produce a variety of complex designs, which are difficult to be produced mechanically. The designs can be further secured by using Intaglio in combination with offset and screen-printing. There are a number of elements, which can be introduced into the design without incurring any extra cost. These design elements are found to be very effective against colour copying. The special design background or intaglio print can introduce a variety of security features.
Computer Graphics: The graphics with 0.08 mm line width with a separation of 0.3 mm as ruling distance will be difficult to be reproduced exactly in analog type of colour copying. In digital computers, 0.06 mm line width will be difficult for reproduction. The colour copiers have a preferential angle or direction. The computer graphics generate intricate designs with the combination of different colours and variations in the line thickness most effectively. They use a laser plotter. Changing the parameters incrementally creates the guilloche patterns. They describe a simple harmonic wave function, which have been imposed on a number of circles with increasing radii. To make the guilloche patterns much more difficult and complicate, one image can be melded with other. They can be interrupted at any point or isolated. With this technique, a star shape can be melded to a circular wave pattern or a circular wave to a larger polygon. This technique is called as "Melding". The fineness achieved by computer graphics is difficult to be reproduced by colour copiers.
Multi-directional lines: The watermark area when carries fine multi-directional lines, becomes difficult to be reproduced as the lines cannot appear perfectly in the copies. The colour copiers have a preferential angle or direction. Lines, which are perpendicular to the movement, will be easily distinguished while lines, which are slanted at an angle different from 90 degrees, will produce a deformation.
See-through register: The simultaneous printing of front and back of a bank note offset design in a single pass has been recognized as a security feature, since, all the colour copiers are not provided with front and back reproduction simultaneously.
Micro lettering: The note designs are filled with micro lettering normally near the portrait area. They are seen as a part of the design with the naked eye but when viewed under a magnifying glass, the micro lettering can be seen clearly. The copies cannot bring the clarity in this area and have a moiré effect. Micro letters are thought of as one of the preventive measures so as not to reproduce micro letters of height fewer than 400 microns with colour copiers.
Bleed off: The notes, which have a bleed off - that is an extension of the design to the edges of the note without leaving margin, will be difficult to be reproduced in the Copiers as they leave white border. The notes need further operation of trimming, which may not be accurate.
Intaglio: Intaglio printing provides durability to the notes with a three-dimensional effect. The machines, which give the raised effect on the designs, are unique and are not available easily to the commercial printers. The three-dimensional raised effect is not possible to get in the colour copying.
Intaglio process is chosen for bank notes because it has peerless quality of resolution, tonality and pictorial strength. Its tactility is fully exploited when brought together with the visible and invisible chromatic abilities. Intaglio inks with unique print characteristics are developed. There are intaglio ink pairs with characteristics of transparency and absorption in the infrared. Magnetic substances can also be included in intaglio and letterpress inks. However, they produce relatively strong body colours limiting the colour space to the designers as they contain iron oxide. Now soft magnetic pigments are used in intaglio inks to overcome this difficulty. Intaglio process can also seal a foil a full-face intaglio over print.
Blind Intaglio: Some countries have started blind intaglio embossing without inking on the bank notes. This blind intaglio on the offset backgrounds carries the same effect as intaglio printing. This is called as PEAK technology. This printing is done with several combinations and sequence with offset, intaglio and screen-printing. PEAK (Printed and Embossed Anticopy Key) - Consists of a background of printed lines overlaid with a blind – embossed structure produced by the intaglio process. This results in latent image.
Latent image: The system of latent image was developed to defeat the purpose of photo colour copiers. The image is not visible in the normal course but when the note is tilted at an angle, the image becomes visible.
The latent image can be incorporated only in the Intaglio design and is considered as a satisfactory security feature. M/s. Bradbury Wilkinson was the pioneer in latent image technology.
There are two opinions about retention of the latent image. M/s. Giori feels that the latent image will not be visible when the notes get soiled and hence does not provide any security feature in the long run, whereas the majority of others feel that the latent image can be seen even after the notes soiled. The latent image is obtained in Intaglio printing. The design variations in the depth of intaglio engravings on different plates reflect a latent image on printing, which can be seen only at an angle. This provides a durable image, which changes with angle. It fully integrates with the conventional bank note printing. Average particle size of colour tones is approximately 10 microns. The digitizer analyses a discrete number of dots per unit of scanned surface. When the digitizer finds two dots or lines separated less than resolution, it produces one dot or line instead of two. An adequately arranged design with rough and fine ruling having 10-15 % of dot area will act as an effective latent image.
OMRON: Arrival of colour copiers into the market, threatened the bank note printing industry. The industry and the Interpol together requested the manufacturers of colour copiers to find out ways and means by which the colour copier identifies the bank note duplication and rejects or voids it or leaves a mark on the duplicated notes.
A software company – OMRON from Japan have come out with a solution. Accordingly, the bank notes carry a common mark in offset design that enables an optional recognition of security documents by colour copiers. The mark area consists of a common mark, non-mark design element and background. The mark area has certain specifications to ensure the correct recognition of common mark by the Common Mark Recognition system. The common mark has a circle outlining. The design and size of the common mark shall be as per the master film provided by the SSG-2. These common marks have specific size, thickness number (quantity) and placement in the mark area, besides their colour density. The colour copiers are incorporated with a mechanism to recognise these features and classify them as security documents.
