Abstract:
Provided is a pagewidth printer having an elongate bi-lithic printhead unit. The unit includes two integrated circuits laid end-to-end, each having a multitude of micro-electromechanical nozzles for operatively ejecting printing fluid, and an ink distribution plate for feeding printing fluid into apertures defined in the integrated circuits. The unit also includes an ink distribution molding defining a plurality of elongate conduits for distributing printing fluid to the plate along a length of the printhead, and an end plug configured for fitment onto an end of the distribution molding to arrange said conduits in fluid communication with ink reservoirs. The unit further includes a printed circuit board (PCB) configured to relay data signals from a printer controller to respective ends of the integrated circuits.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. patent application Ser. No. 10/913,380 filed on Aug. 9, 2004 all of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present application relates to a method of minimizing absorption of visible light in ink compositions comprising infrared (IR) cyanine-type dyes. It has been developed primarily to avoid undesirable coloration in IR inks and, more particularly, to avoid undesirable coloration of substrates printed or marked with IR inks. 
     CO-PENDING APPLICATIONS 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application: 
                                                 7,278,727   10/913,373   10/913,374   7,367,665   7,138,391       7,153,956   10/913,379   10/913,376   7,122,076                    
The disclosures of these co-pending applications are incorporated herein by cross-reference.
 
     BACKGROUND OF THE INVENTION 
     IR absorbing dyes have numerous applications, such as optical recording systems, thermal writing displays, laser filters, infrared photography, medical applications and printing. Typically, it is desirable for the dyes used in these applications to have strong absorption in the near-IR at the emission wavelengths of semiconductor lasers (e.g. between about 700 and 2000 nm, preferably between about 700 and 1000 nm). In optical recording technology, for example, gallium aluminium arsenide (GaAlAs) and indium phosphide (InP) diode lasers are widely used as light sources. 
     Another important application of IR dyes is in inks, such as printing inks. The storage and retrieval of digital information in printed form is particularly important. A familiar example of this technology is the use of printed, scannable bar codes. Bar codes are typically printed onto tags or labels associated with a particular product and contain information about the product, such as its identity, price etc. Bar codes are usually printed in lines of visible black ink, and detected using visible light from a scanner. The scanner typically comprises an LED or laser (e.g. a HeNe laser, which emits light at 633 nm) light source and a photocell for detecting reflected light. Black dyes suitable for use in barcode inks are described in, for example, WO03/074613. 
     However, in other applications of this technology (e.g. security tagging) it is desirable to have a barcode, or other intelligible marking, printed with an ink that is invisible to the unaided eye, but which can be detected under UV or IR light. 
     An especially important application of detectable invisible ink is in automatic identification systems, and especially “netpage” and “Hyperlabel™” systems. Netpage systems are described in the following patent applications, all of which are incorporated herein by reference. 
     CROSS-REFERENCES 
     Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present application: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
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     The disclosures of all of these co-pending patents/patent applications are incorporated herein by reference. 
     In general, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive web pages. Information is encoded on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen and transmitted to the netpage system. 
     Active buttons and hyperlinks on each page may be clicked with the pen to request information from the network or to signal preferences to a network server. In some forms, text written by hand on a netpage may be automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other forms, signatures recorded on a netpage may be automatically verified, allowing e-commerce transactions to be securely authorized. 
     Netpages are the foundation on which a netpage network is built. They may provide a paper-based user interface to published information and interactive services. 
     A netpage consists of a printed page (or other surface region) invisibly tagged with references to an online description of the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system. 
     Multiple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier. This page ID has sufficient precision to distinguish between a very large number of netpages. 
     Each reference to the page description is encoded in a printed tag. The tag identifies the unique page on which it appears, and thereby indirectly identifies the page description. The tag also identifies its own position on the page. 
     Tags are printed in infrared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter. 
     A tag is sensed by an area image sensor in the netpage pen, and the tag data is transmitted to the netpage system via the nearest netpage printer. The pen is wireless and communicates with the netpage printer via a short-range radio link. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. Tags are error-correctably encoded to make them partially tolerant to surface damage. 
     The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage. 
     Hyperlabel™ is a trade mark of Silverbrook Research Pty Ltd, Australia. In general, Hyperlabel™ systems use an invisible (e.g. infrared) tagging scheme to uniquely identify a product item. This has the significant advantage that it allows the entire surface of a product to be tagged, or a significant portion thereof, without impinging on the graphic design of the product&#39;s packaging or labeling. If the entire surface of a product is tagged (“omnitagged”), then the orientation of the product does not affect its ability to be scanned i.e. a significant part of the line-of-sight disadvantage of visible barcodes is eliminated. Furthermore, if the tags are compact and massively replicated (“omnitags”), then label damage no longer prevents scanning. 
     Thus, hyperlabelling consists of covering a large portion of the surface of a product with optically-readable invisible tags. When the tags utilize reflection or absorption in the infrared spectrum, they are referred to as infrared identification (IRID) tags. Each Hyperlabel™ tag uniquely identifies the product on which it appears. The tag may directly encode the product code of the item, or it may encode a surrogate ID which in turn identifies the product code via a database lookup. Each tag also optionally identifies its own position on the surface of the product item, to provide the downstream consumer benefits of netpage interactivity. 
     Hyperlabels™ are applied during product manufacture and/or packaging using digital printers, preferably inkjet printers. These may be add-on infrared printers, which print the tags after the text and graphics have been printed by other means, or integrated colour and infrared printers which print the tags, text and graphics simultaneously. 
     Hyperlabels™ can be detected using similar technology to barcodes, except using a light source having an appropriate near-IR frequency. The light source may be a laser (e.g. a GaAlAs laser, which emits light at 830 nm) or it may be an LED. 
     From the foregoing, it will be readily apparent that invisible IR detectable inks are an important component of netpage and Hyperlabel™ systems. In order for an IR absorbing ink to function satisfactorily in these systems, it should ideally meet a number of criteria: 
     (i) compatibility with inkjet printers; 
     (ii) compatibility of the IR dye with aqueous solvents used in inkjet inks; 
     (iii) intense absorption in the near infra-red region (e.g. 700 to 1000 nm); 
     (iv) zero or low intensity visible absorption; 
     (v) lightfastness; 
     (vi) thermal stability; 
     (vii) zero or low toxicity; 
     (viii) low-cost manufacture; 
     (ix) adheres well to paper and other media; and 
     (x) no strikethrough and minimal bleeding of the ink on printing. 
     Hence, it would be desirable to develop IR dyes and ink compositions fulfilling at least some and preferably all of the above criteria. Such inks are desirable to complement netpage and Hyperlabel™ systems. 
     Some IR dyes are commercially available from various sources, such as Epolin Products, Avecia Inks and H.W. Sands Corp. 
     In addition, the prior art describes various IR dyes. U.S. Pat. No. 5,460,646, for example, describes an infrared printing ink comprising a colorant, a vehicle and a solvent, wherein the colorant is a silicon (IV) 2,3-naphthalocyanine bis-trialkylsilyloxide. 
     U.S. Pat. No. 5,282,894 describes a solvent-based printing ink comprising a metal-free phthalocyanine, a complexed phthalocyanine, a metal-free naphthalocyanine, a complexed naphthalocyanine, a nickel dithiolene, an aminium compound, a methine compound or an azulenesquaric acid. 
     However, none of the prior art dyes can be formulated into ink compositions suitable for use in netpage or Hyperlabel™ systems. In particular, commercially available and/or prior art inks suffer from one or more of the following problems: absorption at wavelengths unsuitable for detection by near-IR sensors; poor solubility or dispersibility in aqueous solvent systems; or unacceptably high absorption in the visible part of the spectrum. 
     In a typical netpage, there may be a large number of hyperlinks on one page and correspondingly relatively large areas of the page printed with IR ink. In the Hyperlabel™ system, the majority of a product&#39;s packaging may be printed with the invisible ink. Thus, it is especially desirable that the ink used is invisible to the unaided eye and contains minimal residual colour. 
     Moreover, inkjet printing is the preferred means for generating netpages and Hyperlabels™. Inkjet printing is preferred primarily for its high-speed and low cost. Inkjet inks are typically water-based for reasons of low cost, low toxicity and low flammability. In thermal bubble-jet printers, the ink needs to be rapidly vaporized during the printing process. This rapid vaporization of the ink during the printing process necessitates a water-based ink composition. Accordingly, it is desirable that the IR dyes used in netpage and Hyperlabel™ inks are suitable for formulating into aqueous ink compositions and are compatible with inkjet printers. 
     A further essential requirement of IR dyes used in netpage systems is that they must absorb IR radiation at a frequency complementary to the frequency of the IR sensor in the netpage pen. Preferably, the ink should contain a dye, which absorbs strongly at the frequency of the IR sensor. Accordingly, the dyes used in netpage systems should absorb strongly in the near-IR region—that is, 700 to 1000 nm, preferably 750 to 900 nm, more preferably 780 to 850 nm. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a method of minimizing absorption of visible light in an ink composition comprising an IR-absorbing macrocyclic cyanine dye, comprising preselecting said dye such that it includes at least one moiety for reducing intermolecular interactions between adjacent dye molecules. 
     In a second aspect, the present invention provides a method of minimizing absorption of visible light in an inkjet ink comprising an IR-absorbing macrocyclic cyanine dye, said method comprising preselecting said dye such that it includes a moiety for reducing intermolecular interactions between adjacent dye molecules. 
     In a third aspect, the present invention provides a method of minimizing visible coloration of a substrate having an IR-absorbing macrocyclic cyanine dye disposed thereon, said method comprising preselecting said dye such that it includes a moiety for reducing intermolecular interactions between adjacent dye molecules. 
     In a fourth aspect, there is provided a method of enabling entry of data into a computer system via a printed form, the form containing human-readable information and machine-readable coded data, the coded data being indicative of an identity of the form and of a plurality of reference points of the form, the method including the steps of: 
     receiving, in the computer system and from a sensing device, indicating data regarding the identity of the form and a position of the sensing device relative to the form, the sensing device, when placed in an operative position relative to the form, generating the indicating data using at least some of the coded data; 
     identifying, in the computer system and from the indicating data, at least one field of the form; and 
     interpreting, in the computer system, at least some of the indicating data as it relates to the at least one field, 
     wherein said coded data comprises an ink composition in which visible absorption is minimized by a method as described above. 
     In a fifth aspect, there is provided a method of enabling entry of data into a computer system via a printed form, the form containing human-readable information and machine-readable coded data, the coded data being indicative of at least one field of the form, the method including the steps of: 
     receiving, in the computer system and from a sensing device, indicating data regarding the at least one field and including movement data regarding movement of the sensing device relative to the form, the sensing device, when moved relative to the form, generating the data regarding said at least one field using at least some of the coded data and generating the data regarding its own movement relative to the form; and 
     interpreting, in the computer system, at least some of said indicating data as it relates to said at least one field, 
     wherein said coded data comprises an ink composition in which visible absorption is minimized by a method as described above. 
     In an sixth aspect, there is provided a method of enabling entry of data into a computer system via a product item, the product item having a printed surface containing human-readable information and machine-readable coded data, the coded data being indicative of an identity of the product item, the method including the steps of: 
     (a) receiving, in the computer system and from a sensing device, indicating data regarding the identity of the product item, the sensing device, when placed in an operative position relative to the product item, generating the indicating data using at least some of the coded data; and
 
(b) recording, in the computer system and using the indicating data, information relating to the product item,
 
wherein said coded data comprises an ink composition in which visible absorption is minimized by a method as described above.
 
     In a seventh aspect, there is provided a method of enabling retrieval of data from a computer system via a product item, the product item having a printed surface containing human-readable information and machine-readable coded data, the coded data being indicative of an identity of the product item, the method including the steps of: 
     (a) receiving, in the computer system and from a sensing device, indicating data regarding the identity of the product item, the sensing device, when placed in an operative position relative to the product item, generating the indicating data using at least some of the coded data;
 
(b) retrieving, in the computer system and using the indicating data, information relating to the product item; and
 
(c) outputting, from the computer system and to an output device, the information relating to the product item, the output device selected from the group comprising a display device and a printing device,
 
wherein said coded data comprises an ink composition in which visible absorption is minimized by a method as described above.
 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic of a the relationship between a sample printed netpage and its online page description; 
         FIG. 2  is a schematic view of a interaction between a netpage pen, a Web terminal, a netpage printer, a netpage relay, a netpage page server, and a netpage application server, and a Web server; 
         FIG. 3  illustrates a collection of netpage servers, Web terminals, printers and relays interconnected via a network; 
         FIG. 4  is a schematic view of a high-level structure of a printed netpage and its online page description; 
         FIG. 5   a  is a plan view showing the interleaving and rotation of the symbols of four codewords of the tag; 
         FIG. 5   b  is a plan view showing a macrodot layout for the tag shown in  FIG. 5   a;    
         FIG. 5   c  is a plan view showing an arrangement of nine of the tags shown in  FIGS. 5   a  and  5   b , in which targets are shared between adjacent tags; 
         FIG. 5   d  is a plan view showing a relationship between a set of the tags shown in  FIG. 5   a  and a field of view of a netpage sensing device in the form of a netpage pen; 
         FIG. 6  is a perspective view of a netpage pen and its associated tag-sensing field-of-view cone; 
         FIG. 7  is a perspective exploded view of the netpage pen shown in  FIG. 6 ; 
         FIG. 8  is a schematic block diagram of a pen controller for the netpage pen shown in  FIGS. 6 and 7 ; 
         FIG. 9  is a perspective view of a wall-mounted netpage printer; 
         FIG. 10  is a section through the length of the netpage printer of  FIG. 9 ; 
         FIG. 10   a  is an enlarged portion of  FIG. 10  showing a section of the duplexed print engines and glue wheel assembly; 
         FIG. 11  is a detailed view of the ink cartridge, ink, air and glue paths, and print engines of the netpage printer of  FIGS. 9 and 10 ; 
         FIG. 12  is an exploded view of an ink cartridge; 
         FIG. 13  is a schematic view of the structure of an item ID; 
         FIG. 14  is a schematic view of the structure of an omnitag; 
         FIG. 15  is a schematic view of a pen class diagram; 
         FIG. 16  is a schematic view of the interaction between a product item, a fixed product scanner, a hand-held product scanner, a scanner relay, a product server, and a product application server; 
         FIG. 17  is a perspective view of a bi-lithic printhead; 
         FIG. 18  an exploded perspective view of the bi-lithic printhead of  FIG. 17 ; 
         FIG. 19  is a sectional view through one end of the bi-lithic printhead of  FIG. 17 ; 
         FIG. 20  is a longitudinal sectional view through the bi-lithic printhead of  FIG. 17 ; 
         FIGS. 21(   a ) to  21 ( d ) show a side elevation, plan view, opposite side elevation and reverse plan view, respectively, of the bi-lithic printhead of  FIG. 17 ; 
         FIGS. 22(   a ) to  22 ( c ) show the basic operational principles of a thermal bend actuator; 
         FIG. 23  shows a three dimensional view of a single ink jet nozzle arrangement constructed in accordance with  FIG. 22 ; 
         FIG. 24  shows an array of the nozzle arrangements shown in  FIG. 23 ; 
         FIG. 25  is a schematic cross-sectional view through an ink chamber of a unit cell of a bubble forming heater element actuator; 
         FIG. 26  shows an absorption spectrum of a dye prepared according to Example 1. 
         FIG. 27  shows an absorption spectrum of a dye prepared according to Example 2. 
         FIG. 28  shows an absorption spectrum of a dye prepared according to Example 3. 
         FIG. 29  ( 1   a - d ) show reflectance spectra of ink solutions comprising sulfonated Vanadyl octabutoxyphthalocyanine on plain paper (80 gsm). 
     
    
    
     DETAILED DESCRIPTION 
     IR-Absorbing Dye 
     As used herein, the term “IR-absorbing dye” means a dye substance, which absorbs infrared radiation and which is therefore suitable for detection by an infrared sensor. Preferably, the IR-absorbing dye absorbs in the near infrared region, and preferably has a λ max  in the range of 700 to 1000 nm, more preferably 750 to 900 nm, more preferably 780 to 850 nm. Dyes having a λ max  in this range are particularly suitable for detection by semiconductor lasers, such as a gallium aluminium arsenide diode laser. 
     It has been recognized by the present inventors that IR-absorbing dye compounds of the prior art absorb, at least to some extent, in the visible region of the spectrum. Indeed, the vast majority of IR-absorbing dye compounds known in the prior art are black. This visible absorption is clearly undesirable in “invisible” IR inks, especially IR inks for use in netpage or Hyperlabel™ systems. 
     It has further been recognized by the present inventors that the presence of visible bands in the IR spectra of IR-absorbing dye compounds, and particularly IR-absorbing metal-ligand complexes, is mainly due to intermolecular interactions between adjacent molecules. 
     Typically, IR-absorbing compounds comprise a π-system which forms a substantially planar moiety in at least part of the molecule. There is a natural tendency for planar π-systems in adjacent molecules to stack on top of each other via intermolecular π-interactions, known as π-π stacking. Hence, IR-absorbing compounds have a natural tendency to group together via intermolecular π-interactions, producing relatively weakly bound dimers, trimers etc. Without wishing to be bound by theory, it is understood by the present inventors that π-π stacking of IR-absorbing compounds contributes significantly to the production of visible absorption bands in their IR spectra, which would not otherwise be present in the corresponding monomeric compounds. This visible absorption is understood to be due to broadening of IR absorption bands when π-systems stack on top of each other and π-orbitals interact, producing small changes in their respective energy levels. Broadening of IR absorption bands is undesirable in two respects: firstly, it reduces the intensity of absorption in the IR region; secondly, the IR absorption band tends to tail into the visible region, producing highly coloured compounds. 
     Furthermore, the formation of coloured dimers, trimers etc. via π-π interactions occurs both in the solid state and in solution. However, it is a particular problem in the solid state, where there are no solvent molecules to disrupt the formation of extended π-stacked oligomers. IR dyes having acceptable solution characteristics may still be intensely coloured solids when printed onto paper. The ideal “invisible” IR dye should remain invisible when the solvent has evaporated or wicked into the paper. 
     Additionally, the interaction of π-orbitals with local charges or partially charged atoms, such as ions, can be large and this may introduce additional absorption in the visible region. 
     None of the prior art addresses the problem visible absorption in IR inks by designing and synthesizing dye molecules specifically adapted to reduce intermolecular interactions in the form of π-π stacking. 
     Specific examples of moieties suitable for reducing intermolecular interactions are described in more detail below. However, it will be appreciated from the above that any moiety or group that can interfere sufficiently with the intermolecular π-π interactions of adjacent dye molecules will be suitable for minimizing visible absorption, and will therefore be suitable for use in the present invention. 
     Preferably, the moiety configured to reduce intermolecular interactions reduces these interactions by a steric repulsive effect. Hence, by providing a dye molecule having suitably positioned sterically repulsive group(s), it is possible to increase the distance between potentially interacting π-systems, thereby minimizing π-π stacking. 
     Preferably, the moiety configured to reduce intermolecular interactions is positioned at the periphery of the dye molecule, or at least at the periphery of the substantially planar π-system. Typically, intermolecular π-interactions result from overlapping planes of π-systems. By positioning the moiety at the periphery of the dye molecule, the moiety has a maximum effect in reducing the degree of overlap. 
     Generally, it is preferable to configure the dye molecule such that the average distance between the π-systems of adjacent molecules is greater than about 3.5 Å, more preferably greater than about 4 Å, and more preferably greater than about 5 Å. This preferred distance between the π-systems of adjacent molecules is based on theoretical calculations. From theoretical studies by the present inventors, it is understood that π-π interactions are significant at a distance of 3.5 Å or less. 
     Preferably, the moiety for reducing intermolecular interactions extends out of the plane of the substantially planar π-system. Macrocyclic cyanines, for example, typically comprise a π-system which forms a major plane of the molecule. This major plane is usually comprised of conjugated heteroaromatic and/or aromatic rings. The moiety is preferably configured, or can at least fold into a conformation, such that it extends out of this plane and exert steric repulsion on neighbouring dye molecules. The greater the moiety extends out of the plane of the π-system, the greater the reduction in intermolecular interactions will be. 
     Preferably, the moiety for reducing intermolecular interactions has three-dimensional structure. By “three-dimensional structure”, it is meant that the moiety occupies a volume of three-dimensional space in all conformations. Preferably, the moiety is a three-dimensional C 3-30  hydrocarbyl or C 3-30  hydrocarbylene group. 
     As will be apparent to the person skilled in the art, the exact nature of the three-dimensional hydrocarbyl or hydrocarbylene group is not crucial to the present invention, provided that the group has sufficient three-dimensional structure to inhibit intermolecular interactions. However, preferred moieties suitable for reducing intermolecular interactions are C 3-30  bridged cyclic groups. As mentioned above, such groups are preferably positioned at the periphery of the π-system to maximize their overlap-inhibiting effect. 
     As an alternative (or in addition to) the dye molecule comprising a three-dimensional hydrocarbyl or hydrocarbylene group, such as a bridged cyclic group, it may comprise one or more polymeric substituents for reducing intermolecular interactions. In the context of the present invention, the term “polymeric” is used to describe a group having 2 or more repeating monomer units. For example, the polymeric substituent for reducing intermolecular interactions may comprise from 2 to 5000 repeating monomer units, more preferably 2 to 1000, more preferably 2 to 100, and more preferably 2 to 50. 
     Without wishing to be bound by theory, it is understood by the present inventors that polymeric substituents interfere with π-π interactions by folding into conformations where at least part of the polymer is positioned between π-systems of adjacent dye molecules. This inhibits intermolecular π-π interactions and hence polymeric substituents can reduce the propensity for π-systems to overlap and interact. 
     It will be readily apparent that the exact nature of the polymeric substituent(s) is not crucial provided that it is able to provide steric repulsion. Accordingly, the substituent may comprise any type of polymer, such as polyethers, polyesters, polyamides, polyurethanes, polyalkenes etc. 
     The polymeric group may comprise a plurality of polymer chains in the form of a dendrimer—that is, a central core or template having a plurality of polymer chains radiating therefrom. The branched nature of dendrimer molecules means that their polymeric chains are able to occupy a large volume in three-dimensional space. This large three-dimensional volume is advantageous for increasing the steric repulsion between respective dye molecules and, hence, reducing intermolecular interactions. 
     In addition to providing steric repulsion, the polymeric substituent may confer additional properties on the dye molecule. For example, with appropriate selection of the polymeric substituent, it may be used to impart hydrophilic properties on the molecule. Polymeric substituents comprising repeating units of ethylene glycol (a PEG chain) are particularly suitable for providing hydrophilicity, as well as reducing intermolecular π-interactions. 
     Preferably, the dye molecule comprises a hydrophilic group. A hydrophilic group is preferred for imparting water-dispersibility or water-solubility on the dye molecule. The dye molecules of the present invention are intended for use in inkjet ink compositions, preferably aqueous inkjet ink compositions. Hence, the provision of a hydrophilic group is one means for allowing the dye molecules of the present invention to be dispersed in an aqueous inkjet ink composition. 
     Preferably, the hydrophilic group is selected from a hydrophilic polymer chain; an ammonium group; an acid group including salts thereof; or a sulfonamide group. 
     An example of a hydrophilic polymeric chain is a PEG chain, which may comprise from 2 to 5000 repeating units of ethylene glycol. Other hydrophilic polymer chains will be readily apparent to the person skilled in the art. The hydrophilic polymer chain may be a substituent (or part of a substituent) on the dye molecule. 
     An ammonium group may be present as a substituent comprising a group of general formula —N + (R a )(R b )(R c ) or —U, wherein R a , R b , R c  may be the same or different and are independently selected from H, C 1-8  alkyl (e.g. methyl, ethyl, cyclohexyl, cyclopentyl, tert-butyl, iso-propyl etc.), C 6-12  arylalkyl (e.g. benzyl, phenylethyl etc.) or C 6-12  aryl (e.g. phenyl, naphthyl etc.); and U is pyridinium, imidazolinium or pyrrolinium. Alternatively, the ammonium group may be present in the form of a quaternarized N atom in the dye molecule. For example, a heteroaromatic N atom in the dye molecule may be quaternarized with a C 1-8  alkyl or a C 6-12  arylalkyl group, in accordance with known procedures. 
     An acid group (including salts thereof) may be of formula —CO 2 Z, —SO 3 Z, —OSO 3 Z, —PO 3 Z 2  or —OPO 3 Z 2 , wherein Z is H or a water-soluble cation. Preferably, Z is selected from Li + , Na + , K +  or NH 4   + . Methods of introducing acid groups, such as those described above, will be well known to the person skilled in the art. For example, a carboxylic acid group may be introduced by oxidation of an olefinic or hydroxyl group in the dye molecule. Alternatively, a sulfonic acid group (—SO 3 H) may be introduced to an aromatic moiety in the dye molecule by sulfonation using, for example, oleum or chlorosulfonic acid. Conversion of the acid group to its salt form can be effected using, for example, a metal hydroxide reagent (e.g. LiOH, NaOH or KOH) or a metal bicarbonate (e.g. NaHCO 3 ). Non-metal salts may also be prepared using, for example, ammonium hydroxides (e.g. Bu 4 NOH, NH 4 OH etc.). 
     A sulfonamide group may be present as a substituent comprising a group of general formula —SO 2 NR p R q , wherein R p  and R q  are independently selected from H, C 1-8  alkyl (e.g. methyl, ethyl, cyclohexyl, cyclopentyl, tert-butyl, iso-propyl etc.), —(CH 2 CH 2 O) e R e  (wherein e is an integer from 2 to 5000 and R e  is H, C 1-8  alkyl or C(O)C 1-8  alkyl), C 6-12  arylalkyl (e.g. benzyl, phenylethyl etc.) or C 6-12  aryl (e.g. phenyl, methoxyphenyl etc.). 
     The problem of absorption in the visible part of the spectrum is a particular problem in IR-absorbing metal-cyanine dyes. Metal-cyanine dye molecules are known in the art (e.g. metal phthalocyanines and metal naphthalocyanines). Hence, the present invention, in its preferred form, reduces intermolecular interactions in IR-absorbing metal-cyanine dyes. 
     As used herein, the term “metal” includes any metal or semimetal (such as Si or Ge) capable of forming a metal-ligand complex. Some examples of such metals are Si, Ge, Ga, Mg, Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Sn, Pb, Zr, Pd and Pt. The metal is preferably Si, Ge, Al, Mn Ti, V, Zn or Sn. 
     The metal-ligand complex may have any ligand coordination structure, such as a tetra-coordinate (e.g. square planar), penta-coordinate (e.g. square pyramidal) or hexa-coordinate (e.g. octahedral) structure. The structure of the metal-ligand complex will depend on the nature of the ligand and the metal, as well as the oxidation state of the metal. 
     Preferably, the metal-cyanine complex comprises at least one multidentate ligand. By “multidentate ligand”, it is meant a ligand having a plurality of coordinating atoms. 
     Multidentate ligands are preferred since their complexes with metals are usually more thermodynamically stable than their monodentate counterparts. Moreover, multidentate ligands can form an extended IR-absorbing π-system with the central metal atom of the metal-ligand complex via its coordinating heteroatoms and π-bond(s) in the ligand conjugated with the coordinating heteroatoms. 
     In certain embodiments of the present invention, the dye molecule is a metal-ligand complex having an equatorial tetradentate macrocyclic cyanine ligand. Especially in such cases, vacant axial position(s) of the complex may be used to introduce further functional groups into the metal-ligand complex. Accordingly, in some forms of the present invention, the metal-ligand dye is preselected such that at least one axial ligand comprises a group for reducing intermolecular interactions. The axial ligand may comprise, for example, one or more polymeric groups. As already described above, polymeric groups can exist in conformations which reduce the propensity for π-systems of adjacent dye molecules to interact. Hence, polymeric axial ligands can further assist in reducing intermolecular interactions. 
     In order for the axial ligand to have maximum steric repulsion, it may comprise a Preferably, the axial ligand adopts a conformation (or is configured) such that it effectively “protects” or blocks a π-face of the dye molecule. An axial ligand, which can form an “umbrella” over the π-system and reduce intermolecular interactions between adjacent dye molecules is particularly suitable for use in the present invention. 
     In order for the axial ligand to have maximum steric repulsion, it may comprise a plurality of chains, such as polymer chains, in the form of a dendrimer—that is, a central core or template having a plurality of chains radiating therefrom. Dendrimers will be well known to the skilled person and are described in more detail below. 
     Notwithstanding the advantageous use of axial ligand(s) to reduce intermolecular interactions, the axial ligand(s) may also provide the dye molecule with hydrophilicity. For example, if the axial ligand(s) comprise one or more hydrophilic groups, they will provide the dye molecule with water-dispersibility. As mentioned above, water dispersibility is advantageous, since the dye molecules may be used in aqueous inkjet ink compositions. Examples of hydrophilic groups are PEG chains, ammonium groups and acid groups (including salts thereof). Accordingly, an axial ligand comprising a dendrimer with hydrophilic group(s) will impart the dual properties of (1) reducing intermolecular interactions, and (2) increasing water-dispersibility of the dye molecule. 
     In a preferred method of the present invention, the dye is preselected from a complex of formula (I) or a compound of formula (II): 
                                
wherein
 
Q 1 , Q 2 , Q 3  and Q 4  are the same or different and are independently selected from a C 3-20  arylene group or a C 3-20  heteroarylene group, said C 3-20  arylene or
 
C 3-20  heteroarylene group including at least one group suitable for reducing intermolecular interactions;
 
M is selected from Si(A 1 )(A 2 ), Ge(A 1 )(A 2 ), Ga(A 1 ), Mg, Al(A 1 ), TiO, Ti(A 1 )(A 2 ), ZrO, Zr(A 1 )(A 2 ), VO, V(A 1 )(A 2 ), Mn, Mn(A 1 ), Fe, Fe(A 1 ), Co, Ni, Cu, Zn, Sn, Sn(A 1 )(A 2 ), Pb, Pb(A 1 )(A 2 ), Pd and Pt;
 
A 1  and A 2  are axial ligands, which may be the same or different, and are selected from —OH, halogen, —OR 3 , a hydrophilic ligand and/or a ligand suitable for reducing intermolecular interactions;
 
R 3  is a selected from C 1-12  alkyl, C 5-12  aryl, C 5-12  arylalkyl or Si(R x )(R y )(R z ); and
 
R x , R y  and R z  may be the same or different and are selected from C 1-12  alkyl, C 5-12  aryl, C 5-12  arylalkyl, C 1-12  alkoxy, C 5-12  aryloxy or C 5-12  arylalkoxy.
 
     Q 1 , Q 2 , Q 3  and Q 4  may be the same or different from each other. Generally, macrocyclic cyanine dyes are symmetrical structures synthesized by a cascaded coupling of vicinal cyano groups to form a macrocyclic ring. For example, the dye of formula (I) above may be prepared by a cascaded base-catalysed coupling of four dicyano compounds of general formula (1) or four imidine compounds of formula (2): 
     
       
                 
         
             
             
         
      
     
     The cascaded base-catalysed reaction may be facilitated by metal templating, or it may proceed in the absence of a metal. Accordingly, by the nature of this preferred synthesis of macrocyclic cyanine compounds, the groups represented as Q 1 , Q 2 , Q 3  and Q 4  will usually be the same or at least have the same core structural units. However, in cases where the compound is functionalized after macrocycle formation, the groups represented by Q 1 , Q 2 , Q 3  and Q 4  may be different. For example, functionalization of aromatic moieties subsequent to macrocycle formation may not occur entirely symmetrically, in which case Q 1 , Q 2 , Q 3  and Q 4  may be different from each other by virtue of different numbers of substituents. 
     Preferably, the groups represented as Q 1 , Q 2 , Q 3  and Q 4  are selected from an arylene or heteroarylene group of formula (i) to (vii) below: 
                                
wherein:
 
R 1  and R 2  may be the same or different and are selected from hydrogen, hydroxyl, C 1-12  alkyl, C 1-12  alkyl bearing a hydrophilic or hydrophilizable group, C 1-12  alkoxy, C 1-12  alkoxy bearing a hydrophilic or hydrophilizable group, amino, C 1-12  alkylamino, di(C 1-12  alkyl)amino, halogen, cyano, thiol, C 1-12  alkylthio, nitro, carboxy, C 1-12  alkylcarbonyl, C 1-12  alkoxycarbonyl, C 1-12  alkylcarbonyloxy or C 1-12  alkylcarbonylamino;
 
T is selected from a substituent comprising a polymeric chain or a C 6-30  hydrocarbyl group having three-dimensional structure;
 
W is a hydrophilic group;
 
E is selected from —OH, —O − , C 1-6  alkyl, carboxy-C 1-6  alkyl, sulfo-C 1-6  alkyl, C 1-6  alkoxy, C 5-12  arylalkyl, C 1-6  alkylcarbonyl, C 5-12  arylalkylcarbonyl, C 1-6  alkoxycarbonyl or C 5-12  arylalkoxycarbonyl;
 
m is 0, 1 or 2;
 
n is 0, 1 or 2; and
 
p is 0, 1 or 2.
 
     Preferably, Q 1 , Q 2 , Q 3  and Q 4  are of formula (v). 
     The groups represented by R 1  and R 2  are primarily for modifying or “tuning” the wavelength of λ max  of the dye. Electron-donating substituents (e.g. alkoxy, alkylamino) at these positions can produce a red-shift in the dye. Conversely electron-withdrawing substituents at these positions (e.g. cyano, carboxy, nitro) can produce a blue-shift in the dye. By varying these substituents, the dye may be “tuned” to the frequency of a particular laser detector. 
     In a preferred embodiment of the present invention, R 1  and R 2  are both C 1-8  alkoxy groups, preferably butoxy. Butoxy substituents advantageously shift the λ max  towards longer wavelengths in the near infrared, which are preferable for detection by commercially available lasers. 
     In an alternative preferred embodiment R 1  and R 2  are C 1-12  alkoxy groups bearing a hydrophilic or hydrophilizable group. C 1-12  alkoxy groups bearing a hydrophilic or hydrophilizable group are advantageous since they provide the dual functions of (i) tuning the absorption frequency of the dye, and (ii) providing hydrophilicity to aid water-dispersibility. The hydrophilic group may be a hydrophilic polymer chain; an ammonium group; an acid group including salts thereof, or a sulfonamide group, as defined above. The hydrophilizable group may be a hydroxyl, protected hydroxyl, amino, protected amino, thiol, protected thiol, cyano, ester, halogen or alkenyl group. Such groups may be readily converted into hydrophilic groups. For example, hydroxyl groups may be oxidized to carboxylic acid groups (including salts thereof); hydroxyl groups may be coupled to PEG chains; amino groups may be quaternarized using, for example, methyl iodide; thiol groups may be oxidized to sulfonic acid groups (including salts thereof) or sulfonamides; cyano and ester groups may be hydrolysed to carboxylic acid groups (including salts thereof); and alkenyl groups may be oxidatively cleaved (e.g. by ozonolysis or permanganate oxidation) to provide carboxylic acid groups (including salts thereof). In the case of protected heteroatoms, the protecting group is removed before conversion to a hydrophilic group. Hence, R 1  and R 2  both may be a hydroxyalkoxy group such as —O(CH 2 ) 4 OH. 
     The group(s) represented by T reduces intermolecular interactions between dye molecules. The suffix n in T n  and the suffix p in T p  indicate the number of T substituents. In cases where there are two T substituents, these may be joined to form a cyclic structure. Preferably, T is a substituent comprising a C 3-30  bridged carbocycle, such as those described above. Alternatively, T is a substituent comprising a polymeric chain, such as those described above. In all cases, the dye comprises at least one moiety suitable for reducing intermolecular interactions. 
     The group(s) represented by W, when present, imparts hydrophilicity to the dye molecule. The suffix m in T m  indicates the number of W substituents. Each of Q 1 , Q 2 , Q 3  and Q 4  may have different numbers of W substituents arising, for example, from unsymmetrical sulfonations. Preferably, W is selected from a substituent comprising a PEG chain; a substituent comprising an ammonium group; a substituent comprising an acid group, including salts thereof; or a substituent comprising a sulfonamide group. W may be any one of the preferred hydrophilic groups described above. Preferably, W is —SO 3 H or a water-soluble salt thereof, such as Li, Na + , K + , NH 4   +  etc. Sulfonic acid groups may be easily introduced into any of aromatic structures (i) to (v) by sulfonation using, for example, oleum or chlorosulfonic acid. Alternatively (or in addition), hydrophilicity may be imparted into the dye molecule by quaternarizing an N atom. This is shown in heteroaromatic moieties (vi) and (vii). 
     Preferably, the dye molecule contains a central metal atom and corresponds to a compound of formula (I). Metal-ligand dye molecules of formula (I) are preferred, since metal atoms (or ions) can be used to tune the absorption λ max  of the molecule to a preferred wavelength. For example, certain metals such as Mn, V and Sn can produce large red-shifts in the λ max . In this context, red-shift means a shift of λ max  towards longer wavelengths as compared to the metal-free compound. 
     The degree of red-shift may be influenced by the oxidation state of the metal. High oxidation states (e.g. V(IV), Mn(III) and Sn(IV)) will tend to produce large red-shifts, while low oxidation states (e.g. Mn (II) and Sn(II)) will tend to produce smaller red-shifts. 
     Metal atoms having one or more axial ligands are also preferred in the present invention. As described above, axial ligands may be used as a handle for introducing additional or supplemental functionalities into the dye molecule. Accordingly, M is preferably Si(A 1 )(A 2 ), Ge(A 1 )(A 2 ), Ga(A 1 ), Al(A 1 ), Mn(A 1 ), Fe(A 1 ) or Sn(A 1 )(A 2 ). Si(A 1 )(A 2 ) is particularly preferred due it low cost and low toxicity. Mn(A 1 ) is also preferred since it offers the advantages of a large red-shift in addition to its potential for functionalizing the dye molecule via its axial ligand. 
     In cases where there are two axial ligands, these may be on opposite faces or they may be intermolecular. The geometry of the ligands is generally dictated by the metal and its preferred bonding geometry. 
     A 1  and A 2  may be selected to add axial steric bulk to the dye molecule, thereby reducing intermolecular interactions even further. 
     Alternatively (or in addition), A 1  and/or A 2  may be selected to add hydrophilicity to the dye molecule. Hence, A 1  and/or A 2  may include a hydrophilic group, such as any one of the groups defined as W above. 
     In order to introduce axial steric bulk and/or increase hydrophilicity, A 1  and/or A 2  are preferably dendrimers. In one preferred form A 1  and/or A 2  is a ligand of formula (IIIa): 
                                
wherein:
 
C 1  represents a core unit having two or more branching positions; each P 1  is independently selected from H, a hydrophilic moiety or a branched moiety;
 
g 1  is an integer from 2 to 8;
 
q 1  is 0 or an integer from 1 to 6;
 
each p 1  is independently selected from 0 or an integer from 1 to 6;
 
     Preferably, the core unit C 1  is selected from a C atom, an N atom, a Si atom, a C 1-8  alkyl residue, a C 3-8  cycloalkyl residue, or a phenyl residue. The core unit C 1  has at least two branching positions, the number of branching positions corresponding to the value of g 1 . Hence, an axial ligand having 3 branching positions and a carbon atom core (i.e. g 1 =3; C 1 ═C atom) may be, for example, a pentaerythritol derivative of formula (A): 
     
       
                 
         
             
             
         
      
     
     Each P 1  group in formula (IIIa) may be the same or different. For example, in a pentaerythritol derivative (having three branching positions), there may be two arms bearing terminal hydroxyl groups (—CH 2 OH; P 1 ═H) and one arm bearing a sulfate group (—CH 2 OSO 3 Z; P 1 ═SO 3 Z). 
     Preferably, P 1  is a hydrophilic moiety. The hydrophilic moiety may be an acid group (including salts thereof), a sulfonamide group, a hydrophilic polymer chain or an ammonium group. 
     Accordingly, P 1  may comprise a hydrophilic polymer chain, such as a PEG chain. Hence, in some embodiments, P 1  may be of formula: (CH 2 CH 2 O) v R 6 , wherein v is an integer from 2 to 5000 (preferably 2 to 1000, preferably 2 to 100) and R 6  is H, C 1-6  alkyl or C(O)C 1-8  alkyl. 
     Alternatively, P 1  may comprise an acid group (including salts thereof), such as sulfonic acids, sulfates, phosphonic acids, phosphates, carboxylic acids, carboxylates etc. Hence, in some embodiments P 1  may be of formula: SO 3 Z, PO 3 Z 2 , C 1-12  alkyl-CO 2 Z, C 1-12  alkyl-SO 3 Z or C 1-12 -alkyl-PO 3 Z 2 , C 1-12  alkyl-OSO 3 Z or C 1-12 -alkyl-OPO 3 Z 2  wherein Z is H or a water-soluble cation. Examples of water-soluble cations are Li + , Na + , K + , NH 4   +  etc. 
     Alternatively, P 1  may comprise an ammonium group, such as a quaternary ammonium group. Hence, in some embodiments P 1  may be of formula: C 1-12 -alkyl-N + (R a )(R b )(R c ) or C 1-12  alkyl-U, wherein R a , R b , R c  may be the same or different and are independently selected from H, C 1-8  alkyl (e.g. methyl, ethyl, cyclohexyl, cyclopentyl, tert-butyl, iso-propyl etc.) or C 6-12  arylalkyl (e.g. benzyl, phenylethyl etc. or C 6-12  aryl (e.g. phenyl, naphthyl etc.) and U is pyridinium, imidazolinium or pyrrolinium. 
     Alternatively, P 1  may comprise a sulfonamide group, such as a group of general formula —SO 2 NR p R q , wherein R p  and R q  are independently selected from H, C 1-8  alkyl (e.g. methyl, ethyl, cyclohexyl, cyclopentyl, tert-butyl, iso-propyl etc.), —(CH 2 CH 2 O) e R e  (wherein e is an integer from 2 to 5000 and R e  is H, C 1-8  alkyl or C(O)C 1-8  alkyl), C 6-12  arylalkyl (e.g. benzyl, phenylethyl etc.) or C 6-12  aryl (e.g. phenyl, methoxyphenyl etc.). 
     Branched structures such as those described above are generally known as dendrimers. Dendrimers are advantageous since their branched chains maximize the effective three-dimensional volume of the axial ligand and, in addition, provide the potential for introducing a plurality of hydrophilic groups into the dye molecule. The pentaerythritol structure shown in formula (A) is an example of a simple dendrimer suitable for use in the present invention. Further examples are triethanolamine derivatives (B), phloroglucinol derivatives (C), and 3,5-dihydroxybenzyl alcohol derivatives (D): 
     
       
                 
         
             
             
         
      
     
     In an alternative embodiment, one or more of the P 1  groups is itself a branched moiety. The branched moiety may be any structure adding further branching to the axial ligand, such as a moiety of formula (IIIb): 
                                
wherein:
 
C 2  represents a core unit having two or more branching positions;
 
P 2  is H or a hydrophilic moiety;
 
g 2  is an integer from 2 to 8;
 
q 2  is 0 or an integer from 1 to 6;
 
p 2  is 0 or an integer from 1 to 6;
 
     Preferred forms of C 2  and P 2  correspond to the preferred forms of C 1  and P 1  described above. A specific example of an axial ligand, wherein P 1  is a branched moiety of formula (IIIb) is dipentaerythritol derivative (E): 
     
       
                 
         
             
             
         
      
     
     Alternatively, the branched moiety may comprise multiple randomized branched chains, based on motifs of core units linked by alkylene or ether chains. It will be readily understood that randomized dendrimer structures may be rapidly built up by, for example, successive etherifications of pentaerythritol with further pentaerythritol, 3,5-dihydroxybenzyl alcohol or triethanolamine moieties. One or more terminal hydroxyl groups on the dendrimer may be capped with hydrophilic groups, such as any of the hydrophilic groups above described. The extent of hydrophilic capping may be used to control the water-solubility of the dye molecule. 
     It will be appreciated that randomized branched structures cannot be readily illustrated using precise structural formulae. However, all branched dendrimer-like structures are contemplated within the scope of the above definitions of A 1  and A 2 . 
     The term “hydrocarbyl” is used herein to refer to monovalent groups consisting generally of carbon and hydrogen. Hydrocarbyl groups thus include alkyl, alkenyl and alkynyl groups (in both straight and branched chain forms), carbocyclic groups (including polycycloalkyl groups such as bicyclooctyl and adamantyl) and aryl groups, and combinations of the foregoing, such as alkylcycloalkyl, alkylpolycycloalkyl, alkylaryl, alkenylaryl, alkynylaryl, cycloalkylaryl and cycloalkenylaryl groups. Similarly, the term “hydrocarbylene” refers to divalent groups corresponding to the monovalent hydrocarbyl groups described above. 
     Unless specifically stated otherwise, up to four —C—C— and/or —C—H moieties in the hydrocarbyl group may be optionally interrupted by one or more moieties selected from —O—; —NR w —; —S—; —C(O)—; —C(O)O—; —C(O)NR w —; —S(O)—; —SO 2 —; —SO 2 O—; —SO 2 NR w —; where R w  is a group selected from H, C 1-12  alkyl, C 6-12  aryl or C 6-12  arylalkyl. 
     Unless specifically stated otherwise, where the hydrocarbyl group contains one or more —C═C— moieties, up to four —C═C— moieties may optionally be replaced by —C═N—. Hence, the term “hydrocarbyl” may include moieties such as heteroaryl, ether, thioether, carboxy, hydroxyl, alkoxy, amine, thiol, amide, ester, ketone, sulfoxide, sulfonate, sulfonamide etc. 
     Unless specifically stated otherwise, the hydrocarbyl group may comprise up to four substituents independently selected from halogen, cyano, nitro, a hydrophilic group as defined above (e.g. —SO 3 H, —SO 3 K, —CO 2 Na, —NH 3   + , —NMe 3   +  etc.) or a polymeric group as defined above (e.g. a polymeric group derived from polyethylene glycol). 
     As used herein, the term “bridged cyclic group” includes C 4-30  carbocycles (preferably C 6-20  carbocycles) containing 1, 2, 3 or 4 bridging atoms. Examples of bridged carbocyclic groups are bornyl and triptycenyl, and derivatives thereof. The term “bridged cyclic group” also includes bridged polycyclic groups, including groups such as adamantanyl and tricyclo[5.2.1.0]decanyl, and derivatives thereof. 
     Unless specifically stated otherwise, the term “bridged cyclic group” also includes bridged carbocycles wherein 1, 2, 3 or 4 carbon atoms are replaced by heteroatoms selected from N, S or O (i.e. bridged heterocycles). When it is stated that a carbon atom in a carbocycle is replaced by a heteroatom, what is meant is that —CH— is replaced by —N—, —CH 2 — is replaced by —O—, or —CH 2 — is replaced by —S—. Hence, the term “bridged cyclic group” includes bridged heterocyclic groups, such as quinuclidinyl and tropanyl. Unless specifically stated otherwise, any of the bridged cyclic groups may be optionally substituted with 1, 2, 3 or 4 of the substituents described below. 
     The term “aryl” is used herein to refer to an aromatic group, such as phenyl, naphthyl or triptycenyl. C 6-12  aryl, for example, refers to an aromatic group having from 6 to 12 carbon atoms, excluding any substituents. The term “arylene”, of course, refers to divalent groups corresponding to the monovalent aryl groups described above. Any reference to aryl implicitly includes arylene, where appropriate. 
     The term “heteroaryl” refers to an aryl group, where 1, 2, 3 or 4 carbon atoms are replaced by a heteroatom selected from N, O or S. Examples of heteroaryl (or heteroaromatic) groups include pyridyl, benzimidazolyl, indazolyl, quinolinyl, isoquinolinyl, indolinyl, isoindolinyl, indolyl, isoindolyl, furanyl, thiophenyl, pyrrolyl, thiazolyl, imidazolyl, oxazolyl, isoxazolyl, pyrazolyl, isoxazolonyl, piperazinyl, pyrimidinyl, piperidinyl, morpholinyl, pyrrolidinyl, isothiazolyl, triazolyl, oxadiazolyl, thiadiazolyl, pyridyl, pyrimidinyl, benzopyrimidinyl, benzotriazole, quinoxalinyl, pyridazyl, coumarinyl etc. The term “heteroarylene”, of course, refers to divalent groups corresponding to the monovalent heteroaryl groups described above. Any reference to heteroaryl implicitly includes heteroarylene, where appropriate. 
     Unless specifically stated otherwise, aryl, arylene, heteroaryl and heteroarylene groups may be optionally substituted with 1, 2, 3, 4 or 5 of the substituents described below. 
     Where reference is made to optionally substituted groups (e.g. in connection with bridged cyclic groups, aryl groups or heteroaryl groups), the optional substituent(s) are independently selected from C 1-8  alkyl, C 1-8  alkoxy, —(OCH 2 CH 2 ) d OR d  (wherein d is an integer from 2 to 5000 and R d  is H, C 1-8  alkyl or C(O)C 1-8  alkyl), cyano, halogen, amino, hydroxyl, thiol, —SR v , —NR u R v , nitro, phenyl, phenoxy, —CO 2 R v , —C(O)R v , —OCOR v , —SO 2 R v , —OSO 2 R v , —SO 2 OR v , —NHC(O)R v , —CONR u R v , —CONR u R v , —SO 2 NR u R v , wherein R u  and R V  are independently selected from hydrogen, C 1-12  alkyl, phenyl or phenyl-C 1-8  alkyl (e.g. benzyl). Where, for example, a group contains more than one substituent, different substituents can have different R u  or R v  groups. For example, a naphthyl group may be substituted with three substituents: —SO 2 NHPh, —CO 2 Me group and —NH 2 . 
     The term “alkyl” is used herein to refer to alkyl groups in both straight and branched forms, The alkyl group may be interrupted with 1, 2 or 3 heteroatoms selected from O, N or S. The alkyl group may also be interrupted with 1, 2 or 3 double and/or triple bonds. However, the term “alkyl” usually refers to alkyl groups having no heteroatom interruptions or double or triple bond interruptions. Where “alkenyl” groups are specifically mentioned, this is not intended to be construed as a limitation on the definition of “alkyl” above. 
     Where reference is made to, for example, C 1-12  alkyl, it is meant the alkyl group may contain any number of carbon atoms between 1 and 12. Unless specifically stated otherwise, any reference to “alkyl” means C 1-12  alkyl, preferably C 1-6  alkyl. 
     The term “alkyl” also includes cycloalkyl groups. As used herein, the term “cycloalkyl” includes cycloalkyl, polycycloalkyl, and cycloalkenyl groups, as well as combinations of these with linear alkyl groups, such as cycloalkylalkyl groups. The cycloalkyl group may be interrupted with 1, 2 or 3 heteroatoms selected from O, N or S. However, the term “cycloalkyl” usually refers to cycloalkyl groups having no heteroatom interruptions. Examples of cycloalkyl groups include cyclopentyl, cyclohexyl, cyclohexenyl, cyclohexylmethyl and adamantyl groups. 
     The term “arylalkyl” refers to groups such as benzyl, phenylethyl and naphthylmethyl. 
     The term “halogen” or “halo” is used herein to refer to any of fluorine, chlorine, bromine and iodine. Usually, however, refers to chlorine or fluorine substituents. 
     Where reference is made to “a substituent comprising . . . ” (e.g. “a substituent comprising a hydrophilic group”, “a substituent comprising an acid group (including salts thereof)”, “a substituent comprising a polymeric chain” etc.), the substituent in question may consist entirely or partially of the group specified. For example, “a substituent comprising an acid group (including salts thereof)” may be of formula —(CH 2 ) j —SO 3 K, wherein j is 0 or an integer from 1 to 6. Hence, in this context, the term “substituent” may be, for example, an alkyl group, which has a specified group attached. However, it will be readily appreciated that the exact nature of the substituent is not crucial to the desired functionality, provided that the specified group is present. 
     Chiral compounds described herein have not been given stereo-descriptors. However, when compounds may exist in stereoisomeric forms, then all possible stereoisomers and mixtures thereof are included (e.g. enantiomers, diastereomers and all combinations including racemic mixtures etc.). 
     Likewise, when compounds may exist in a number of regioisomeric forms, then all possible regioisomers and mixtures thereof are included. 
     For the avoidance of doubt, the term “a” (or “an”), in phrases such as “comprising a”, means “at least one” and not “one and only one”. Where the term “at least one” is specifically used, this should not be construed as having a limitation on the definition of “a”. 
     Throughout the specification, the term “comprising”, or variations such as “comprise” or “comprises”, should be construed as including a stated element, integer or step, but not excluding any other element, integer or step. 
     Inkjet Inks 
     The method of the present invention is particularly suitable for use with inkjet ink compositions, preferably water-based inkjet inks. Hence, the present invention provides a method of minimizing absorption of visible light in an inkjet ink comprising an IR-absorbing macrocyclic cyanine dye, said method comprising preselecting said dye such that it includes at least one moiety for reducing intermolecular interactions between adjacent dye molecules. 
     Water-based inkjet ink compositions are well known in the literature and, in addition to water, may comprise additives, such as co-solvents, biocides, sequestering agents, humectants, pH adjusters, viscosity modifiers, penetrants, wetting agents, surfactants etc. 
     Co-solvents are typically water-soluble organic solvents. Suitable water-soluble organic solvents include C 1-4  alkyl alcohols, such as ethanol, methanol, butanol, propanol, and 2-propanol; glycol ethers, such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, ethylene glycol mono-isopropyl ether, diethylene glycol mono-isopropyl ether, ethylene glycol mono-n-butyl ether, diethylene glycol mono-n-butyl ether, triethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-isopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol mono-isopropyl ether, propylene glycol mono-n-butyl ether, and dipropylene glycol mono-n-butyl ether; formamide, acetamide, dimethyl sulfoxide, sorbitol, sorbitan, glycerol monoacetate, glycerol diacetate, glycerol triacetate, and sulfolane; or combinations thereof. 
     Other useful water-soluble organic solvents include polar solvents, such as 2-pyrrolidone, N-methylpyrrolidone, ε-caprolactam, dimethyl sulfoxide, sulfolane, morpholine, N-ethylmorpholine, 1,3-dimethyl-2-imidazolidinone and combinations thereof. 
     The inkjet ink may contain a high-boiling water-soluble organic solvent which can serve as a wetting agent or humectant for imparting water retentivity and wetting properties to the ink composition. Such a high-boiling water-soluble organic solvent includes one having a boiling point of 180° C. or higher. Examples of the water-soluble organic solvent having a boiling point of 180° C. or higher are ethylene glycol, propylene glycol, diethylene glycol, pentamethylene glycol, trimethylene glycol, 2-butene-1,4-diol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, tripropylene glycol monomethyl ether, dipropylene glycol monoethyl glycol, dipropylene glycol monoethyl ether, dipropylene glycol monomethyl ether, dipropylene glycol, triethylene glycol monomethyl ether, tetraethylene glycol, triethylene glycol, diethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, tripropylene glycol, polyethylene glycols having molecular weights of 2000 or lower, 1,3-propylene glycol, isopropylene glycol, isobutylene glycol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, glycerol, erythritol, pentaerythritol and combinations thereof. 
     The total water-soluble organic solvent content in the inkjet ink is preferably about 5 to 50% by weight, more preferably 10 to 30% by weight, based on the total ink composition. 
     Other suitable wetting agents or humectants include saccharides (including monosaccharides, oligosaccharides and polysaccharides) and derivatives thereof (e.g. maltitol, sorbitol, xylitol, hyaluronic salts, aldonic acids, uronic acids etc.) 
     The inkjet ink may also contains a penetrant for accelerating penetration of the aqueous ink into the recording medium. Suitable penetrants include polyhydric alcohol alkyl ethers (glycol ethers) and/or 1,2-alkyldiols. Examples of suitable polyhydric alcohol alkyl ethers are ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol mono-n-propyl ether, ethylene glycol mono-isopropyl ether, diethylene glycol mono-isopropyl ether, ethylene glycol mono-n-butyl ether, diethylene glycol mono-n-butyl ether, triethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, 1-methyl-1-methoxybutanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-isopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, dipropylene glycol mono-isopropyl ether, propylene glycol mono-n-butyl ether, and dipropylene glycol mono-n-butyl ether. Examples of suitable 1,2-alkyldiols are 1,2-pentanediol and 1,2-hexanediol. The penetrant may also be selected from straight-chain hydrocarbon diols, such as 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, and 1,8-octanediol. Glycerol may also be used as a penetrant. 
     The amount of penetrant is preferably in the range of 1 to 20% by weight, more preferably 1 to 10% by weight, based on the total ink composition. 
     The inkjet ink may also contain a surface active agent, especially an anionic surface active agent and/or a nonionic surface active agent. Useful anionic surface active agents include sulfonic acid types, such as alkanesulfonic acid salts, α-olefinsulfonic acid salts, alkylbenzenesulfonic acid salts, alkylnaphthalenesulfonic acids, acylmethyltaurines, and dialkylsulfosuccinic acids; alkylsulfuric ester salts, sulfated oils, sulfated olefins, polyoxyethylene alkyl ether sulfuric ester salts; carboxylic acid types, e.g., fatty acid salts and alkylsarcosine salts; and phosphoric acid ester types, such as alkylphosphoric ester salts, polyoxyethylene alkyl ether phosphoric ester salts, and glycerophosphoric ester salts. Specific examples of the anionic surface active agents are sodium dodecylbenzenesulfonate, sodium laurate, and a polyoxyethylene alkyl ether sulfate ammonium salt. 
     Suitable nonionic surface active agents include ethylene oxide adduct types, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, and polyoxyethylene alkylamides; polyol ester types, such as glycerol alkyl esters, sorbitan alkyl esters, and sugar alkyl esters; polyether types, such as polyhydric alcohol alkyl ethers; and alkanolamide types, such as alkanolamine fatty acid amides. Specific examples of nonionic surface active agents are ethers such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether, polyoxyethylene dodecylphenyl ether, polyoxyethylene alkylallyl ether, polyoxyethylene oleyl ether, polyoxyethylene lauryl ether, and polyoxyalkylene alkyl ethers (e.g. polyoxyethylene alkyl ethers); and esters, such as polyoxyethylene oleate, polyoxyethylene oleate ester, polyoxyethylene distearate, sorbitan laurate, sorbitan monostearate, sorbitan mono-oleate, sorbitan sesquioleate, polyoxyethylene mono-oleate, and polyoxyethylene stearate. Acetylene glycol surface active agents, such as 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 3,6-dimethyl-4-octyne-3,6-diol or 3,5-dimethyl-1-hexyn-3-ol, may also be used. 
     The inkjet ink may contain a pH adjuster for adjusting its pH to 7 to 9. Suitable pH adjusters include basic compounds, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate, lithium carbonate, sodium phosphate, potassium phosphate, lithium phosphate, potassium dihydrogenphosphate, dipotassium hydrogenphosphate, sodium oxalate, potassium oxalate, lithium oxalate, sodium borate, sodium tetraborate, potassium hydrogenphthalate, and potassium hydrogentartrate; ammonia; and amines, such as methylamine, ethylamine, diethylamine, trimethylamine, triethylamine, tris(hydroxymethyl)aminomethane hydrochloride, triethanolamine, diethanolamine, diethylethanolamine, triisopropanolamine, butyldiethanolamine, morpholine, and propanolamine. 
     The inkjet ink may also include a biocide, such as benzoic acid, dichlorophene, hexachlorophene, sorbic acid, hydroxybenzoic esters, sodium dehydroacetate, 1,2-benthiazolin-3-one, 3,4-isothiazolin-3-one or 4,4-dimethyloxazolidine. 
     The inkjet ink may also contain a sequestering agent, such as ethylenediaminetetraacetic acid (EDTA). 
     The inkjet ink may also contain a singlet oxygen quencher. The presence of singlet oxygen quencher(s) in the ink reduces the propensity for the IR-absorbing dye to degrade. The quencher consumes any singlet oxygen generated in the vicinity of the dye molecules and, hence, minimizes their degradation. An excess of singlet oxygen quencher is advantageous for minimizing degradation of the dye and retaining its IR-absorbing properties over time. Preferably, the singlet oxygen quencher is selected from ascorbic acid, 1,4-diazabicyclo-[2.2.2]octane (DABCO), azides (e.g. sodium azide), histidine or tryptophan. 
     Inkjet Printers 
     The inkjet ink in the method described above may be contained in an ink reservoir in fluid communication with a printhead of an inkjet printer. 
     Inkjet printers, such as thermal bubble-jet and piezoelectric printers, are well known in the art and will form part of the skilled person&#39;s common general knowledge. The printer may be a high-speed inkjet printer. The printer is preferably a pagewidth printer. Preferred inkjet printers and printheads for use in the present invention are described in the following patent applications, all of which are incorporated herein by reference in their entirety. 
                                                 10/302,274   6,692,108   6,672,709   10/303,348   6,672,710       6,669,334   10/302,668   10/302,577   6,669,333   10/302,618       10/302,617   10/302,297                    
Printhead
 
     A Memjet printer generally has two printhead integrated circuits that are mounted adjacent each other to form a pagewidth printhead. Typically, the printhead ICs can vary in size from 2 inches to 8 inches, so several combinations can be used to produce, say, an A4 pagewidth printhead. For example two printhead ICs of 7 and 3 inches, 2 and 4 inches, or 5 and 5 inches could be used to create an A4 printhead (the notation is 7:3). Similarly 6 and 4 (6:4) or 5 and 5 (5:5) combinations can be used. An A3 printhead can be constructed from 8 and 6-inch printhead integrated circuits, for example. For photographic printing, particularly in camera, smaller printheads can be used. It will also be appreciated that a single printhead integrated circuit, or more than two such circuits, can also be used to achieve the required printhead width. 
     A preferred printhead embodiment of the printhead will now be described with reference to  FIGS. 17 and 18 . A printhead  420  takes the form of an elongate unit. As best shown in  FIG. 18 , the components of the printhead  420  include a support member  421 , a flexible PCB  422 , an ink distribution molding  423 , an ink distribution plate  424 , a MEMS printhead comprising first and second printhead integrated circuits (ICs)  425  and  426 , and busbars  427 . 
     The support member  421  is can be formed from any suitable material, such as metal or plastic, and can be extruded, molded or formed in any other way. The support member  421  should be strong enough to hold the other components in the appropriate alignment relative to each other whilst stiffening and strengthening the printhead as a whole. 
     The flexible PCB extends the length of the printhead  420  and includes first and second electrical connectors  428  and  429 . The electrical connectors  428  and  429  correspond with flexible connectors (not shown). The electrical connectors include contact areas  450  and  460  that, in use, are positioned in contact with corresponding output connectors from a SoPEC chip (not shown). Data from the SoPEC chip passes along the electrical connectors  428  and  429 , and is distributed to respective ends of the first and second printhead ICs  425  and  426 . 
     As shown in  FIG. 19 , the ink distribution molding  423  includes a plurality of elongate conduits  430  that distribute fluids (ie, colored inks, infrared ink and fixative) and pressurized air from the air pump along the length of the printhead  420  ( FIG. 18 ). Sets of fluid apertures  431  ( FIG. 20 ) disposed along the length of the ink distribution molding  423  distribute the fluids and air from the conduits  430  to the ink distribution plate  424 . The fluids and air are supplied via nozzles  440  formed on a plug  441  ( FIG. 21 ), which plugs into a corresponding socket (not shown) in the printer. 
     The distribution plate  424  is a multi-layer construction configured to take fluids provided locally from the fluid apertures  431  and distribute them through smaller distribution apertures  432  into the printhead ICs  425  and  426  (as shown in  FIG. 20 ). 
     The printhead ICs  425  and  426  are positioned end to end, and are held in contact with the distribution plate  424  so that ink from the smaller distribution apertures  432  can be fed into corresponding apertures (not shown) in the printhead ICs  425  and  426 . 
     The busbars  427  are relatively high-capacity conductors positioned to provide drive current to the actuators of the printhead nozzles (described in detail below). As best shown in  FIG. 20 , the busbars  427  are retained in position at one end by a socket  433 , and at both ends by wrap-around wings  434  of the flexible PCB  422 . The busbars also help hold the printhead ICs  425  in position. 
     As shown best in  FIG. 18 , when assembled, the flexible PCB  422  is effectively wrapped around the other components, thereby holding them in contact with each other. Notwithstanding this binding effect, the support member  421  provides a major proportion of the required stiffness and strength of the printhead  420  as a whole. 
     Two forms of printhead nozzles (“thermal bend actuator” and “bubble forming heater element actuator”), suitable for use in the printhead described above, will now be described. 
     Thermal Bend Actuator 
     In the thermal bend actuator, there is typically provided a nozzle arrangement having a nozzle chamber containing ink and a thermal bend actuator connected to a paddle positioned within the chamber. The thermal actuator device is actuated so as to eject ink from the nozzle chamber. The preferred embodiment includes a particular thermal bend actuator which includes a series of tapered portions for providing conductive heating of a conductive trace. The actuator is connected to the paddle via an arm received through a slotted wall of the nozzle chamber. The actuator arm has a mating shape so as to mate substantially with the surfaces of the slot in the nozzle chamber wall. 
     Turning initially to  FIGS. 22(   a )-( c ), there is provided schematic illustrations of the basic operation of a nozzle arrangement of this embodiment. A nozzle chamber  501  is provided filled with ink  502  by means of an ink inlet channel  503  which can be etched through a wafer substrate on which the nozzle chamber  501  rests. The nozzle chamber  501  further includes an ink ejection port  504  around which an ink meniscus forms. 
     Inside the nozzle chamber  501  is a paddle type device  507  which is interconnected to an actuator  508  through a slot in the wall of the nozzle chamber  501 . The actuator  508  includes a heater means e.g.  509  located adjacent to an end portion of a post  510 . The post  510  is fixed to a substrate. 
     When it is desired to eject a drop from the nozzle chamber  501 , as illustrated in  FIG. 22(   b ), the heater means  509  is heated so as to undergo thermal expansion. Preferably, the heater means  509  itself or the other portions of the actuator  508  are built from materials having a high bend efficiency where the bend efficiency is defined as: 
     
       
         
           
             
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     A suitable material for the heater elements is a copper nickel alloy which can be formed so as to bend a glass material. 
     The heater means  509  is ideally located adjacent the end portion of the post  510  such that the effects of activation are magnified at the paddle end  507  such that small thermal expansions near the post  510  result in large movements of the paddle end. 
     The heater means  509  and consequential paddle movement causes a general increase in pressure around the ink meniscus  505  which expands, as illustrated in  FIG. 22(   b ), in a rapid manner. The heater current is pulsed and ink is ejected out of the port  504  in addition to flowing in from the ink channel  503 . 
     Subsequently, the paddle  507  is deactivated to again return to its quiescent position. The deactivation causes a general reflow of the ink into the nozzle chamber. The forward momentum of the ink outside the nozzle rim and the corresponding backflow results in a general necking and breaking off of the drop  512  which proceeds to the print media. The collapsed meniscus  505  results in a general sucking of ink into the nozzle chamber  502  via the ink flow channel  503 . In time, the nozzle chamber  501  is refilled such that the position in  FIG. 22(   a ) is again reached and the nozzle chamber is subsequently ready for the ejection of another drop of ink. 
       FIG. 23  illustrates a side perspective view of the nozzle arrangement.  FIG. 24  illustrates sectional view through an array of nozzle arrangement of  FIG. 23 . In these figures, the numbering of elements previously introduced has been retained. 
     Firstly, the actuator  508  includes a series of tapered actuator units e.g.  515  which comprise an upper glass portion (amorphous silicon dioxide)  516  formed on top of a titanium nitride layer  517 . Alternatively a copper nickel alloy layer (hereinafter called cupronickel) can be utilized which will have a higher bend efficiency. 
     The titanium nitride layer  517  is in a tapered form and, as such, resistive heating takes place near an end portion of the post  510 . Adjacent titanium nitride/glass portions  515  are interconnected at a block portion  519  which also provides a mechanical structural support for the actuator  508 . 
     The heater means  509  ideally includes a plurality of the tapered actuator unit  515  which are elongate and spaced apart such that, upon heating, the bending force exhibited along the axis of the actuator  508  is maximized. Slots are defined between adjacent tapered units  515  and allow for slight differential operation of each actuator  508  with respect to adjacent actuators  508 . 
     The block portion  519  is interconnected to an arm  520 . The arm  520  is in turn connected to the paddle  507  inside the nozzle chamber  501  by means of a slot e.g.  522  formed in the side of the nozzle chamber  501 . The slot  522  is designed generally to mate with the surfaces of the arm  520  so as to minimize opportunities for the outflow of ink around the arm  520 . The ink is held generally within the nozzle chamber  501  via surface tension effects around the slot  522 . 
     When it is desired to actuate the arm  520 , a conductive current is passed through the titanium nitride layer  517  via vias within the block portion  519  connecting to a lower CMOS layer  506  which provides the necessary power and control circuitry for the nozzle arrangement. The conductive current results in heating of the nitride layer  517  adjacent to the post  510  which results in a general upward bending of the arm  20  and consequential ejection of ink out of the nozzle  504 . The ejected drop is printed on a page in the usual manner for an inkjet printer as previously described. 
     An array of nozzle arrangements can be formed so as to create a single printhead. For example, in  FIG. 24  there is illustrated a partly sectioned various array view which comprises multiple ink ejection nozzle arrangements of  FIG. 23  laid out in interleaved lines so as to form a printhead array. Of course, different types of arrays can be formulated including full color arrays etc. 
     The construction of the printhead system described can proceed utilizing standard MEMS techniques through suitable modification of the steps as set out in U.S. Pat. No. 6,243,113 entitled “Image Creation Method and Apparatus (IJ 41)” to the present applicant, the contents of which are fully incorporated by cross reference. 
     Bubble Forming Heater Element Actuator 
     With reference to  FIG. 25 , the unit cell  1001  of a bubble forming heater element actuator comprises a nozzle plate  1002  with nozzles  1003  therein, the nozzles having nozzle rims  1004 , and apertures  1005  extending through the nozzle plate. The nozzle plate  1002  is plasma etched from a silicon nitride structure which is deposited, by way of chemical vapor deposition (CVD), over a sacrificial material which is subsequently etched. 
     The printhead also includes, with respect to each nozzle  1003 , side walls  1006  on which the nozzle plate is supported, a chamber  1007  defined by the walls and the nozzle plate  1002 , a multi-layer substrate  1008  and an inlet passage  1009  extending through the multi-layer substrate to the far side (not shown) of the substrate. A looped, elongate heater element  1010  is suspended within the chamber  1007 , so that the element is in the form of a suspended beam. The printhead as shown is a microelectromechanical system (MEMS) structure, which is formed by a lithographic process. 
     When the printhead is in use, ink  1011  from a reservoir (not shown) enters the chamber  1007  via the inlet passage  1009 , so that the chamber fills. Thereafter, the heater element  1010  is heated for somewhat less than 1 micro second, so that the heating is in the form of a thermal pulse. It will be appreciated that the heater element  1010  is in thermal contact with the ink  1011  in the chamber  1007  so that when the element is heated, this causes the generation of vapor bubbles in the ink. Accordingly, the ink  1011  constitutes a bubble forming liquid. 
     The bubble  1012 , once generated, causes an increase in pressure within the chamber  1007 , which in turn causes the ejection of a drop  1016  of the ink  1011  through the nozzle  1003 . The rim  1004  assists in directing the drop  1016  as it is ejected, so as to minimize the chance of a drop misdirection. 
     The reason that there is only one nozzle  1003  and chamber  1007  per inlet passage  1009  is so that the pressure wave generated within the chamber, on heating of the element  1010  and forming of a bubble  1012 , does not effect adjacent chambers and their corresponding nozzles. 
     The increase in pressure within the chamber  1007  not only pushes ink  1011  out through the nozzle  1003 , but also pushes some ink back through the inlet passage  1009 . However, the inlet passage  1009  is approximately 200 to 300 microns in length, and is only approximately 16 microns in diameter. Hence there is a substantial viscous drag. As a result, the predominant effect of the pressure rise in the chamber  1007  is to force ink out through the nozzle  1003  as an ejected drop  1016 , rather than back through the inlet passage  9 . 
     As shown in  FIG. 25 , the ink drop  1016  is being ejected is shown during its “necking phase” before the drop breaks off. At this stage, the bubble  1012  has already reached its maximum size and has then begun to collapse towards the point of collapse  1017 . 
     The collapsing of the bubble  1012  towards the point of collapse  1017  causes some ink  1011  to be drawn from within the nozzle  1003  (from the sides  1018  of the drop), and some to be drawn from the inlet passage  1009 , towards the point of collapse. Most of the ink  1011  drawn in this manner is drawn from the nozzle  1003 , forming an annular neck  1019  at the base of the drop  16  prior to its breaking off. 
     The drop  1016  requires a certain amount of momentum to overcome surface tension forces, in order to break off. As ink  1011  is drawn from the nozzle  1003  by the collapse of the bubble  1012 , the diameter of the neck  1019  reduces thereby reducing the amount of total surface tension holding the drop, so that the momentum of the drop as it is ejected out of the nozzle is sufficient to allow the drop to break off. 
     When the drop  1016  breaks off, cavitation forces are caused as reflected by the arrows  1020 , as the bubble  1012  collapses to the point of collapse  1017 . It will be noted that there are no solid surfaces in the vicinity of the point of collapse  1017  on which the cavitation can have an effect. 
     Inkjet Cartridges 
     The inkjet ink in the method described above may be contained in an ink cartridge. Ink cartridges for inkjet printers are well known in the art and are available in numerous forms. Preferably, the inkjet ink cartridges are replaceable. Inkjet cartridges suitable for use in the present invention are described in the following patent applications, all of which are incorporated herein by reference in their entirety.
         U.S. Pat. No. 6,428,155, Ser. No. 10/171,987
 
In one preferred form, the ink cartridge comprises:
       

     a housing defining a plurality of storage areas wherein at least one of the storage areas contains colorant for printing information that is visible to the human eye and at least one of the other storage areas contains an inkjet ink as described above. 
     Preferably, each storage area is sized corresponding to the expected levels of use of its contents relative to the intended print coverage for a number of printed pages. 
     There now follows a brief description of a typical ink cartridge.  FIG. 12  shows the complete assembly of the replaceable ink cartridge  627 . It has bladders or chambers for storing fixative  644 , adhesive  630 , and cyan  631 , magenta  632 , yellow  633 , black  634  and infrared  635  inks. The cartridge  627  also contains a micro air filter  636  in a base molding  637 . As shown in  FIG. 9 , the micro air filter  636  interfaces with an air pump  638  inside the printer via a hose  639 . This provides filtered air to the printheads  705  to prevent ingress of micro particles into the Memjet™ printheads  705  which may clog the nozzles. By incorporating the air filter  636  within the cartridge  627 , the operational life of the filter is effectively linked to the life of the cartridge. This ensures that the filter is replaced together with the cartridge rather than relying on the user to clean or replace the filter at the required intervals. Furthermore, the adhesive and infrared ink are replenished together with the visible inks and air filter thereby reducing how frequently the printer operation is interrupted because of the depletion of a consumable material. 
     The cartridge  627  has a thin wall casing  640 . The ink bladders  631  to  635  and fixitive bladder  644  are suspended within the casing by a pin  645  which hooks the cartridge together. The single glue bladder  630  is accommodated in the base molding  637 . This is a fully recyclable product with a capacity for printing and gluing 3000 pages (1500 sheets). 
     Substrates 
     As mentioned above, the method of the present invention is especially suitable for use in connection with Hyperlabel™ and netpage systems. Such systems are described in detail in the patent applications listed above, all of which are incorporated herein by reference in their entirety. 
     Hence, the present invention provides a method of minimizing visible coloration of a substrate having an IR-absorbing macrocyclic cyanine dye disposed thereon, said method comprising preselecting said dye such that it includes a moiety for reducing intermolecular interactions between adjacent dye molecules. 
     Preferably, the substrate comprises an interface surface. Preferably, the dye is disposed in the form of coded data suitable for use in netpage and/or Hyperlabel™ systems. For example, the coded data may be indicative of the identity of a product item. Preferably, the coded data is disposed over a substantial portion of an interface surface of the substrate (e.g. greater than 20%, greater than 50% or greater than 90% of the surface). 
     Preferably, the substrate is IR reflective so that the dye disposed thereon may be detected by a sensing device. The substrate may be comprised of any suitable material such as plastics (e.g. polyolefins, polyesters, polyamides etc.), paper, metal or combinations thereof. 
     For netpage applications, the substrate is preferably a paper sheet. For Hyperlabel™ applications, the substrate is preferably a tag, a label, a packaging material or a surface of a product item. Typically, tags and labels are comprised of plastics, paper or combinations thereof. 
     In accordance with Hyperlabel™ applications of the invention, the substrate may be an interactive product item adapted for interaction with a user via a sensing device and a computer system, the interactive product item comprising: 
     a product item having an identity; 
     an interface surface associated with the product item and having disposed thereon information relating to the product item and coded data indicative of the identity of the product item, wherein said coded data comprise an IR-absorbing dye as described above. 
     Netpage and Hyperlabel™ 
     Netpage applications of this invention are described generally in the fourth and fifth aspects of the invention above. Hyperlabel™ applications of this invention are described generally in the sixth and seventh aspects of the invention above. 
     There now follows a detailed overview of netpage and Hyperlabel™. (Note: Memjet™ and Hyperlabel™ are trade marks of Silverbrook Research Pty Ltd, Australia). It will be appreciated that not every implementation will necessarily embody all or even most of the specific details and extensions discussed below in relation to the basic system. However, the system is described in its most complete form to reduce the need for external reference when attempting to understand the context in which the preferred embodiments and aspects of the present invention operate. 
     In brief summary, the preferred form of the netpage system employs a computer interface in the form of a mapped surface, that is, a physical surface which contains references to a map of the surface maintained in a computer system. The map references can be queried by an appropriate sensing device. Depending upon the specific implementation, the map references may be encoded visibly or invisibly, and defined in such a way that a local query on the mapped surface yields an unambiguous map reference both within the map and among different maps. The computer system can contain information about features on the mapped surface, and such information can be retrieved based on map references supplied by a sensing device used with the mapped surface. The information thus retrieved can take the form of actions which are initiated by the computer system on behalf of the operator in response to the operator&#39;s interaction with the surface features. 
     In its preferred form, the netpage system relies on the production of, and human interaction with, netpages. These are pages of text, graphics and images printed on ordinary paper, but which work like interactive web pages. Information is encoded on each page using ink which is substantially invisible to the unaided human eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen and transmitted to the netpage system. 
     In the preferred form, active buttons and hyperlinks on each page can be clicked with the pen to request information from the network or to signal preferences to a network server. In one embodiment, text written by hand on a netpage is automatically recognized and converted to computer text in the netpage system, allowing forms to be filled in. In other embodiments, signatures recorded on a netpage are automatically verified, allowing e-commerce transactions to be securely authorized. 
     As illustrated in  FIG. 1 , a printed netpage  1  can represent an interactive form which can be filled in by the user both physically, on the printed page, and “electronically”, via communication between the pen and the netpage system. The example shows a “Request” form containing name and address fields and a submit button. The netpage consists of graphic data  2  printed using visible ink, and coded data  3  printed as a collection of tags  4  using invisible ink. The corresponding page description  5 , stored on the netpage network, describes the individual elements of the netpage. In particular it describes the type and spatial extent (zone) of each interactive element (i.e. text field or button in the example), to allow the netpage system to correctly interpret input via the netpage. The submit button  6 , for example, has a zone  7  which corresponds to the spatial extent of the corresponding graphic  8 . 
     As illustrated in  FIG. 2 , the netpage pen  101 , a preferred form of which is shown in  FIGS. 6 and 7  and described in more detail below, works in conjunction with a personal computer (PC), Web terminal  75 , or a netpage printer  601 . The netpage printer is an Internet-connected printing appliance for home, office or mobile use. The pen is wireless and communicates securely with the netpage network via a short-range radio link  9 . Short-range communication is relayed to the netpage network by a local relay function which is either embedded in the PC, Web terminal or netpage printer, or is provided by a separate relay device  44 . The relay function can also be provided by a mobile phone or other device which incorporates both short-range and longer-range communications functions. 
     In an alternative embodiment, the netpage pen utilises a wired connection, such as a USB or other serial connection, to the PC, Web terminal, netpage printer or relay device. 
     The netpage printer  601 , a preferred form of which is shown in  FIGS. 9 to 11  and described in more detail below, is able to deliver, periodically or on demand, personalized newspapers, magazines, catalogs, brochures and other publications, all printed at high quality as interactive netpages. Unlike a personal computer, the netpage printer is an appliance which can be, for example, wall-mounted adjacent to an area where the morning news is first consumed, such as in a user&#39;s kitchen, near a breakfast table, or near the household&#39;s point of departure for the day. It also comes in tabletop, desktop, portable and miniature versions. 
     Netpages printed at their point of consumption combine the ease-of-use of paper with the timeliness and interactivity of an interactive medium. 
     As shown in  FIG. 2 , the netpage pen  101  interacts with the coded data on a printed netpage  1  (or product item  201 ) and communicates the interaction via a short-range radio link  9  to a relay. The relay sends the interaction to the relevant netpage page server  10  for interpretation. In appropriate circumstances, the page server sends a corresponding message to application computer software running on a netpage application server  13 . The application server may in turn send a response which is printed on the originating printer. 
     In an alternative embodiment, the PC, Web terminal, netpage printer or relay device may communicate directly with local or remote application software, including a local or remote Web server. Relatedly, output is not limited to being printed by the netpage printer. It can also be displayed on the PC or Web terminal, and further interaction can be screen-based rather than paper-based, or a mixture of the two. 
     The netpage system is made considerably more convenient in the preferred embodiment by being used in conjunction with high-speed microelectromechanical system (MEMS) based inkjet (Memjet™) printers. In the preferred form of this technology, relatively high-speed and high-quality printing is made more affordable to consumers. In its preferred form, a netpage publication has the physical characteristics of a traditional newsmagazine, such as a set of letter-size glossy pages printed in full color on both sides, bound together for easy navigation and comfortable handling. 
     The netpage printer exploits the growing availability of broadband Internet access. Cable service is available to 95% of households in the United States, and cable modem service offering broadband Internet access is already available to 20% of these. The netpage printer can also operate with slower connections, but with longer delivery times and lower image quality. Indeed, the netpage system can be enabled using existing consumer inkjet and laser printers, although the system will operate more slowly and will therefore be less acceptable from a consumer&#39;s point of view. In other embodiments, the netpage system is hosted on a private intranet. In still other embodiments, the netpage system is hosted on a single computer or computer-enabled device, such as a printer. 
     Netpage publication servers  14  on the netpage network are configured to deliver print-quality publications to netpage printers. Periodical publications are delivered automatically to subscribing netpage printers via pointcasting and multicasting Internet protocols. Personalized publications are filtered and formatted according to individual user profiles. 
     A netpage printer can be configured to support any number of pens, and a pen can work with any number of netpage printers. In the preferred implementation, each netpage pen has a unique identifier. A household may have a collection of colored netpage pens, one assigned to each member of the family. This allows each user to maintain a distinct profile with respect to a netpage publication server or application server. 
     A netpage pen can also be registered with a netpage registration server  11  and linked to one or more payment card accounts. This allows e-commerce payments to be securely authorized using the netpage pen. The netpage registration server compares the signature captured by the netpage pen with a previously registered signature, allowing it to authenticate the user&#39;s identity to an e-commerce server. Other biometrics can also be used to verify identity. A version of the netpage pen includes fingerprint scanning, verified in a similar way by the netpage registration server. 
     Although a netpage printer may deliver periodicals such as the morning newspaper without user intervention, it can be configured never to deliver unsolicited junk mail. In its preferred form, it only delivers periodicals from subscribed or otherwise authorized sources. In this respect, the netpage printer is unlike a fax machine or e-mail account which is visible to any junk mailer who knows the telephone number or email address. 
     1 Netpage System Architecture 
     Each object model in the system is described using a Unified Modeling Language (UML) class diagram. A class diagram consists of a set of object classes connected by relationships, and two kinds of relationships are of interest here: associations and generalizations. An association represents some kind of relationship between objects, i.e. between instances of classes. A generalization relates actual classes, and can be understood in the following way: if a class is thought of as the set of all objects of that class, and class A is a generalization of class B, then B is simply a subset of A. The UML does not directly support second-order modelling—i.e. classes of classes. 
     Each class is drawn as a rectangle labelled with the name of the class. It contains a list of the attributes of the class, separated from the name by a horizontal line, and a list of the operations of the class, separated from the attribute list by a horizontal line. In the class diagrams which follow, however, operations are never modelled. 
     An association is drawn as a line joining two classes, optionally labelled at either end with the multiplicity of the association. The default multiplicity is one. An asterisk (*) indicates a multiplicity of “many”, i.e. zero or more. Each association is optionally labelled with its name, and is also optionally labelled at either end with the role of the corresponding class. An open diamond indicates an aggregation association (“is-part-of”), and is drawn at the aggregator end of the association line. 
     A generalization relationship (“is-a”) is drawn as a solid line joining two classes, with an arrow (in the form of an open triangle) at the generalization end. 
     When a class diagram is broken up into multiple diagrams, any class which is duplicated is shown with a dashed outline in all but the main diagram which defines it. It is shown with attributes only where it is defined. 
     1.1 Netpages 
     Netpages are the foundation on which a netpage network is built. They provide a paper-based user interface to published information and interactive services. 
     A netpage consists of a printed page (or other surface region) invisibly tagged with references to an online description of the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the input elements on the page, including buttons, hyperlinks, and input fields. A netpage allows markings made with a netpage pen on its surface to be simultaneously captured and processed by the netpage system. 
     Multiple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier. This page ID has sufficient precision to distinguish between a very large number of netpages. 
     Each reference to the page description is encoded in a printed tag. The tag identifies the unique page on which it appears, and thereby indirectly identifies the page description. The tag also identifies its own position on the page. Characteristics of the tags are described in more detail below. 
     Tags are printed in infrared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter. 
     A tag is sensed by an area image sensor in the netpage pen, and the tag data is transmitted to the netpage system via the nearest netpage printer. The pen is wireless and communicates with the netpage printer via a short-range radio link. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. Tags are error-correctably encoded to make them partially tolerant to surface damage. 
     The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage. 
     The relationship between the page description, the page instance, and the printed netpage is shown in  FIG. 4 . The printed netpage may be part of a printed netpage document  45 . The page instance is associated with both the netpage printer which printed it and, if known, the netpage user who requested it. 
     As shown in  FIG. 4 , one or more netpages may also be associated with a physical object such as a product item, for example when printed onto the product item&#39;s label, packaging, or actual surface. 
     1.2 Netpage Tags 
     1.2.1 Tag Data Content 
     In a preferred form, each tag identifies the region in which it appears, and the location of that tag within the region. A tag may also contain flags which relate to the region as a whole or to the tag. One or more flag bits may, for example, signal a tag sensing device to provide feedback indicative of a function associated with the immediate area of the tag, without the sensing device having to refer to a description of the region. A netpage pen may, for example, illuminate an “active area” LED when in the zone of a hyperlink. 
     As will be more clearly explained below, in a preferred embodiment, each tag contains an easily recognized invariant structure which aids initial detection, and which assists in minimizing the effect of any warp induced by the surface or by the sensing process. The tags preferably tile the entire page, and are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. It is important that the pen recognize the page ID and position on every interaction with the page, since the interaction is stateless. 
     In a preferred embodiment, the region to which a tag refers coincides with an entire page, and the region ID encoded in the tag is therefore synonymous with the page ID of the page on which the tag appears. In other embodiments, the region to which a tag refers can be an arbitrary subregion of a page or other surface. For example, it can coincide with the zone of an interactive element, in which case the region ID can directly identify the interactive element. 
     In the preferred form, each tag contains 120 bits of information. The region ID is typically allocated up to 100 bits, the tag ID at least 16 bits, and the remaining bits are allocated to flags etc. Assuming a tag density of 64 per square inch, a 16-bit tag ID supports a region size of up to 1024 square inches. Larger regions can be mapped continuously without increasing the tag ID precision simply by using abutting regions and maps. The 100-bit region ID allows 2 100  (˜10 30  or a million trillion trillion) different regions to be uniquely identified. 
     1.2.2 Tag Data Encoding 
     In one embodiment, the 120 bits of tag data are redundantly encoded using a (15, 5) Reed-Solomon code. This yields 360 encoded bits consisting of 6 codewords of 15 4-bit symbols each. The (15, 5) code allows up to 5 symbol errors to be corrected per codeword, i.e. it is tolerant of a symbol error rate of up to 33% per codeword. 
     Each 4-bit symbol is represented in a spatially coherent way in the tag, and the symbols of the six codewords are interleaved spatially within the tag. This ensures that a burst error (an error affecting multiple spatially adjacent bits) damages a minimum number of symbols overall and a minimum number of symbols in any one codeword, thus maximising the likelihood that the burst error can be fully corrected. 
     Any suitable error-correcting code can be used in place of a (15, 5) Reed-Solomon code, for example: a Reed-Solomon code with more or less redundancy, with the same or different symbol and codeword sizes; another block code; or a different kind of code, such as a convolutional code (see, for example, Stephen B. Wicker, Error Control Systems for Digital Communication and Storage, Prentice-Hall 1995, the contents of which a herein incorporated by reference thereto). 
     In order to support “single-click” interaction with a tagged region via a sensing device, the sensing device must be able to see at least one entire tag in its field of view no matter where in the region or at what orientation it is positioned. The required diameter of the field of view of the sensing device is therefore a function of the size and spacing of the tags. 
     1.2.3 Tag Structure 
       FIG. 5   a  shows a tag  4 , in the form of tag  726  with four perspective targets  17 . The tag  726  represents sixty 4-bit Reed-Solomon symbols  747 , for a total of 240 bits. The tag represents each “one” bit by the presence of a mark  748 , referred to as a macrodot, and each “zero” bit by the absence of the corresponding macrodot.  FIG. 5   c  shows a square tiling  728  of nine tags, containing all “one” bits for illustrative purposes. It will be noted that the perspective targets are designed to be shared between adjacent tags.  FIG. 5   d  shows a square tiling of 16 tags and a corresponding minimum field of view  193 , which spans the diagonals of two tags. 
     Using a (15, 7) Reed-Solomon code, 112 bits of tag data are redundantly encoded to produce 240 encoded bits. The four codewords are interleaved spatially within the tag to maximize resilience to burst errors. Assuming a 16-bit tag ID as before, this allows a region ID of up to 92 bits. 
     The data-bearing macrodots  748  of the tag are designed to not overlap their neighbors, so that groups of tags cannot produce structures that resemble targets. This also saves ink. The perspective targets allow detection of the tag, so further targets are not required. 
     Although the tag may contain an orientation feature to allow disambiguation of the four possible orientations of the tag relative to the sensor, the present invention is concerned with embedding orientation data in the tag data. For example, the four codewords can be arranged so that each tag orientation (in a rotational sense) contains one codeword placed at that orientation, as shown in  FIG. 5   a , where each symbol is labelled with the number of its codeword (1-4) and the position of the symbol within the codeword (A-O). Tag decoding then consists of decoding one codeword at each rotational orientation. Each codeword can either contain a single bit indicating whether it is the first codeword, or two bits indicating which codeword it is. The latter approach has the advantage that if, say, the data content of only one codeword is required, then at most two codewords need to be decoded to obtain the desired data. This may be the case if the region ID is not expected to change within a stroke and is thus only decoded at the start of a stroke. Within a stroke only the codeword containing the tag ID is then desired. Furthermore, since the rotation of the sensing device changes slowly and predictably within a stroke, only one codeword typically needs to be decoded per frame. 
     It is possible to dispense with perspective targets altogether and instead rely on the data representation being self-registering. In this case each bit value (or multi-bit value) is typically represented by an explicit glyph, i.e. no bit value is represented by the absence of a glyph. This ensures that the data grid is well-populated, and thus allows the grid to be reliably identified and its perspective distortion detected and subsequently corrected during data sampling. To allow tag boundaries to be detected, each tag data must contain a marker pattern, and these must be redundantly encoded to allow reliable detection. The overhead of such marker patterns is similar to the overhead of explicit perspective targets. Various such schemes are described in the present applicants&#39; co-pending PCT application PCT/AU01/01274 filed 11 Oct. 2001. 
     The arrangement  728  of  FIG. 5   c  shows that the square tag  726  can be used to fully tile or tesselate, i.e. without gaps or overlap, a plane of arbitrary size. 
     Although in preferred embodiments the tagging schemes described herein encode a single data bit using the presence or absence of a single undifferentiated macrodot, they can also use sets of differentiated glyphs to represent single-bit or multi-bit values, such as the sets of glyphs illustrated in the present applicants&#39; co-pending PCT application PCT/AU01/01274 filed 11 Oct. 2001. 
     1.3 The Netpage Network 
     In a preferred embodiment, a netpage network consists of a distributed set of netpage page servers  10 , netpage registration servers  11 , netpage ID servers  12 , netpage application servers  13 , netpage publication servers  14 , Web terminals  75 , netpage printers  601 , and relay devices  44  connected via a network  19  such as the Internet, as shown in  FIG. 3 . 
     The netpage registration server  11  is a server which records relationships between users, pens, printers, applications and publications, and thereby authorizes various network activities. It authenticates users and acts as a signing proxy on behalf of authenticated users in application transactions. It also provides handwriting recognition services. As described above, a netpage page server  10  maintains persistent information about page descriptions and page instances. The netpage network includes any number of page servers, each handling a subset of page instances. Since a page server also maintains user input values for each page instance, clients such as netpage printers send netpage input directly to the appropriate page server. The page server interprets any such input relative to the description of the corresponding page. 
     A netpage ID server  12  allocates document IDs  51  on demand, and provides load-balancing of page servers via its ID allocation scheme. 
     A netpage printer uses the Internet Distributed Name System (DNS), or similar, to resolve a netpage page ID  50  into the network address of the netpage page server handling the corresponding page instance. 
     A netpage application server  13  is a server which hosts interactive netpage applications. A netpage publication server  14  is an application server which publishes netpage documents to netpage printers. 
     Netpage servers can be hosted on a variety of network server platforms from manufacturers such as IBM, Hewlett-Packard, and Sun. Multiple netpage servers can run concurrently on a single host, and a single server can be distributed over a number of hosts. Some or all of the functionality provided by netpage servers, and in particular the functionality provided by the ID server and the page server, can also be provided directly in a netpage appliance such as a netpage printer, in a computer workstation, or on a local network. 
     1.4 The Netpage Printer 
     The netpage printer  601  is an appliance which is registered with the netpage system and prints netpage documents on demand and via subscription. Each printer has a unique printer ID  62 , and is connected to the netpage network via a network such as the Internet, ideally via a broadband connection. 
     Apart from identity and security settings in non-volatile memory, the netpage printer contains no persistent storage. As far as a user is concerned, “the network is the computer”. Netpages function interactively across space and time with the help of the distributed netpage page servers  10 , independently of particular netpage printers. 
     The netpage printer receives subscribed netpage documents from netpage publication servers  14 . Each document is distributed in two parts: the page layouts, and the actual text and image objects which populate the pages. Because of personalization, page layouts are typically specific to a particular subscriber and so are pointcast to the subscriber&#39;s printer via the appropriate page server. Text and image objects, on the other hand, are typically shared with other subscribers, and so are multicast to all subscribers&#39; printers and the appropriate page servers. 
     The netpage publication server optimizes the segmentation of document content into pointcasts and multicasts. After receiving the pointcast of a document&#39;s page layouts, the printer knows which multicasts, if any, to listen to. 
     Once the printer has received the complete page layouts and objects that define the document to be printed, it can print the document. 
     The printer rasterizes and prints odd and even pages simultaneously on both sides of the sheet. It contains duplexed print engine controllers  760  and print engines utilizing Memjet™ printheads  350  for this purpose. 
     The printing process consists of two decoupled stages: rasterization of page descriptions, and expansion and printing of page images. The raster image processor (RIP) consists of one or more standard DSPs  757  running in parallel. The duplexed print engine controllers consist of custom processors which expand, dither and print page images in real time, synchronized with the operation of the printheads in the print engines. 
     Printers not enabled for IR printing have the option to print tags using IR-absorptive black ink, although this restricts tags to otherwise empty areas of the page. Although such pages have more limited functionality than IR-printed pages, they are still classed as netpages. 
     A normal netpage printer prints netpages on sheets of paper. More specialised netpage printers may print onto more specialised surfaces, such as globes. Each printer supports at least one surface type, and supports at least one tag tiling scheme, and hence tag map, for each surface type. The tag map  811  which describes the tag tiling scheme actually used to print a document becomes associated with that document so that the document&#39;s tags can be correctly interpreted. 
       FIG. 2  shows the netpage printer class diagram, reflecting printer-related information maintained by a registration server  11  on the netpage network. 
     1.5 The Netpage Pen 
     The active sensing device of the netpage system is typically a pen  101 , which, using its embedded controller  134 , is able to capture and decode IR position tags from a page via an image sensor. The image sensor is a solid-state device provided with an appropriate filter to permit sensing at only near-infrared wavelengths. As described in more detail below, the system is able to sense when the nib is in contact with the surface, and the pen is able to sense tags at a sufficient rate to capture human handwriting (i.e. at 200 dpi or greater and 100 Hz or faster). Information captured by the pen is encrypted and wirelessly transmitted to the printer (or base station), the printer or base station interpreting the data with respect to the (known) page structure. 
     The preferred embodiment of the netpage pen operates both as a normal marking ink pen and as a non-marking stylus. The marking aspect, however, is not necessary for using the netpage system as a browsing system, such as when it is used as an Internet interface. Each netpage pen is registered with the netpage system and has a unique pen ID  61 .  FIG. 14  shows the netpage pen class diagram, reflecting pen-related information maintained by a registration server  11  on the netpage network. 
     When either nib is in contact with a netpage, the pen determines its position and orientation relative to the page. The nib is attached to a force sensor, and the force on the nib is interpreted relative to a threshold to indicate whether the pen is “up” or “down”. This allows a interactive element on the page to be ‘clicked’ by pressing with the pen nib, in order to request, say, information from a network. Furthermore, the force is captured as a continuous value to allow, say, the full dynamics of a signature to be verified. 
     The pen determines the position and orientation of its nib on the netpage by imaging, in the infrared spectrum, an area  193  of the page in the vicinity of the nib. It decodes the nearest tag and computes the position of the nib relative to the tag from the observed perspective distortion on the imaged tag and the known geometry of the pen optics. Although the position resolution of the tag may be low, because the tag density on the page is inversely proportional to the tag size, the adjusted position resolution is quite high, exceeding the minimum resolution required for accurate handwriting recognition. 
     Pen actions relative to a netpage are captured as a series of strokes. A stroke consists of a sequence of time-stamped pen positions on the page, initiated by a pen-down event and completed by the subsequent pen-up event. A stroke is also tagged with the page ID  50  of the netpage whenever the page ID changes, which, under normal circumstances, is at the commencement of the stroke. 
     Each netpage pen has a current selection  826  associated with it, allowing the user to perform copy and paste operations etc. The selection is timestamped to allow the system to discard it after a defined time period. The current selection describes a region of a page instance. It consists of the most recent digital ink stroke captured through the pen relative to the background area of the page. It is interpreted in an application-specific manner once it is submitted to an application via a selection hyperlink activation. 
     Each pen has a current nib  824 . This is the nib last notified by the pen to the system. In the case of the default netpage pen described above, either the marking black ink nib or the non-marking stylus nib is current. Each pen also has a current nib style  825 . This is the nib style last associated with the pen by an application, e.g. in response to the user selecting a color from a palette. The default nib style is the nib style associated with the current nib. Strokes captured through a pen are tagged with the current nib style. When the strokes are subsequently reproduced, they are reproduced in the nib style with which they are tagged. 
     Whenever the pen is within range of a printer with which it can communicate, the pen slowly flashes its “online” LED. When the pen fails to decode a stroke relative to the page, it momentarily activates its “error” LED. When the pen succeeds in decoding a stroke relative to the page, it momentarily activates its “ok” LED. 
     A sequence of captured strokes is referred to as digital ink. Digital ink forms the basis for the digital exchange of drawings and handwriting, for online recognition of handwriting, and for online verification of signatures. 
     The pen is wireless and transmits digital ink to the netpage printer via a short-range radio link. The transmitted digital ink is encrypted for privacy and security and packetized for efficient transmission, but is always flushed on a pen-up event to ensure timely handling in the printer. 
     When the pen is out-of-range of a printer it buffers digital ink in internal memory, which has a capacity of over ten minutes of continuous handwriting. When the pen is once again within range of a printer, it transfers any buffered digital ink. 
     A pen can be registered with any number of printers, but because all state data resides in netpages both on paper and on the network, it is largely immaterial which printer a pen is communicating with at any particular time. 
     A preferred embodiment of the pen is described in greater detail below, with reference to  FIGS. 6 to 8 . 
     1.6 Netpage Interaction 
     The netpage printer  601  receives data relating to a stroke from the pen  101  when the pen is used to interact with a netpage  1 . The coded data  3  of the tags  4  is read by the pen when it is used to execute a movement, such as a stroke. The data allows the identity of the particular page and associated interactive element to be determined and an indication of the relative positioning of the pen relative to the page to be obtained. The indicating data is transmitted to the printer, where it resolves, via the DNS, the page ID  50  of the stroke into the network address of the netpage page server  10  which maintains the corresponding page instance  830 . It then transmits the stroke to the page server. If the page was recently identified in an earlier stroke, then the printer may already have the address of the relevant page server in its cache. Each netpage consists of a compact page layout maintained persistently by a netpage page server (see below). The page layout refers to objects such as images, fonts and pieces of text, typically stored elsewhere on the netpage network. 
     When the page server receives the stroke from the pen, it retrieves the page description to which the stroke applies, and determines which element of the page description the stroke intersects. It is then able to interpret the stroke in the context of the type of the relevant element. 
     A “click” is a stroke where the distance and time between the pen down position and the subsequent pen up position are both less than some small maximum. An object which is activated by a click typically requires a click to be activated, and accordingly, a longer stroke is ignored. The failure of a pen action, such as a “sloppy” click, to register is indicated by the lack of response from the pen&#39;s “ok” LED. 
     There are two kinds of input elements in a netpage page description: hyperlinks and form fields. Input through a form field can also trigger the activation of an associated hyperlink. 
     2 Netpage Pen Description 
     2.1 Pen Mechanics 
     Referring to  FIGS. 6 and 7 , the pen, generally designated by reference numeral  101 , includes a housing  102  in the form of a plastics moulding having walls  103  defining an interior space  104  for mounting the pen components. The pen top  105  is in operation rotatably mounted at one end  106  of the housing  102 . A semi-transparent cover  107  is secured to the opposite end  108  of the housing  102 . The cover  107  is also of moulded plastics, and is formed from semi-transparent material in order to enable the user to view the status of the LED mounted within the housing  102 . The cover  107  includes a main part  109  which substantially surrounds the end  108  of the housing  102  and a projecting portion  110  which projects back from the main part  109  and fits within a corresponding slot  111  formed in the walls  103  of the housing  102 . A radio antenna  112  is mounted behind the projecting portion  110 , within the housing  102 . Screw threads  113  surrounding an aperture  113 A on the cover  107  are arranged to receive a metal end piece  114 , including corresponding screw threads  115 . The metal end piece  114  is removable to enable ink cartridge replacement. 
     Also mounted within the cover  107  is a tri-color status LED  116  on a flex PCB  117 . The antenna  112  is also mounted on the flex PCB  117 . The status LED  116  is mounted at the top of the pen  101  for good all-around visibility. 
     The pen can operate both as a normal marking ink pen and as a non-marking stylus. An ink pen cartridge  118  with nib  119  and a stylus  120  with stylus nib  121  are mounted side by side within the housing  102 . Either the ink cartridge nib  119  or the stylus nib  121  can be brought forward through open end  122  of the metal end piece  114 , by rotation of the pen top  105 . Respective slider blocks  123  and  124  are mounted to the ink cartridge  118  and stylus  120 , respectively. A rotatable cam barrel  125  is secured to the pen top  105  in operation and arranged to rotate therewith. The cam barrel  125  includes a cam  126  in the form of a slot within the walls  181  of the cam barrel. Cam followers  127  and  128  projecting from slider blocks  123  and  124  fit within the cam slot  126 . On rotation of the cam barrel  125 , the slider blocks  123  or  124  move relative to each other to project either the pen nib  119  or stylus nib  121  out through the hole  122  in the metal end piece  114 . The pen  101  has three states of operation. By turning the top  105  through 90° steps, the three states are:
         stylus  120  nib  121  out   ink cartridge  118  nib  119  out, and   neither ink cartridge  118  nib  119  out nor stylus  120  nib  121  out       

     A second flex PCB  129 , is mounted on an electronics chassis  130  which sits within the housing  102 . The second flex PCB  129  mounts an infrared LED  131  for providing infrared radiation for projection onto the surface. An image sensor  132  is provided mounted on the second flex PCB  129  for receiving reflected radiation from the surface. The second flex PCB  129  also mounts a radio frequency chip  133 , which includes an RF transmitter and RF receiver, and a controller chip  134  for controlling operation of the pen  101 . An optics block  135  (formed from moulded clear plastics) sits within the cover  107  and projects an infrared beam onto the surface and receives images onto the image sensor  132 . Power supply wires  136  connect the components on the second flex PCB  129  to battery contacts  137  which are mounted within the cam barrel  125 . A terminal  138  connects to the battery contacts  137  and the cam barrel  125 . A three volt rechargeable battery  139  sits within the cam barrel  125  in contact with the battery contacts. An induction charging coil  140  is mounted about the second flex PCB  129  to enable recharging of the battery  139  via induction. The second flex PCB  129  also mounts an infrared LED  143  and infrared photodiode  144  for detecting displacement in the cam barrel  125  when either the stylus  120  or the ink cartridge  118  is used for writing, in order to enable a determination of the force being applied to the surface by the pen nib  119  or stylus nib  121 . The IR photodiode  144  detects light from the IR LED  143  via reflectors (not shown) mounted on the slider blocks  123  and  124 . 
     Rubber grip pads  141  and  142  are provided towards the end  108  of the housing  102  to assist gripping the pen  101 , and top  105  also includes a clip  142  for clipping the pen  101  to a pocket. 
     3.2 Pen Controller 
     The pen  101  is arranged to determine the position of its nib (stylus nib  121  or ink cartridge nib  119 ) by imaging, in the infrared spectrum, an area of the surface in the vicinity of the nib. It records the location data from the nearest location tag, and is arranged to calculate the distance of the nib  121  or  119  from the location tab utilising optics  135  and controller chip  134 . The controller chip  134  calculates the orientation of the pen and the nib-to-tag distance from the perspective distortion observed on the imaged tag. 
     Utilising the RF chip  133  and antenna  112  the pen  101  can transmit the digital ink data (which is encrypted for security and packaged for efficient transmission) to the computing system. 
     When the pen is in range of a receiver, the digital ink data is transmitted as it is formed. When the pen  101  moves out of range, digital ink data is buffered within the pen  101  (the pen  101  circuitry includes a buffer arranged to store digital ink data for approximately 12 minutes of the pen motion on the surface) and can be transmitted later. 
     The controller chip  134  is mounted on the second flex PCB  129  in the pen  101 .  FIG. 8  is a block diagram illustrating in more detail the architecture of the controller chip  134 .  FIG. 8  also shows representations of the RF chip  133 , the image sensor  132 , the tri-color status LED  116 , the IR illumination LED  131 , the IR force sensor LED  143 , and the force sensor photodiode  144 . 
     The pen controller chip  134  includes a controlling processor  145 . Bus  146  enables the exchange of data between components of the controller chip  134 . Flash memory  147  and a 512 KB DRAM  148  are also included. An analog-to-digital converter  149  is arranged to convert the analog signal from the force sensor photodiode  144  to a digital signal. 
     An image sensor interface  152  interfaces with the image sensor  132 . A transceiver controller  153  and base band circuit  154  are also included to interface with the RF chip  133  which includes an RF circuit  155  and RF resonators and inductors  156  connected to the antenna  112 . 
     The controlling processor  145  captures and decodes location data from tags from the surface via the image sensor  132 , monitors the force sensor photodiode  144 , controls the LEDs  116 ,  131  and  143 , and handles short-range radio communication via the radio transceiver  153 . It is a medium-performance (˜40 MHz) general-purpose RISC processor. 
     The processor  145 , digital transceiver components (transceiver controller  153  and baseband circuit  154 ), image sensor interface  152 , flash memory  147  and 512 KB DRAM  148  are integrated in a single controller ASIC. Analog RF components (RF circuit  155  and RF resonators and inductors  156 ) are provided in the separate RF chip. 
     The image sensor is a CCD or CMOS image sensor. Depending on tagging scheme, it has a size ranging from about 100×100 pixels to 200×200 pixels. Many miniature CMOS image sensors are commercially available, including the National Semiconductor LM9630. 
     The controller ASIC  134  enters a quiescent state after a period of inactivity when the pen  101  is not in contact with a surface. It incorporates a dedicated circuit  150  which monitors the force sensor photodiode  144  and wakes up the controller  134  via the power manager  151  on a pen-down event. 
     The radio transceiver communicates in the unlicensed 900 MHz band normally used by cordless telephones, or alternatively in the unlicensed 2.4 GHz industrial, scientific and medical (ISM) band, and uses frequency hopping and collision detection to provide interference-free communication. 
     In an alternative embodiment, the pen incorporates an Infrared Data Association (IrDA) interface for short-range communication with a base station or netpage printer. 
     In a further embodiment, the pen  101  includes a pair of orthogonal accelerometers mounted in the normal plane of the pen  101  axis. The accelerometers  190  are shown in  FIGS. 7 and 8  in ghost outline. 
     The provision of the accelerometers enables this embodiment of the pen  101  to sense motion without reference to surface location tags, allowing the location tags to be sampled at a lower rate. Each location tag ID can then identify an object of interest rather than a position on the surface. For example, if the object is a user interface input element (e.g. a command button), then the tag ID of each location tag within the area of the input element can directly identify the input element. 
     The acceleration measured by the accelerometers in each of the x and y directions is integrated with respect to time to produce an instantaneous velocity and position. 
     Since the starting position of the stroke is not known, only relative positions within a stroke are calculated. Although position integration accumulates errors in the sensed acceleration, accelerometers typically have high resolution, and the time duration of a stroke, over which errors accumulate, is short. 
     3 Netpage Printer Description 
     3.1 Printer Mechanics 
     The vertically-mounted netpage wallprinter  601  is shown fully assembled in  FIG. 9 . It prints netpages on Letter/A4 sized media using duplexed 8½″ Memjet™ print engines  602  and  603 , as shown in  FIGS. 10 and 10   a . It uses a straight paper path with the paper  604  passing through the duplexed print engines  602  and  603  which print both sides of a sheet simultaneously, in full color and with full bleed. 
     An integral binding assembly  605  applies a strip of glue along one edge of each printed sheet, allowing it to adhere to the previous sheet when pressed against it. This creates a final bound document  618  which can range in thickness from one sheet to several hundred sheets. 
     The replaceable ink cartridge  627 , shown in  FIG. 12  coupled with the duplexed print engines, has bladders or chambers for storing fixative, adhesive, and cyan, magenta, yellow, black and infrared inks. The cartridge also contains a micro air filter in a base molding. The micro air filter interfaces with an air pump  638  inside the printer via a hose  639 . This provides filtered air to the printheads to prevent ingress of micro particles into the Memjet™ printheads  350  which might otherwise clog the printhead nozzles. By incorporating the air filter within the cartridge, the operational life of the filter is effectively linked to the life of the cartridge. The ink cartridge is a fully recyclable product with a capacity for printing and gluing 3000 pages (1500 sheets). 
     Referring to  FIG. 10 , the motorized media pick-up roller assembly  626  pushes the top sheet directly from the media tray past a paper sensor on the first print engine  602  into the duplexed Memjet™ printhead assembly. The two Memjet™ print engines  602  and  603  are mounted in an opposing in-line sequential configuration along the straight paper path. The paper  604  is drawn into the first print engine  602  by integral, powered pick-up rollers  626 . The position and size of the paper  604  is sensed and full bleed printing commences. Fixative is printed simultaneously to aid drying in the shortest possible time. 
     The paper exits the first Memjet™ print engine  602  through a set of powered exit spike wheels (aligned along the straight paper path), which act against a rubberized roller. These spike wheels contact the ‘wet’ printed surface and continue to feed the sheet  604  into the second Memjet™ print engine  603 . 
     Referring to  FIGS. 10 and 10   a , the paper  604  passes from the duplexed print engines  602  and  603  into the binder assembly  605 . The printed page passes between a powered spike wheel axle  670  with a fibrous support roller and another movable axle with spike wheels and a momentary action glue wheel. The movable axle/glue assembly  673  is mounted to a metal support bracket and it is transported forward to interface with the powered axle  670  via gears by action of a camshaft. A separate motor powers this camshaft. 
     The glue wheel assembly  673  consists of a partially hollow axle  679  with a rotating coupling for the glue supply hose  641  from the ink cartridge  627 . This axle  679  connects to a glue wheel, which absorbs adhesive by capillary action through radial holes. A molded housing  682  surrounds the glue wheel, with an opening at the front. Pivoting side moldings and sprung outer doors are attached to the metal bracket and hinge out sideways when the rest of the assembly  673  is thrust forward. This action exposes the glue wheel through the front of the molded housing  682 . Tension springs close the assembly and effectively cap the glue wheel during periods of inactivity. 
     As the sheet  604  passes into the glue wheel assembly  673 , adhesive is applied to one vertical edge on the front side (apart from the first sheet of a document) as it is transported down into the binding assembly  605 . 
     4 Product Tagging 
     Automatic identification refers to the use of technologies such as bar codes, magnetic stripe cards, smartcards, and RF transponders, to (semi-)automatically identify objects to data processing systems without manual keying. 
     For the purposes of automatic identification, a product item is commonly identified by a 12-digit Universal Product Code (UPC), encoded machine-readably in the form of a printed bar code. The most common UPC numbering system incorporates a 5-digit manufacturer number and a 5-digit item number. Because of its limited precision, a UPC is used to identify a class of product rather than an individual product item. The Uniform Code Council and EAN International define and administer the UPC and related codes as subsets of the 14-digit Global Trade Item Number (GTIN). 
     Within supply chain management, there is considerable interest in expanding or replacing the UPC scheme to allow individual product items to be uniquely identified and thereby tracked. Individual item tagging can reduce “shrinkage” due to lost, stolen or spoiled goods, improve the efficiency of demand-driven manufacturing and supply, facilitate the profiling of product usage, and improve the customer experience. 
     There are two main contenders for individual item tagging: optical tags in the form of so-called two-dimensional bar codes, and radio frequency identification (RFID) tags. For a detailed description of RFID tags, refer to Klaus Finkenzeller,  RFID Handbook , John Wiley &amp; Son (1999), the contents of which are herein incorporated by cross-reference. Optical tags have the advantage of being inexpensive, but require optical line-of-sight for reading. RFID tags have the advantage of supporting omnidirectional reading, but are comparatively expensive. The presence of metal or liquid can seriously interfere with RFID tag performance, undermining the omnidirectional reading advantage. Passive (reader-powered) RFID tags are projected to be priced at 10 cents each in multi-million quantities by the end of 2003, and at 5 cents each soon thereafter, but this still falls short of the sub-one-cent industry target for low-price items such as grocery. The read-only nature of most optical tags has also been cited as a disadvantage, since status changes cannot be written to a tag as an item progresses through the supply chain. However, this disadvantage is mitigated by the fact that a read-only tag can refer to information maintained dynamically on a network. 
     The Massachusetts Institute of Technology (MIT) Auto-ID Center has developed a standard for a 96-bit Electronic Product Code (EPC), coupled with an Internet-based Object Name Service (ONS) and a Product Markup Language (PML). Once an EPC is scanned or otherwise obtained, it is used to look up, possibly via the ONS, matching product information portably encoded in PML. The EPC consists of an 8-bit header, a 28-bit EPC manager, a 24-bit object class, and a 36-bit serial number. For a detailed description of the EPC, refer to Brock, D. L.,  The Electronic Product Code  ( EPC ), MIT Auto-ID Center (January 2001), the contents of which are herein incorporated by cross-reference. The Auto-ID Center has defined a mapping of the GTIN onto the EPC to demonstrate compatibility between the EPC and current practices Brock, D. L.,  Integrating the Electronic Product Code  ( EPC )  and the Global Trade Item Number  ( GTIN ), MIT Auto-ID Center (November 2001), the contents of which are herein incorporated by cross-reference. The EPC is administered by EPCglobal, an EAN-UCC joint venture. 
     EPCs are technology-neutral and can be encoded and carried in many forms. The Auto-ID Center strongly advocates the use of low-cost passive RFID tags to carry EPCs, and has defined a 64-bit version of the EPC to allow the cost of RFID tags to be minimized in the short term. For detailed description of low-cost RFID tag characteristics, refer to Sarma, S.,  Towards the  5 c Tag , MIT Auto-ID Center (November 2001), the contents of which are herein incorporated by cross-reference. For a description of a commercially-available low-cost passive RFID tag, refer to 915  MHz RFID Tag , Alien Technology (2002), the contents of which are herein incorporated by cross-reference. For detailed description of the 64-bit EPC, refer to Brock, D. L.,  The Compact Electronic Product Code , MIT Auto-ID Center (November 2001), the contents of which are herein incorporated by cross-reference. 
     EPCs are intended not just for unique item-level tagging and tracking, but also for case-level and pallet-level tagging, and for tagging of other logistic units of shipping and transportation such as containers and trucks. The distributed PML database records dynamic relationships between items and higher-level containers in the packaging, shipping and transportation hierarchy. 
     4.1 Omnitagging in the Supply Chain 
     Using an invisible (e.g. infrared) tagging scheme to uniquely identify a product item has the significant advantage that it allows the entire surface of a product to be tagged, or a significant portion thereof, without impinging on the graphic design of the product&#39;s packaging or labelling. If the entire product surface is tagged, then the orientation of the product doesn&#39;t affect its ability to be scanned, i.e. a significant part of the line-of-sight disadvantage of a visible bar code is eliminated. Furthermore, since the tags are small and massively replicated, label damage no longer prevents scanning. 
     Omnitagging, then, consists of covering a large proportion of the surface of a product item with optically-readable invisible tags. Each omnitag uniquely identifies the product item on which it appears. The omnitag may directly encode the product code (e.g. EPC) of the item, or may encode a surrogate ID which in turn identifies the product code via a database lookup. Each omnitag also optionally identifies its own position on the surface of the product item, to provide the downstream consumer benefits of netpage interactivity described earlier. 
     Omnitags are applied during product manufacture and/or packaging using digital printers. These may be add-on infrared printers which print the omnitags after the text and graphics have been printed by other means, or integrated color and infrared printers which print the omnitags, text and graphics simultaneously. Digitally-printed text and graphics may include everything on the label or packaging, or may consist only of the variable portions, with other portions still printed by other means. 
     4.2 Omnitagging 
     As shown in  FIG. 13 , a product&#39;s unique item ID  215  may be seen as a special kind of unique object ID  210 . The Electronic Product Code (EPC)  220  is one emerging standard for an item ID. An item ID typically consists of a product ID  214  and a serial number  213 . The product ID identifies a class of product, while the serial number identifies a particular instance of that class, i.e. an individual product item. The product ID in turn typically consists of a manufacturer number  211  and a product class number  212 . The best-known product ID is the EAN.UCC Universal Product Code (UPC)  221  and its variants. 
     As shown in  FIG. 14 , an omnitag  202  encodes a page ID (or region ID)  50  and a two-dimensional (2D) position  86 . The region ID identifies the surface region containing the tag, and the position identifies the tag&#39;s position within the two-dimensional region. Since the surface in question is the surface of a physical product item  201 , it is useful to define a one-to-one mapping between the region ID and the unique object ID  210 , and more specifically the item ID  215 , of the product item. Note, however, that the mapping can be many-to-one without compromising the utility of the omnitag. For example, each panel of a product item&#39;s packaging could have a different region ID  50 . Conversely, the omnitag may directly encode the item ID, in which case the region ID contains the item ID, suitably prefixed to decouple item ID allocation from general netpage region ID allocation. Note that the region ID uniquely distinguishes the corresponding surface region from all other surface regions identified within the global netpage system. 
     The item ID  215  is preferably the EPC  220  proposed by the Auto-ID Center, since this provides direct compatibility between omnitags and EPC-carrying RFID tags. 
     In  FIG. 14  the position  86  is shown as optional. This is to indicate that much of the utility of the omnitag in the supply chain derives from the region ID  50 , and the position may be omitted if not desired for a particular product. 
     For interoperability with the netpage system, an omnitag  202  is a netpage tag  4 , i.e. it has the logical structure, physical layout and semantics of a netpage tag. 
     When a netpage sensing device such as the netpage pen  101  images and decodes an omnitag, it uses the position and orientation of the tag in its field of view and combines this with the position encoded in the tag to compute its own position relative to the tag. As the sensing device is moved relative to a Hyperlabelled surface region, it is thereby able to track its own motion relative to the region and generate a set of timestamped position samples representative of its time-varying path. When the sensing device is a pen, then the path consists of a sequence of strokes, with each stroke starting when the pen makes contact with the surface, and ending when the pen breaks contact with the surface. 
     When a stroke is forwarded to the page server  10  responsible for the region ID, the server retrieves a description of the region keyed by region ID, and interprets the stroke in relation to the description. For example, if the description includes a hyperlink and the stroke intersects the zone of the hyperlink, then the server may interpret the stroke as a designation of the hyperlink and activate the hyperlink. 
     4.3 Omnitag Printing 
     An omnitag printer is a digital printer which prints omnitags onto the label, packaging or actual surface of a product before, during or after product manufacture and/or assembly. It is a special case of a netpage printer  601 . It is capable of printing a continuous pattern of omnitags onto a surface, typically using a near-infrared-absorptive ink. In high-speed environments, the printer includes hardware which accelerates tag rendering. This typically includes real-time Reed-Solomon encoding of variable tag data such as tag position, and real-time template-based rendering of the actual tag pattern at the dot resolution of the printhead. 
     The printer may be an add-on infrared printer which prints the omnitags after text and graphics have been printed by other means, or an integrated color and infrared printer which prints the omnitags, text and graphics simultaneously. Digitally-printed text and graphics may include everything on the label or packaging, or may consist only of the variable portions, with other portions still printed by other means. Thus an omnitag printer with an infrared and black printing capability can displace an existing digital printer used for variable data printing, such as a conventional thermal transfer or inkjet printer. 
     For the purposes of the following discussion, any reference to printing onto an item label is intended to include printing onto the item packaging in general, or directly onto the item surface. Furthermore, any reference to an item ID  215  is intended to include a region ID  50  (or collection of per-panel region ids), or a component thereof. 
     The printer is typically controlled by a host computer, which supplies the printer with fixed and/or variable text and graphics as well as item ids for inclusion in the omnitags. The host may provide real-time control over the printer, whereby it provides the printer with data in real time as printing proceeds. As an optimisation, the host may provide the printer with fixed data before printing begins, and only provide variable data in real time. The printer may also be capable of generating per-item variable data based on parameters provided by the host. For example, the host may provide the printer with a base item ID prior to printing, and the printer may simply increment the base item ID to generate successive item ids. Alternatively, memory in the ink cartridge or other storage medium inserted into the printer may provide a source of unique item ids, in which case the printer reports the assignment of items ids to the host computer for recording by the host. 
     Alternatively still, the printer may be capable of reading a pre-existing item ID from the label onto which the omnitags are being printed, assuming the unique ID has been applied in some form to the label during a previous manufacturing step. For example, the item ID may already be present in the form of a visible 2D bar code, or encoded in an RFID tag. In the former case the printer can include an optical bar code scanner. In the latter case it can include an RFID reader. 
     The printer may also be capable of rendering the item ID in other forms. For example, it may be capable of printing the item ID in the form of a 2D bar code, or of printing the product ID component of the item ID in the form of a ID bar code, or of writing the item ID to a writable or write-once RFID tag. 
     4.4 Omnitag Scanning 
     Item information typically flows to the product server in response to situated scan events, e.g. when an item is scanned into inventory on delivery; when the item is placed on a retail shelf, and when the item is scanned at point of sale. Both fixed and hand-held scanners may be used to scan omnitagged product items, using both laser-based 2D scanning and 2D image-sensor-based scanning, using similar or the same techniques as employed in the netpage pen. 
     As shown in  FIG. 16 , both a fixed scanner  254  and a hand-held scanner  252  communicate scan data to the product server  251 . The product server may in turn communicate product item event data to a peer product server (not shown), or to a product application server  250 , which may implement sharing of data with related product servers. For example, stock movements within a retail store may be recorded locally on the retail store&#39;s product server, but the manufacturer&#39;s product server may be notified once a product item is sold. 
     4.5 Omnitag-Based Netpage Interactions 
     A product item whose labelling, packaging or actual surface has been omnitagged provides the same level of interactivity as any other netpage. 
     There is a strong case to be made for netpage-compatible product tagging. Netpage turns any printed surface into a finely differentiated graphical user interface akin to a Web page, and there are many applications which map nicely onto the surface of a product. These applications include obtaining product information of various kinds (nutritional information; cooking instructions; recipes; related products; use-by dates; servicing instructions; recall notices); playing games; entering competitions; managing ownership (registration; query, such as in the case of stolen goods; transfer); providing product feedback; messaging; and indirect device control. If, on the other hand, the product tagging is undifferentiated, such as in the case of an undifferentiated 2D barcode or RFID-carried item ID, then the burden of information navigation is transferred to the information delivery device, which may significantly increase the complexity of the user experience or the required sophistication of the delivery device user interface. 
     The invention will now be described with reference to the following examples. However, it will of course be appreciated that this invention may be embodied in many other forms without departing from the scope of the invention, as defined in the accompanying claims. 
     EXAMPLES 
     In the following examples, uv-visible spectra are reported conventionally by stating an absorption wavelength first, followed by the corresponding log ε max  in parentheses. For example, “760 (5.11)” denotes an absorption at 760 nm having a log ε max  of 5.11. 
     Example 1 
     
       
                 
         
             
             
         
      
     
     Lithium metal (78.2 mg; 11 mmol) was added portionwise to a boiling solution of the phthalonitrile (203 mg; 0.49 mmol) in n-butanol (10 mL). After 45 min the reaction mixture was cooled, and diluted with water (20 mL) and acetic acid (5 mL). The dark mixture was poured into water (200 mL) and extracted with chloroform (3×150 mL). The combined extracts were dried (Na 2 SO 4 ) and the solvents were removed under high vacuum. The crude phthalocyanine was purified by column chromatography [alumina (activity grade I), toluene] to give the pure product as an apple-green powder (90 mg; 47%). λ max  760 (5.11), 732 (5.07), 693 (4.54), 660 (4.48), 402 (4.61), 331 (4.62);  1 H NMR spectrum (CDCl 3 ) δ −0.20 (2H, s, NH), 1.16 (24H, t, J=7.3 Hz, 8×CH 3 ), 1.69 (16H, sxt, J=7.3 Hz, 8×CH 2 ), 2.20 (16H, qnt, J=7.3 Hz, 8×CH 2 ), 4.93 (16H, t, J=7.3 Hz, 8×CH 2 O), 6.50 (8H, s, 8×CH), 7.10-7.13 (16H, m, Ar—H), 7.63-7.65 (16H, m, Ar—H). 
     Example 2 
     
       
                 
         
             
             
         
      
     
     H 2 Pc(dib) 4 (OBu) 8  (113 mg; 0.071 mmol) was suspended in dry DMF (5 mL) and then vanadyl acetylacetonate (97 mg; 0.37 mmol) and tributylamine (500 μL) were added consecutively with stirring. The resulting mixture was heated under reflux overnight, cooled and diluted with dichloromethane (200 mL). The solution was washed with water (100 mL), HCl (0.1 M; 2×100 mL) and saturated NaHCO 3  (100 mL), and dried (MgSO 4 ). Removal of the solvent left a dark green solid that was dissolved in toluene and purified by column chromatography on neutral alumina (toluene). The first green band contained the product and removal of the solvent afforded a green powder (90 mg; 76%).  1 H NMR (CDCl 3 ) (all signals are broad) δ1.15-1.26; 1.67-1.76; 2.1-2.4; 4.9-5.0; 6.4-6.7; 7.10-7.23; 7.64-7.70; λ max  768 (5.2), 688 (4.6), 401 (4.6), 357 (4.6), 331 (4.7) nm. 
     Example 3 
     
       
                 
         
             
             
         
      
     
     VOPc(dib) 4 (OBu) 8  (22.7 mg; 13.7 μmol) in oleum (1.5 mL) was stirred at room temperature for 1 h. The deep blue solution containing the sulfonated derivative (presumed to be VOPc(dib) 4 (OBu) 8 S 8 ) was first analysed by taking an aliquot and diluting it in DMSO to give a 10 μM solution. The resulting purple solution had λ max  843, 746 nm. The remainder of the reaction solution was quenched by carefully adding it to ice (5 g) and washing with sulfuric acid (98%, 0.5 mL) giving a total volume of 6 mL. Aliquots (4×1 mL) of this solution were diluted to 5 mL with either water (2× samples) or ink-base (2× samples). One of each pair of samples was neutralised with solid NaHCO 3  to pH 7-8 while the other samples remained acidic (Table 1). The final concentration of each sample was ca. 4 mg in 5 mL (ca. 0.1% w/v). Each solution was applied to plain paper (80 gsm) with a brush and reflectance spectra were recorded on a Cary 5E spectrometer with an integrating sphere. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Composition of ink-base 
               
             
          
           
               
                   
                 ingredient 
                 Concentration (v/v) 
               
               
                   
                   
               
             
          
           
               
                   
                 Polyethylene glycol 400 
                 9.0% 
               
               
                   
                 1,2-hexanediol 
                 6.0% 
               
               
                   
                 glycerol 
                 6.0% 
               
               
                   
                 triethylene glycol monomethyl ether 
                 2.0% 
               
               
                   
                 triethylene glycol 
                 1.0% 
               
               
                   
                 Surfynol ® surfactant 
                 0.5% 
               
               
                   
                 0.01M acetic acid 
                 74.5% 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Test solutions for reflectance vis-NIR spectroscopy 
               
             
          
           
               
                 sample 
                 medium 
                 additive 
                 pH 
               
               
                   
               
               
                 1 
                 Water 
                 None 
                 &lt;2 
               
               
                 2 
                 Water 
                 NaHCO 3   
                 7-8 
               
               
                 3 
                 Ink-base 
                 None 
                 &lt;2 
               
               
                 4 
                 Ink-base 
                 NaHCO 3   
                 7-8