PHOTOGRAPH PROCESSING AND COPYING SYSTEMS

Abstract: A digital color photograph printer (mini-lab) and a photograph copier which uses a drop on concurrent drop selection and drop separation demand printing mechanism. The system consists of a color continuous tone scanner, a scanner signal conditioning unit, a digital halftoning unit, a data phasing unit, and a printing mechanism using liquid ink. When copying, continuous tone image information from the image scanner is digitally color corrected, digitally halftoned in real-time and printed by the printing head. The system may be operated by a human operator, or may be fully automatic, producing a copy of a photograph after sensing payment by coin, banknote, credit card, or other means.

Description: PHOTOGRAPH PROCESSING AND COPYING SYSTEMS Field of the Invention The present invention is in the field of computer controlled printing devices. In particular, the field is thermally activated drop on demand (DOD) printing systems. The present invention is an apparatus for printing color photographs from negative strips. Approximately fifty billion color photographs are taken every year. The majority of these photographs are taken using negative transparency film, which is processed and then printed on photographic paper using chemical and optical processes. This work is usually carried out by a processing lab (commonly known as a photo finisher), using largely automated equipment Such processing labs are common in many areas of the world. The film is usually processed in strips of 24 or 36 photographs, and the processed negative film is cut into strips of four or five images, and provided to the customer along with the positive color prints. The chemical printing process can produce high quality results at a low cost per photograph. However, the equipment required is not ideal. Significant quantities of chemicals are required. The color of the resultant print varies with temperature and 'age' of the chemicals used. The images are not digitized, so digital image processing cannot be used to enhance the image. The photograph processing equipment (e.g. a 'mini-lab') is bulky and expensive. To ensure commercial success, a compact digital electronic mini-lab should be substantially lower in cost than chemical mini-labs. Image quality should be equivalent to the consumer. Processing speed should be equivalent or faster, with one second per photograph being a reasonable target. Color correction should be automatic, and preferably should incorporate sophisticated techniques such as color histogram correction. Processing costs should be no more than that of current chemical optical mini-labs. There has been a major obstacle in the development of high performance digital mini-labs. This is the lack of a suitable digital color printing mechanism. In many instances, the consumer requires an extra copy of one or more of the photographs. This requirement may occur at any time after the original photographs are taken. Extra copies are usually made in the following manner; the customer finds the negative strip with the required image, and identifies the correct image in the strip. The customer then takes the strip to the processing lab, which then does a special processing run printing only the requires image or images. Usually, the processing lab requires at least an hour to provide the service, as the processing machine may be busy with other jobs. The customer is required to return later to pick up the copies. This process is unsatisfactory in many respects. Negatives must be provided to the customer. These must be stored, and are often lost. The negatives must be handled carefully, as they are seriously degraded by fingerprints. The correct image must be identified by viewing a small negative image of the required photograph. The negative strips must then be taken to a processing lab, placed in a job queue, and the customer must either wait, or return later to collect the copies. The printing of a few copies is an interruption to the normal workflow of the processing lab, and therefore is usually charged at a premium rate. The duplication of instant print photographs such as 'Polaroids' is more difficult, usually requiring photographing of the original Polaroid print Background of the Inygnύon Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical printing presses, even though this conventional method requires very expensive setup and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems, for example, being able to produce high quality color images at a high-speed and low cost, using standard paper. Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Many types of ink jet printing mechanisms have been invented. These can be categorized as either continuous ink jet (CIJ) or drop on demand (DOD) ink jet Continuous ink jet printing dates back to at least 1929: Hansell, US Pat. No. 1,941,001. Sweet et al US Pat. No. 3,373,437, 1967, discloses an array of continuous ink jet nozzles where ink drops to be printed are selectively charged and deflected towards the recording medium. This technique is known as binary deflection CIJ, and is used by several manufacturers, including Elmjet and Scitex. Hertz et al US Pat. No. 3,416,153, 1966, discloses a method of achieving variable optical density of printed spots in CIJ printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets which pass through a small aperture. This technique is used in ink jet printers manufactured by Iris Graphics. Kyser et al US Pat No. 3,946,398, 1970, discloses a DOD ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Many types of piezoelectric drop on demand printers have subsequently been invented, which utilize piezoelectric crystals in bend mode, push mode, shear mode, and squeeze mode. Piezoelectric DOD printers have achieved commercial success using hot melt inks (for example, Tektronix and Dataproducts printers), and at image resolutions up to 720 dpi for home and office printers (Seiko Epson). Piezoelectric DOD printers have an advantage in being able to use a wide range of inks. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance. Endo et al GB Pat. No. 2,007,162, 1979, discloses an electrothermal DOD ink jet printer which applies a power pulse to an electrothermal transducer (heater) which is in thermal contact with ink in a nozzle. The heater rapidly heats water based ink to a high temperature, whereupon a small quantity of ink rapidly evaporates, forming a bubble. The formation of these bubbles results in a pressure wave which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet™ (trademark of Canon K.K. of Japan), and is used in a wide range of printing systems from Canon, Xerox, and other manufacturers. Vaught et al US Pat. No. 4,490,728, 1982, discloses an electrothermal drop ejection system which also operates by bubble formation. In this system, drops are ejected in a direction normal to the plane of the heater substrate, through nozzles formed in an aperture plate positioned above the heater. This system is known as Thermal Ink Jet and is manufactured by Hewlett-Packard. In this document the term Thermal Ink Jet is used to refer to both the Hewlett- Packard system and systems commonly known as Bubblejet™. Thermal Ink Jet printing typically requires approximately 20 μJ over a period of approximately 2 μs to eject each drop. The 10 Watt active power consumption of each heater is disadvantageous in itself and also necessitates special inks, complicates the driver electronics and precipitates deterioration of heater elements. Other ink jet printing systems have also been described in technical literature, but are not currently used on a commercial basis. For example, U.S. Patent No. 4,275,290 discloses a system wherein the coincident address of predetermined print head nozzles with heat pulses and hydrostatic pressure, allows ink to flow freely to spacer-separated paper, passing beneath the print head. U.S. Patent Nos. 4,737,803; 4,737,803 and 4,748,458 disclose ink jet recording systems wherein the coincident address of ink in print head nozzles with heat pulses and an electrostatically attractive field cause ejection of ink drops to a print sheet Each of the above-described inkjet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved inkjet printing approach, providing advantages for example, as to cost speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables. Summary of the invention My concurrently filed applications, entitled "Liquid Ink Printing Apparatus and System" and "Coincident Drop-Selection, Drop-Separation Printing Method and System" describe new methods and apparatus that afford significant improvements toward overcoming the prior art problems discussed above. Those inventions offer important advantages, e.g., in regard to drop size and placement accuracy, as to printing speeds attainable, as to power usage, as to durability and operative thermal stresses encountered and as to other printer performance characteristics, as well as in regard to manufacturability and the characteristics of useful inks. One important purpose of the present invention is to further enhance the structures and methods described in those applications and thereby contribute to the advancement of printing technology. One object of the present invention is to provide liquid ink printing systems which afford significant advantages toward attaining the above-noted advantages. The invention provides a color photograph printing apparatus comprising: 1) a color film scanner for negative or positive; 2) a scanner signal conditioning unit which converts the scanner data to digital form; 3) a digital image processing system which converts negative image data to positive image data, if appropriate; 4) a digital halftoning unit which converts the continuous tone image data output by the scanner signal processing unit to bi-level image data; 5) a data distribution and timing system which provides the bi-level image data to the printing head at the correct time during a printing operation; and 6) a bi-level color printing mechanism, preferably operating on the concurrent drop selection and separation principle. Another preferred feature of the invention is that the bi-level printing mechanism is a single monolithic printing head which can print to the full width of the photographic print An alternative preferred feature of the invention is that the bi-level printing mechanism is composed of a plurality of monolithic printing heads. Another preferred feature of the invention is the inclusion of a digital color correction apparatus. Another preferred feature of the invention is that the printing ink colors used are CC'MM * YK. Another preferred feature of the invention is that the digital halftoning unit operates using vector error diffusion. An alternative preferred feature of the invention is that the digital halftoning unit operates using independent error diffusion of each printing color. An alternative preferred feature of the invention is that the digital halftoning unit operates using dithering. Another preferred feature of the invention is that the print paper is in the form of pre-cut sheets. An alternative preferred feature of the invention is that the print paper is in the form of a continuous roll, and which incorporates an automatic paper cutter. Brief Description of the Drawings Figure 1(a) shows a simplified block schematic diagram of one exemplary printing apparatus according to the present invention. Figure 1(b) shows a cross section of one variety of nozzle tip in accordance with the invention. Figures 2(a) to 2(f) show fluid dynamic simulations of drop selection. Figure 3(a) shows a finite element fluid dynamic simulation of a nozzle in operation according to an embodiment of the invention. Figure 3(b) shows successive meniscus positions during drop selection and separation. Figure 3(c) shows the temperatures at various points during a drop selection cycle. Figure 3(d) shows measured surface tension versus temperature curves for various ink additives. Figure 3(e) shows the power pulses which are applied to the nozzle heater to generate the temperature curves of figure 3(c) Figure 4 shows a block schematic diagram of print head drive circuitry for practice of the invention. Figure 5 shows projected manufacturing yields for an A4 page width color print head embodying features of the invention, with and without fault tolerance. Figure 6 shows a simplified schematic diagram of a high performance color photograph printer using a printing mechanism according to the present invention. Figure 7(a) shows a side view of major component placement in one possible configuration of the scanner section of the digital 'mini-lab' . Figure 7(b) shows a side view of major component placement in one possible configuration of the printer section of the digital 'mini-lab' . Figure 8 shows a perspective view of one possible configuration of the digital 'mini-lab'. Figure 9 shows a simplified schematic diagram of a portable printer using LIFT printing technology. Figure 10 shows a side view of major component placement in one possible configuration of the photograph copier. Figure 11 (a) shows a perspective view of the photograph copier with the copy lid closed. Figure 11(b) shows a perspective view of the photograph copier with the copy lid open. Detailed Description of Preferred Embodiments One aspect of the invention is a high speed color photograph printing system (mini-lab) which uses a concurrent drop selection and drop separation printing mechanism. The system consists of a color continuous tone scanner which is the width of the color negative transparency, a scanner signal conditioning unit a digital image processing unit, a digital halftoning unit a data phasing unit and a printing mechanism using liquid ink. The printing head is the same width as the photograph to be printed. When printing a photograph, continuous tone image information from the image scanner is digitally halftoned in real-time and printed by the printing head. Another aspect of the invention is a high speed color photograph copier which uses a drop on demand concurrent drop selection and drop separation printing mechanism. The system consists of a color continuous tone scanner the width of a photograph, a scanner signal conditioning unit, a digital halftoning unit (preferably using vector error diffusion), a data phasing unit and a printing mechanism using liquid ink. The printing head is also the same width as the photograph. When copying, continuous tone image information from the image scanner is digitally halftoned in real-time and printed by the printing head. The system may be operated by a human operator, or may be fully automatic, producing a copy of a photograph after sensing payment by coin, banknote, credit card, or other means. In one general aspect, the invention constitutes a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an alternative means is provided to cause separation of the selected drops from the body of ink. The separation of drop selection means from drop separation means significantly reduces the energy required to select which ink drops are to be printed. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles. The drop selection means may be chosen from, but is not limited to, the following list: 1) Electrothermal reduction of surface tension of pressurized ink 2) Electrothermal bubble generation, with insufficient bubble volume to cause drop ejection 3) Piezoelectric, with insufficient volume change to cause drop ejection 4) Electrostatic attraction with one electrode per nozzle The drop separation means may be chosen from, but is not limited to, the following list: 1) Proximity (recording medium in close proximity to print head) 2) Proximity with oscillating ink pressure 3) Electrostatic attraction 4) Magnetic attraction The table "DOD printing technology targets" shows some desirable characteristics of drop on demand printing technology. The table also lists some methods by which some embodiments described herein, or in other of my related applications, provide improvements over the prior art. DOD printing technology targets In thermal ink jet (TU) and piezoelectric ink jet systems, a drop velocity of approximately 10 meters per second is preferred to ensure that the selected ink drops overcome ink surface tension, separate from the body of the ink, and strike the recording medium. These systems have a very low efficiency of conversion of electrical energy into drop kinetic energy. The efficiency of TD systems is approximately 0.02%). This means that the drive circuits for TU print heads must switch high currents. The drive circuits for piezoelectric ink jet heads must either switch high voltages, or drive highly capacitive loads. The total power consumption of pagewidth TO ptintheads is also very high. An 800 dpi A4 full color pagewidth TU print head printing a four color black image in one second would consume approximately 6 kW of electrical power, most of which is converted to waste heat. The difficulties of removal of this amount of heat precludes the production of low cost, high speed, high resolution compact pagewidth TU systems. One important feature of embodiments of the invention is a means of significantly reducing the energy required to select which ink drops are to be printed. This is achieved by separating the means for selecting ink drops from the means for ensuring that selected drops separate from the body of ink and form dots on the recording medium. Only the drop selection means must be driven by individual signals to each nozzle. The drop separation means can be a field or condition applied simultaneously to all nozzles. The table "Drop selection means" shows some of the possible means for selecting drops in accordance with the invention. The drop selection means is only required to create sufficient change in the position of selected drops that the drop separation means can discriminate between selected and unselected drops. Drop selection means Other drop selection means may also be used. The preferred drop selection means for water based inks is method 1: "Electrothermal reduction of surface tension of pressurized ink". This drop selection means provides many advantages over other systems, including; low power operation (approximately 1% of TU), compatibility with CMOS VLSI chip fabrication, low voltage operation (approx. 10 V), high nozzle density, low temperature operation, and wide range of suitable ink formulations. The ink must exhibit a reduction in surface tension with increasing temperature. The preferred drop selection means for hot melt or oil based inks is method 2: 'Εlectrothermal reduction of ink viscosity, combined with oscillating ink pressure". This drop selection means is particularly suited for use with inks which exhibit a large reduction of viscosity with increasing temperature, but only a small reduction in surface tension. This occurs particularly with non-polar ink carriers with relatively high molecular weight This is especially applicable to hot melt and oil based inks. The table "Drop separation means" shows some of the possible methods for separating selected drops from the body of ink, and ensuring that the selected drops form dots on the printing medium. The drop separation means discriminates between selected drops and unselected drops to ensure that unselected drops do not form dots on the printing medium. Drop separation means Other drop separation means may also be used. The preferred drop separation means depends upon the intended use. For most applications, method 1: "Electrostatic attraction", or method 2: "AC electric field" are most appropriate. For applications where smooth coated paper or film is used, and very high speed is not essential, method 3: "Proximity" may be appropriate. For high speed, high quality systems, method 4: 'Transfer proximity" can be used. Method 6: "Magnetic attraction" is appropriate for portable printing systems where the print medium is too rough for proximity printing, and the high voltages required for electrostatic drop separation are undesirable. There is no clear 'best' drop separation means which is applicable to all circumstances. Further details of various types of printing systems according to the present invention are described in the following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'A Liquid ink Fault Tolerant (LIFT) printing mechanism' (Filing no.: PN2308); 'Electrothermal drop selection in LIFT printing' (Filing no.: PN2309); 'Drop separation in LIFT printing by print media proximity' (Filing no.: PN2310); 'Drop size adjustment in Proximity LIFT printing by varying head to media distance' (Filing no.: PN2311); 'Augmenting Proximity LIFT printing with acoustic ink waves' (Filing no.: PN2312); 'Electrostatic drop separation in LEFT printing' (Filing no.: PN2313); 'Multiple simultaneous drop sizes in Proximity LIFT printing' (Filing no.: PN2321); 'Self cooling operation in thermally activated print heads' (Filing no.: PN2322); and 'Thermal Viscosity Reduction LIFT printing' (Filing no. : PN2323). A simplified schematic diagram of one preferred printing system according to the invention appears in Figure 1(a). An image source 52 may be raster image data from a scanner or computer, or outline image data in the form of a page description language (PDL), or other forms of digital image representation. This image data is converted to a pixel-mapped page image by the image processing system 53. This may be a raster image processor (RIP) in the case of PDL image data, or may be pixel image manipulation in the case of raster image data. Continuous tone data produced by the image processing unit 53 is halftoned. Halftoning is performed by the Digital Halftoning unit 54. Halftoned bitmap image data is stored in the image memory 72. Depending upon the printer and system configuration, the image memory 72 may be a full page memory, or a band memory. Heater control circuits 71 read data from the image memory 72 and apply time- varying electrical pulses to the nozzle heaters (103 in figure 1(b)) that are part of the print head 50. These pulses are applied at an appropriate time, and to the appropriate nozzle, so that selected drops will form spots on the recording medium 51 in the appropriate position designated by the data in the image memory 72. The recording medium 51 is moved relative to the head 50 by a paper transport system 65, which is electronically controlled by a paper transport control system 66, which in turn is controlled by a microcontroller 315. The paper transport system shown in figure 1(a) is schematic only, and many different mechanical configurations are possible. In the case of pagewidth print heads, it is most convenient to move the recording medium 51 past a stationary head 50. However, in the case of scanning print systems, it is usually most convenient to move the head 50 along one axis (the sub-scanning direction) and the recording medium 51 along the orthogonal axis (the main scanning direction), in a relative raster motion. The microcontroller 315 may also control the ink pressure regulator 63 and the heater control circuits 71. For printing using surface tension reduction, ink is contained in an ink reservoir 64 under pressure. In the quiescent state (with no ink drop ejected), the ink pressure is insufficient to overcome the ink surface tension and eject a drop. A constant ink pressure can be achieved by applying pressure to the ink reservoir 64 under the control of an ink pressure regulator 63. Alternatively, for larger printing systems, the ink pressure can be very accurately generated and controlled by situating the top surface of the ink in the reservoir 64 an appropriate distance above the head 50. This ink level can be regulated by a simple float valve (not shown). For printing using viscosity reduction, ink is contained in an ink reservoir 64 under pressure, and the ink pressure is caused to oscillate. The means of producing this oscillation may be a piezoelectric actuator mounted in the ink channels (not shown). When properly arranged with the drop separation means, selected drops proceed to form spots on the recording medium 51, while unselected drops remain part of the body of ink. The ink is distributed to the back surface of the head 50 by an ink channel device 75. The ink preferably flows through slots and/or holes etched through the silicon substrate of the head 50 to the front surface, where the nozzles and actuators are situated. In the case of thermal selection, the nozzle actuators are electrothermal heaters. In some types of printers according to the invention, an external field 74 is required to ensure that the selected drop separates from the body of the ink and moves towards the recording medium 51. A convenient external field 74 is a constant electric field, as the ink is easily made to be electrically conductive. In this case, the paper guide or platen 67 can be made of electrically conductive material and used as one electrode generating the electric field. The other electrode can be the head 50 itself. Another embodiment uses proximity of the print medium as a means of discriminating between selected drops and unselected drops. For small drop sizes gravitational force on the ink drop is very small; approximately 10 "4 of the surface tension forces, so gravity can be ignored in most cases. This allows the print head 50 and recording medium 51 to be oriented in any direction in relation to the local gravitational field. This is an important requirement for portable printers. Figure 1(b) is a detail enlargement of a cross section of a single microscopic nozzle tip embodiment of the invention, fabricated using a modified CMOS process. The nozzle is etched in a substrate 101, which may be silicon, glass, metal, or any other suitable material. If substrates which are not semiconductor materials are used, a semiconducting material (such as amorphous silicon) may be deposited on the substrate, and integrated drive transistors and data distribution circuitry may be formed in the surface semiconducting layer. Single crystal silicon (SCS) substrates have several advantages, including: 1 ) High performance drive transistors and other circuitry can be fabricated in SCS; 2) Print heads can be fabricated in existing facilities (fabs) using standard VLSI processing equipment; 3) SCS has high mechanical strength and rigidity; and 4) SCS has a high thermal conductivity. In this example, the nozzle is of cylindrical form, with the heater 103 forming an annulus. The nozzle tip 104 is formed from silicon dioxide layers 102 deposited during the fabrication of the CMOS drive circuitry. The nozzle tip is passivated with silicon nitride. The protruding nozzle tip controls the contact point of the pressurized ink 100 on the print head surface. The print head surface is also hydrophobized to prevent accidental spread of ink across the front of the print head. Many other configurations of nozzles are possible, and nozzle embodiments of the invention may vary in shape, dimensions, and materials used. Monolithic nozzles etched from the substrate upon which the heater and drive electronics are formed have the advantage of not requiring an orifice plate. The elimination of the orifice plate has significant cost savings in manufacture and assembly. Recent methods for eliminating orifice plates include the use of 'vortex' actuators such as those described in Domoto et al US Pat No. 4,580,158, 1986, assigned to Xerox, and Miller et al US Pat. No. 5,371,527, 1994 assigned to Hewlett-Packard. These, however are complex to actuate, and difficult to fabricate. The preferred method for elimination of orifice plates for print heads of the invention is incorporation of the orifice into the actuator substrate. This type of nozzle may be used for print heads using various techniques for drop separation. Operation with Electrostatic Drop Separation As a first example, operation using thermal reduction of surface tension and electrostatic drop separation is shown in figure 2. Figure 2 shows the results of energy transport and fluid dynamic simulations performed using FIDAP, a commercial fluid dynamic simulation software package available from Fluid Dynamics Inc., of Illinois, USA. This simulation is of a thermal drop selection nozzle embodiment with a diameter of 8 μm, at an ambient temperature of 30°C. The total energy applied to the heater is 276 nJ, applied as 69 pulses of 4 nJ each. The ink pressure is 10 kPa above ambient air pressure, and the ink viscosity at 30°C is 1.84 cPs. The ink is water based, and includes a sol of 0.1% palmitic acid to achieve an enhanced decrease in surface tension with increasing temperature. A cross section of the nozzle tip from the central axis of the nozzle to a radial distance of 40 μm is shown. Heat flow in the various materials of the nozzle, including silicon, silicon nitride, amorphous silicon dioxide, crystalline silicon dioxide, and water based ink are simulated using the respective densities, heat capacities, and thermal conductivities of the materials. The time step of the simulation is 0.1 μs. Figure 2(a) shows a quiescent state, just before the heater is actuated. An equilibrium is created whereby no ink escapes the nozzle in the quiescent state by ensuring that the ink pressure plus external electrostatic field is insufficient to overcome the surface tension of the ink at the ambient temperature. In the quiescent state, the meniscus of the ink does not protrude significantly from the print head surface, so the electrostatic field is not significantly concentrated at the meniscus. Figure 2(b) shows thermal contours at 5°C intervals 5 μs after the start of the heater energizing pulse. When the heater is energized, the ink in contact with the nozzle tip is rapidly heated. The reduction in surface tension causes the heated portion of the meniscus to rapidly expand relative to the cool ink meniscus. This drives a convective flow which rapidly transports this heat over part of the free surface of the ink at the nozzle tip. It is necessary for the heat to be distributed over the ink surface, and not just where the ink is in contact with the heater. This is because viscous drag against the solid heater prevents the ink directly in contact with the heater from moving. Figure 2(c) shows thermal contours at 5°C intervals 10 μs after the start of the heater energizing pulse. The increase in temperature causes a decrease in surface tension, disturbing the equilibrium of forces. As the entire meniscus has been heated, the ink begins to flow. Figure 2(d) shows thermal contours at 5°C intervals 20 μs after the start of the heater energizing pulse. The ink pressure has caused the ink to flow to a new meniscus position, which protrudes from the print head. The electrostatic field becomes concentrated by the protruding conductive ink drop. Figure 2(e) shows thermal contours at 5°C intervals 30 μs after the start of the heater energizing pulse, which is also 6 μs after the end of the heater pulse, as the heater pulse duration is 24 μs. The nozzle tip has rapidly cooled due to conduction through the oxide layers, and conduction into the flowing ink. The nozzle tip is effectively 'water cooled' by the ink. Electrostatic attraction causes the ink drop to begin to accelerate towards the recording medium. Were the heater pulse significantly shorter Qess than 16 μs in this case) the ink would not accelerate towards the print medium, but would instead return to the nozzle. Figure 2(f) shows thermal contours at 5°C intervals 26 μs after the end of the heater pulse. The temperature at the nozzle tip is now less than 5°C above ambient temperature. This causes an increase in surface tension around the nozzle tip. When the rate at which the ink is drawn from the nozzle exceeds the viscously limited rate of ink flow through the nozzle, the ink in the region of the nozzle tip 'necks' , and the selected drop separates from the body of ink. The selected drop then travels to the recording medium under the influence of the external electrostatic field. The meniscus of the ink at the nozzle tip then returns to its quiescent position, ready for the next heat pulse to select the next ink drop. One ink drop is selected, separated and forms a spot on the recording medium for each heat pulse. As the heat pulses are electrically controlled, drop on demand ink jet operation can be achieved. Figure 3(a) shows successive meniscus positions during the drop selection cycle at 5 μs intervals, starting at the beginning of the heater energizing pulse. Figure 3(b) is a graph of meniscus position versus time, showing the movement of the point at the centre of the meniscus. The heater pulse starts 10 μs into the simulation. Figure 3(c) shows the resultant curve of temperature with respect to time at various points in the nozzle. The vertical axis of the graph is temperature, in units of 100°C. The horizontal axis of the graph is time, in units of 10 μs. The temperature curve shown in figure 3(b) was calculated by FIDAP, using 0.1 μs time steps. The local ambient temperature is 30 degrees C. Temperature histories at three points are shown: A - Nozzle tip: This shows the temperature history at the circle of contact between the passivation layer, the ink, and air. B - Meniscus midpoint: This is at a circle on the ink meniscus midway between the nozzle tip and the centre of the meniscus. C - Chip surface: This is at a point on the print head surface 20 μm from the centre of the nozzle. The temperature only rises a few degrees. This indicates that active circuitry can be located very close to the nozzles without experiencing performance or lifetime degradation due to elevated temperatures. Figure 3(e) shows the power applied to the heater. Optimum operation requires a sharp rise in temperature at the start of the heater pulse, a maintenance of the temperature a little below the boiling point of the ink for the duration of the pulse, and a rapid fall in temperature at the end of the pulse. To achieve this, the average energy applied to the heater is varied over the duration of the pulse. In this case, the variation is achieved by pulse frequency modulation of 0.1 μs sub-pulses, each with an energy of 4 nJ. The peak power applied to the heater is 40 mW, and the average power over the duration of the heater pulse is 11.5 mW. The sub-pulse frequency in this case is 5 Mhz. This can readily be varied without significantly affecting the operation of the print head. A higher sub-pulse frequency allows finer control over the power applied to the heater. A sub-pulse frequency of 13.5 Mhz is suitable, as this frequency is also suitable for minimizing the effect of radio frequency interference (RFI). Inks with a negative temperature coefficient of surface tension The requirement for the surface tension of the ink to decrease with increasing temperature is not a major restriction, as most pure liquids and many mixtures have this property. Exact equations relating surface tension to temperature for arbitrary liquids are not available. However, the following empirical equation derived by Ramsay and Shields is satisfactory for many liquids: Where γris the surface tension at temperature T, k is a constant, T c is the critical temperature of the liquid, M is the molar mass of the liquid, x is the degree of association of the liquid, and p is the density of the Uquid. This equation indicates that the surface tension of most liquids falls to zero as the temperature reaches the critical temperature of the liquid. For most liquids, the critical temperature is substantially above the boiling point at atmospheric pressure, so to achieve an ink with a large change in surface tension with a small change in temperature around a practical ejection temperature, the admixture of surfactants is recommended. The choice of surfactant is important For example, water based ink for thermal ink jet printers often contains isopropyl alcohol (2-propanol) to reduce the surface tension and promote rapid drying. Isopropyl alcohol has a boiling point of 82.4°C, lower than that of water. As the temperature rises, the alcohol evaporates faster than the water, decreasing the alcohol concentration and causing an increase in surface tension. A surfactant such as 1-Hexanol (b.p. 158°C) can be used to reverse this effect and achieve a surface tension which decreases shghtly with temperature. However, a relatively large decrease in surface tension with temperature is desirable to maximize operating latitude. A surface tension decrease of 20 mN/m over a 30°C temperature range is preferred to achieve large operating margins, while as little as lOmN/m can be used to achieve operation of the print head according to the present invention. Inks With Large -Aγ. Several methods may be used to achieve a large negative change in surface tension with increasing temperature. Two such methods are: 1) The ink may contain a low concentration sol of a surfactant which is solid at ambient temperatures, but melts at a threshold temperature. Particle sizes less than 1,000 Λ are desirable. Suitable surfactant melting points for a water based ink are between 50°C and 90°C, and preferably between 60°C and 80°C. 2) The ink may contain an oil water microemulsion with a phase inversion temperature (PIT) which is above the maximum ambient temperature, but below the boiling point of the ink. For stability, the PIT of the microemulsion is preferably 20°C or more above the maximum non-operating temperature encountered by the ink. A PIT of approximately 80°C is suitable. Inks with Surfactant Sols Inks can be prepared as a sol of small particles of a surfactant which melts in the desired operating temperature range. Examples of such surfactants include carboxylic acids with between 14 and 30 carbon atoms, such as: As the melting point of sols with a small particle size is usually slightly less than of the bulk material, it is preferable to choose a carboxylic acid with a melting point slightly above the desired drop selection temperamre. A good example is Arachidic acid. These carboxylic acids are available in high purity and at low cost. The amount of surfactant required is very small, so the cost of adding them to the ink is insignificant A mixture of carboxylic acids with slightly varying chain lengths can be used to spread the melting points over a range of temperatures. Such mixtures will typically cost less than the pure acid. It is not necessary to restrict the choice of surfactant to simple unbranched carboxylic acids. Surfactants with branched chains or phenyl groups, or other hydrophobic moieties can be used. It is also not necessary to use a carboxylic acid. Many highly polar moieties are suitable for the hydrophilic end of the surfactant. It is desirable that the polar end be ionizable in water, so that the surface of the surfactant particles can be charged to aid dispersion and prevent flocculation. In the case of carboxylic acids, this can be achieved by adding an alkali such as sodium hydroxide or potassium hydroxide. Preparation of Inks with Surfactant Sols The surfactant sol can be prepared separately at high concentration, and added to die ink in the required concentration. An example process for creating the surfactant sol is as follows: 1) Add the carboxylic acid to purified water in an oxygen free atmosphere. 2) Heat the mixture to above the melting point of the carboxylic acid. The water can be brought to a boil. 3) Ultrasonicate the mixture, until the typical size of the carboxylic acid droplets is between lOOA and 1 ,000λ. 4) Allow the mixture to cool. 5) Decant the larger particles from the top of the mixture. 6) Add an alkali such as NaOH to ionize the carboxyhc acid molecules on the surface of the particles. A pH of approximately 8 is suitable. This step is not absolutely necessary, but helps stabilize the sol. 7) Centrifuge the sol. As the density of die carboxyhc acid is lower than water, smaller particles will accumulate at the outside of the centrifuge, and larger particles in the centre. 8) Filter the sol using a microporous filter to eliminate any particles above 5000 A. 9) Add the surfactant sol to the ink preparation. The sol is required only in very dilute concentration. The ink preparation will also contain either dye(s) or pigment(s), bactericidal agents, agents to enhance the electrical conductivity of the ink if electrostatic drop separation is used, humectants, and other agents as required. Anti-foaming agents will generally not be required, as there is no bubble formation during the drop ejection process. Cationic surfactant sols Inks made witii anionic surfactant sols are generally unsuitable for use with cationic dyes or pigments. This is because the cationic dye or pigment may precipitate or flocculate with the anionic surfactant To allow the use of cationic dyes and pigments, a cationic surfactant sol is required. The family of alkylamines is suitable for this purpose. Various suitable alkylamines are shown in the following table: The method of preparation of cationic surfactant sols is essentially similar to that of anionic surfactant sols, except that an acid instead of an alkali is used to adjust the pH balance and increase the charge on the surfactant particles. A pH of 6 using HC1 is suitable. Microemulsion Based Inks An alternative means of achieving a large reduction in surface tension as some temperature threshold is to base the ink on a microemulsion. A microemulsion is chosen with a phase inversion temperature (PIT) around the desired ejection threshold temperature. Below the PIT, the microemulsion is oil in water (O W), and above the PIT the microemulsion is water in oil (W/O). At low temperatures, the surfactant forming the microemulsion prefers a high curvature surface around oil, and at temperatures significandy above the PIT, the surfactant prefers a high curvature surface around water. At temperatures close to the PIT, the microemulsion forms a continuous 'sponge' of topologically connected water and oil. There are two mechanisms whereby this reduces the surface tension. Around the PIT, the surfactant prefers surfaces with very low curvature. As a result, surfactant molecules migrate to the ink/air interface, which has a curvature which is much less than the curvature of the oil emulsion. This lowers the surface tension of the water. Above the phase inversion temperature, the microemulsion changes from O/W to W/O, and therefore the ink/air interface changes from water/air to oil/air. The oil/air interface has a lower surface tension. There is a wide range of possibilities for the preparation of microemulsion based inks. For fast drop ejection, it is preferable to chose a low viscosity oil. In many instances, water is a suitable polar solvent. However, in some cases different polar solvents may be required. In these cases, polar solvents with a high surface tension should be chosen, so that a large decrease in surface tension is achievable. The surfactant can be chosen to result in a phase inversion temperature in the desired range. For example, surfactants of the group poly(oxyethylene)alkylphenyl ether (ethoxylated alkyl phenols, general formula: C n H 2n+ ιC H 6 (CH 2 CH 2 θ) m OH) can be used. The hydrophilicity of the surfactant can be increased by increasing m, and the hydrophobicity can be increased by increasing n. Values of m of approximately 10, and n of approximately 8 are suitable. Low cost commercial preparations are the result of a polymerization of various molar ratios of ethylene oxide and alkyl phenols, and the exact number of oxyethylene groups varies around the chosen mean. These commercial preparations are adequate, and highly pure surfactants with a specific number of oxyethylene groups are not required. The formula for this surfactant is C 8 Hι 7 C 4 H6(CH 2 CH 2 O)_OH (average n= 10). Synonyms include Octoxynol-10, PEG-10 octyl phenyl ether and POE (10) octyl phenyl ether The HLB is 13.6, the melting point is 7°C, and the cloud point is 65°C. Commercial preparations of this surfactant are available under various brand names. Suppliers and brand names are listed in the following table: These are available in large volumes at low cost (less than one dollar per pound in quantity), and so contribute less than 10 cents per liter to prepared microemulsion ink with a 5% surfactant concentration. Other suitable ethoxylated alkyl phenols include those listed in the foUowing table: Microemulsion based inks have advantages other than surface tension control: 1) Microemulsions are thermodynamicaUy stable, and will not separate. Therefore, the storage time can be very long. This is especiaUy significant for office and portable printers, which may be used sporadically. 2) The microemulsion wiU form spontaneously with a particular drop size, and does not require extensive stirring, centrifuging, or filtering to ensure a particular range of emulsified oil drop sizes. 3) The amount of oil contained in the ink can be quite high, so dyes which are soluble in oU or soluble in water, or both, can be used. It is also possible to use a mixture of dyes, one soluble in water, and the other soluble in oil, to obtain specific colors. 4) Oil miscible pigments are prevented from flocculating, as they are trapped in the oil microdroplets. 5) The use of a microemulsion can reduce the mixing of different dye colors on the surface of the print medium. 6) The viscosity of microemulsions is very low. 7) The requirement for humectants can be reduced or eliminated. Dves and pigments in microemulsion based inks OU in water mixtures can have high oil contents - as high as 40% - and still form O/W microemulsions. This aUows a high dye or pigment loading. Mixtures of dyes and pigments can be used. An example of a microemulsion based ink mixture with both dye and pigment is as foUows: 1) 70% water 2) 5% water soluble dye 3) 5% surfactant 4) 10% oil 5) 10% oil miscible pigment The foUowing table shows the nine basic combinations of colorants in the oU and water phases of the microemulsion that may be used. The ninth combination, with no colorants, is useful for printing transparent coatings, UV ink, and selective gloss highUghts. As many dyes are amphiphiUc, large quantities of dyes can also be solubilized in the oU-water boundary layer as this layer has a very large surface area. It is also possible to have multiple dyes or pigments in each phase, and to have a mixture of dyes and pigments in each phase. When using multiple dyes or pigments the absorption spectrum of the resultant ink wUl be the weighted average of the absorption spectra of the different colorants used. This presents two problems: 1) The absorption spectrum wUl tend to become broader, as the absorption peaks of both colorants are averaged. This has a tendency to 'muddy' the colors. To obtain briUiant color, careful choice of dyes and pigments based on their absorption spectra, not just their human-perceptible color, needs to be made. 2) The color of the ink may be different on different substrates. If a dye and a pigment are used in combination, the color of the dye wiU tend to have a smaUer contribution to the printed ink color on more absorptive papers, as the dye will be absorbed into the paper, whUe the pigment wiU tend to 'sit on top' of the paper. This may be used as an advantage in some circumstances. Surfactants with a Krafft point in the drop selection temperature range For ionic surfactants there is a temperature (the Krafft point) below which the solubility is quite low, and the solution contains essentiaUy no miceUes. Above the Krafft temperature miceUe formation becomes possible and there is a rapid increase in solubility of the surfactant If the critical miceUe concentration (CMC) exceeds the solubility of a surfactant at a particular temperature, then the minimum surface tension wiU be achieved at the point of maximum solubility, rather than at the CMC. Surfactants are usuaUy much less effective below the Krafft point. This factor can be used to achieve an increased reduction in surface tension with increasing temperature. At ambient temperatures, only a portion of the surfactant is in solution. When the nozzle heater is turned on, die temperature rises, and more of the surfactant goes into solution, decreasing the surface tension. A surfactant should be chosen with a Krafft point which is near t e top of the range of temperatures to which the ink is raised. This gives a maximum margin between the concentration of surfactant in solution at ambient temperatures, and the concentration of surfactant in solution at the drop selection temperature. The concentration of surfactant should be approximately equal to the CMC at the Krafft point. In this manner, the surface tension is reduced to the maximum amount at elevated temperatures, and is reduced to a minimum amount at ambient temperatures. The following table shows some commerciaUy avaUable surfactants with Krafft points in the desired range. Surfactants with a cloud point in the drop selection temnerature ranee Non-ionic surfactants using polyoxyethylene (POE) chains can be used to create an ink where the surface tension faUs with increasing temperature. At low temperatures, the POE chain is hydrophiUc, and maintains the surfactant in solution. As the temperature increases, the structured water around die POE section of the molecule is disrupted, and the POE section becomes hydrophobic. The surfactant is increasingly rejected by the water at higher temperatures, resulting in increasing concentration of surfactant at the air/ink interface, thereby lowering surface tension. The temperature at which the POE section of a nonionic surfactant becomes hydrophilic is related to the cloud point of that surfactant POE chains by themselves are not particularly suitable, as the cloud point is generally above 100°C Polyoxypropylene (POP) can be combined with POE in POE POP block copolymers to lower the cloud point of POE chains without introducing a strong hydrophobicity at low temperatures. Two main configurations of symmetrical POE/POP block copolymers are avaUable. These are: 1) Surfactants with POE segments at the ends of the molecules, and a POP segment in the centre, such as the poloxamer class of surfactants (generically CAS 9003-11-6) 2) Surfactants with POP segments at the ends of the molecules, and a POE segment in the centre, such as the meroxapol class of surfactants (generically also CAS 9003-11-6) Some commercially avaUable varieties of poloxamer and meroxapol with a high surface tension at room temperature, combined with a cloud point above 40°C and below 100°C are shown in the following table: (CH.CH.O). n OH Poloxamer 124 Pluronic L44 HO(CH 2 CH 2 O). 11 - 45.3 65°C (CHCH 3 CH-O)_ 21 - (CH-CH.O)_ π OH Other varieties of poloxamer and meroxapol can readUy be synthesized using weU known techniques. Desirable characteristics are a room temperature surface tension which is as high as possible, and a cloud point between 40°C and 100°C, and preferably between 60°C and 80°C. Meroxapol [HO(CHCH 3 CH_O) x (CH 2 CH 2 O) y (CHCH 3 CH 2 O)_OH] varieties where the average x and z are approximately 4, and the average y is approximately 15 may be suitable. If salts are used to increase the electrical conductivity of the ink, then the effect of this salt on the cloud point of the surfactant should be considered. The cloud point of POE surfactants is increased by ions that disrupt water structure (such as I " ), as this makes more water molecules available to form hydrogen bonds with the POE oxygen lone pairs. The cloud point of POE surfactants is decreased by ions that form water structure (such as Cl " , OH " ), as fewer water molecules are available to form hydrogen bonds. Bromide ions have relatively Utde effect The ink composition can be 'tuned' for a desired temperamre range by altering the lengths of POE and POP chains in a block copolymer surfactant, and by changing the choice of salts (e.g Cl " to Br to I " ) that are added to increase electrical conductivity. NaCl is likely to be the best choice of salts to increase ink conductivity, due to low cost and non-toxicity. NaCl shghtly lowers the cloud point of nonionic surfactants . Hot Melt Inks The ink need not be in a Uquid state at room temperature. SoUd 'hot melt' inks can be used by heating the printing head and ink reservoir above the melting point of the ink. The hot melt ink must be formulated so that the surface tension of the molten ink decreases with temperature. A decrease of approximately 2 mN/m wiU be typical of many such preparations using waxes and other substances. However, a reduction in surface tension of approximately 20 mN/m is desirable in order to achieve good operating margins when relying on a reduction in surface tension rather than a reduction in viscosity. The temperature difference between quiescent temperature and drop selection temperature may be greater for a hot melt ink than for a water based ink, as water based inks are constrained by die boiling point of the water. The ink must be Uquid at the quiescent temperature. The quiescent temperature should be higher than the highest ambient temperature likely to be encountered by die printed page. T he quiescent temperature should also be as low as practical, to reduce the power needed to heat the print head, and to provide a maximum margin between the quiescent and the drop ejection temperatures. A quiescent temperature between 60°C and 90°C is generaUy suitable, though other temperatures may be used. A drop ejection temperature of between 160°C and 200°C is generaUy suitable. There are several methods of achieving an enhanced reduction in surface tension with increasing temperamre. 1) A dispersion of microfine particles of a surfactant with a melting point substantially above the quiescent temperature, but substantially below the drop ejection temperature, can be added to d e hot melt ink while in the Uquid phase. 2) A polar/non-polar microemulsion with a PIT which is preferably at least 20°C above the melting points of both the polar and non-polar compounds. To achieve a large reduction in surface tension with temperamre, it is desirable that the hot melt ink carrier have a relatively large surface tension (above 30 mN/m) when at the quiescent temperamre. This generally excludes alkanes such as waxes. Suitable materials wiU generaUy have a strong intermolecular attraction, which may be achieved by multiple hydrogen bonds, for example, polyols, such as Hexanetetrol, which has a melting point of 88°C. Surface tension reduction of various solutions Figure 3(d) shows the measured effect of temperamre on the surface tension of various aqueous preparations containing the foUowing additives: 1) 0.1% sol of Stearic Acid 2) 0.1% sol of Palmitic acid 3) 0.1 % solution of Pluronic 10R5 (trade mark of BASF) 4) 0.1 % solution of Pluronic L35 (trade mark of BASF) 5) 0.1% solution of Pluronic L44 (trade mark of BASF) Inks suitable for printing systems of the present invention are described in die foUowing Australian patent specifications, the disclosure of which are hereby incoφorated by reference: 'Ink composition based on a microemulsion' (FUing no.: PN5223, filed on 6 September 1995); 'Ink composition containing surfactant sol' (Filing no.: PN5224, filed on 6 September 1995); 'Ink composition for DOD printers with Krafft point near the drop selection temperature sol' (Filing no.: PN6240, filed on 30 October 1995); and 'Dye and pigment in a microemulsion based ink' (Filing no.: PN6241, filed on 30 October 1995). Operation U inp Reduction of Viscosity As a second example, operation of an embodiment using thermal reduction of viscosity and proximity drop separation, in combination with hot melt ink, is as foUows. Prior to operation of the printer, sohd ink is melted in the reservoir 64. The reservoir, ink passage to the print head, ink channels 75, and print head 50 are maintained at a temperamre at which the ink 100 is Uquid, but exhibits a relatively high viscosity (for example, approximately 100 cP). The Ink 100 is retained in the nozzle by the surface tension of the ink. The ink 100 is formulated so that the viscosity of the ink reduces with increasing temperamre. The ink pressure osciUates at a frequency which is an integral multiple of the drop ejection frequency from the nozzle. The ink pressure osciUation causes osciUations of the ink meniscus at the nozzle tips, but this osciUation is smaU due to d e high ink viscosity. At the normal operating temperamre, these osciUations are of insufficient ampUtude to result in drop separation. When the heater 103 is energized, the ink forming the selected drop is heated, causing a reduction in viscosity to a value which is preferably less than 5 cP. The reduced viscosity results in the ink meniscus moving further during the high pressure part of the ink pressure cycle. The recording medium 51 is arranged sufficientiy close to the print head 50 so that the selected drops contact the recording medium 51, but sufficientiy far away that the unselected drops do not contact the recording medium 51. Upon contact with the recording medium 51 , part of d e selected drop freezes, and attaches to the recording medium. As die ink pressure faUs, ink begins to move back into the nozzle. The body of ink separates from the ink which is frozen onto the recording medium. The meniscus of the ink 100 at the nozzle tip then returns to low amplitude osciUation. The viscosity of the ink increases to its quiescent level as remaining heat is dissipated to die bulk ink and print head. One ink drop is selected, separated and forms a spot on die recording medium 51 for each heat pulse. As the heat pulses are electricaUy controUed, drop on demand inkjet operation can be achieved. Manufacturing of Print Heads Manufacturing processes for monolithic print heads in accordance witii the present invention are described in the following AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incorporated by reference: 'A monoUthic LIFT printing head' (Filing no.: PN2301); 'A manufacturing process for monoUthic LIFT printing heads' (FUing no.: PN2302); 'A self-aUgned heater design for LIFT print heads' (Filing no.: PN2303); 'Integrated four color LIFT print heads' (FUing no.: PN2304); 'Power requirement reduction in monolithic LIFT printing heads' (FUing no.: PN2305); ' A manufacturing process for monoUthic LIFT print heads using anisotropic wet etching' (Filing no.: PN2306); 'Nozzle placement in monoUthic drop-on-demand print heads' (Filing no.: PN2307); 'Heater structure for monoUthic LIFT print heads' (FUing no.: PN2346); 'Power supply connection for monolithic LIFT print heads' (Filing no.: PN2347); 'External connections for Proximity LIFT print heads' (Filing no.: PN2348); and 'A self- aligned manufacturing process for monolithic LIFT print heads' (Filing no.: PN2349); and 'CMOS process compatible fabrication of LIFT print heads' (FUing no.: PN5222, 6 September 1995). 'A manufacturing process for LIFT print heads with nozzle rim heaters' (Filing no.: PN6238, 30 October 1995); 'A modular LIFT print head' (FUing no.: PN6237, 30 October 1995); 'Method of increasing packing density of printing nozzles' (Filing no.: PN6236, 30 October 1995); and 'Nozzle dispersion for reduced electrostatic interaction between simultaneously printed droplets' (FUing no.: PN6239, 30 October 1995). Control of Print Heads Means of providing page image data and controlling heater temperamre in print heads of the present invention is described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Integrated drive circuitry in LIFT print heads' (FUing no.: PN2295); 'A nozzle clearing procedure for Liquid Ink Fault Tolerant (LIFT) printing' (FUing no.: PN2294); 'Heater power compensation for temperature in LIFT printing systems' (Filing no.: PN2314); 'Heater power compensation for tiiermal lag in LIFT printing systems' (Filing no.: PN2315); 'Heater power compensation for print density in LIFT printing systems' (Filing no.: PN2316); 'Accurate control of temperamre pulses in printing heads' (FUing no.: PN2317); 'Data distribution in monoUthic LIFT print heads' (Filing no.: PN2318); 'Page image and fault tolerance routing device for LIFT printing systems' (Filing no.: PN2319); and 'A removable pressurized liquid ink cartridge for LIFT printers' (Filing no.: PN2320). Image Processing for Print Heads An objective of printing systems according to the invention is to attain a print quality which is equal to that which people are accustomed to in quality color pubhcations printed using offset printing. This can be achieved using a print resolution of approximately 1,600 dpi. However, 1,600 dpi printing is difficult and expensive to achieve. Similar results can be achieved using 800 dpi printing, with 2 bits per pixel for cyan and magenta, and one bit per pixel for yeUow and black. This color model is herein caUed CC'MM' YK. Where high quahty monochrome image printing is also required, two bits per pixel can also be used for black. This color model is herein caUed CC'MM'YKK'. Color models, halftoning, data compression, and real-time expansion systems suitable for use in systems of this invention and other printing systems are described in the foUowing AustraUan patent specifications filed on 12 April 1995, die disclosure of which are hereby incoφorated by reference: 'Four level ink set for bi-level color printing' (Filing no.: PN2339); 'Compression system for page images' (Filing no.: PN2340); 'Real-time expansion apparatus for compressed page images' (FUing no.: PN2341); and 'High capacity compressed document image storage for digital color printers' (FUing no.: PN2342); 'Improving JPEG compression in the presence of text' (Filing no.: PN2343); 'An expansion and halftoning device for compressed page images' (Filing no.: PN2344); and 'Improvements in image halftoning' (Filing no.: PN2345). Applications Using Print Heads According to this Invention Printing apparatus and methods of this invention are suitable for a wide range of appUcations, including (but not limited to) die foUowing: color and monochrome office printing, short run digital printing, high speed digital printing, process color printing, spot color printing, offset press supplemental printing, low cost printers using scanning print heads, high speed printers using pagewidth print heads, portable color and monochrome printers, color and monochrome copiers, color and monochrome facsimile machines, combined printer, facsimUe and copying machines, label printing, large format plotters, photographic dupUcation, printers for digital photographic processing, portable printers incoφorated into digital 'instant' cameras, video printing, printing of PhotoCD images, portable printers for 'Personal Digital Assistants', waUpaper printing, indoor sign printing, biUboard printing, and fabric printing. Printing systems based on this invention are described in the foUowing AustraUan patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'A high speed color office printer with a high capacity digital page image store' (Filing no.: PN2329); 'A short run digital color printer with a high capacity digital page image store' (Filing no.: PN2330); 'A digital color printing press using LIFT printing technology' (FUing no.: PN2331); 'A modular digital printing press ' (Filing no. : PN2332) ; 'A high speed digital fabric printer' (FUing no.: PN2333); 'A color photograph copying system' (Filing no.: PN2334); 'A high speed color photocopier using a LIFT printing system' (Filing no.: PN2335); 'A portable color photocopier using LIFT printing technology' (Filing no.: PN2336); 'A photograph processing system using LIFT printing technology' (Filing no.: PN2337); 'A plain paper facsimile machine using a LIFT printing system' (Filing no.: PN2338); 'A PhotoCD system with integrated printer' (Filing no.: PN2293); 'A color plotter using LIFT printing technology' (FUing no.: PN2291); 'A notebook computer with integrated LIFT color printing system' (F ing no.: PN2292); 'A portable printer using a LIFT printing system' (FUing no.: PN2300); 'Fax machine with on-line database interrogation and customized magazine printing' (FUing no.: PN2299); 'Miniature portable color printer' (FUing no.: PN2298); 'A color video printer using a LIFT printing system' (FUing no.: PN2296); and 'An integrated printer, copier, scanner, and facsimile using a LIFT printing system' (FUing no.: PN2297) Compensation of Print Heads for Environmental Conditions It is desirable that drop on demand printing systems have consistent and predictable ink drop size and position. Unwanted variation in ink drop size and position causes variations in the optical density of d e resultant print, reducing the perceived print quaUty. These variations should be kept to a smaU proportion of the nominal ink drop volume and pixel spacing respectively. Many environmental variables can be compensated to reduce their effect to insignificant levels. Active compensation of some factors can be achieved by varying the power applied to the nozzle heaters. An optimum temperamre profile for one print head embodiment involves an instantaneous raising of the active region of die nozzle tip to die ejection temperamre, maintenance of this region at the ejection temperature for the duration of die pulse, and instantaneous cooling of die region to die ambient temperature. This optimum is not achievable due to the stored heat capacities and thermal conductivities of the various materials used in die fabrication of the nozzles in accordance with the invention. However, improved performance can be achieved by shaping die power pulse using curves which can be derived by iterative refinement of finite element simulation of the print head. The power appUed to the heater can be varied in time by various techniques, including, but not limited to: 1 ) Varying the voltage appUed to die heater 2) Modulating the widtii of a series of short pulses (PWM) 3) Modulating the frequency of a series of short pulses (PFM) To obtain accurate results, a transient fluid dynamic simulation with free surface modeling is required, as convection in the ink, and ink flow, significantiy affect on the temperamre achieved with a specific power curve. By the incoφoration of appropriate digital circuitry on the print head substrate, it is practical to individuaUy control the power appUed to each nozzle. One way to achieve this is by 'broadcasting' a variety of different digital pulse trains across the print head chip, and selecting the appropriate pulse train for each nozzle using multiplexing circuits. An example of the environmental factors which may be compensated for is Usted in the table "Compensation for environmental factors". This table identifies which environmental factors are best compensated globaUy (for the entire print head), per chip (for each chip in a composite multi-chip print head), and per nozzle. Compensation for environmental factors Most appUcations wUl not require compensation for aU of tiiese variables. Some variables have a minor effect, and compensation is only necessary where very high image quaUty is required. Print head drive circuits Figure 4 is a block schematic diagram showing electronic operation of an example head driver circuit in accordance with this invention. This control circuit uses analog modulation of die power supply voltage applied to the print head to achieve heater power modulation, and does not have individual control of the power applied to each nozzle. Figure 4 shows a block diagram for a system using an 800 dpi pagewidth print head which prints process color using the CC'MM'YK color model. The print head 50 has a total of 79,488 nozzles, with 39,744 main nozzles and 39,744 redundant nozzles. The main and redundant nozzles are divided into six colors, and each color is divided into 8 drive phases. Each drive phase has a shift register which converts the serial data from a head control ASIC 400 into parallel data for enabling heater drive circuits. There is a total of 96 shift registers, each providing data for 828 nozzles. Each shift register is composed of 828 shift register stages 217, the outputs of which are logicaUy anded widi phase enable signal by a nand gate 215. The ouφut of the nand gate 215 drives an inverting buffer 216, which in turn controls the drive transistor 201. The drive transistor 201 actuates the electrothermal heater 200, which may be a heater 103 as shown in figure 1(b). To maintain the shifted data valid during the enable pulse, the clock to the shift register is stopped die enable pulse is active by a clock stopper 218, which is shown as a single gate for clarity, but is preferably any of a range of weU known gUtch free clock control circuits. Stopping the clock of the shift register removes the requirement for a paraUel data latch in the print head, but adds some complexity to the control circuits in the Head Control ASIC 400. Data is routed to eitiier the main nozzles or the redundant nozzles by the data router 219 depending on d e state of the appropriate signal of the fault status bus. The print head shown in figure 4 is simplified, and does not show various means of improving manufacturing yield, such as block fault tolerance. Drive circuits for different configurations of print head can readily be derived from me apparatus disclosed herein. Digital information representing patterns of dots to be printed on the recording medium is stored in die Page or Band memory 1513, which may be the same as the Image memory 72 in figure 1(a). Data in 32 bit words representing dots of one color is read from me Page or Band memory 1513 using addresses selected by die address mux 417 and control signals generated by the Memory Interface 418. These addresses are generated by Address generators 411, which forms part of the 'Per color circuits' 410, for which there is one for each of the six color components. The addresses are generated based on die positions of the nozzles in relation to the print medium. As die relative position of the nozzles may be different for different print heads, the Address generators 411 are preferably made programmable. The Address generators 411 normally generate the address corresponding to d e position of the main nozzles. However, when faulty nozzles are present, locations of blocks of nozzles containing faults can be marked in die Fault Map RAM 412. The Fault Map RAM 412 is read as the page is printed. If the memory indicates a fault in the block of nozzles, the address is altered so that the Address generators 411 generate d e address corresponding to the position of the redundant nozzles. Data read from me Page or Band memory 1513 is latched by die latch 413 and converted to four sequential bytes by the multiplexer 414. Timing of these bytes is adjusted to match that of data representing other colors by the FIFO 415. This data is tiien buffered by die buffer 430 to form the 48 bit main data bus to the print head 50. The data is buffered as the print head may be located a relatively long distance from die head control ASIC. Data from the Fault Map RAM 412 also forms the input to the FIFO 416. The timing of this data is matched to die data output of die FIFO 415, and buffered by die buffer 431 to form the fault status bus. The programmable power supply 320 provides power for d e head 50. The voltage of the power supply 320 is controUed by die DAC 313, which is part of a RAM and DAC combination (RAMDAC) 316. The RAMDAC 316 contains a dual port RAM 317. The contents of the dual port RAM 317 are programmed by d e MicrocontroUer 315. Temperamre is compensated by changing the contents of the dual port RAM 317. These values are calculated by d e microcontroUer 315 based on temperature sensed by a diermal sensor 300. The thermal sensor 300 signal connects to the Analog to Digital Converter (ADC) 311. The ADC 311 is preferably incoφorated in the MicrocontroUer 315. The Head Control ASIC 400 contains control circuits for thermal lag compensation and print density. Thermal lag compensation requires that the power supply voltage to the head 50 is a rapidly time-varying voltage which is synchronized with the enable pulse for the heater. This is achieved by programming the programmable power supply 320 to produce this voltage. An analog time varying programming voltage is produced by die DAC 313 based upon data read from me dual port RAM 317. The data is read according to an address produced by the counter 403. The counter 403 produces one complete cycle of addresses during d e period of one enable pulse. This synchronization is ensured, as d e counter 403 is clocked by die system clock 408, and die top count of die counter 403 is used to clock d e enable counter 404. The count from the enable counter 404 is then decoded by die decoder 405 and buffered by die buffer 432 to produce die enable pulses for the head 50. The counter 403 may include a prescaler if the number of states in the count is less than the number of clock periods in one enable pulse. Sixteen voltage states are adequate to accurately compensate for the heater thermal lag. These sixteen states can be specified by using a four bit connection between die counter 403 and die dual port RAM 317. However, these sixteen states may not be linearly spaced in time. To aUow non-linear timing of tiiese states the counter 403 may also include a ROM or otiier device which causes die counter 403 to count in a non-linear fashion. Alternatively, fewer than sixteen states may be used. For print density compensation, die printing density is detected by counting the number of pixels to which a drop is to be printed ('on' pixels) in each enable period. The 'on' pixels are counted by die On pixel counters 402. There is one On pixel counter 402 for each of the eight enable phases. The number of enable phases in a print head in accordance with the invention depend upon d e specific design. Four, eight, and sixteen are convenient numbers, though there is no requirement that me number of enable phases is a power of two. The On Pixel Counters 402 can be composed of combinatorial logic pixel counters 420 which determine how many bits in a nibble of data are on. This number is then accumulated by die adder 421 and accumulator 422. A latch 423 holds die accumulated value vaUd for die duration of the enable pulse. The multiplexer 401 selects the output of the latch 423 which corresponds to the current enable phase, as determined by the enable counter 404. The output of the multiplexer 401 forms part of the address of die dual port RAM 317. An exact count of the number of 'on' pixels is not necessary, and the most significant four bits of this count are adequate. Combining the four bits of thermal lag compensation address and die four bits of print density compensation address means mat the dual port RAM 317 has an 8 bit address. This means that the dual port RAM 317 contains 256 numbers, which are in a two dimensional array. These two dimensions are time (for thermal lag compensation) and print density. A third dimension - temperature - can be included. As the ambient temperamre of the head varies only slowly, the microcontroUer 315 has sufficient time to calculate a matrix of 256 numbers compensating for thermal lag and print density at die current temperamre. PeriodicaUy (for example, a few times a second), the microcontroUer senses the current head temperamre and calculates this matrix. The clock to the print head 50 is generated from me system clock 408 by the Head clock generator 407, and buffered by die buffer 406. To faciUtate testing of the Head control ASIC, JTAG test circuits 499 may be included. Comparison with thermal ink iet technology The table "Comparison between Thermal ink jet and Present Invention" compares the aspects of printing in accordance witii the present invention witii thermal inkjet printing technology. A direct comparison is made between d e present invention and thermal inkjet technology because both are drop on demand systems which operate using thermal actuators and Uquid ink. Although they may appear simUar, the two technologies operate on different principles. Thermal ink jet printers use the foUowing fundamental operating principle. A thermal impulse caused by electrical resistance heating results in the explosive formation of a bubble in Uquid ink. Rapid and consistent bubble formation can be achieved by superheating the ink, so that sufficient heat is transferred to die ink before bubble nucleation is complete. For water based ink, ink temperatures of approximately 280°C to 400°C are required. The bubble formation causes a pressure wave which forces a drop of ink from the aperture with high velocity. The bubble tiien coUapses, drawing ink from the ink reservoir to re-fill die nozzle. Thermal ink jet printing has been highly successful commerciaUy due to d e high nozzle packing density and d e use of weU established integrated circuit manufacturing techniques. However, thermal ink jet printing technology faces significant technical problems including multi-part precision fabrication, device yield, image resolution, 'pepper' noise, printing speed, drive transistor power, waste power dissipation, satelUte drop formation, thermal stress, differential thermal expansion, kogation, cavitation, rectified diffusion, and difficulties in ink formulation. Printing in accordance with die present invention has many of the advantages of thermal ink jet printing, and completely or substantiaUy eliminates many of the inherent problems of thermal ink jet technology. Comparison between Thermal inkjet and Present Invention Yield and Fault Tolerance In most cases, monoUthic integrated circuits cannot be repaired if tiiey are not completely functional when manufacmred. The percentage of operational devices which are produced from a wafer run is known as the yield. Yield has a direct influence on manufacturing cost. A device witii a yield of 5% is effectively ten times more expensive to manufacture than an identical device with a yield of 50%. There are three major yield measurements: 1) Fab yield 2) Wafer sort yield 3) Final test yield For large die, it is typically die wafer sort yield which is the most serious Umitation on total yield. FuU pagewidth color heads in accordance with this invention are very large in comparison with typical VLSI circuits. Good wafer sort yield is critical to the cost-effective manufacture of such heads. Figure 5 is a graph of wafer sort yield versus defect density for a monolidύc fuU widtii color A4 head embodiment of die invention. The head is 215 mm long by 5 mm wide. The non fault tolerant yield 198 is calculated according to Muφhy's method, which is a widely used yield prediction method. Witii a defect density of one defect per square cm, Muφhy's method predicts a yield less tiian 1 %. This means that more than 99% of heads fabricated would have to be discarded. This low yield is highly undesirable, as the print head manufacturing cost becomes unacceptably high. Muφhy' s method approximates the effect of an uneven distribution of defects. Figure 5 also includes a graph of non fault tolerant yield 197 which expUcidy models the clustering of defects by introducing a defect clustering factor. The defect clustering factor is not a controllable parameter in manufacturing, but is a characteristic of die manufacturing process. The defect clustering factor for manufacturing processes can be expected to be approximately 2, in which case yield projections closely match Muφhy's method. A solution to die problem of low yield is to incoφorate fault tolerance by including redundant functional units on the chip which are used to replace faulty functional units. In memory chips and most Wafer Scale Integration (WSI) devices, die physical location of redundant sub-units on the chip is not important. However, in printing heads the redundant sub-unit may contain one or more printing actuators. These must have a fixed spatial relationship to die page being printed. To be able to print a dot in die same position as a faulty actuator, redundant actuators must not be displaced in the non-scan direction. However, faulty actuators can be replaced with redundant actuators which are displaced in the scan direction. To ensure that the redundant actuator prints the dot in d e same position as the faulty actuator, the data timing to the redimdant actuator can be altered to compensate for d e displacement in die scan direction. To aUow replacement of all nozzles, there must be a complete set of spare nozzles, which results in 100% redundancy. The requirement for 100% redundancy would normaUy more than double die chip area, dramaticaUy reducing the primary yield before substituting redundant units, and tiuis eliminating most of the advantages of fault tolerance. However, with print head embodiments according to tiiis invention, the minimum physical dimensions of die head chip are determined by die width of die page being printed, the fragUity of the head chip, and manufacturing constraints on fabrication of ink channels which supply ink to the back surface of the chip. The minimum practical size for a full widtii, fuU color head for printing A4 size paper is approximately 215 mm x 5 mm. This size aUows the inclusion of 100% redundancy without significandy increasing chip area, when using 1.5 μm CMOS fabrication technology. Therefore, a high level of fault tolerance can be included widiout significandy decreasing primary yield. When fault tolerance is included in a device, standard yield equations cannot be used. Instead, die mechanisms and degree of fault tolerance must be specificaUy analyzed and included in d e yield equation. Figure 5 shows the fault tolerant sort yield 199 for a fuU width color A4 head which includes various forms of fault tolerance, the modeling of which has been included in the yield equation. This graph shows projected yield as a function of both defect density and defect clustering. The yield projection shown in figure 5 indicates tiiat thoroughly implemented fault tolerance can increase wafer sort yield from under 1 % to more than 90% under identical manufacturing conditions. This can reduce the manufacturing cost by a factor of 100. Fault tolerance is highly recommended to improve yield and reliabiUty of print heads containing thousands of printing nozzles, and thereby make pagewiddi printing heads practical. However, fault tolerance is not to be taken as an essential part of the present invention. Fault tolerance in drop-on-demand printing systems is described in die following Australian patent specifications filed on 12 April 1995, the disclosure of which are hereby incoφorated by reference: 'Integrated fault tolerance in printing mechanisms' (FUing no.: PN2324); 'Block fault tolerance in integrated printing heads' (Filing no.: PN2325); 'Nozzle duplication for fault tolerance in integrated printing heads' (FUing no.: PN2326); 'Detection of faulty nozzles in printing heads' (FUing no.: PN2327); and 'Fault tolerance in high volume printing presses' (Filing no.: PN2328). Photograph processing systems using printing The table "Example product specifications" shows die specifications of one possible configuration of a high quaUty color photograph processing system. Example product specifications Figure 6 shows a schematic process diagram of a high speed color photograph printing system (mini-lab) using printing technology according to die present invention. The blocks in this diagram represent discrete functions, irrespective of tiieir implementations. Some of the blocks are electronic hardware, some are computer software, some are electromechanical units, and some are mechanical units. Some of the blocks are subsystems, which may include electronic hardware, software, mechanics, and optics. The operator places the strip of negative film 520 in the negative transport mechanism 521. The negative transport mechanism 521 moves the negative strip 520 relative to the scanner 502. This scanner can be composed of a color Unear CCD, one or more focusing lenses, a Ught source, and an optical patii. A suitable configuration includes a monoUthic linear CCD sensor containing three lines of optical sensors. If these sensor lines are the same length as die width of the negative film to be scanned (approximately 25 mm for standard 35 mm film), then no reduction or enlargement optics are required. The three sensor lines can be fabricated witii integral optical filters, one for each of the red, green, and blue additive primary colors. The technology required to manufacture such scanners is weU known. A scanner resolution of 2400 dpi is adequate, as tiiis wiU provides a resolution approximately equivalent to the size of film grain in 35 mm film. An array CCD may be used instead of a linear CCD, in which case no relative movement is required. However, array CCD's of optimum resolution are currentiy too expensive to be practical. This wUl change in the future, and an area scanning CCD element designed for high definition television is likely to be suitable. The output of the scanner 502 is connected to a scanner signal conditioning unit 503. This unit amplifies and filters the CCD analog signal, converts the signal to digital form, and provides CCD sensitivity compensation to adjust for differences in individual CCD sensors. To preserve die dynamic range of the negative film, and thereby allow color correction, brightness correction, and saturation correction over a wide range widiout causing color quantization artifacts, the analog ouφut of the linear CCD should be converted to 12 bits per color component. Lower color resolution can be used, witii a subsequent reduction in color quaUty or color correction range. The ouφut of the scanner signal conditioning unit 503 is raster-ordered continuous tone image data, witii typically 36 bits per pixel. The ouφut of the scanner signal conditioning unit 503 is connected to a digital image processing unit 519. The primary function of this unit is color correction, but the unit may also be used for digital special effects. Many digital effects can be performed, and many are weU known to the art. The color correction process can be performed in real-time, and may incoφorate color look-up tables for each color as weU as matrix multiply operations. Alternatively, more general color correction techniques may be used, such as a subsampled color space look-up table with tri-linear inteφolation between the color samples. The color correction process can include a conversion from the negative image of the negative film, to the positive image typically required for die photographic print. The color correction process may also include gamma correction, and correction for film stock type. The ouφut of the digital image processing unit 519 is raster-ordered continuous tone image data, witii typicaUy 24 bits per pixel. This is digitaUy halftoned by die vector error diffusion unit 504. A vector error diffusion algorithm is used to achieve a high image quaUty. This operates by selecting die closest printable color in three dimensional color space to the desired color. The difference between the desired coior and tiiis printable Color is determined. This difference is then diffused to neighboring pixels. The vector error diffusion unit 504 accepts a raster ordered continuous tone input image and generates a bi-level ouφut with 7 bits per pixel (one bit for each of Cyan, Magenta, YeUow, and Black, as weU as quarter density inks of Cyan, Magenta, and Black). Alternatively, the color components can be independenUy error diffused, although diis provides an image of substantiaUy lower quality. It is also possible to dither the continuous tone image to obtain a bi-level image. In this case, a computer optimized stochastic dispersed dot ordered dither is recommended. This data is tiien processed by die data phasing and fault tolerance system 506. This unit provides die appropriate delays to synchronize the print data witii the offset positions of the nozzle of the printing head. It also provides alternate data patiis for fault tolerance, to compensate for blocked nozzles, faulty nozzles or faulty circuits in the head. The monoUthic printing head 50 prints the image 60 composed of a multimde of ink drops onto a recording medium 51. This medium wUl typically be paper, but can also be overhead transparency film, cloth, or most other substantially flat surfaces which wiU accept ink drops. The bi-level image processed by die data phasing and fault tolerance circuit 506 provides die pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and die movement of the paper. When the data has been loaded into die shift registers, it is presented in parallel to the heater driver circuits 57. At the correct time, these driver circuits wiU electronically connect the corresponding heaters 58 witii the voltage pulse generated by die pulse shaper circuit 61 and d e voltage regulator 62. The heaters 58 heat the tip of die nozzles 59, reducing die attraction of the ink to the nozzle surface material. Ink drops 60 escape from the nozzles in a pattem which corresponds to the digital impulses which have been appUed to the heater driver circuits. The pressure of the ink in the nozzle is important, and die pressure in the ink reservoir 64 is regulated by die pressure regulator 63. The ink drops 60 fall under die influence of gravity or another field type towards the paper 51. During printing, the paper is continuaUy moved relative to the print head by die paper transport system 65. As the print head is the fuU widtii of die paper used, it is only necessary to move the paper in one direction, and die print head can remain fixed. The paper may be suppUed as pre-cut sheets, in which case the paper transport mechanism must acquire and transport the sheets individually past the printing head. However, it is difficult to manufacture high speed cut-sheet paper transport mechanisms which are not prone to paper jamming. To avoid tiiis problem, die paper may be provided in rolls. In tiiis case, an automatic paper cutting blade is required. The various subsystems are coordinated under die control of one or more control microcomputers 511, which also provide die user interface of the system. There are many possible physical configurations of this aspect of the invention. One configuration convenient to use in photograph processing shops involves spUtting the negative scanning and printing portions of the device into two separate units. Digital information representing the photographs is transferred from die scanner to die printer using a digital data link operating in real time. Figure 7(a) shows a side view of die scanner section 628 of the digital 'mini-lab', showing possible placement of some of the major modules. Strips of film negative 520 are inserted into the scanner unit and pass through a negative scanner 502. The negative scanner consists of a Ught source, focusing optics, and an electronic image sensor, such as a linear CCD. The signal from the scanner 502 is processed by control electronics 900. Data representing the scanned images is transmitted to die printer section 629 of the digital 'mini-lab' shown in figure 7(b). A cutting device 569 can be incoφorated to cut die fuU length negative (typicaUy 24 or 36 frames) into strips, as is conventional for photo processing. Figure 7(b) shows a side view of die printer section 629 of the digital 'mini-lab', showing possible placement of some of the major modules. Ink reservoirs 574 can be placed at die top of die printing unit, aUowing gravity feed of ink to the head 50 witii sufficient pressure to aUow operation of the nozzle. This depends upon nozzle diameter, but is approximately 11,200 Pa above ambient air pressure for an 11 mm nozzle. A drop of approximately 1.16 meters from the ink level in die reservoirs to the printing surface of the nozzle is required if gravity feed is to be used. When photographs are to be printed, paper from die paper roll 575 is moved past die head 50 by paper transport mechanisms 65. Data representing the scanned image is received from die scanner section 628 of the device and processed by an electronic subsystems 900. This subsystem apphes any image enhancement and color correction required, then digitally halftones the image data. This data is then used to control the pattern of dots printed by die head 50. The printed photographs 51 are cut from the roll by the cutter 569. Figure 8 shows a perspective view of the digital 'mini-lab'. The negative fUm 520 is shown inserted into die scanner section 628 which is connected by a cable 627 to the printer section 629. The amount of paper left on the paper roll 575 can be seen through a window in the printer section. Figure 9 shows a schematic process diagram of a high speed color photograph copier using LIFT printing technology. The operator places the photograph 516 in the photograph transport mechanism 517. This system has a means of signaUng the control microcomputer 511 that a photograph is ready to be scanned. This may be an optical sensor, a mechanical sensor, a push-button, or other means. If the device is required to operate and accept payment wid out operator assistance, (commonly known as 'coin operated'), tiien a payment detection mechanism 518 is required. This mechanism signals the microcomputer 511 with the payment amount every time payment is inserted. A coin detection unit, a bank-note detection system, credit card system, or other automatic payment system may be used. Such mechanisms are weU known. When sufficient payment has been deposited, die system is ready to print a photograph copy. The photograph transport mechanism 517 moves the photograph 516 relative to die scanner 502. This may be achieved by moving die photograph past a fixed linear CCD, moving a linear CCD past the photograph, or by some other means. This scanner can be composed of a color linear CCD, one or more focusing lenses, an optical path, and a means of scanning the optical path of the CCD relative to d e object being scanned. A suitable configuration includes a monoUthic linear CCD sensor containing three lines of optical sensors. If these sensor lines are the same length as the width of the largest photograph to be scanned, tiien no reduction optics are required. The tiiree sensor lines can be fabricated with integral optical filters, one for each of the red, green, and blue additive primary colors. The technology required to manufacture such scanners is weU known. A scanner resolution of 400 dpi is adequate, and eliminates the need for complex sampUng rate conversions to match the 800 dpi resolution of the printer. An array CCD may be used instead of a Unear CCD, in which case no relative movement is required. However, array CCD's of optimum resolution are currendy too expensive to be practical. This will change in the future, and an area scanning CCD element designed for high definition television is likely to be suitable. The ouφut of the scanner 502 is connected to a scanner signal conditioning unit 503. This unit amplifies and filters the CCD analog signal, converts the signal to digital form, provides CCD sensitivity compensation to adjust for differences in individual CCD sensors, and provides image processing functions such as color correction. The ouφut of the Scanner signal conditioning unit 503 is raster-ordered continuous tone image data, with typicaUy 24 bits per pixel. This is digitaUy halftoned by die vector error diffusion unit 504. A vector error diffusion algorithm is used to achieve a high image quaUty. This operates by selecting the closest printable color in three dimensional color space to the desired color. The difference between die desired color and tiiis printable color is determined. This difference is then diffused to neighboring pixels. The vector error diffusion unit 504 accepts a raster ordered continuous tone input image and generates a bi-level ouφut witii 6 bits per pixel (one bit for each of C,C',M,M',Y and K). Alternatively, the color components can be independendy error diffused, although tiiis provides an image of substantiaUy lower quaUty. It is also possible tc dither die continuous tone image to obtain a bi-level image. In this case, a computer optimized stochastic dispersed dot ordered dither is recommended. This data is then processed by die data phasing and fault tolerance system 506. This unit provides the appropriate delays to synchronize the print data with the offset positions of the nozzle of the printing head. It also provides alternate data patiis for fault tolerance, to compensate for blocked nozzles, faulty nozzles or faulty circuits in the head. The monoUthic printing head 50 prints the image 60 composed of a multimde of ink drops onto a recording medium 51. This medium wUl typically be paper, but can also be overhead transparency film, cloth, or most other substantiaUy flat surfaces which wiU accept ink drops. The bi-level image processed by the data phasing and fault tolerance circuit 506 provides die pixel data in the correct sequence to the data shift registers 56. Data sequencing is required to compensate for the nozzle arrangement and die movement of the paper. When the data has been loaded into the shift registers, it is presented in parallel to the heater driver circuits 57. At the correct time, these driver circuits will electronically connect the corresponding heaters 58 with the voltage pulse generated by die pulse shaper circuit 61 and die voltage regulator 62. The heaters 58 heat the tip of the nozzles 59, reducing the attraction of the ink to the nozzle surface material. Ink drops 60 escape from the nozzles in a pattern which corresponds to the digital impulses which have been appUed to the heater driver circuits. The pressure of the ink in the nozzle is important and d e pressure in die ink reservoir 64 is regulated by d e pressure regulator 63. The ink drops 60 fall under the influence of gravity or another field type towards die paper 51. During printing, the paper is continuaUy moved relative to the print head by the paper transport system 65. As the print head is the full width of the paper used, it is only necessary to move the paper in one direction, and die print head can remain fixed. The various subsystems are coordinated under die control of one or more control microcomputers 511, which also provide die user interface of the system. There are many possible physical configurations of this aspect of the invention. Figure 10 shows a side view of die photograph copier, showing possible placement of some of the major modules. To make a photograph copy, the original photograph 516 is placed on the copy glass 519. The image of one line of the photograph at a time is presented via three scanning mirrors forming part the photograph transport system 517 and a lens to the linear CCD scanner 502. The signal from the CCD 502 is processed by control electronics 900. Bi-level image data derived from die photograph image is used to control activation of the ink nozzles in the head 50. Sheets of recording material 51 (preferably coated paper) are picked up by the paper pick-up roUer 912 from the paper and ink cartridge 910 and passed through die paper transport roUers 65 and under die head 50, whereupon die ejected ink drops form an image related to die original photograph image. Figure 11 (a) shows a perspective view of the photograph copier with the paper and ink cartridge 910 inserted, and die copy lid down. Visible are the printed paper 51, die ouφut paper tray 911, and die user interface control buttons 901. Figure 11(b) shows a perspective view of the photograph copier with the paper and ink cartridge 910 removed, and the copy lid up, exposing the copy glass 519. The foregoing describes embodiments of the present invention. Modifications, obvious to those skiUed in the art, can be made tiiereto without departing from the scope of die invention.

Claims: I Claim; 1. A color photograph printing apparatus including: (a) a color negative film scanner to produce an electronic representation of the image on negative film; (b) a scanner signal conditioning unit which converts the scanner data to digital form; (c) a digital image processing system which converts negative image data to positive image data; (d) a digital halftoning unit which converts the continuous tone image data ouφut by die scanner signal processing unit to bi-level image data; (e) a data distribution and timing system which provides die bi- level image data to die printing head at die correct time during a printing operation; and (f) a bi-level color printing mechanism. 2. A color photograph printing apparatus as claimed in claim 1 where the bi-level printing mechanism includes a printer having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated witii said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, whUe aUowing ink to be retained in non-selected nozzles. 3. A color photograph printing apparatus as claimed in claim 2 where the print head is a single monoUthic printing head which can print to die full widtii of the photographic print. 4. A color photograph printing apparatus as claimed in claim 2 where the is composed of a plurality of monolitiiic printing heads. 5. A color photograph printing apparatus as claimed in claim 1 where die bi-level printing mechanism includes a printer having a print head comprising: (a) a plurality of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while allowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 6. A color photograph printing apparatus as claimed in claim 5 where the print head is a single monoUthic printing head which can print to the full widtii of the photographic print. 7. A color photograph printing apparatus as claimed in claim 5 where die is composed of a plurality of monolithic printing heads. 8. A color photograph printing apparatus as claimed in claim 1 where die bi-level printing mechanism includes a printer having a print head comprising: (a) a plurahty of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, while allowing ink to be retained in non-selected nozzles. 9. A color photograph printing apparatus as claimed in claim 8 where d e print head is a single monotitiiic printing head which can print to the full widtii of the photographic print. 10. A color photograph printing apparatus as claimed in claim 8 where die is composed of a plurality of monoUdiic printing heads. 11. A color photograph printing apparatus as claimed in claim 1 which includes a digital color correction apparatus. 12. A color photograph printing apparatus as claimed in claim 1 where die printing ink colors used are CC'MM'YK. 13. A color photograph printing apparatus as claimed in claim 1 where the digital halftoning unit operates using vector error diffusion. 14. A color photograph printing apparatus as claimed in claim 1 where the digital halftoning unit operates using independent error diffusion of each printing color. 15. A color photograph printing apparatus as claimed in claim 1 where the digital halftoning unit operates using didiering. 16. A color photograph printing apparatus as claimed in claim 1 where the print paper is in the form of pre-cut sheets. 17. A color photograph printing apparatus as claimed in claim 1 where die print paper is in the form of a continuous roll, and which incoφorates an automatic paper cutter. 18. A color photograph printer substantially as herein described, witii reference to the accompanying diagrams. 19. A color photograph copier including a printer having a print head comprising: (a) a pluraUty of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (d) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non- selected nozzles; and (e) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, whUe aUowing ink to be retained in non-selected nozzles. 20. The color photograph copier as claimed in Claim 19 where the photograph copying operation is automaticaUy enabled by a payment sensing device. 21. A color photograph copier including a printer having a print head comprising: (a) a pluraUty of drop-emitter nozzles; (b) a body of ink associated with said nozzles; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, while aUowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 22. The color photograph copier as claimed in Claim 21 where the photograph copying operation is automatically enabled by a payment sensing device. 23. A color photograph copier including a printer having a print head comprising: (a) a pluraUty of drop-emitter nozzles; (b) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperamre range; (c) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non- selected nozzles; and (d) drop separating means for causing ink from selected nozzles to separate as drops from the body of ink, whUe aUowing ink to be retained in non-selected nozzles. 24. The color photograph copier as claimed in Claim 23 where the photograph copying operation is automatically enabled by a payment sensing device. 25. A color photograph copying apparatus including: (a) a color image scanner which produces data representing a photograph to be copied; (b) a scanner signal conditioning unit which converts the scanner data to digital form, and may provide various digital signal processing functions; (c) a digital halftoning unit which converts the continuous tone image data ouφut by said scanner signal processing unit to bi-level image data; (d) a data distribution and timing system which provides die bi- level image data to die printing head at die correct time during a printing operation; and (e) a bi-level color printing mechanism including a printer having a print head comprising: (1) a pluraUty of drop-emitter nozzles; (2) a body of ink associated witii said nozzles; (3) pressure means for subjecting ink in said body of ink to a pressure of at least 2% above ambient pressure, at least during drop selection and separation; (4) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (5) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, whUe aUowing ink to be retained in non-selected nozzles. 26. A color photograph copying apparatus as claimed in claim 25 where d e print head is a single monoUthic printing head which can print to the fuU widtii of die paper. 27. A color photograph copying apparatus as claimed in claim 25 where the print head is composed of a pluraUty of monoUthic LIFT printing heads. 28. A color photograph copying apparatus as claimed in claim 25 where the photograph copying operation is automaticaUy enabled by a payment sensing device. 29. A color photograph copying apparatus as claimed in claim 25 where the digital halftoning unit operates using vector eπor diffusion. 30. A color photograph copying apparatus as claimed in claim 25 where the digital halftoning unit operates using independent error diffusion of each printing color. 31. A color photograph copying apparatus as claimed in claim 25 where the digital halftoning unit operates using dithering. 32. A color photograph copying apparatus including: (a) a color image scanner which produces data representing a photograph to be copied; (b) a scanner signal conditioning unit which converts the scanner data to digital form, and may provide various digital signal processing functions; (c) a digital halftoning unit which converts the continuous tone image data ouφut by said scanner signal processing unit to bi-level image data; (d) a data distribution and timing system which provides the bi- level image data to die printing head at die correct time during a printing operation; and (e) a bi-level color printing mechanism including a printer having a print head comprising: (1) a plurality of drop-emitter nozzles; (2) a body of ink associated with said nozzles; (3) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (4) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, whUe aUowing ink to be retained in non-selected nozzles, said drop selecting means being capable of producing said difference in miniscus position in the absence of said drop separation means. 33. A color photograph copying apparatus as claimed in claim 32 where the print head is a single monoUthic printing head which can print to the full widtii of die paper. 34. A color photograph copying apparatus as claimed in claim 32 where die print head is composed of a plurality of monoUdiic LIFT printing heads. 35. A color photograph copying apparatus as claimed in claim 32 where the photograph copying operation is automaticaUy enabled by a payment sensing device. 36. A color photograph copying apparatus as claimed in claim 32 where die digital halftoning unit operates using vector error diffusion. 37. A color photograph copying apparatus as claimed in claim 32 where the digital halftoning unit operates using independent enor diffusion of each printing color. 38. A color photograph copying apparatus as claimed in claim 32 where the digital halftoning unit operates using dithering. 39. A color photograph copying apparatus including: (a) a color image scanner which produces data representing a photograph to be copied; (b) a scanner signal conditioning unit which converts the scanner data to digital form, and may provide various digital signal processing functions; (c) a digital halftoning unit which converts the continuous tone image data ouφut by said scanner signal processing unit to bi-level image data; (d) a data distribution and timing system which provides die bi- level image data to die printing head at die correct time during a printing operation; and (e) a bi-level color printing mechanism including a printer having a print head comprising: (1) a plurality of drop-emitter nozzles; (2) a body of ink associated with said nozzles, said ink exhibiting a surface tension decrease of at least 10 mN/m over a 30°C temperature range; (2) drop selection means for selecting predetermined nozzles and generating a difference in meniscus position between ink in selected and non-selected nozzles; and (4) drop separating means for causing ink from selected nozzles to separate as drops from die body of ink, whtie allowing ink to be retained in non-selected nozzles. 40. A color photograph copying apparatus as claimed in claim 39 where the print head is a single monolitiiic printing head which can print to the full width of the paper. 41. A color photograph copying apparatus as claimed in claim 39 where the print head is composed of a pluraUty of monoUdiic LIFT printing heads. 42. A color photograph copying apparatus as claimed in claim 39 where the photograph copying operation is automatically enabled by a payment sensing device. 43. A color photograph copying apparatus as claimed in claim 39 where the digital halftoning unit operates using vector enor diffusion. 44. A color photograph copying apparatus as claimed in claim 39 where the digital halftoning unit operates using independent enor diffusion of each printing color. 45. A color photograph copying apparatus as claimed in claim 39 where die digital halftoning unit operates using dithering. 46. A color photograph copier substantially as herein described, witii reference to the accompanying diagrams.

Inventors: SILVERBROOK KIA, SILVERBROOK, KIA
Applicants: EASTMAN KODAK CO, SILVERBROOK KIA, EASTMAN KODAK COMPANY, SILVERBROOK, KIA
Application year: 1996
Main classification: H04N1
IPC codes: B41J2, G07F17, H04N1