A 3-Colour Ink Jet Plotter for Computer Ggaphics

BIT 13 (1973), 181--195

A 3-COLOUR INK JET PLOTTER FOR COMPUTER GRAPHICS BORIS SMEDS Abstract. A new plotter for hard copy colour display of digital computer outputs has been developed. The plotter uses three intensity modulated ink jets with the colours red, yellow and blue, which plot the information on a normal A 4 paper (210 × 297 ram) fastened to a rotating drum. The ink jets are individually controlled from a magnetic tape unit which is loaded with a tape prepared by the computer. The total plotting time is 85 seconds independent of the complexity of the picture t~ be plotted. The total information contents plotted in one 85 second run is 3.7.10 e bits. 1. Introduction.

A t present, c o m p u t e r o u t p u t s are n o r m a l l y p r e s e n t e d in a l p h a n u m e r i cal form. Due to the great speed a n d growing use of c o m p u t e r s these results t e n d t o flood t h e user, owing t o the fact t h a t this f o r m of pres e n t a t i o n is n o t easily interpreted. Therefore, results f r o m c o m p u t e r s are m o r e f r e q u e n t l y p r e s e n t e d in graphical form, e.g., on t h e screen of a c a t h o d e r a y tube. H o w e v e r , it is often also desirable t o o b t a i n such graphic d a t a in the f o r m of h a r d copy. This is usually done b y means of a linep r i n t e r [1], a mechanical x-y-plotter, or a microfilm recorder [2]. A disadv a n t a g e with the line p r i n t e r m e t h o d is t h e limitation in resolution due to t h e size of the individual characters used as picture elements. This gives v e r y large pictures which often h a v e to be p h o t o g r a p h i c a l l y reduced. T h e generation of m o r e complicated figures or curves b y m e a n s of a n x-y-plotter is quite t i m e consuming if finer details are to be represented or different areas e n h a n c e d b y cross-hatching. F u r t h e r , the accessibility of the graphic display g e n e r a t e d b y a n x-y-plotter would be v e r y m u c h increased if the i n f o r m a t i o n could be p r e s e n t e d in different coloul~ [3]. Because of the obvious a d v a n t a g e of a graphical display in colour a n d t h e n e e d of a high-speed h a r d c o p y p r i n t e r for such displays, the application of electrically controlled ink jets for this purpose has been investigated b y the a u t h o r . As a result of this a new p l o t t e r has been Received Jan. 12, 1973.

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developed which allows the plotting of 980 × 1250 bits of information for each of the three colours red, yellow, and blue on an A4 sized paper fastened to a rotating drum. While an x-y-plotter uses the calligraphic method to plot the graphic display [4], the present method produces a raster display similar to that of a TV picture. The raster is built up of 1250 parallel lines in each colour, each line consisting of 980 individual points. Since the raster display is plotted line b y line at high speed, it is not possible to reverse the direction of printing or to return the printing head to a point already passed earlier during the plotting operation as can be done with the x-y-plotter. On the other hand, if complex displays are to be plotted, the present plotter is usually much faster than an x-y-plotter because of its constant plotting time of 85 seconds for an A4 sized page. As mentioned above, three fine jets of ink are used in the printing head of the present plotter. These jets are formed from blue, yellow, and red ink and can be controlled separately b y electric signals. Before describing the present plotter a short survey of the electric control of these ink jets will be given.

2. The basic principle for the intensity m o d u l a t i o n of an ink jet.

Fine jets of ink have been used for more than 10 years for the recording of electrical signals [5]. Hertz et al. [61 achieved the intensity modulation of such ink jets b y using the fact that an ink jet can be transformed into a spray of fine droplets b y the application of an electric field near the nozzle producing the jet as shown below. Fig.la shows how a fine ink jet J is produced b y forcing ink under

a

b

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Fig.1. (a) B y forcing i n k u n d e r h i g h p r e s s u r e t h r o u g h t h e fine nozzle N , a n i n k j e t J is f o r m e d w h i c h g e n e r a t e s a r e c o r d i n g t r a c e o n t h e r e c o r d i n g p a p e r . T h i s j e t c a n be t r a n s f o r m e d i n t o a s p r a y of s m a l l d r o p l e t s b y a p p l y i n g a s u i t a b l e v o l t a g e V to t h e c o n t r o l electrode E a s s h o w n i n (b). I n t h a t case n o t r a c e is o b t a i n e d o n t h e p a p e r . T h i s allows ~ n i n t e n s i t y m o d u l a t i o n of t h e trace. Since t h e d r o p l e t s i n t h e s p r a y a r e electrically c h a r g e d , t h e y c a n be p r e v e n t e d f r o m r e a c h i n g t h e r e c o r d i n g p a p e r b y a n electric D C field b e t w e e n t h e electrodes D a n d B (c).

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high pressure through a small nozzle N. The jet travels through a ringshaped electrode E towards a recording paper where it generates a trace. If a voltage V of several hundred volts is placed between the control electrode E and a metal surface in contact with the ink, the jet is broken up into a spray of electrically charged droplets as shown in Fig.lb. Obviously, no distinct trace is generated on the paper as long as the voltage V is applied to the control electrode E. An on-off modulation of the trace can thus be obtained b y switching the voltage V between zero and some 500 volts. Hertz et al. [6] showed that the upper frequency limit of this modulation process is about 100 k H z for a 10 # m diameter ink jet. In the device shown in Fig.lb the ink spray still reaches the recording paper, thus generating a continuous background on the paper. Various methods of removing the spray have been investigated b y Hertz et al. [6] and [7]. One of these methods is shown in Fig.lc. It makes use of the fact that the droplets of the spray are all charged with the same sign, while the drops of the undisturbed jet carry practically no charge. B y arranging an electric field between the electrodes D and B through which the jet has to pass, all charged drops are removed while the undisturbed jet is not affected b y the field. To collect the ink deposited on the electrodes, one or both of them is made from porous material, which absorbs the ink quickly. The ink is continually removed from this material either b y the influence of gravity or b y a suction pump. Using electrodes D and B, about 10 mm in length and 4 mm apart, the field strength has to be about 400 Volts/mm for the efficient interception of the charged drops in the spray.

3. Principle of the colour plotter. The principle of the graphic plotter is shown in Fig.2. The plotter consists of four different units: the plotting unit, a magnetic tape transport, a buffer memory and a control unit. In the following a short survey of these units will be given. The plotting unit consists of a drum which carries the recording paper. The drum can carry an A4 sized paper with its long side parallel to the drum axis. This drum is rotated b y a synchronous motor M S with a constant speed of 1000 rpm. A recording head carrying 3 electrically on/off modulated ink jets with different colours can be moved along the drum b y means of stepper motor M and a lead screw. During the plotting operation the stepper motor M advances the recording head in increments of 0.2 mm for each drum rotation. In this

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BORIS SMEDS

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Fig.2. Basic units of the colour plotter. The information to be plotted is computed in a suitable form b y a computer and stored on a magnetic tape. This tape is then read by a magnetic tape transport into a buffer merory. The read-in and read-out periods of the memory are governed b y control circuits, and the read-out signals are used to modulate the intensity of the three ink jets (red, yellow and blue) mounted in front of the rotating d r u m carrying the recording paper.

w a y the entire surface of the A4 recording paper is covered b y the ink jets in parallel lines perpendicular to the drum axis, the line density being 5 lines per millimeter. 1250 lines are printed on the paper b y each colour in one plotting operation, and the plotting time is 85 seconds for a complete picture. B y suitable control of the ink jets a picture consisting of raster lines in the eolours yellow, red, and blue can be built up. The electrical signals necessary to control the ink jets are generated b y a computer and stored on magnetic tape in a suitable form. One picture is stored on the tape as one file with a number of blocks each storing 3 x 1024 bits of data for 3 raster lines in the three different colours. Of these 1024 bits 980 are being used for individual control of 980 different points in each raster line. Since for each colour 1250 lines are plotted, the total amount of information displayed in one picture is 3 × 1250 × 980 = 3.7 million bits. After the computation is completed the tape is transferred to the tape transport connected to the plotter. This tape transport reads the tape block b y block for each revolution of the drum during the plotting operation, thereby generating the picture on the plotter. The fact that the tape speed and the drum surface speed are not synchronized makes it necessary to store the information from one block in a buffer memory before the information is printed b y the plotter. This buffer memory is built up of shift registers and is divided into two halves, each containing three 1024-bit shift registers for the three colours. At the same time as data from one half of the memory are read out to control the ink jets of the recording head, the other half is refilled with new data from the tape transport. Thus, during the print out operation the two halves of the buffer memory are filled from the tape transport and read out b y the plotter alternatively for each drum rotation.

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To synchronize the transfer of data from the tape to the buffer memory, the read in and read out clocks controlling the shift registers and the stepper motor with the continuous drum rotation, a control unit is added. Among other things, this control unit starts the rotation of the drum and actuates the stepper motor which moves the recording head in 0.2 mm increments along the drum surface for each revolution of the drum. In the following, the different units comprising the plotter system will be described in more detail.

4. Plotting unit. The plotting unit is built up as a detached unit separated from the magnetic tape transport and the electronic control circuits with the buffer memory, which are housed in a 19 inch rack. The function of the plotting unit is shown b y the system block diagram in Fig.3 and will be described in the following. TO CONTROLCIRCUITS CABLE RECEIVER/DRIVER .3 SUCTIOH PUMP I

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Fig.3. Block diagram of plotting unit. The recording paper is attached to a rotating drum. A recording head with three on/off modulated ink jets moves along the drum so t h a t the entire paper can be covered with ink in the colours yellow, red and blue. All necessary control of the ink jets is carried out b y logic signals (Cf. Figures 3 and 5).

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A rotating drum is used to carry the recording paper. This peper is a nm~nal untreated A4 paper size 210 × 297 mm. The length of the drum is 310 mm and its diameter 89 mm which allows the paper to be fastened to the drum with its long side parallel to the drum axis. The drum is rotated by a 6-pole synchronous motor with a constant speed of 1000 rpm. This results in a drum surface speed of 4.66 m/s and a period of revolution of 60 ms. A recording head with the three electrically on/off modulated ink jets is mounted on the lead screw in front of the drum. By turning the lead screw the recording head can be moved along the whole length of the drum. With a lead screw pitch of 1.6 mm a 45 degree turn of the lead screw results in a 0.2 mm movement of the recording head. In order to obtain a fast and exact positioning of the recording head a stepper motor is used to drive the lead screw. The motor used is of the variable reluctance type with a stepping angle of 15 degrees. Thus for an 0.2 mm movement of the recording head the stepper motor is turned 3 steps. During the recording the head is advanced in increments of 0.2 mm for each drum revolution starting at the left end of the drum. When the recording is finished the stepper motor is reversed and the recording head is moved back to the starting point at the left end of the drum. An 0.2 mm movement of the recording head is made in 15 ms. Since the circumference of the drum is larger t h a n the width of the paper the actual recording takes only 43 ms of the 60 ms period of the drum revolution. Therefore, the recording head can be moved during the time between the recording of two successive lines. The starting point of each line must be positioned at the same height on the paper for each revolution of the drum. This is achieved by means of a trigger disc mounted on the drum axis. This disc has a small slit so t h a t light from a lamp can fall on a phototransistor which, after amplification, gives a trigger pulse each time the slit passes between the lamp and the phototransistor. The recording head uses three electrically on/off modulated ink jets to produce coloured recording traces on the paper. Fig.4 shows how the nozzles producing the ink jets and the control electrodes are arranged. Three jets of ink are produced by forcing ink under high pressure (here 3 MN/m ~= 30 bar) through the nozzles N, the diameter of which was 10 #m. Directly after leaving the nozzles each ink jet passes through a hole in the control electrodes E connected to the control amplifiers (Fig.3). The slightly conductive ink in the tubes leading to the nozzles is connected to ground. If a voltage of about + 400 V is applied to a control electrode it is found t h a t the ink jet is transformed into a spray of fine charged

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l~ig.4. E l e c t r o d e a r r a n g e m e n t in t h e r e c o r d i n g h e a d . I n k is forced t h r o u g h t h e nozzles N a n d f o r m s a s t r e a m of d r o p l e t s t h a t p a s s e s t h r o u g h t h e holes in t h e control electrodes E a n d t h e electrode f r o n t F . T h e nozzles a r e v i b r a t e d b y m e a n s of a n u l t r a s o n i c c r y s t a l C to o b t a i n u n i f o r m sized drops. If a c o n t r o l v o l t a g e of a b o u t 400 V is a p p l i e d to t h e c o n t r o l electrode E w i t h r e s p e c t to t h e c o n d u c t i v e i n k t h e j e t is d i s p e r s e d i n t o a s p r a y . T h i s c h a r g e d i n k s p r a y is p r e v e n t e d f r o m r e a c h i n g t h e recording p a p e r b y a D C field b e t w e e n t h e electrode D a n d t h e electrode b o t t o m B. B y c h a n g i n g t h e control electrode v o l t a g e b e t w e e n 0 a n d 400 V t h e j e t c a n be m o d u l a t e d on/off w i t h a n u p p e r f r e q u e n c y l i m i t of 100 k H z . T h r e e i n k j e t s y s t e m s u s i n g yellow, r e d a n d b l u e i n k a r e a r r a n g e d side b y side a s s h o w n in t h e figure, t h e electrodes D a n d B b e i n g c o m m o n to all t h r e e s y s t e m s .

droplets [7]. With a nozzle diameter of 10 /~m and a ring-formed electrode with an inner diameter of 2 mm placed 2 mm in front of the nozzle, this phenomenon is observable with signal voltages as low as 100 volts and increases continually as the signal voltage is raised. The spray, as well as the ink jet, is directed towards the recording paper. As this will deteriorate the recording in many eases the droplets of the spray have to be intercepted. In the present case the constant DC-field method shown in Fig.lc was chosen. B y arranging an electric field between the electrodes D and B through which the jet has to pass, all charged drops are removed while the undisturbed jet is not affected b y the field [8]. To collect the ink deposited on the electrodes, both of them are made from porous material which absorbs the ink quickly. The ink is continually removed from this material b y a suction pump (Fig.3). Using a signal voltage of 400 volts, a distance of 4 mm between the electrodes D and B, and a DC voltage of 1.5 kV, almost all spray drops were prevented from reaching the paper. Due to this fact practically no background is created on the recording paper, which is important for the present application of the ink jet method. However, M~nsson [9] has

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shown that this method to eliminate background on the recording results in a slight blurring of the recordings since small extra drops are deposited somewhat out of line from the main trace due to the action of the electric field. This could be prevented using some of the other means to decrease background as described by M~nsson [9]. Recording lines produced b y these ink jets are often interrupted by short unrecorded lengths. M£nsson [9] has shown that this depends on the fact that drops differing in size merge on their way to the recording paper. This can be prevented b y producing ink drops of uniform size. The nozzles are, therefore, vibrated by means of piezoelectric crystals C which are glued to the nozzles (Fig.4). The vibration frequency is about 1 MHz which is about the average drop formation frequency without vibration. Since the ink jets must be individually controlled the electrode system includes 3 different control electrodes E. The nozzles N are mounted 10 m m apart from each other. This causes a 10 m m displacement of the three coloured lines recorded simultaneously. This fact should be considered when generating the magnetic tape. Thus, when recording a trace with all three colours in the same line the recording of the red line must be retarded by 50 lines and the blue line by 100 lines relative to the recording of the yellow line. This is further explained below when the data format is described.

5. Control circuits and m e m o r y buffer.

This unit is housed in the same rack as the tape transport and power supplies. I t is used to carry out the necessary formatting of data read from the tape transport, and to give suitable commands to the tape transport and the plotting unit. The operation of the circuits will he described in reference to Fig.5, showing a system block diagram of the control circuits and memory buffer. A recording is started from the control panel by pressing the "PLOT" button. This sets a flip flop in the tape and plotter control and starts the drum and ink pumps in the plotting unit. At the same time the first block is read from the tape transport. The data are strobed into the input multiplexer in form of 9-bit words, where 1 bit is a parity bit. The frequency of the incoming data bytes is 30 kHz. The parity is checked, and a parity error will be indicated on the control panel. A parallel to serial conversion is made in the input multiplexer and data are transferred to one of the shift registers, e.g., register A, through the shift register selector starting with the most significant bit in each 8-bit word.

A ~-COLOUR IlgK JET PLOTTER FOR COMPUTER GRAPHICS TO MAGNETIC TAPE TRANSPORT

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Fig.5. Block d i a g r a m of buffer m e m o r y a n d control circuits. Manual c o m m a n d s are given b y m e a n s of p u s h b u t t o n s on the control panel. These c o m m a n d s are e v a l u a t e d b y t h e tape a n d plotter control which t r a n s m i t s suitable control signals to the t a p e t r a n s p o r t a n d the plotting unit. T h e information on the m a g n e t i c t a p e is read in 8 parallel bits plus a p a r i t y bit a n d is converted to a serial f o r m b y t h e i n p u t multiplexer a n d stored in one of the shift registers, e.g. register A storing 3 × 1024 bits of information for one d r u m revolution. A t t h e s a m e t i m e t h e d a t a stored in t h e other shift register B is strobed out via t h e o u t p u t control to the p l o t t i n g u n i t a n d controls the on/off m o d u l a t i o n of t h e ink jets. I n t h e n e x t cycle d a t a from t h e m a g n e t i c t a p e are stored in register B a n d t h e previously recorded d a t a in register A are used to control t h e ink jets. I n this w a y t h e d a t a can be printed continuously w i t h o u t waiting for new d a t a to be read from the m a g netic tape.

The incoming data bytes are counted in the tape and plotter control and only the first 384 bytes, equal to 3072 bits, will be stored in one of the two shift registers. The last 3 bytes completing the standard block length of 387 bytes generated by the UNIVAC 1108 will be omitted. This reading of a block takes place in 12.5 ms plus the time to accelerate and move the tape in the block gap. In order not to lose the information stored in the dynamic MOS shift registers the data are recirculated until the output transfer starts. After a time delay of 5 seconds from pressing the " P L O T " button the output is started. This delay is enough for the drum to accelerate to a constant speed and for the ink pumps to reach full ink pressure (Fig.3).

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Now the drum-trigger pulses from the plotting unit are gated to the tape and plotter control via the plotting unit interface. The drum trigger pulse causes a new block of data to be read from the tape recorder. This new block will be stored in shift register B. In shift register A where the first block is stored, the recirculation is terminated and data are strobed out to the plotting unit and controls the three ink jets. This output transfer is made with a frequency of 23.3 kHz or 466 times the 50 Hz AC line frequency used to drive the drum motor MS. This choice of frequency gives a distance of 0.2 mm between two consecutive points in a line. The 23.3 kHz signal is obtained with a phase locked oscillator which is locked to the line frequency. In t h a t way a change in the line frequency will not cause a distortion in the picture t h a t is displayed. After the first line is recorded with data from shift register A the recording head is incremented by a signal from the plotter control to the stepper motor M. B y t h a t time the shift register B has stored the d a t a for the second line and the data in register A can be expelled as the third block on the tape is now stored in register A. This sequence is repeated until the whole picture is recorded. The last block in a file is a file mark character and thus much shorter than a normal block. This last block is detected and resets the flipflop t h a t was set by the " P L O T " command. Now the drum stops and the recording head is moved back to the starting position. The print out can also be stopped in advance by a manual " P L O T B R E A K O F F " command from the control panel. Normally, the pictures are plotted in sequence from the tape. If a picture shall be plotted twice this can be accomplished by pressing the " R E T U R N " button on the control panel. This causes the tape transport to wind the tape one file backwards. In a similar way a picture can be skipped by pressing the "ADVANCE" button on the control pane]. A file counter is included in tlm control panel so t h a t the operator can see which picture is going to be read from the tape in the next print-out cycle.

6. Data format.

The pictures have been generated by a UNIVAC 1108 computer [10], [11]. The information to be displayed is specified in terms of a coordinate system with 1024 rows and 1250 columns for each of the three colours, Due to the large amount of memory capacity needed to store the whole coordinate system, a mass storage is used to store the picture during the generation, and only a small working area is stored in the core memory during generation of t h a t part of the picture. After gene-

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ration the picture is transferred column b y column to a magnetic tape. The tape is 0.5 inch standard computer tape on 10.5 inch tape reels or smaller. Data are stored as 8-bit bytes plus a parity bit on 9 tracks, with a recording density of 800 bytes per inch (NRZI). A picture is stored in one ~ile on the tape. A file consists of a number of blocks. Each of these blocks contains information for one line segment per colour in the picture. Figure 6 shows the data format in block number h r. The first group of 128 bytes corresponds to the yellow line segment in column h r. Since the red jet is displaced 50 lines from the yellow jet the second group of

LRC CRC DUMMY

BLUE

RED

YELL OW

L INE SEGMEN T AIR, N-tO0

N-50

N

FORWARD TAPE MOTION

Fig.6. D a t a f o r m a t of block n u m b e r N o n t h e m a g n e t i c t a p e . E a c h block s t o r e s inform a t i o n for t h r e e r a s t e r lines in t h e d i f f e r e n t colours yellow, r e d a n d blue. E a c h line is d i v i d e d into 1024 p o i n t s ; t h u s 128 8-bit b y t e s a r e s t o r e d for e a c h colour. D u e to t h e p h y sical d i s p l a c e m e n t of t h e i n k j e t s in t h e r e c o r d i n g device t h e r e d a n d b l u e i n f o r m a t i o n in block n u m b e r N will be r e c o r d e d 50 a n d 100 lines r e s p e c t i v e l y to t h e left of t h e yellow line (of. Fig.7).

128 bytes corresponds to the red line segment in column N-50. The blue jet is displaced 50 lines away, and thus the third group of 128 bytes corresponds to the blue line segment in column/V-100. At the end of the block there can be a number of d u m m y bytes, which do not contain any useful information. The last data b y t e is followed b y the check characters CRC and LRC. The last block in a file is a file mark character. Figure 7 shows the format of the picture and where the information from the tape is placed. A binary 1 results in a dot on the paper. Consecutive binary ones result in a vertical line on the paper. The distance between the lines is 0.2 mm, giving a line density of 5 lines/mm. The distance between consecutive addressable points in each line is 0.2 ram. The size of the paper is A4 (210 × 297 ram). The useful plotting area is 196 × 250 mm with 980 × 1250 addressable points for each colour. 1024 points are stored on the tape for each line segment b u t the last 44 points fall outside the usable plotting area and should be left blank. The position of the bytes in block N is shown in the figure. The first byte comes at the bottom of the yellow line segment no. N starting with the Most Significant Bit (MSB) and ending with the Least Significant Bit (LSB). Then consecutive bytes form the vertical line, until the

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Fig.7. I m a g e f o r m a t . Tho p i c t u r e is b u i l t u p of 1250 r a s t e r lines each in a n y c o m b i n a t i o n of the three colours yellow, red a n d blue. E a c h r a s t e r line is s t o r e d as 1024 points, b u t only 980 p o i n t s m a y be used. This results in a p l o t t i n g a r e a of 196 × 250 ram. Moreover a 20 m m wide a r e a a t the left a n d r i g h t m a r g i n can be recorded w i t h a limited n u m b e r of eolours.

upper limit is reached at byte 123, which partly falls outside the picture. In the same w a y bytes 129 to 251 form the red line segment no. N-50, and bytes 257 to 379 form the blue line segment no. hr-100. Due to the displacement of the ink jets a 100-line-segment area of blue and a 50-line-segment area of red could be plotted outside the left margin. These areas are normally used for identification purposes. Outside the right margin a 50-line-segment area of red and a 100-line-segment area of yellow could also be plotted.

7. Results.

The purpose of the present development was to show that it is possible to obtain pictorial output in colours from a computer in the form of hard copy by using electrically modulated ink jets. Because of the high speed of the recording process used it has been found that the new plotter is much faster than the conventional mechanical x-y-plotters commonly used for graphic output especially when the picture contains m a n y details or cross-hatched areas. Compared with microfilm recorders, the present equipment seems to be more simple in construction

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and does not require development and magnification of the film. Finally it is obvious that the eolour capability of the new plotter in m a n y eases adds a new dimension to the information content of the picture. Two examples obtained with the present plotter are shown in Figures 8 and 9, which demonstrate its use in cartography and pseudocolour imaging. The maps in Fig.8 show a result from an inventory of the buffdings in certain real estate blocks in the center of Malm~, which was used in connection with the planning of city reconstruction. All variables were recorded and separately coded into six classes. The higher values (classes 4 and 5) indicate buildings most suitable for demolition. The picture was kindly supplied b y Mikael Jern. In Fig.9 a density diagram of a chromosome is shown after processing in a computer [12]. In this case the image is converted from the data obtained b y scanning a black and white picture into a psedo-colour image b y using the ink jet plotter. The picture was kindly supplied b y Thomas Gustafsson. The plotter has been in use in the laboratory from J a n u a r y until September 1972. During that period software [11] and hardware were tested together and further developed. At the time of writing the system has been in use at the Lund University Computing Center for 3 months. The colour chart shown in Fig.10 is supplied b y M. Jern. This picture illustrates the ability to combine the three colours in different line and dot densities. I t also indicates the accuracy in line and dot position obtained with the plotter. In this picture three errors typical for the present plotter can be detected. First of all a variation of the vertical dot position is noticed at the upper and lower edge of the squares. This is caused b y a change of the velocity of the ink jets which is due to a 10% variation in ink pressure generated b y the ink pumps presently used. Since the ink droplets have to cover a distance of about 25 m m before reaching the recording paper on the rotating drum, a velocity decrease from 25 to 24 m/s will cause a change in the drop transit time from 1 to 1.04 ms. Since the surface velocity of the drum is 4.66 m/s, the drum moves almost 0.2 mm in 40 #s, i.e. the time difference in the transit time at maximum and minimum ink pressure. This causes the variation in vertical dot position observed in Fig. 10. As can be seen from the Figures 8 to 10 the distance between the individual scanning lines varies somewhat in a periodic fashion across the recording. This is partly due to the lead screw advancing the recording head in the horizontal direction, which has a pitch of 1.6 millimeters

194

BORIS 8MEDS

equivalent to 8 scanning lines. Therefore, sometimes an 8 line periodicity can be detected due to mechanical inaccuracies in this screw. Also other changes in line density are caused b y mechanical errors. However, it should be noted that neither the horizontal nor the vertical errors are cumulative. Finally, it is important that the three jets are not misaligned. When properly adjusted all points with the same coordinates must fall on the same point independent of the colour. This error can be analysed only on an original recording and not on the offset copies shown here (Fig. 8 to 10) because additional errors can be introduced in the offset printing process. Obviously, this error can be caused b y misalignment of the jets; however, it can also be introduced b y changes in the ink pressure, which causes variations in the speed of the jets. Experience with the present equipment has shown that no change in the direction of the jets can be detected as long as the glass nozzles producing the jets are properly fixed. The ink pressure, however, does change due to faultiness in the pumps as explained earlier. However, in agreement with the calculation given above, this error is too small to impair the picture quality seriously. Pictorial output in colours from computers has been obtained earlier on colour television displays [3], [4]. In that case different shades of eolour were obtained b y adding the three colours red, green and blue in the form of small matrix points. However, when producing coloured images in the form of hard copy on paper, subtractive colours have to be used as well known from colour photography [13]. In this case, the colours should absorb the colours red, green and blue from the white light illuminating the image and thus appear as cyan, magenta and yellow. The inks used in the present equipment are not an ideal colour triple. While the yellow and red ink is very close to the desired yellow and magenta, a blue colour was substituted for a correct cyan. This limits the number of eolour shades that can be obtained b y mixing the three inks used. Another disadvantage of the inks used up till now is that the colours show some tendency to fade as they are exposed to sunlight.

A cknowled~ements. The author wishes to thank Professor Hellnmth Hertz of the Department of Electrical Measurements, Lund Institute of Technolog~~, for his stimulating guidance and many valuable discussions. The collaboration with fil.kand. Mikael Jern has been very fruitful and his work with the development of the software for picture generation is gratefully acknowledged.

A S-COLOUR INK JET PLOTTER FOR COMPUTER GRAPHICS

195

The interest in this work shown by all colleagues and their assistance are very much appreciated. Financial support has been given by the National Swedish Council for Building Research and Lund University Computing Center. The English language has been examined by Robert Brogden. REFERENCES 1. I.D.G. Mac Leod, Pictorial Output with a Line Printer, I E E E Transactions on Computers, Vol. C-19, N u m b e r 2, Febr. 1970, pp. 160-162. 2. M.R. Schroeder, Images from Computers, I E E E Spectrum, Vol. 6, N u m b e r 3, March 1969, pp. 66-78. 3. H.C. Andrews, A.G. Tescher, R.P. Kruger, Image processing by digital computer, I E E E Spectrum, Vol. 9, N u m b e r 7, J u l y 1972, pp. 20-32. 4. I.E. Sutherland, Computer Displays, Scientific American, Vol. 222, N u m b e r 6, J u n e 1970, pp. 56-81. 5. F.J. Kamphoefner, Ink Jet Printing, I E E E Transactions on Electron Devices, Vol. ED-19, N u m b e r 4, April 1972, pp. 584-593. 6. C.H. Hertz, /~. M~nsson, S.I. Simonsson, A Method for the Intensity Modulation of a Recording Ink Jet and its Applications, Acta Universitatis Lundensis, Seetio II, No. 15, 1967, pp. 1-16. 7. C.H. Hertz, A. Mhnsson, Electric Control of Fluid Jets and its application to recording devices, The Review of Scientific I n s t r u m e n t s , Vol. 43, ]Number 3, March 1972, pp. 413-416. 8. C.H. Hertz, S.L Simonsson, Intensity modulation of ink-jet oscillographs, Med. & Biol. Engng., Vol. 7, 1969, pp. 337-340.9. A. MAnsson, Electric Control of Fluid Jets, Theory and Application to Image Generation, Report 1/1972 Dept. of Electrical Measurements, Lund I n s t i t u t e of Technology, Sweden, 1972. 10. L.A. BergstrSm, C.H. Hertz, M. J e r n , B. Smeds, Computer Graphics in Planning 7. Hard Copy ColorDisplay System Using Ink Jets, Principle and Applications, Departm e n t s of Building Function Analysis & Electrical Measurements a n d Lund University Computing Center. Sweden. 1972. i1. M. J e r n , Programvara f6r fdrgbildskrivare, Lunds Universitets Datacentral, Lund, Sweden. 1972. (In Swedish). 12. T. Fleischmann, T. Gustafsson, C.H. H~kansson a n d Albert Levan, The fluorescent

pattern of normal chromosomes in biopsies of malignant lymphomas, and its computer display, Hereditas 70: 75-88, 1972. 13. A. Weissberger, The theory of the photographic process (T.H. James, Editor), Ch.17, pp. 382-383, The Macmillan Company, New York, 1966.

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