GB2253481A - Fibre-optic colour-balance monitor - Google Patents

Abstract

Monitoring the colour balance of a product (e.g. in a colour offset printing machine) that can consist of a complex two-dimensional colour pattern involves a plurality of sensors positioned across the product and each comprising at least one illuminating optical fibre and at least two pick-up optical fibres 16. Filters 36 associated with each of the pick-up fibres 16 separate the reflected light into different colour components. Photodetectors 38 convert the light intensity from each pick-up fibre 16 into analogue electrical signals fed to a microprocessor-based signal processing module for conversion to digital form and subsequent analysis for monitoring colour balance. If there is a change in the colour balance, a visual and/or acoustic alarm may be automatically activated. Measurements are coordinated to the rotary position of the printing roll. <IMAGE>

Description

FIBRE OPTIC COLOUR BALANCE MONITORS The present invention relates to fibre optic colour balance monitors which may be capable of measuring colour balance in printing operations.

The operator of a colour offset printing machine makes adjustments during set-up to provide the proper balance among the different inks required for realistic colour rendition. After this initial "tuning", the press produces copies at rates of up to twenty copies per second. If the ink delivery changes for any component colour during the press run, the resulting colour rendition for the prints is degraded and significant amounts of material and time may be wasted.

There are commercially available instruments such as colourimeters which are intended specifically for the measurement of colour balance. For example, US Patent Nos US-A-3 885 878, US-A-3 999 860, US-A-4 019 819, US-A-Il 150 898, US-A-4 464 05it, and US-A-4 909 633 relate to various colourimeters, some of which employ fibre optics.

There still exists a need for a continuous method for monitoring the process during the run so that corrective action can be taken in a timely manner as the colour balance deviates from acceptable limits.

Several factors place the desired monitoring function beyond the capabilities of the prior art. Firstly, the desired system must have colour resolution over a wide area where the colour may vary since the prints to be monitored may well constitute a very complex twodimensional pattern, a part of which may be in colour and the other part in black and white. Secondly, the system needs to be flexible and convenient to initialise since patterns are different with each press run. Finally, the desired system needs to be very fast since prints pass at rates of up to twenty copies per second.

According to one aspect of the invention there is provided a fibre optic colour balance monitor for monitoring colour balance on a product, the monitor comprising: a light source; a plurality of fibre optic sensors positioned across the product, each of said fibre optic sensors having at least one optical fibre connected to said light source for illuminating the product, and at least two optical fibres situated therebetween for receiving reflected light from the product; a filter connected to each of said at least two optical fibres receiving reflected light for separating the reflected light into two different colour components; a photodetector connected to each filter for providing a different colour component signal from each of said at least two optical fibres; and a signal processor receiving said colour component signals for characterising the reflected light to determine colour balance.

According to another aspect of the invention there is provided a fibre optic method for monitoring colour balance on a product, the method comprising the steps of: positioning a plurality of fibre optic sensors across the product, each of the fibre optic sensors having at least one optical fibre connected to a light source for illuminating the product, and at least two optical fibres situated therebetween for receiving reflected light from the product; separating the reflected light transmitted by the at least two optical fibres into at least two different colour components; detecting the at least two different colour components and converting them to analogue signals; and processing the analogue signals into digital signals for characterising the reflected light to determine colour balance.

A preferred embodiment of the invention finds particular applicability to a printing machine having a printing roll. An additional feature involves a rotary encoder in communication with the signal processor for coordinating light measurements with locations on the printing roll. A further aspect provides for mounting plates with a purge air supply to keep the fibre optic sensors clean for accurate characterisation.

The preferred embodiment provides a monitor for a printing process which allows corrective action to be taken in a timely manner when colour balance deviates from acceptable limits, which is capable of measuring colour differences in complex two-dimensional patterns, which is flexible and convenient to initialise and can monitor prints at high rates, and which is simple in design, rugged in construction, and economical to manufacture.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an embodiment of the present invention; Figure 2 is a cross-sectional illustration of a fibre optic sensor measurement point in the embodiment of Figure 1; Figure 3 is a sectional illustration on a line III-III in Figure 2; Figure Il is a schematic illustration of a four-position fibre optic sensor detector assembly; Figure 5 is a sectional representation of a detector assembly located on a circuit board; Figure 6 is a block diagram of a four-position signal processing module; Figures 7a to 7d represent four different lighting conditions of four differently filtered detector channels;; Figures 8a and 8b show the ratios of the three filtered channels to the unfiltered channel; and Figures 9a to 9c show the ratio of the yellow to cyan channel for white, yellow and cyan illumination.

Referring to Figure 1, a plurality of fibre optic sensors 2 are positioned in a fibre optic pick-up assembly 3 which extends across the width of a product such as a printed page carried by a printing roll 6.

A light source or illuminator 4, such as a multicolour laser or a wellregulated high intensity lamp, transmits light through optical fibres 18 for illuminating the printed page on the printing roll 6. Light is reflected from the printed page and is received by other pick-up optical fibres 16 by means of which it is delivered to a filter/detector assembly 8 in the form of an array of filters and detectors. A filter is associated with each of the receiving or pickup optical fibres 16 to separate the reflected light into separate colour components. Photodetectors in the filter/detector assembly 8 convert the light intensity from each optical fibre into analogue electrical signals which are then supplied to a microprocessor-based signal processing system 10 for conversion to a digital output and subsequent analysis by software 9.The subsequent analysis indicates any change in colour balance for actuating a visual and/or acoustic alarm.

A rotary encoder 12 mounted to a shaft 14 of the printing roll 6 generates pulses to allow the electronics in the signal processing system 10 to coordinate the several light measurements from the fibre optic sensors 2 with their location on the print.

Figure 2 illustrates the preferred arrangement of each fibre optic sensor 2 in the fibre optic pick-up assembly 3 In Figure 2, a single fibre optic sensor 2 represents a single measurement point along the fibre optic pick-up assembly 3.

In the preferred embodiment, for each measurement point, there are four pick-up or receiving optical fibres 16 and two illuminating optical fibres 18. Whereas many types of optical fibre are suitable for use in the present invention, low cost, 0.5 mm diameter acrylic fibres are preferably used for both illumination and pick-up. The optical fibres 16, 18 are fastened into a mounting plate 20 by gluing or any other fastening means. Preferably, the illuminating fibres 18 are positioned on opposite sides of a cluster or group of the pick-up fibres 16 as best seen in Figure 3. In the preferred embodiment, the illuminating fibres 18 are recessed about 2 mm in the mounting plate 20 to minimise the amount of light which reaches the pick-up fibres 16 without the light having been reflected back from the printed page.

All of the optical fibres 16, 18 can view the print surface 26 directly, without the use of lenses, thus simplifying the fabrication of the fibre optic sensor 2. Nonetheless, if lenses are desired, they may be emploved. A cover plate 22 protects the ends of the optical fibres 16, 18 from dust or other interfering particles but still allows the light to pass therethrough.

The optical fibres 16, 18 positioned in the fibre mounting plate 20 along with the cover plate 22 as shown in Figures 2 and 3 make up a single fibre optic sensor 2. Each of these fibre optic sensors 2 comprises a single measurement point in the fibre optic pick-up assembly 3.

The optical fibres 16, 18 view a target area 24 on the print surface 26 of the page on the printing roll 6. Immediately above the print surface 26 are located upper and lower purge air plates 28, 30, respectively. Both purge air plates 28, 30 have apertures 32, 34 which are about 1 cm in diameter and are in optical alignment with the optical fibres 16, 18. The apertures 32, 34 provide a path for both the measured light as well as purge air from a purge air supply (not shown). A relatively low flow of air is sufficient to keep the cover plate 22 clean. Additional plates 27, 29 may optionally be employed to extend on either side of the vicinity of the target area 24, the apertures 32, 34, the upper and lower purge air plates 28, 30, and up to the cover plate 22 for shading the target area 24 from outside light sources such as room lights or sunlight.All of these features may be manufactured into a single fibre optic pick-up assembly 3.

Next, referring to Figure 4, the four pick-up fibres 16 of each fibre optic sensor 2 transmit the reflected light from the respective target area 24 on the print surface 26 to the filter/detector assembly 8. Figure 4 depicts an arrangement for four fibre optic sensors 2 located at four positions in the fibre optic pick-up assembly 3. Each fibre optic sensor position has filters 36 labelled f1 to f4 and four detectors 38 labelled d1 to d4. The light from the four pick-up fibres 16 associated with a given measurement point or fibre optic sensor position should essentially be identical since all of the fibres view the same target area 24.However, at the optical fibre termination point constituted by the filter/detector assembly 8, the filters 36 (f1 to fIl) separate the reflected light into four distinct colour components. These four separate colour components are transmitted to the detectors 38 (d1 to d4) which correspond to the filters 36 (f1 to f4) to characterise the light reflected from the print surface 26.

Typical colour printing applications involve the use of four ink colours. So as to simplify the task of identifying which of the inks is experiencing a deviation from normal conditions, four colour channels are chosen for each measurement point.

Figure 5 illustrates one version of the manner in which the pickup optical fibres 16 may terminate at the detectors 38. The pick-up optical fibres 16 are fastened into a fibre mounting board 40 by glue or other fastening means so that the fibre mounting board 40 communicates with a circuit board 42 holding the detectors 38 which may be silicon photodetectors. The detector area is large enough so that lensing is optional between each pick-up fibre 16 and the detector 38.

The major components of the signal processing module or system 10 are shown in Figure 6. These components include a multiplexer 44, an amplifier 46, an analogue-to-digital converter 48 and a microprocessor 50. The microprocessor 50 is associated with random access memory (RAM) 52 and read only memory (ROM) 54 with operator interface capability 56, such as a push button, keyboard or some other means, with appropriate connections or port 58 for an ipterface such as an RS232 and a connection 60 for the signal from the rotary position encoder 12 and relays 62 for visual and/or audio alarms.

Sixteen detector inputs are built on to a single circuit board 42 for holding sixteen detectors 38 as well as respective preamplifiers 64 (Figures 4 and 5). This provides the respective signal processing system 10 with the ability to process measurements from four fibre optic sensor positions. Additional measurement points are accommodated by simply adding more of the self-contained signal processing system modules. Five signal processing system modules are required for an anticipated twenty point printing production system.

Returning to Figure 6, the sixteen detector output signals are multiplexed, amplified, and passed through the analogue-to-digital converter 48 to the microprocessor 50. The rotary position encoder 12 pulses every 10 mm of print advance. Each pulse triggers the system 10 to scan the detectors 38 and store the resultant measurements in the RAM 52. Typically, sixty measurements are accumulated in memory from each detector 38 as a 600 mm length of print passes under the pick-up fibres 16. For sixteen channels (four measurement points), a print rate of 20 pages per second, and an average page length of 600 mm, each signal processing system module samples at 19,200 Hz.

At the end of the scanned print, the system 10 switches to the analysis mode to determine whether the colour balance is within desired limits. If an out-of-limits condition is detected, the appropriate alarm is switched on by the respective relay 62. After the analysis is completed, the system 10 switches back to the measurement mode and scans another print. Since measurements are suspended during the analysis, the system 10 is capable of only checking every other passing print.

The signal processing system 10 is controlled by the software 9 resident in the ROM 54. The signal processing system 10 includes the RS-232 standard serial communication port 58 which is optional for system operation, but which may be useful for communicating with programmable controllers or a desk-top computer. For example, the system 10 may be programmed to transmit results of each analysis to another computer for storage as a record of press performance problems, or used as an input to a statistical process control (SPC) program to provide information for press maintenance scheduling.

The radiant power delivered by one of the pick-up fibres 16 to its associated detector 38 is expressed as follows: P = P ( , f1, , y, m, c, B, I, w, 1) (1) where, is the fraction of the total reflected light that falls on to the fibre 16, f1 is the fraction of the light that is transmitted by the ith colour filter 36 (i = 1 to 4), is the diameter of the target area 24, y is the relative amount of yellow ink, m is the relative amount of magenta ink, c is the relative amount of cyan ink, B is the relative amount of black ink, I is the radiant power density incident on the surface of the print, and w, 1 are the width and length coordinates of the target area 24.

Measurements are taken at twenty different values of w, but for any given detector 38 channel, w is constant. On the other hand, the 1 coordinate changes with time as the print travels past the measurement point.

The factor f1 is constant for any given fibre 16/filter 36/detector 38 set. Furthermore, & , m and I are each held constant so that, as successive prints pass under each measurement point, the four viewing detectors 38 (d1 to d4) generate time varying signals which are repetitive for successive copies of the same print under nominal conditions.

These four detector signals at each measurement point are expressed as: Dy = D1 (,m,c,B,t), Dm = D2 (y,m,c,B,t), Dc = D3 (y,m,c,B,t), and DB = D4 (y,m,c,B,t), (2) where the dominant parameters are underlined.

These time dependent signals are different for each new print loaded into the press. However, as long as the same print is being copied, the time dependence of the signals remains the same. The variables y, m, c and B are the relative "ink-flow" factors that it is desired to monitor and the four measurements of Equation (2) are sufficient to determine the four ink-flow unknowns.

The minimum change in the variables, y, m, c, or B necessary to provide a detectable change in the detector output signals (system sensitivity) depends on how constant the factors 6 , f, I are held and on the inherent detector noise floor.

In practice, the press operator signals the signal processing system 10 via the operator interface capability 56 that the currently measured trace is to be used as the standard once he has satisfactorily tuned the colour balance. Then, while in the analysis mode, the microprocessor 50 determines a deviation factor for each channel on its module. The deviation factor is defined as: s (t) = D(t) - Do(t), (3) where, D(t) is the currently measured trace, and Do(t) is the standard trace for each colour at each measurement location.

Several potential alarm criteria may be used. For each colour at each measurement position along the print width, the deviation function 6 (t) will consist of a series of sixty numbers, one for each of the sixty measurements spaced every 10 mm along the print length. It is possible, and it may be desirable, to alarm if any of the sixty deviation values for a point exceeds a set limit. However, since expected problems are likely to affect more than just a single, isolated location, the sensitivity to variations in print colour rendition is increased by summing several or all of the values for a given measurement position, over the length of the print.

In a simulation example, four plastic optical fibres were mounted together, 5 cm above a movable platform holding a page of coloured newsprint. Each fibre delivered light to a separate silicon photodetector. One of the detectors received the light directly while the other three received light that was filtered. Yellow, magenta, and cyan filters were used. The signals from the four photodetectors were input to a computer-based data acquisition system along with the signal from a sensor that monitored the position of the movable platform.

Data was recorded simultaneously on all five channels as the platform was dragged to scan the newsprint past the optical-fibre pick-up.

Figures 7a to 7d show the results of four such scans. In the first (Figure 7a), the newsprint was illuminated directly by a lamp mounted beside the fibre pick-up. In each of the other scans (Figures 7b to 7d), a coloured filter was placed in front of the lamp to simulate a change in the colour balance.

The data in Figures 7a to 7d demonstrate that the changes in colour balance induced by the filters are clearly detectable from the unfiltered, white light results. Although the general shape of the position response function from the same filtered signal from each fibre is about the same irrespective of the colour of illumination, the output signal levels change significantly with illumination colour.

Using the white light data as the standard, the deviation in the detected signal levels for all three filtered channels is significant, indicating that the fibre optic colour balance monitor is capable of detecting changes in colour balance.

Individual deviations from the four detector signals for each measurement point provide sufficient sensitivity for alarm signalling.

An advantage of the microprocessor 50 electronics is their flexibility in accommodating a wide range of more complex analysis schemes.

As an example, Figures 8a and 8b show the ratios of the three filtered channels to the unfiltered channel. The results are from the same test presented in Figures 7a to 7d. Again, using the white light results as the standard, it is apparent from the ratios that the deviation caused by the magenta filtering of the light source is very large. In addition to the three ratio traces shown in Figures 8a and 8b, three additional ratios are possible among the filtered channels.

Figures 9a to 9c show the ratios of the yellow to cyan channel for white, yellow, and cyan illumination. The signal level data used to generate the graphical plots of Figures 7a to 7d, 8a, 8b, and 9a to 9c are summarised in Tables 1 and 2 below.

While a specific embodiment of the present invention has been shown and described in detail to illustrate the application and principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

Table 1 Detector Signal Maximum, Minimum, and Average Magnitudes for Different Lighting Conditions Y M W C Y/W M/W C/W Y/M C/M Y/C UNFILTERED LAMP UNFILTERED LAMP Maximum 1.30 0.50 2.56 0.37 0.65 0.25 0.17 2.66 0.88 4.80 Minimum 0.19 0.09 0.41 0.06 0.42 0.18 0.42 0.13 0.13 2.11 0.55 2.68 Average 0.83 0.32 1.63 0.23 0.52 0.20 0.14 2.52 0.71 3.60 LAMP THROUGH YELLOW FILTER Maximum 1.02 0.37 1.75 0.18 0.72 0.28 0.12 2.77 0.64 7.26 Minimum 0.16 0.07 0.30 0.03 0.47 0.19 0.10 2.14 0.37 4.06 Average 0.63 0.24 1.08 0.11 0.59 0.22 0.10 2.63 0.47 5.64 LAMP THROUGH MAGENTA FILTER Maximum 0.63 0.31 1.10 0.12 0.76 0.36 014 2.18 0.51 7.24 Minimum 0.10 0.06 0.20 0.02 0.48 0.26 0.10 1.70 0.30 3.62 Average 0.41 0.20 0.70 0.08 0.59 0.30 0.11 1.98 0.38 5.30 LAMP THROUGH CYAN FILTER Maximum 0.15 0.06 0.58 0.20 0.30 0.19 0.41 2.62 3.40 0.81 Minimum 0.02 0.01 0.09 0.04 0.21 0.10 0.33 1.37 2.06 0.55 Average 0.10 0.04 0.37 0.13 0.26 0.12 0.35 2.20 3.02 0.73 Table 2 Fractional Change in Detector Signals Caused by Filtering the Illuminating Lamp (Y/W) (M/W) (C/W) (Y/M)9 (C/M) (Y/C) Y/Yo M/Mo W/Wo C/Co (Y/W)o (M/W)o (C/W)o (Y/M)o (C/M)o (Y/C)o LAMP THROUGH YELLOW FILTER LAMP THROUGH YELLOW FILTER Maximum 0.78 0.75 0.68 0.49 1.11 1.12 0.76 1.04 0.72 1.51 Minimum 0.82 0.83 0.73 0.54 1.14 1.04 0.73 1.02 0.67 1.52 Average 0.76 0.73 0.67 0.48 1.14 1.09 0.73 1.04 0.67 1.57 LAMP THROUGH MAGENTA FILTER Maximum 0.49 0.63 0.43 0.33 1.18 1.40 0.83 0.82 0.58 1.51 Minimum 0.54 0.67 0.49 0.38 1.14 1.43 0.76 0.81 0.55 1.35 Average 0.49 0.63 0.43 0.33 1.14 1.45 0.78 0.79 0.54 1.47 LAMP THROUGH CYAN FILTER Maximum 0.11 0.13 0.23 0.54 0.47 0.74 2.51 0.98 3.87 0.17 Minimum 0.10 0.16 0.22 0.61 0.51 0.54 2.48 0.65 3.76 0.20 Average 0.12 0.13 0.23 0.56 0.50 0.59 2.47 0.87 4.28 0.20

Claims (16)

1. CLAIMS 1. A fibre optic colour balance monitor for monitoring colour balance on a product, the monitor comprising: a light source; a plurality of fibre optic sensors positioned across the product, each of said fibre optic sensors having at least one optical fibre connected to said light source for illuminating the product, and at least two optical fibres situated therebetween for receiving reflected light from the product; a filter connected to each of said at least two optical fibres receiving reflected light for separating the reflected light into two different colour components; a photodetector connected to each filter for providing a different colour component signal from each of said at least two optical fibres; and a signal processor receiving said colour component signals for characterising the reflected light to determine colour balance.
2. 2. A monitor according to claim 1, wherein the product is an output of a printing machine having a printing roll.
3. 3. A monitor according to claim 2, comprising a rotary encoder in communication with said signal processor for coordinating light measurements with locations on the printing roll.
4. 4. A monitor according to claim 1, claim 2 or claim 3, comprising a mounting plate for said plurality of fibre optic sensors, said mounting plate extending across the product to be monitored.
5. 5. A monitor according to claim 4, wherein the illuminating optical fibres are recessed in said mounting plate for minimising external light from being transmitted directly to at least four optical fibres.
6. 6. A monitor according to claim 5, wherein said illuminating optical fibres are recessed by approximately 2 mm.
7. 7. A monitor according to claim 4, claim 5 or claim 6, comprising at least one purge air plate positioned between said mounting plate and the product to be monitored, the or each purge air plate having an aperture in optical alignment with each of said fibre optic sensors.
8. 8. A monitor according to claim 7, comprising means for supplying purge air to the or each purge air plate.
9. 9. A monitor according to any one of the preceding claims, wherein said signal processor is set to a predetermined colour balance for tracking a repetitive pattern and identifying any deviations in that pattern.
10. 10. A monitor according to claim 9, wherein said signal processor includes an alarm to be activated when the deviations exceed preset limits.
11. 11. A monitor according to claim 9 or claim 10, comprising means for controlling printing in communication with and responsive to said signal processor for bringing the deviations back to said predetermined colour balance.
12. 12. A fibre optic colour balance monitor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
13. 13. A fibre optic method for monitoring colour balance on a product, the method comprising the steps of: positioning a plurality of fibre optic sensors across the product, each of the fibre optic sensors having at least one optical fibre connected to a light source for illuminating the product, and at least two optical fibres situated therebetween for receiving reflected light from the product; separating the reflected light transmitted by the at least two optical fibres into at least two different colour components; detecting the at least two different colour components and converting them to analogue signals; and processing the analogue signals into digital signals for characterising the reflected light to determine colour balance.
14. 14. A method according to claim 13, comprising the step of coordinating light measurements with location on the product.
15. 15. A method according to claim 13 or claim 14, comprising the step of purging air between the product and each of the plurality of fibre optic sensors.
16. 16. A fibre optic method for monitoring colour balance on a product, the method being substantially as hereinbefore described.