GB2316741A - Apparatus for measuring percentage dot in printing and the like applications - Google Patents

Abstract

Apparatus for determining the percentage dot in half-tone images incorporates a CCD camera, the output of which is processed by electronic circuitry in such a way that the circuitry counts the number of light sensitive elements of the CCD array providing a "black" level signal, derives from that count a value for percentage dot and displays the percentage dot values arrived at on a display. The circuitry may include means for determining or adjusting the threshold CCD signal below which a "black" level will be assumed and above which a "white" level will be assumed. The apparatus is preferably housed within a housing 16 which can be manipulated by hand and which has aligned windows 18, 22, in its lower and upper surfaces through which the half-tone image being monitored can be viewed and an angled half-silvered mirror 20, between the windows 18 and 22, arranged to direct some of the light from the image being monitored through a lens system 24 onto CCD array 26.

Description

DESCRIPTION OF INVENTION Title: "Apparatus for measuring percentage dot in printing and the like applications" THIS INVENTION relates to apparatus suitable for determining the percentage dot in half-tone images or on printing plates for printing such images.

In the printing and graphic arts industry, graphic images, for example photographic images, are commonly produced by a so-called "half-tone" printing system in which the varying tones of the original image are translated into minute dots of uniform tone but varying size. Monochrome half-tone printing is effected by a single half-tone imprint ink of a single colour, typically black. Colour printing is conventionally achieved by printing, in superimposed relationship, a plurality of individual monochrome "half-tone" images in inks of different colours, for example in the three primary colours, or in the three primary colours plus black.

Typically, the dots in such a half-tone image are arranged on a rectangular grid of parallel regularly disposed rows and columns (although printing systems utilising randomly distributed dots or dots distributed according to some nonregular system are also known). It is necessary, for monitoring graphic printing processes to be able to assess the proportion of the area covered by a selected part of a half-tone image which is covered by the respective ink, or the equivalent (for example black silver in a photographic transparency forming the immediate precursor of a half-tone printing plate. (This proportion, expressed as a percentage, is referred to generally as the percentage dot).

Half-tone printing plates are made traditionally by a photographic process using photographic film as an intermediary, although modern technology is now capable of exposing such plates directly. Independently of the technology being used it is essential for control to be able to measure the percentage dot.

There are many instruments on the market for measuring percentage dot. Known instruments suffer from several limitations. Thus known instruments are essentially densitometers and use photo sensors to measure overall density of the image area to be checked, and convert the measured density to percentage dot by using standard conversion equations. Such known arrangements are prone to inaccuracy, however, particularly when used to read very low values of percentage dot corresponding to very low image densities.

Typically inspection of half-tone images, including measuring the percentage dot in various portions of such images is carried out using photographic transparencies bearing such images which may be, for example, the photographic precursors of the printing plates to be produced or which may themselves be produced photographically from such precursors, such transparencies being arranged upon a so-called light table comprising a light-diffusive glass plate arranged above an array of electric lamps, whereby the light table forms a diffusive light source directly below the transparency. In such an arrangement the instrument for measuring percentage dot is arranged above the transparency, over the area of interest, to receive light directed through such area of the transparency from the light table.

Measurement of percentage dot using a known densitometer and a light table requires calibration of the densitometer and is also subject to the inconvenience that, because, in practice, the illumination over the surface of the light table is not entirely uniform, the densitometer must be calibrated for one position only on the light table, so that the transparency being checked must be moved on the light table to allow different parts of the image to be inspected, whilst the densitometer is kept stationary relative to the light table. These known densitometers are subject to inaccuracies arising from changes in ambient light. This becomes a major problem with large areas of film, which is often used up to 6' x 4'.

It would be convenient, in some instances, to measure percentage dot directly on the face of a printing plate, rather than on a photographic transparency from which the plate is derived, or from a print taken from the plate. However, because of the very low contrast on the printing plate material it is impossible to measure the density, and hence percentage dot, on plate material using known densitometers.

It is an object of the present invention to provide an improved apparatus suitable for determining the percentage dot in printed images or on printing plates for printing such images.

According to the invention there is provided apparatus suitable for determining the percentage dot in half-tone images, for example printed images or on printing plates for printing such images or images otherwise formed on film, paper or other media, comprising means for optically sensing the light output from each image element of an array of such image elements forming an image area, to be measured, of the printed or otherwise formed image on the medium concerned, or printing plate surface, for example, and for assigning to each said image element a binary value according to whether the respective image element is sensed as relatively dark or relatively light, and counting means for counting the number of such binary values which represent a relatively dark value (or the number of such binary values which represent a relatively light value) and means for calculating, from the resultant count, the proportion of such image elements which yield a relatively low brightness, (or alternatively the proportion of such image elements which yield a relatively high brightness), and for displaying the calculated proportion, for example in terms of percentage dot.

An embodiment of the present invention is described below by way of example with reference to the accompanying drawings in which: FIGURE 1 is a schematic view illustrating the operation of an apparatus embodying the invention, FIGURE 2 is a schematic view, in vertical section, showing apparatus embodying the invention in use, and FIGURE 3 is a plan view from above of the apparatus illustrated in Figure 2.

Referring to Figures 2 and 3, there is indicated schematically at 10 a conventional light table, for use in inspecting photographic films, such films, in the present context, being half-tone photographic transparencies. Such a transparency is indicated schematically at 14 in Figures 2 and 3. In Figure 3, the area of the transparency 14 is indicated by a rectangular array of rows and columns of dots, but it will be appreciated that the size and pitch of the dots of any half-tone image likely to be inspected on the light table in practice will be several orders of magnitude smaller than the dots shown on the transparency 14. Thus, typically, the dot pitch may correspond to 150 dots per inch whilst the size of the dots will vary appreciably from one part of the overall image to another.

In Figures 2 and 3, the reference 12 represents a percentage-dot-measuring apparatus embodying the present invention and which may, for example, be a hand-held apparatus which can be moved manually over a half-tone transparency 14 located on the light table 10 to measure the percentage dot in different areas of the transparency.

In the arrangement illustrated, the apparatus 12 comprises a casing 16 provided with a window 18 in its underside, a half-silvered mirror 20 within the casing 16 above the window 18, the mirror 20 being angled, as shown, with respect to the horizontal, for example at 450 to the horizontal to reflect some of the light from the window 18 into an optical system, represented by lens 24 in Figure 2, which optical system in turn forms, on a light sensitive CCD array 26, an image of a desired part (e.g. the central part) of the portion of the print 14 within the window 18.

The half-silvered mirror 20 also allows some of the light from the window 18 to pass upwardly through a viewing window 22 on the upper side of the casing so that the operator can see the region of the transparency being monitored. The CCD array 26 may be of the type used in socalled camcorders. The CCD array device 26 is in turn connected with electrical circuitry (not shown) the characteristics of which will be evident from the description below.

The CCD array 26 is connected in electrical circuitry which operates the CCD array in manner known per se to derive, for each picture element or pixel of the CCD array an analog electrical signal corresponding to the illumination of that element by the respective portion of the image projected onto the CCD array by the lens system 24. These analog signals are then digitised by a portion of said circuitry which, in effect, compares the value of each such signal with a threshold value and generates a corresponding digital value, i.e. a signal representing a "1" or Or "0" according to whether the respective signal monitored is above or below said threshold value. The digitised signals are then passed to counting circuitry which, in effect, counts the number of "1" signals received in a "frame" and calculates from that number a percentage value, the circuitry driving a display, e.g. an LCD display, 28 on the top of the casing 16 for display of the percentage dot in numerals.

The device is provided with means allowing for the setting of the threshold value referred to, either manually or automatically. It is possible, for example, for the device to determine the threshold value as the average brightness, over the sample area, in a part of the halftone image which the operator judges to be of medium density. Since, apart from these CCD pixels which extend across an edge of a dot, the illumination of any single pixel element of the CCD array will correspond either to full "black" or full "white", the percentage dot measurement derived in the manner described above is relatively insensitive to the threshold value setting. If desired, the apparatus may include automatic calibration means arranged, for example, to set said threshold value at a level midway between the highest and lowest brightness level sensed by individual CCD pixels.

In the arrangement shown in Figures 2 and 3, the circuitry driving the CCD array and processing the signals thereof and driving the display 28 is contained within the casing 16 and may be powered by a self-contained battery or via an electrical cable, not shown, extending to the device. However, it will be appreciated that some or most of such circuitry may be disposed remotely from the casing 16 and connected therewith by an appropriate cable, for example. The apparatus preferably also has provision for inspecting, for example, prints or plates, in reflected light (as opposed to light transmitted through a transparency, for example). Thus, for example, a light source may be incorporated within the casing 16 so as to direct light through the half-silvered mirror and through the window 18 onto an area of a print or plate to be checked.

It will be appreciated that the "pixel" density of the CCD array 26 is substantially greater than the dot density of the image of the respective parts of the halftone transparency projected onto the CCD array by the lens system 24, so that the dot pitch of the dot array on the portion of the half-tone image projected onto the CCD array is several times greater, for example, 100 times greater, than the pitch between "pixel" elements of the CCD array.

That is to say the resolution of the CCD array is significantly greater than the minimum required to resolve separate dots in the portion of the half-tone image projected onto the CCD array bearing lens 24. Clearly the higher the resolution of the device, relative to the dot pitch, the less sensitive the device will be to the threshold value setting and therefore to variations in background illumination or light table illumination, because there will be a smaller proportional area provided by pixels lying at the edges of dots.

In the preferred embodiment, the image projected onto the CCD array is the image of an area of the half-tone transparency of approximately 3 mm diameter, whilst the image-receiving area of the CCD array may comprise an array of upwards of 1000 pixels x 1000 pixels. The apparatus is preferably capable, at least, of resolving the dots in a 1% dot and a 99% dot (i.e. in regions almost completely clear and regions almost completely black) at 200 line screen.

As a further refinement, the circuitry associated with the CCD array 26 may include computer or microprocessor means capable of deducing, from the signals received from the CCD array, the orientation, relative to the casing 16, of the rows and columns of dots in the halftone image. By way of example, the processing circuitry associated with the CCD array may include a conventional desk-top or other computer having an appropriate interface facility, which may be a dedicated input module or card, connected by a flexible lead forming a parallel or serial link, to the circuitry within the casing 12, the computer being programmed with dedicated software adapted to discriminate between various orientations of dots as represented by the signals derived from the device 12, to determine the orientation of the rows of columns and dots.

The facility for determining such orientation is of particular significance in relation to colour printing, where it is necessary, for optimum effects, to ensure that, for example, the angle of orientation between the rows of dots in the red half-plate image is at a predetermined angle with respect to the orientation of the rows of dots in the yellow half-plate image, that the orientation of the rows of dots in the blue half-plate image is at a predetermined angle with respect to the orientation of the rows of dots in the yellow and red half-plate images and so on. Thus, for example, by operation of control means (not shown) in the casing 16 or associated with the computer referred to, the display 28, or a display driven by the computer referred to, may be arranged to display, in terms of angular measure, the orientation of such rows of dots in the image being inspected relative to the casing 12, which may, for the purposes of such measurement, be abutted against an aligning edge (not shown) upstanding from the table and against which the transparency being inspected, and the casing 12, may both be aligned.

The computing means, and its associated software, is also preferably capable of providing a measurement of and readout of the dot spatial frequency, for example in terms of lines per centimetre or any other convenient measure.

With the manual or automatic calibration facility referred to above, the apparatus is readily adapted to, or is self-adapting to, measurement of percentage dot directly on a half-tone printing plate, or even on a printed paper page for, despite the lower contrast (i.e. less difference between full "black" and full "white" as sensed by the device, as long as the device can discriminate between areas of the plate which are intended to print "black" and areas which are intended to print "white", after the digitisation stage the operation of the device is precisely the same and is unaffected by the lack of overall contrast on such plate material or on such printed page.

For the same reasons, and given the above-noted calibration or self-calibrating facilities of the device, it is substantially unaffected by the variations in ambient light.

Similarly, when used with a light table as illustrated the accuracy of the device is very little affected by variations in illumination in one area of a light table compared with another.

Whilst, as described in relation to the figures, the device 12 is a hand-held mobile device, it will be appreciated that a similarly-operating device may be fixedly mounted using an appropriate bracket or mounting arrangement connected with the table 10 or may be mounted on a mechanical traversing mechanism supported from the table 10, for example.

Claims (5)

1. Apparatus suitable for determining the percentage dot in images on film, paper, plate or other media, comprising means for optically sensing the light output from each image element of an array of such image elements forming an image area, to be measured and for assigning to each said image element a binary value according to whether the respective image element is sensed as relatively dark or relatively light, and counting means for counting the number of such binary values which represent a relatively dark value (or the number of such binary values which represent a relatively light value) and means for calculating, from the resultant count, the proportion of such image elements which yield a relatively low brightness, (or alternatively the proportion of such image elements which yield a relatively high brightness), and for displaying the calculated proportion, for example in terms of percentage dot.

2. Apparatus according to claim 1 wherein said means for sensing the light output from each said image element includes a CCD array device and an optical system for forming, on said CCD array device, an image of said image area to be measured.

3. Apparatus according to claim 1 wherein said means for counting includes a computer and wherein the computer is also programmed to provide an indication of the orientation of a regular array of dots extending across such image area.

4. Apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

5. Any novel feature or combination of features disclosed herein.