WO2008035072A1

WO2008035072A1 - Test equipment for screenprinting

Info

Publication number: WO2008035072A1
Application number: PCT/GB2007/003561
Authority: WO
Inventors: Christopher Charles Brighty
Original assignee: LAURIE MULANEY ASSOCIATES Ltd
Current assignee: LAURIE MULANEY ASSOCIATES Ltd
Priority date: 2006-09-19
Filing date: 2007-09-19
Publication date: 2008-03-27
Anticipated expiration: 2009-03-19
Also published as: GB0618408D0; WO2008035072A9

Abstract

The present invention concerns improvements in and relating to screenprinting and, amongst other things, provides a test apparatus and method for measuring the percentage printing area (percentage dot) of a stencilled screen for screenprinting. The stencilled screen (4) is formed from a mesh on which a stencil has been applied, the mesh having a series of holes. A first portion (2) of the mesh has a plurality of holes not obscured by the stencil and a second portion (3) of the mesh has any number of holes obscured by the stencil. The apparatus comprises a source of illumination (12), an image capture means (40), and a computing means linked to the image capture means. The source of illumination (12) illuminates the first portion (2) of the screen (4), and the image capture means captures at least one image of the illuminated first portion of the screen. The computing means determines the number of unobscured holes in the first portion. The source of illumination (12) is then used to illuminate the second portion (3) of the screen (4), and the image capture means captures at least one image of the illuminated second portion of the screen. The computing means then determines the number of unobscured holes in the second portion, and then assesses from the number of the unobscured holes in the first and second portions the percentage dot in the second portion of the screen.

Description

Test Equipment for Screenprinting

BACKGROUND

a. Field of the Invention

The present invention concerns improvements in and relating to screenprinting and, amongst other things, provides a test apparatus and method for measuring the percentage printing area (percentage dot) of a stencilled screen for screenprinting.

b. Related Art

Modern colour printing is a high quality business, and ever more printing companies are obliged to introduce testing to ensure that what they print is what the customer requires. This has lead to increased demand for accurate test equipment.

The majority of image printing, i.e. reproduction of photographic or other graphic images, is carried out using what is known as the 'half tone' system where the wide range of tones in the original image are represented in the printed image by myriad microscopic dots of uniform tone but varying size to fool the eye into seeing the different tones. Four colour printing a monochrome image formed by this 'half tone' approach in one of the primary colours is supplemented by superimposed half tone images in each of the other three primary colours. Densitometry by measuring overall light transmittance through or reflectance from a selected part of the substrate is one of the commonest ways in which the print quality for 'half-tone' printing is assessed. Densitometry values are normally converted by standard conversion equations to percentage dot values, which represent the proportion of the area covered by a selected part of a half tone image that is covered by the respective ink. Screenprinting, also called silkscreening, or serigraphy, is a printmaking technique that creates a sharp-edged image using a stencil. A screenprint or serigraph is an image created using this technique. Screenprinting technology used to be ubiquitous in the field of graphic image printing, but has lost market share and is now predominantly used commercially for the preparation of banners/ posters where the high density pigmentation and resistance to light-degradation of the printed image is particularly valued. Silkscreening is also popular in the printing of ceramic tiles, wall paper and fabrics.

A screen is a mesh usually of a fabric material, but in some instances may be a metal screen. A fabric mesh is made of a piece of porous, finely woven fabric, usually polyester, stretched over a supporting frame. Areas of the screen are blocked off with a non-permeable material to form a stencil, which is a negative of the image to be printed; that is, the open spaces are where the ink will appear on the printed substrate.

In screenprinting the ink is forced, by pressing down of a squeegee or similar, through the 'holes' created in the weave of the screen and is transferred to the substrate (paper etc.). Where these holes are blocked by the stencil, ink is not transferred to the substrate.

There are several ways to create a stencil for screenprinting, but the photo emulsion technique is most common. A photographic emulsion is coated over the screen, and then exposed with an ultraviolet (uv) light source which polymerises and hardens the exposed areas of the emulsion. The coated and exposed screen is washed thoroughly to remove areas of emulsion that were not exposed to the uv light to produce a stencilled screen that contains a negative stencil image on the mesh.

Photographic stencilled screens can reproduce images with a high level of detail, and can be reused for tens of thousands of copies and are capable of printing on a wide range of substrates. However, unlike mainstream print technologies such as photolithography which have gained in popularity over recent years, there is no reliable testing system for determining the print quality for screenprinting and particularly for determining the percentage printing area, conventionally referred as the "percentage dot", in a stencilled screen for screenprinting.

The term "percentage dot" derives from the conventional half tone printing process in which tones between full black and full white are represented by a two- dimensional array of dots, the tone being determined by the size of the dots in relation to the spaces between dots. The same principle may be applied in screenprinting, although the elements of the array are not necessarily dots but may be areas of different shapes in the screen mesh not covered by the stencil. The term "percentage dot" as used herein is, accordingly, intended to be understood in this wider sense.

A raster image processor (RIP) is software used in a printing system which produces a bitmap. The bitmap is then sent to a printing device for output. RIPs are electronic devices which receive a description of the page or other graphical output and then generate a "hardware bitmap output" that is used to enable or disable each pixel on a real-time output device such as an optical projector for exposing a screenprinting emulsion.

It is usually necessary to calibrate the RIP in order to calibrate or match the screenprinted product against a proof for a print run. This may be because of variations in the mesh material or variations in the sizes and pitches of the holes in the mesh, or because of the desire to achieve a particular aesthetic effect in the printed product.

Densitometry, which is the quantitative measurement of optical density over a given area, can be carried out on the printed product but not meaningfully on the stencilled screen and is, accordingly, of only very limited or no value in calibrating the Raster Interface Processor (RIP) against a proof for a print run. The screen in screenprinting needs to be read if the errors in dot gain and loss arising from it are - A -

to be effectively and efficiently eliminated/ adjusted for in the printing process. However, existing test apparatus cannot reliably read such screens, for example because the screens do not have a straightforward tone curve but rather a distorted asymmetric tone curve.

It is an object of the invention to provide a more convenient test apparatus and method for measuring the percentage dot of a screen for screenprinting.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of using a measurement apparatus to assess the percentage dot in a stencilled screen for screenprinting, said screen being formed from a mesh on which a stencil has been applied, the mesh having a series of holes therethrough a first portion of the mesh having a plurality of holes not obscured by the stencil and a second portion of the mesh having any number of holes obscured by the stencil, and the apparatus comprising a source of illumination, an image capture means, and a computing means linked to the image capture means, the method comprising the steps of: i) using the source of illumination to illuminate the first portion of the screen; ii) using the image capture means to capture at least one image of said illuminated first portion of the screen; iii) using the computing means to determine the number of unobscured holes in the first portion; iv) using the source of illumination to illuminate the second portion of the screen; v) using the image capture means to capture at least one image of said illuminated second portion of the screen; vi) using the computing means to determine the number of unobscured holes in the second portion; and vii) using the computing means to assess from the number of said unobscured holes in said first and second portions the percentage dot in the second portion of the screen. The determination of the percentage dot using this method enables the tone curve distorting effect of the screen mesh or weave to be eliminated. Thus accurate percentage dot figures are obtained and may be input into the RIP for screenprinting so that the stencil may thenceforth be eliminated as a cause of variables in the screenprinting process.

The computing means preferably determines the number of unobscured holes by identifying and counting individual unobscured holes in said portions of the mesh.

In a preferred embodiment of the invention, the unobscured holes are counted by determining a threshold value for the image intensity of an unobscured hole and then comparing the image intensity of a plurality of image pixels against the threshold value. When the mesh is back lit the holes will have a bright image intensity, and if the mesh is front lit the holes will have a dark image intensity. In the former case the test for the presence of a hole will be if the image intensity is above the threshold, and in the latter case the test will be if the image intensity is below the threshold.

The method may comprise the steps of subdividing the image of the first portion into a plurality of different patches, determining a plurality of threshold values, one for each one of these patches, and then comparing the image intensity for each pixel against the threshold value for the patch corresponding to that pixel. This provides significant advantages against a system having a single or average threshold value, because of the ability of the different threshold values to compensate at least partially for differences in illumination or image quality across the imaged area of the stencilled screen.

The computing means may determine the number of the unobscured holes in the first and second portions by examining a square or rectangular array of pixel values in raster fashion starting at one corner of the array and working towards another corner of the array. In order to unambiguously detect the presence and the location of a hole, the computing means may be arranged to perform a local search of pixel values around each pixel value for which the comparison against the threshold value indicates the presence of a hole.

The computing means therefore preferably identifies the presence of a hole by locating a pixel having (for back lit illumination) a local maximum image intensity value.

In a preferred embodiment of the invention, the image capture means is provided on a first side of the stencilled screen and steps i) and iv) involve illuminating the stencilled screen from an opposite second side of the screen so that a portion of said illumination passes through any unobscured holes and into the image capture means.

Also according to the invention, there is provided a measurement apparatus for assessing the percentage dot in a stencilled screen for screenprinting, said screen being formed from a mesh on which a stencil has been applied to obscure holes in said mesh, the apparatus comprising a source of illumination for illuminating a portion of the screen, an image capture means arranged to capture an image from the illuminated portion of the screen, and a computing means linked to the image capture means, wherein the computing means is arranged to count the number of holes in a first captured image of a first portion of the screen and in a second captured image of a second portion of the screen and to assess from the number of said unobscured holes in said first and second portions the percentage dot in the second portion of the screen.

The computing means may then be arranged to determine from one captured image a threshold value for the presence of holes and to use this threshold value in the determination of the presence of holes in another captured image. The measurement apparatus may be arranged to subdivide each captured image into a plurality of patches and to perform the calculation of the threshold separately for each of the patches.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which

Figure 1 is a side view of the overall appearance of a measurement apparatus according to a preferred embodiment of the invention, for assessing the percentage dot in a stencilled screen for screenprinting;

Figure 2 is a schematic block diagram of the apparatus of Figure 1; and

Figures 3 and 4 are, respectively, examples of a 100% screen and a 50% stencilled screen image after image processing by the measurement apparatus of Figures 1 and 2.

DETAILED DESCRIPTION

Referring to Figure 1 , a measurement apparatus 1 for assessing the percentage dot in a stencilled screen 4 for screenprinting, comprises on an upper side 6 of the screen a hand-held measuring unit 8 and on the other lower side 10 of the screen 4 a separate source of even illumination in the form of a light table 12 or other suitable lighting means. In this example, the light table 12 includes a white light source 14 which projects optical radiation 16 through a light diffusing glass tabletop 18 to provided even illumination 20 to the screen 4 from the lower side 10 opposing the measuring unit 8. In other applications it may be desirable to use coloured light.

The measuring unit 8 is activated in a manner reminiscent of a stapler. The unit 8 has a flat base or foot 22 which is placed on the upper side 6 of the stencilled screen 4. A measuring head 24 is pivotably joined to the foot 22 at a pivoting joint 26 at one end 28 of the foot 22. The other end 30 of the foot 22 surrounds an aperture 32 in the foot into which optical radiation 34 which has been transmitted through the screen 4 passes. The aperture has a conical inner wall 33.

The measuring unit 8 is used by first placing the foot 22 on the screen 4 so that a first measurement portion of the screen, indicated by the circled area labelled 2 in Figure 1 , is surrounded by the aperture 32 in the foot. Although not shown in the drawings, the aperture 32 has in its peripheral regions segments of graticules or cross hairs to aid in the correct placement of the measuring unit with respect to the portions of the screen 4 to be measured.

Once the unit 8 is correctly placed, the person performing the percentage dot measurement presses down on an upper surface 36 of the measuring head 24 in order to rotate this downwards towards the foot 22. The measuring head 24 has at an end 38 opposite the pivoting joint 26 an imaging system 40 for collecting and imaging the transmitted optical radiation 34.

With reference now also to Figure 2, the imaging system 40 includes a magnifying means in the form of an objective lens 42 for collecting and focussing onto an image sensor, here a CCD detector array 44, the transmitted optical radiation 34. The detector array 44 is a conventional CCD having a detector area of one-third inch square (8.47 mm square), which is arranged to image an area of the screen about 3 mm square. The lens 42 therefore provides a magnification of about 2.8.

The lens 42 is held within a circular bezel 46 that extends downwards from a lower surface 48 of the measuring head 24. The bezel 46 has a conical peripheral wall 49 which matches the conical inner wall 33 of the aperture 32 so that as the measuring head 24 is rotated downwards, the bezel 46 enters and is accurately located in three dimensions with respect to the foot 22 and hence the portion 2 of the stencilled screen 4 to be measured. At the same time, a rod 50, which extends downwards from the lower surface 48 of the measuring head 24 adjacent to the bezel 46, comes into contact with a circular plinth 52 on an upper surface 54 of the foot 22. The rod 50 is connected to a microswitch 56 within a protective outer casing 58 of the measuring head 24.

Once the microswitch 56 is activated, the transmitted optical radiation 34 is analysed in the manner described below. Results are displayed visually to the user by a liquid crystal display (LCD) panel 59 in the upper surface 36 of the measuring head 24, and may also be downloaded to external equipment by means of an optional data port 61 in the casing 58.

With reference now particularly to Figure 2, the measuring head 24 includes inside the casing 58, in addition to the lens 42 various electronic components 59, 64, 68, 74 that are powered by a power source 62. The power source may be any convenient source of electrical power, but in this example is a disposable battery.

The circuitry comprises an electronic framegrabber 64 for receiving and storing an electronic image representing the illuminated screen, which is, connected to the detector array 44. The framegrabber 64 has an associated memory 66 for the capture and temporary storage of images from the detector array 44. The framegrabber 64 is also connected to a computing means 68 including a central processing unit (CPU) 70 having its own associated memory 72 for performing computation on the images. A switch unit 74, controlled by the microswitch 56, is connected to the CPU in order to initiate the measurement sequence. The CPU 70 is also connected to the LCD panel 59 for displaying the status of the apparatus, images obtained, and analysis curves. The computing means 68 is completed by a memory 76 for program and data storage, for example a flash memory or an EPROM unit connected to the CPU 70 for holding the software required for the controlled operation of the apparatus, and the data port 61, which is here a serial port for downloading data to an external computer (not shown).

In operation, on power up for the first time, the CPU 70 initialises its memory 72 and the framegrabber 64. It then enters a standby mode, monitoring the microswitch 56. When the user presses the measuring head 24 to the foot 22 and so activates the microswitch 56 to take a measurement, the CPU 70 activates the LCD panel 59 to display the status of the unit and then instructs the framegrabber 64 to capture a single frame into its memory 66. Once the framegrabber 64 has signalled that it has completed this, the stored image is made available to the CPU 70 which then processes the image.

A dot area value is calculated by the CPU 70 and this is displayed on the LCD panel 59 and a message sent to the serial port 61. If no further activity takes place for 30 seconds, the unit re-enters the standby mode to conserve power.

It will be appreciated that the analysis of the grabbed image is the critical part of the system. The image is grabbed as a rectangular monochrome image composed of pixels arranged in an array having 250 rows and 240 columns. The CCD 44 outputs for each pixel an analogue signal which is digitised by the framegrabber as an 8 bit value that represents one of 256 greyscales.

To assess the percentage printing area/ percentage dot, a calibration measurement is first conducted on an area or portion of the screen 4 having no holes obscured by the stencil. Subsequent measurements are then done on portions of the screen formed with a required screen pattern in order to determine the percentage dot for each of those portions. For example, a rectangular test patch may be formed on the screen 4 immediately adjacent a section of the screen on which images, text or other graphical content is to be formed for printing onto a substrate. The test patch can be segmented into distinct portions over which known stencil patterns have been formed in steps from none or zero to fully covered. One way of doing this is to apply the stencil in each portion as dots with different sizes, as with a half tone image. A portion with no coverage of stencil is referred to as 100%, a portion with half coverage is called 50%, and a portion with full coverage is called 0%. When the measuring head 24 performs a measurement of any selected patch of the screen, the unit's CPU 70 first determines in a calibration step the number of possible 'holes' in an image area and then determines how many are not blocked by the stencil and converts these measurements into a percentage dot value. Whether in the calibration step or in a subsequent measurement, a hole in the screen is detected by the relative amount of light that passes compared to a calculated threshold value.

Calibration

The calibration is carried out on both a 0% and a 100% portion of a test strip on the screen 4. Firstly, an image is stored and then the 250 by 240 pixel image is divided into patches, in this example 10 pixels by 10 pixels. For each patch the average of the 100 pixel values is calculated and the maximum value within the patch is determined, so that a threshold value can be calculated for each patch. The main benefit of performing the measurement in this way is that this helps deal with imperfections or variations in the average level of illumination across the image, which can be due to the light source 12, or from the inherent imaging performance of the optical system 40.

The threshold can be calculated in various different ways. In this example, the average values for the 0% and the 100% patches and the maximum value for the 100% patch are each weighted and summed to generate a threshold that is above half of the maximum value. This is done for each patch; currently the average of these two values is used. This calculated number is the threshold for the patch. The calculated threshold value is written to the memory as the pixel reference threshold for the whole 10 by 10 patch. This process is repeated for each patch. A measurement cycle is then performed across the array of pixels to determine the number of holes in the screen 4. The CPU 70 compares in raster fashion the detected light level for each pixel against the threshold for the relevant patch for each pixel. This is done by starting with the value for the pixel at row 1 and column 1 of the 250 row by 240 column array. If the value for the pixel is below the threshold then the CPU 70 then considers the pixel at row 1 and column 2, and so on to the end of the row and then on to row 2 column 1 in raster fashion until a pixel is found for which the detected light level is above the threshold. At this point the CPU then performs a local search of each of the pixels in a 5 by 5 array centered on that found pixel to see if any of those pixels have a higher detected light level. If the 5 by 5 search area overlaps an edge, then of course the local search is limited to just those pixels within the overall 250 by 240 array.

At this point, a pixel having a local maximum detected light level will have been found by the CPU 70. If this local maximum pixel is at an edge of the 5 by 5 local search area, then a second local search is performed with that local maximum pixel being centered on the second 5 by 5 pixel local search area.

This process could be repeated as necessary, but in the present example, in which the 100% screen is imaged with a nominal approximately1600 holes spread over the CCD array 44, it has been found that it is not necessary to do this in order to locate the true local maximum detected light level, and hence the centre of a clear hole in the mesh as imaged by the CCD detector array 44.

Although it would in principle be possible after locating a pixel at the centre of a detected hole to perform an intelligent search that omits pixel locations already determined not to be the centre of an imaged hole, it is in fact quicker just to complete the search blindly by comparing each successive pixel against the local patch threshold in raster fashion up until the pixel at row 250 and column 240.

When the maximum value is found it is marked with a top of the range value, which in the 8 bit system used is 255 (FF hexadecimal) and the others that are tested in this mode are marked with 00 to reduce the time taken to perform the entire process. An example of a 100% screen image showing as bright each pixel above threshold is shown in Figure 3. For each group of bright pixels there will generally be one pixel marked with the value 255, with all other pixels being marked with the value 0. When the whole image has been compared with each pixel threshold reference, the CPU 70 then performs a count of the number of pixel locations for which there was a local maximum in the detected light level. In this example, this simply entails counting the number pixels that have been marked with the value 255. As mentioned above, in this example for the 100% screen, over a 3 mm square area, this is expected to be approximately 1600 holes. This integer value is referred to here as "X".

Measurement

An image of the a second portion of the screen away from the calibration area, and having at least some holes obscured by the stencil, is then captured by the imaging system 40 and stored by the frame grabber 64.

Then, using the same measurement process described above for calibration, starting at the row 1 column 1 corner of the captured image, the CPU 70 compares each of the pixel values against the pixel threshold for the respective patch that was determined during calibration. The hole locations are determined as described above. An example of a 50% screen image showing as bright each pixel above threshold is shown in Figure 4. For each group of bright pixels there will generally be one pixel marked with the value 255, with all other pixels being marked with the value 0.

Following this, the number of pixels marked with the value 255 is counted. This integer value is referred to here as "Y".

The percentage dot for this second portion is calculated by Y ÷ X * 100.

The measurement process is then continued for as many different portions of the screen as is required, indicated generally in Figure 1 by numeral 3, each having a different screen pattern, in order to generate sufficient data to determine if the percentage dot for different screen patterns is correct in order to reproduce the printed image correctly. If not, then the hardware bitmap output used to expose the screenprinting emulsion is adjusted and further measurements are taken until the determined percentage dot for each different portion is corrected so that images are reproduced correctly in the screenprinting process.

The invention therefore provides a reliable and convenient method and apparatus for assessing the percentage dot in a stencilled screen for screenprinting.

Claims

1. A method of using a measurement apparatus to assess the percentage dot in a stencilled screen for screenprinting, said screen being formed from a mesh on which a stencil has been applied, the mesh having a series of holes therethrough a first portion of the mesh having a plurality of holes not obscured by the stencil and a second portion of the mesh having any number of holes obscured by the stencil, and the apparatus comprising a source of illumination, an image capture means, and a computing means linked to the image capture means, the method comprising the steps of: i) using the source of illumination to illuminate the first portion of the screen; ii) using the image capture means to capture at least one image of said illuminated first portion of the screen; iii) using the computing means to determine the number of unobscured holes in the first portion; iv) using the source of illumination to illuminate the second portion of the screen; v) using the image capture means to capture at least one image of said illuminated second portion of the screen; vi) using the computing means to determine the number of unobscured holes in the second portion; vii) using the computing means to assess from the number of said unobscured holes in said first and second portions the percentage dot in the second portion of the screen.

2. A method as claimed in Claim 1 , in which the computing means determines the number of unobscured holes by identifying and counting individual unobscured holes in said portions of the mesh.

3. A method as claimed in Claim 2, in which said unobscured holes are counted by determining a threshold value for the image intensity of an unobscured hole and then comparing the image intensity of a plurality of image pixels against said threshold value.

4. A method as claimed in Claim 3, in which the method comprises the steps of subdividing the image of the first portion into a plurality of different patches, determining a plurality of threshold values, one for each one of said patches, and then comparing the image intensity for each pixel against the threshold value for the patch corresponding to said pixel.

5. A method as claimed in any preceding claim, in which the computing means determines the number of said unobscured holes in said first and second portions by examining a square or rectangular array of pixel values in raster fashion starting at one corner of said array and working towards another corner of said array.

6. A method as claimed in Claim 5, when dependent from Claim 3 or Claim 4, in which the computing means performs a local search of pixel values around each pixel value for which said comparison against said threshold value indicates the presence of a hole.

7. A method as claimed in Claim 7, in which the computing means identifies the presence of a hole by locating a pixel having a local maximum image intensity value.

8. A method as claimed in any preceding claim, in which the image capture means is provided on a first side of the stencilled screen and steps i) and iv) involve illuminating said stencilled screen from an opposite second side of the screen so that a portion of said illumination passes through any unobscured holes and into the image capture means.

9. A measurement apparatus for assessing the percentage dot in a stencilled screen for screenprinting, said screen being formed from a mesh on which a stencil has been applied to obscure holes in said mesh, the apparatus comprising a source of illumination for illuminating a portion of the screen, an image capture means arranged to capture an image from the illuminated portion of the screen, and a computing means linked to the image capture means, wherein the computing means is arranged to count the number of holes in a first captured image of a first portion of the screen and in a second captured image of a second portion of the screen and to assess from the number of said unobscured holes in said first and second portions the percentage dot in the second portion of the screen.

10. A measurement apparatus as claimed in Claim 9, in which the computing means is arranged to determine from one captured image a threshold value for the presence of holes and to use this threshold value in the determination of the presence of holes in another captured image.

11. A measurement apparatus as claimed in Claim 10, in which the measurement apparatus is arranged to subdivide each captured image into a plurality of patches and to perform said calculation of said threshold separately for each of said patches.