GB2503052B - Method of communicating calibration data and colour calibration article - Google Patents

Method of communicating calibration data and colour calibration article Download PDF

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Publication number
GB2503052B
GB2503052B GB1216019.8A GB201216019A GB2503052B GB 2503052 B GB2503052 B GB 2503052B GB 201216019 A GB201216019 A GB 201216019A GB 2503052 B GB2503052 B GB 2503052B
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Prior art keywords
calibration
image
colour
calibration data
communicating
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GB2503052A (en
GB201216019D0 (en
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Gouch Martin
Revie Craig
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FFEI Ltd
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FFEI Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/00002—Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for
    • H04N1/00026—Methods therefor
    • H04N1/00045—Methods therefor using a reference pattern designed for the purpose, e.g. a test chart
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/00002—Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for
    • H04N1/00007—Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for relating to particular apparatus or devices
    • H04N1/00023—Colour systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/00002—Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for
    • H04N1/00071—Diagnosis, testing or measuring; Detecting, analysing or monitoring not otherwise provided for characterised by the action taken
    • H04N1/00082—Adjusting or controlling
    • H04N1/00087—Setting or calibrating
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/56—Processing of colour picture signals
    • H04N1/60—Colour correction or control
    • H04N1/603—Colour correction or control controlled by characteristics of the picture signal generator or the picture reproducer
    • H04N1/6033—Colour correction or control controlled by characteristics of the picture signal generator or the picture reproducer using test pattern analysis

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Health & Medical Sciences (AREA)
  • Biomedical Technology (AREA)
  • General Health & Medical Sciences (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Character Input (AREA)
  • Color Image Communication Systems (AREA)
  • Measuring And Recording Apparatus For Diagnosis (AREA)
  • Facsimile Image Signal Circuits (AREA)

Description

Method of communicating calibration data and colour calibration article
The invention relates to a method of communicating calibration data and acolour calibration article. In particular, the invention relates to the calibration ofan imaging device such as a colour scanner.
When using a colour scanner it is often necessary to be able to convert colourimages produced by the scanner so that they match the result produced byanother scanner. In some cases it may also be necessary to provide an accuratecolour estimate of the material being scanned. This is usually done using ascanner calibration or characterisation process to determine the colourcharacteristics of the scanner. Knowledge of these characteristics can then beused to define a transform that can be applied to images to match anotherscanner or to create a “true-colour” image.
In many cases absolute colour accuracy is not required and pseudo-colourimages are used. Even in these cases, however, it is important to know thecolour characteristics of the scanner so that a consistent “pseudo-colour” imagecan be produced for a given scanned sample.
One common calibration method makes use of standard profiles such as ICCprofiles defined by the International Color Consortium. ICC profiles are data filesthat provide a means to map RGB pixel values produced by a colour device toCIEXYZ or CIELab standard colour values. These colour values are based on astandard colour observer defined by the Commission International de I'Eclairage(CIE). A traditional ICC scanner profiling method is shown in Figure 1. The ICC profilingmethod typically makes use of a colour characterisation target 1, typically madefrom a similar substrate material to the sample material being scanned with thescanner. Usually the target 1 comprises a set of colour patches 2 of a wide spectral range of colours which are positioned on the target 1 (step S1). Thecolours of the colour patches 2 have known colour measurement values orstandard colour values obtained by measuring each colour patch 2 with aspectrophotometer or other suitable instrument (S2). These values are usuallystored in the form of measurement data 3 in a data file (S3).
To calibrate the scanner (step S4), the target 1 is scanned (S5) by the scanneroperator to produce an RGB image 11 of the target 1, from which a set of RGBpixel values for each of the colour patches 2 can be obtained. These pixel valuesare then used in conjunction with the known measurement data 3 to build acolour conversion or mapping table which relates the RGB to standard colourvalues. The colour conversion table is then used to create (S6) an ICC profile 4which, in turn, can be used by a colour imaging application or can be useddirectly to estimate the colour of subsequently scanned images. A disadvantage of such traditional profiling methods is that the scanner operator(i.e. the user) is required to manually locate the digital file containing themeasurement data for the target. Selecting an inappropriate file, for example afile containing measurement data fora different target, is a common mistake andcan result in very poor calibration.
Accordingly, there is a need for a technique which addresses the above issue.
In accordance with a first aspect of the present invention there is provided amethod of communicating calibration data relating to a colour calibration article,the article having a number of coloured regions and a calibration element forobtaining the calibration data, the method comprising: acquiring an image of thearticle including the calibration image using an imaging device; and automaticallyperforming the steps of obtaining the calibration data from the acquired imageand using the calibration data to obtain colour data, wherein the colour datarepresents the colour of the coloured regions within the said article.
The colour calibration article may also be referred to as a self-contained colourcharacterisation target or a self-contained calibration chart. The calibrationarticle may also be a self-contained calibration slide where it is intended for usewith a medical scanner for example. The colour calibration article allows forautomation of the calibration process of an imaging device such as a scanner.
The coloured regions may include a wide range of colours suitable forcharacterising the target. The coloured regions selected to form the range maydepend upon the intended use of the scanner. The coloured regions may becoloured patches for example.
The calibration data, also referred to as measurement data, may be dataobtained by measuring the coloured regions of the colour calibration article witha spectrophotometer or using other techniques known in the art and which aresuitable for accurately characterising an imaging device. Communicating thecalibration data in accordance with the first aspect of the invention enables theimaging device to automatically obtain the true colours of the coloured regions.
The main advantage of the present invention over traditional calibrationtechniques (such as the method shown in Figure 1) is that, since the scan of thetarget includes measurement data or a reference to the measurement data(encoded on the target in the form of an image, barcode or similar), there is noneed for the user to locate a digital file containing the measurement data for thetarget. This avoids selecting an inappropriate file and ensures that the calibrationis accurate. A further advantage of the present invention is that the methodprovided may be easily understood and performed by scanner operators whichare familiar with traditional scanner profiling methods.
The calibration element is a calibration image containing the calibration data, forexample by encoding the calibration data in the calibration image. The image may be in the form of a barcode for example. Preferably, the barcode is aQRCode or similar 2-D barcode, however a set of 1-D barcodes or any otherencoding method may also be used.
The calibration element may be included in the calibration article using anysuitable method, for example by printing, adhesion or photo-imaging. In the lattertechnique, a substrate of the article may be coated with a photo-sensitivematerial and then the calibration element may be added to the calibration articleby imaging onto the photo-sensitive material using laser for example. This isparticularly advantageous when large numbers of coloured areas are used, asthe calibration article may be made smaller.
Preferably, the coloured regions are formed from a similar material to the samplematerial being imaged by the imaging device. For example, where the material isstained biological material, including stained cells, the coloured regions may beformed from a predetermined biological stained material.
The method of communicating calibration data according to the invention has anumber of applications. In the examples described in detail below, the imagingdevice is a scanner and thus the method may be used to calibrate the scanner.In other applications, the imaging device may be a digital camera for exampleand this invention may also be used to calibrate such devices. A computer program (software) product may comprise program code meansadapted in use to perform the method of communicating calibration dataaccording to the invention, when the computer program code is executed upon acomputer device. Advantageously, the imaging device may be configured withthe aid of the computer program to automatically perform the steps of obtainingthe calibration data from the calibration element (for example by scanning the article using image recognition software) and using the calibration data to obtainthe colour data (for example using image processing software)
In accordance with a second aspect of the present invention, there is provided asystem for communicating calibration data relating to a colour calibration article,the article having a number of coloured regions and a calibration element forobtaining the calibration data, the system comprising: an imaging deviceconfigured to acquire an image of the article including the calibration image; andmeans for automatically performing the steps of obtaining the calibration datafrom the acquired image and using the calibration data to obtain colour data,wherein the colour data represents the colour of the coloured regions within thesaid article.
Typically the apparatus in accordance with the second aspect of the invention isadapted to perform the invention in accordance with the first aspect. Furtheradvantageous features are set out in the appended dependent claims.
Examples of methods in accordance with the invention will now be describedwith reference to the following figures:
Figure 1 is a schematic representation of a traditional scanner profiling method;
Figure 2 shows a variant of a method of communicating calibration data inaccordance with the invention, using a ‘measurements model’; and
Figure 3 shows another schematic representation of a method of communicatingcalibration data in accordance with the variant shown in Figure 2.
Figure 2 illustrates the calibration process of a scanner using a self-containedcolour characterisation target in accordance with the invention. It will be appreciated however that the method may be applied to calibrate any imagingdevice such as a digital camera.
As shown in Figure 2, the creation of the self-contained colour characterisationtarget includes three main steps (S10, S20, S30). In the first step, S10, a set ofcolour patches 20 are placed on a target 10 using methods known in the art. Thetarget 10 may be a slide made from any suitable material which may betransparent for example. Preferably the set of patches 20 is carefully selected toensure that the range of colours is spread over the entire range of colours thatare to be scanned. The patches 20 form a mosaic of patches or calibration chart.Ideally, but not necessarily, the patches 20 are manufactured from a materialthat is similar to the material to be scanned.
In the second step, S20, the coloured patches 20 are measured with aspectrophotometer or similar instrument to generate calibration data ormeasurement data comprising a standard set of wavelengths. The measurementdata may be spectral radiance, spectral transmittance or spectral reflectanceinformation for example using 32 spectral channels per patch. Alternatively,other forms of measurement data may be used such as CIEXYZ or Cl ELabvalues, which in some circumstances may provide adequate means tocharacterise the scanner.
In the third step S30, the measurement data is encoded as an image 30 in theform of a barcode. Preferably, the barcode is a QRCode or similar 2-D barcode, however a set of 1-D barcodes may also be used for this purpose. Alternatively,the measurement data 30 may be encoded in an image in some other form.
The image 30 is then positioned on the target 10. Preferably the image 30 isprinted on the target, on the same surface as the coloured patches 20 andadjacent to them to aid detection. Preferably, an inkjet printer or other similarmarking system is used to print the barcode 30 on the target 10. Where thetarget 10 is transparent (i.e. made from a transmissive material), the image 30may be printed on a reflective material, which is then added to the target 10forming an identification area on the target. The reflective material may be asticker which adheres to the target 10 for example.
Alternatively, other methods for adding the image 30 to the target 10 may beused. These methods include imaging on high-resolution film and then addingthe film to the target 10 by coating the target 10 with a photo-sensitive materialand then imaging onto this material using a scanner laser beam or some othersuitable device. These alternative methods may be required in particular whenlarge numbers of patches 20 are used.
The next steps illustrated in the bottom half of Figure 2 represent the scannercalibration, S40 according to the invention. The scanner calibration S40 includesthe steps of scanning the target, S50, to obtain an image 101 of the target 10and processing the image 101 produced by the scan, S60, to generate a colourprofile 40 of the scanner.
As described above, the coloured target patches 20 and the measurement datafor these patches are included on the target 10. In a preferred embodiment, thescanner scans the target 10 to produce a single image 101. The scanner may beconfigured to be able to distinguish and separate the image 30 containing themeasurement data from the chart area including the colour patches 20.
The scanner may be configured with the aid of calibration software. The scannercalibration software may include image recognition software to detect the colourpatches 20 and the image 10 on the target. In addition, the scanner calibrationsoftware may also include image processing software configured to extract thecalibration data from the image, create a colour profile, and/or calibrate thescanner using the colour profile.
Accordingly, when the target 10 is scanned, S50, the single image 101 producedby the scan includes all of the data needed to create, S60, a colour profile 40,thereby calibrating the scanner. The colour profile 40 may be an ICC colourprofile or other form of colour transform that allows the colour capability of thescanner to be accurately described. The colour profile 40 may be stored on anysuitable digital media.
As can be seen from Figure 2, with the present invention, there is no need for ameasurement data file (item 3 of Figure 1) to be separately identified in order togenerate the colour profile, as required in the traditional profiling method shownin Figure 1. Instead, when scanning the target 10 which includes the image 30characterising the colour chart 20, the scanner (configured by the calibrationsoftware) may automatically perform the steps of obtaining the measurementdata from the image 30 and using the measurement data to create the colourprofile 40.
The feasibility of encoding the measurement data in an image 30 which can beadded on the target 10 is described in the following example. To obtain themeasurement data, the spectrophotometer or other instrument takes samples ofthe colour patches 20. Sampling from 380-720 nm at 10nm intervals for exampleresults in 34 samples. A signal-to-noise, S/N, ratio of 1000:1 is adequate for thispurpose and many instruments can achieve it. If the 34 samples are to beencoded in 3 decimal digits per sample, the instrument would therefore require102 decimal digits for each measurement. It would be appreciated that the required number of samples and digits may vary depending on the specificapplication. A typical (‘version 40’) black-and-white QRCode can encode 7089 digits or 69patch measurements and a 51 digit header. The QRCode in this example is177x177 'modules'. A module is in effect an image pixel that must be imaged sothat it can be read with high certainty. It is desirable that the barcode printed onthe target be no larger than 20x20 mm which requires a printing resolution of8.85/mm or 225/in. If a target 10 with a size 75x25 mm is used, the high-resolution scan area may be 62x25 mm for example. Where the scan resolutionis as high as 4000/mm, an 8-bit RGB image of 62x25x4000x4000x3 bytes (~70Gb) is obtained.
Figure 3 shows the same steps required to create the self-contained colourcharacterisation target (S10, S20, S30) and perform the calibration of thescanner (S40, S50, S60). In addition, Figure 3 shows three optional steps (S70,S71, S61) which may be performed by scanner operators. The steps which maybe performed by the manufacturer of the self-contained colour characterisationtarget (S10, S20, S30 and S60) are represented on the left side of Figure 3,while the steps performed on the scanner operator side (S40, S50, S61, S70,S71) are shown on the right side of Figure 3.
Specifically, to calibrate their device, a scanner operator may for example usethe colour profile 40 with their own software, S70, or may process each scannedimage of the self-contained colour characterisation target using softwaresupplied by the manufacturer of the self-contained colour characterisation target,S71. The software supplied by the manufacturer may be suitable to process ascanned image of the self-contained colour characterisation target and may be‘cloud’ software for example. The software may be supplied by the manufactureron any suitable media such as a CD-ROM and may be provided together withthe self-contained colour characterisation target. Alternatively, adequate third party software may be used to process each scanned image of the self-contained colour characterisation target, S61.
The present invention has many applications, including applications in medicalimaging. For example, the self-contained characterisation target describedabove may be used to calibrate medical image scanners. These may usemedical image formats such as DICOM, SCN, APERIO, Hamamatsu, or OME-TIFF.
Medical image scanners are commonly used to scan slides containing tissuesamples. The material to be scanned therefore is stained biological material,including stained cells. In order to accurately calibrate such scanners, the colourregions or patches of the self-contained characterisation target may be formedfrom biological stained material also referred to as biological stains. Thebiological stains may take a number of different forms including stains which aretissue stains, fluorescent markers and so on. The form in which the stains areprovided preferably mimics their form when in contact with biological material.
The present invention is applicable to other areas of colour scanning and bysimple extension to digital photography, wherein the imaging device is a digitalcamera. Accordingly, self-contained digital camera targets may be created withapplications in studio photography or magazine catalogue imaging for example.With recent advances in digital camera technology it is now easily possible tocapture the details needed to be able to decode the measurement data orreference element, as required by the present invention.
The colour characteristics of a digital camera depend on many factors and it isoften necessary to create a different camera characterisation or ICC profile foreach scene. For some applications it is necessary to create targets of differentmaterials, especially when colour accuracy is important in situations such asphotography for a clothes catalogue or similar. Using a self-containedcharacterisation target as provided by the present invention makes this processmuch simpler.

Claims (15)

1. A method of communicating calibration data relating to a colourcalibration article, the article having a number of coloured regions and acalibration image containing the calibration data, the method comprising: acquiring an image of the article including the calibration image using an imagingdevice; and automatically performing the steps of obtaining the calibration data from theacquired image and using the calibration data to obtain colour data, wherein thecolour data represents the colour of the coloured regions within the said article.
2. A method of communicating calibration data according to claim 1,wherein the calibration image comprises one or more barcodes.
3. A method of communicating calibration data according to claim 2,wherein the calibration image comprises one or more 2-D barcodes.
4. A method of communicating calibration data according to any of claims 1to 3, wherein the calibration image is printed onto the calibration article.
5. A method of communicating calibration data according to any of claims 1to 3, wherein the calibration image is photo-imaged onto the calibration article.
6. A method of communicating calibration data according to any precedingclaim, wherein the colour regions are formed from a predetermined biologicalstain material.
7. A method of communicating calibration data according to any precedingclaim, wherein the imaging device is a scanner.
8. A method of communicating calibration data according to any of claims 1to 6, wherein the imaging device is a digital camera.
9. A computer program product comprising program code means adapted inuse to perform the method of communicating calibration data according to any ofclaims 1 to 8, when the computer program code is executed upon a computerdevice.
10. A system for communicating calibration data relating to a colourcalibration article, the article having a number of coloured regions and acalibration image containing the calibration data, the system comprising: an imaging device configured to acquire an image of the article including thecalibration image; and means for automatically performing the steps of obtaining the calibration datafrom the acquired image and using the calibration data to obtain colour data,wherein the colour data represents the colour of the coloured regions within thesaid article.
11. A system for communicating calibration data according to claim 10,wherein the calibration image comprises one or more barcodes.
12. A system for communicating calibration data according to claim 11,wherein the calibration image comprises one or more 2-D barcodes.
13. A system for communicating calibration data according to any of claims10 to 12, wherein, in use, the calibration image is printed onto the calibrationarticle.
14. A system for communicating calibration data according to any of claims10 to 12, wherein, in use, the calibration image is photo-imaged on thecalibration article.
15. A system for communicating calibration data according to any of claims10 to 14, wherein the colour regions are formed from a predetermined biologicalstain material.
GB1216019.8A 2012-06-13 2012-09-07 Method of communicating calibration data and colour calibration article Active GB2503052B (en)

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CN104359582B (en) * 2014-10-14 2017-04-26 中国计量学院 System and method for onsite verifying or correcting of forced air drying box
EP3744080B1 (en) * 2018-01-26 2023-06-07 Universitat de Barcelona Colour correction
EP4206627A1 (en) * 2021-12-29 2023-07-05 X-Rite, Inc. A color measurement system comprising a calibration plaque

Citations (4)

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EP1184720A2 (en) * 2000-08-09 2002-03-06 Eastman Kodak Company Method and photographic element for calibrating digital images
EP1186943A2 (en) * 2000-08-09 2002-03-13 EASTMAN KODAK COMPANY (a New Jersey corporation) Apparatus for exposing sensitometric and bar code data onto photosensitive media
EP1594301A1 (en) * 2004-05-05 2005-11-09 LaserSoft Imaging AG Calibration of imaging devices for minimizing individual color reproducing errors of such devices
US20070127965A1 (en) * 2005-12-05 2007-06-07 Pagan William G Apparatus, system, and method for modifying print parameters

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1184720A2 (en) * 2000-08-09 2002-03-06 Eastman Kodak Company Method and photographic element for calibrating digital images
EP1186943A2 (en) * 2000-08-09 2002-03-13 EASTMAN KODAK COMPANY (a New Jersey corporation) Apparatus for exposing sensitometric and bar code data onto photosensitive media
EP1594301A1 (en) * 2004-05-05 2005-11-09 LaserSoft Imaging AG Calibration of imaging devices for minimizing individual color reproducing errors of such devices
US20070127965A1 (en) * 2005-12-05 2007-06-07 Pagan William G Apparatus, system, and method for modifying print parameters

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GB201210417D0 (en) 2012-07-25
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