GB2503052A

GB2503052A - Automatically obtaining colour calibration data

Info

Publication number: GB2503052A
Application number: GB1216019.8A
Authority: GB
Inventors: Martin Gouch; Craig Revie
Original assignee: FFEI Ltd
Current assignee: FFEI Ltd
Priority date: 2012-06-13
Filing date: 2012-09-07
Publication date: 2013-12-18
Anticipated expiration: 2032-09-07
Also published as: GB201210417D0; GB201216019D0; GB2503052B

Abstract

A method of communicating calibration data relates to a colour calibration article 10 having a number of coloured regions (color test patches) 20 and a calibration (pattern) element 30 for obtaining the calibration data, e.g., a 2D barcode or other image-encoded data; reference to a location for obtaining the calibration data, such as an Internet (cloud) URL; or an RFID tag device. The method comprises imaging the article with an imaging device (e.g. scanner, digital camera) to produce corresponding device data, automatically obtaining the calibration data from the calibration element and using the calibration data to obtain colour data representing the (true) colour of the coloured patch regions. The calibration pattern may be printed or photo-imaged; color patch regions may be formed from biological stain material.

Description

Method of communicating calibration data and colour calibration article The invention relates to a method of communicating calibration data and a colour calibration article. In particular, the invention relates to the calibration of an imaging device such as a colour scanner.

When using a colour scanner it is often necessary to be able to convert colour images produced by the scanner so that they match the result produced by another scanner. In some cases it may also be necessary to provide an accurate colour estimate of the material being scanned. This is usually done using a scanner calibration or characterisation process to determine the colour characteristics of the scanner. Knowledge of these characteristics can then be used to define a transform that can be applied to images to match another scanner orto create a "true-colour" image.

In many cases absolute colour accuracy is not required and pseudo-colour images are used. Even in these cases, however, it is important to know the colour characteristics of the scanner so that a consistent pseudo-colour" image can be produced for a given scanned sample.

One common calibration method makes use of standard profiles such as ICC profiles defined by the International Color Consortium. ICC profiles are data files that provide a means to map RGB pixel values produced by a colour device to CIEXYZ or CIELab standard colour values. These colour values are based on a standard colour observer defined by the Commission International de l'Eclairage (CIE).

A traditional ICC scanner profiling method is shown in Figure 1. The ICC profiling method typically makes use of a colour characterisation target 1, typically made from a similar substrate material to the sample material being scanned with the scanner. Usually the target I comprises a set of colour patches 2 of a wide spectral range of colours which are positioned on the target I (step SI). The colours of the colour patches 2 have known colour measurement values or standard colour values obtained by measuring each colour patch 2 with a spectrophotometer or other suitable instrument (S2). These values aie usually stored 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 scanner operator to produce an RGB image II of the target I, from which a set of RGB pixel values for each of the colour patches 2 can be obtained. These pixel values are then used in conjunction with the known measurement data 3 to build a colour conversion or mapping table which relates the RGB to standard colour values. The colour conversion table is then used to create (S6) an ICC profile 4 which, in turn, can be used by a colour imaging application or can be used directly 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 the measurement data for the target. Selecting an inappropriate file, for example a file containing measurement data for a different target, is a common mistake and can 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 a method of communicating calibration data relating to a colour calibration article, the article having a number of coloured regions and a calibration element for obtaining the calibration data, the method comprising: imaging the article with an imaging device to produce corresponding device data; and automatically performing the steps of obtaining the calibration data from the calibration element and using the calibration data to obtain colour data, wherein the colour data represents the colour of the coloured regions within the said article.

The colour calibration article may also be referred to as a self-contained colour characterisation target or a self-contained calibration chart. The calibration article may also be a self-contained calibration slide where it is intended for use with a medical scanner for example. The colour calibration article allows for automation of the calibration process of an imaging device such as a scanner.

The coloured regions may include a wide range of colours suitable for characterising the target. The coloured regions selected to form the range may depend upon the intended use of the scanner. The coloured regions may be coloured patches for example.

The calibration data, also referred to as measurement data, may be data obtained by measuring the coloured regions of the colour calibration article with a spectrophotometer or using other techniques known in the art and which are suitable for accurately characterising an imaging device. Communicating the calibration data in accordance with the first aspect of the invention enables the imaging device to automatically obtain the true colours of the coloured regions.

The main advantage of the present invention over traditional calibration techniques (such as the method shown in Figure 1) is that, since the scan of the target 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 no need for the user to locate a digital file containing the measurement data for the target. This avoids selecting an inappropriate file and ensures that the calibration is accurate. A further advantage of the present invention is that the method provided may be easily understood and performed by scanner operators which are familiar with traditional scanner profiling methods.

The calibration element may be indicia such as a calibration image containing the calibration data, for example 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 a ORCode or similar 2-D barcode, however a set of 1-D barcodes or any other encoding method may also be used.

Alternatively, the calibration element may be a reference element or unique identifier for the calibration data, such as a unique resource locator, URL, which may be included on the target in the form of a barcode, REID tag, plain text or any other form. In this variant, instead of including the calibration data directly onto the article, the calibration data is stored elsewhere (e.g. on the internet) and may be accessed via the reference element. This is particularly advantageous where the article includes a large number of coloured regions as less space is required for the reference element than is required for an image encoding the measurement data.

The calibration clement may be included in the calibration article using any suitable method, for example by printing, adhesion or photo-imaging. In the latter technique, a substrate of the article may be coated with a photo-sensitive material and then the calibration element may be added to the calibration article by imaging onto the photo-sensitive material using laser for example. This is particularly advantageous when large numbers of coloured areas are used, as the calibration article may be made smaller.

Preferably, the coloured regions are formed from a similar material to the sample material being imaged by the imaging device. For example, where the material is stained biological material, including stained cells, the coloured regions may be formed from a predetermined biological stained material.

The method of communicating calibration data according to the invention has a number of applications. In the examples described in detail below, the imaging device 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 example and this invention may also be used to calibrate such devices.

A computer program (software) product may comprise program code means adapted in use to perform the method of communicating calibration data according to the invention, when the computer program code is executed upon a computer device. Advantageously, the imaging device may be configured with the aid of the computer program to automatically perform the steps of obtaining the calibration data from the calibration element (for example by scanning the article using image recognition software) and using the calibration data to obtain the colour data (for example using image processing software) In accordance with a second aspect of the present invention, there is provided a system for communicating calibration data relating to a colour calibration article, the article having a number of coloured regions and a calibration element for obtaining the calibration data, the system comprising: an imaging device arranged to image the article to thereby produce corresponding device data; and means for automatically performing the steps of obtaining the calibration data from the calibration element and using the calibration data to obtain colour data, wherein the colour data represents the colour of the coloured regions within the said article.

Typically the apparatus in accordance with the second aspect of the invention is adapted to perform the invention in accordance with the first aspect. Further advantageous features are set out in the appended dependent claims.

Examples of methods in accordance with the invention will now be described with reference to the following figures: Figure 1 is a schematic representation of a traditional scanner profiling method; Figure 2 shows a first variant of a method of communicating calibration data in accordance with the invention, using a measurements model'; Figure 3 shows another schematic representation of a method of communicating calibration data in accordance with the first variant shown in Figure 2; and Figure 4 shows a second variant of method of communicating calibration data in accordance with the invention, using a URL model'.

Figure 2 illustrates the calibration process of a scanner using a self-contained colour characterisation target in accordance with the invention. It will be appreciated however that the method may be applied to calibrate any imaging device such as a digital camera.

As shown in Figure 2, the creation of the self-contained colour characterisation target includes three main steps (Sb, S20, S30). In the first step, 510, a set of colour patches 20 are placed on a target 10 using methods known in the art. The target 10 may be a slide made from any suitable material which may be transparent for example. Preferably the set of patches 20 is carefully selected to ensure that the range of colours is spread over the entire range of colours that are 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 material that is similar to the material to be scanned.

In the second step, S20, the coloured patches 20 are measured with a spectrophotometer or similar instrument to generate calibration data or measurement data comprising a standard set of wavelengths. The measurement data may be spectral radiance, spectral transmittance or spectral reflectance information for example using 32 spectral channels per patch. Alternatively, other forms of measurement data may be used such as CIEXYZ or CIELab values, which in some circumstances may provide adequate means to characterise the scanner.

In the third step S30, the measurement data is encoded as an image 30 in the form of a barcode. Preferably, the barcode is a ORCode or similar 2-D barcode, however a set of I -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 is printed on the target, on the same surface as the coloured patches 20 and adjacent to them to aid detection. Preferably, an inkjet printer or other similar marking system is used to print the barcode 30 on the target 10. Where the target 10 is transparent (i.e. made from a transmissive material), the image 30 may be printed on a reflective material, which is then added to the target 10 forming an identification area on the target. The reflective material may be a sticker which adheres to the target 10 for example.

Alternatively, other methods for adding the image 30 to the target 10 may be used. These methods include imaging on high-resolution film and then adding the film to the target 10 by coating the target 10 with a photo-sensitive material and then imaging onto this material using a scanner laser beam or some other suitable device. These alternative methods may be required in particular when large numbers of patches 20 are used.

The next steps illustrated in the bottom half of Figure 2 represent the scanner calibration, 340 according to the invention. The scanner calibration 340 includes the steps of scanning the target, S50, to obtain an image 101 of the target 10 and processing the image 101 produced by the scan, 360, to generate a colour profile 40 of the scanner.

As described above, the coloured target patches 20 and the measurement data for these patches are included on the target 10. In a preferred embodiment, the scanner scans the target 10 to produce a single image 101. The scanner may be configured to be able to distinguish and separate the image 30 containing the measurement data from the chart area including the colour patches 20.

The scanner may be configured with the aid of calibration software. The scanner calibration software may include image recognition software to detect the colour patches 20 and the image 10 on the target. In addition, the scanner calibration software may also include image processing software configured to extract the calibration data from the image, create a colour profile, and/or calibrate the scanner using the colour profile.

Accordingly, when the target 10 is scanned, 550, the single image 101 produced by 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 colour profile or other form of colour transform that allows the colour capability of the scanner to be accurately described. The colour profile 40 may be stored on any suitable digital media.

As can be seen from Figure 2, with the present invention, there is no need for a measurement data file (item 3 of Figure 1) to be separately identified in order to generate the colour profile, as required in the traditional profiling method shown in Figure 1. Instead, when scanning the target 10 which includes the image 30 characterising the colour chart 20, the scanner (configured by the calibration software) may automatically perform the steps of obtaining the measurement data from the image 30 and using the measurement data to create the colour profile 40.

The feasibility of encoding the measurement data in an image 30 which can be added on the target 10 is described in the following example. To obtain the measurement data, the spectrophotometer or other instrument takes samples of the colour patches 20. Sampling from 380-720 nm at lOnm intervals for example results in 34 samples. A signal-to-noise, S/N, ratio of 1000:1 is adequate for this purpose and many instruments can achieve it. If the 34 samples are to be encoded in 3 decimal digits per sample, the instrument would therefore require 102 decimal digits for each measurement. It would be appreciated that the required number of samples and digits may vary depending on the specific application.

A typical (version 40') black-and-white QRCode can encode 7089 digits or 69 patch measurements and a 51 digit header. The ORCode in this example is 177x177 modules'. A module is in effect an image pixel that must be imaged so that it can be read with high certainty. It is desirable that the barcode printed on the target be no larger than 20x20 mm which requires a printing resolution of 8.85/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 resolution is as high as 4000/mm, an 8-bit RGB image of 62x25x4000x4000x3 bytes (-70 Gb) is obtained.

Figure 3 shows the same steps required to create the self-contained colour characterisation target (510, S20, 530) and perform the calibration of the scanner (540, 550, 560). In addition, Figure 3 shows three optional steps (570, 371, 361) which may be performed by scanner operators. The steps which may be performed by the manufacturer of the self-contained colour characterisation target (Sb, S20, S30 and S60) are represented on the left side of Figure 3, while the steps performed on the scanner operator side (840, 850, 861, 870, 871) are shown on the right side of FigureS.

Specifically, to calibrate their device, a scanner operator may for example use the colour profile 40 with their own software, 570, or may process each scanned image of the self-contained colour characterisation target using software supplied by the manufacturer of the self-contained colour characterisation target, 371. The software supplied by the manufacturer may be suitable to process a scanned image of the self-contained colour characterisation target and may be cloud' software for example. The software may be supplied by the manufacturer on any suitable media such as a CD-ROM and may be provided together with the 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.

Figure 4 shows another example of a method of communicating calibration data in accordance with the invention. In this example, instead of encoding measurement data as an image 30 to be positioned directly on the target 10, the measurement data is stored in a separate location. Measurement data for several targets 10 may be stored in a central location. A reference element or unique identifier 31 for this measurement data stored at the central location may then be printed or imaged onto the target 10 using techniques similar to those used to position the image 30 onto the target, as described above with reference to Figure 2. In a preferred embodiment the measurement data is stored in a data file on a web server and the uniform resource locator, URL, 31 of the data file is printed on the target.

Other alternatives for the reference element 31 are also possible as long as the data may be accessed directly or indirectly by means of a reference element 31 located on the target 10 and which may be imaged together with the target 10.

For example, the reference element could be a radio frequency identification, RFID, tag. A suitable device (which is usually separate from the scanner) may be used to obtain the measurement data using the reference element 31.

In the exemplary method shown in Figure 4, the coloured target patches 20 and a reference element 31 to the measurement data are therefore included on the target 10. In this example, the reference element 31 is in the form of a URL or similar which is encoded by a barcode. This method is particularly useful where large numbers of patches 20 must be used as less space is required for the measurement data URL than is required for an image encoding the measurement data. In this case the measurement data can be stored on the internet and accessed easily by means of the URL when required.

The present invention has many applications, including applications in medical imaging. For example, the self-contained characterisation target described above may be used to calibrate medical image scanners. These may use medical image formats such as DICOM, SCN, APERIO, Hamamatsu, or OME-TIFF.

Medical image scanners are commonly used to scan slides containing tissue samples. The material to be scanned therefore is stained biological material, including stained cells. In order to accurately calibrate such scanners, the colour regions or patches of the self-contained characterisation target may be formed from biological stained material also referred to as biological stains. The biological stains may take a number of different forms including stains which are tissue stains, fluorescent markers and so on. The form in which the stains are provided preferably mimics their form when in contact with biological material.

The present invention is applicable to other areas of colour scanning and by simple extension to digital photography, wherein the imaging device is a digital camera. Accordingly, self-contained digital camera targets may be created with applications in studio photography or magazine catalogue imaging for example.

With recent advances in digital camera technology it is now easily possible to capture the details needed to be able to decode the measurement data or reference element, as required by the present invention.

The colour characteristics of a digital camera depend on many factors and it is often necessary to create a different camera characterisation or ICC profile for each scene. For some applications it is necessary to create targets of different materials, especially when colour accuracy is important in situations such as photography for a clothes catalogue or similar. Using a self-contained characterisation target as provided by the present invention makes this process much simpler.

Claims

CLAIMS1. A method of communicating calibration data relating to a colour calibration article, the article having a number of coloured regions and a calibration element for obtaining the calibration data, the method comprising: imaging the article with an imaging device to produce corresponding device data; and automatically performing the steps of obtaining the calibration data from the calibration element and using the calibration data to obtain colour data, wherein the colour 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 element is a calibration image containing the calibration data.
3. A method of communicating calibration data according to claim 2, wherein the calibration image comprises one or more barcodes.
4. A method of communicating calibration data according to claim 3, wherein the calibration image comprises one or more 2-D barcodes.
5. A method of communicating calibration data according to claim 1, wherein the calibration data is located at a predetermined location and wherein the calibration element is a reference element identifying the predetermined location.
6. A method of communicating calibration data according to claim 5, wherein the reference element comprises a URL.
7. A method of communicating calibration data according to claim 5 or claim 6, wherein the reference element comprises one or more barcodes.
8. A method of communicating calibration data according to claim 5, wherein the reference element comprises a RFID element.
9. A method of communicating calibration data according to any of claims I to 7, wherein the calibration element is printed onto the calibration article.
10. A method of communicating calibration data according to any of claims 1 to 7, wherein the calibration element is photo-imaged onto the calibration article.
11. A method of communicating calibration data according to any preceding claim, wherein the colour regions are formed from a predetermined biological stain material.
12. A method of communicating calibration data according to any preceding claim, wherein the imaging device is a scanner.
13. A method of communicating calibration data according to any of claims 1 to 11, wherein the imaging device is a digital camera.
14. A computer program product comprising program code means adapted in use to perform the method of communicating calibration data according to any of claims I to 13, when the computer program code is executed upon a computer device.
15. A system for communicating calibration data relating to a colour calibration article, the article having a number of coloured regions and a calibration element for obtaining the calibration data, the system comprising: an imaging device arranged to image the article to thereby produce corresponding device data; and means for automatically performing the steps of obtaining the calibration data from the calibration element and using the calibration data to obtain colour data, wherein the colour data represents the colour of the coloured regions within the said article.
16. A system for communicating calibration data according to claim 15, wherein the calibration element is a calibration image containing the calibration data.
17. A system for communicating calibration data according to claim 16.wherein the calibration image comprises one or more barcodes.
18. A system for communicating calibration data according to claim 17, wherein the calibration image comprises one or more 2-D barcodes.
19. A system for communicating calibration data according to claim 14, wherein the calibration data is located at a predetermined location and wherein the calibration element is a reference element identifying the predetermined location.
20. A system for communicating calibration data according to claim 19, wherein the reference element comprises a URL.
21. A system for communicating calibration data according to claim 19 or claim 20, wherein the reference element comprises one or more barcodes.
22. A system for communicating calibration data according to claim 19, wherein the reference element comprises a REID element.
23. A system for communicating calibration data according to any of claims to 21, wherein, in use, the calibration element is printed onto the calibration article.
24. A system for communicating calibration data according to any of claims 15 to 21, wherein, in use, the calibration element is photo-imaged on the calibration article.
25. A system for communicating calibration data according to any of claims to 24, wherein the colour regions are formed from a predetermined biological stain material.AMENDMENT TO THE CLAIMS FILED AS FOLLOWS:-CLAIMS1. A method of communicating calibration data relating to a colour calibration article, the article having a number of coloured regions and a calibration image containing the calibration data, the method comprising: imaging the article with an imaging device to produce corresponding device data; and automatically performing the steps of obtaining the calibration data from the calibration image and using the calibration data to obtain colour data, wherein the colour data represents the colour of the coloured regions within the said article. C')2. A method of communicating calibration data according to claim 1, 0 15 wherein the calibration image comprises one or more barcodes.N r3. 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 I to 3, wherein the calibration image is printed onto the calibration article.5. A method of communicating calibration data according to any of claims I to 3, wherein the calibration image is photo-imaged onto the calibration article.6. A method of communicating calibration data according to any preceding claim, wherein the colour regions are formed from a predetermined biological stain material.7. A method of communicating calibration data according to any preceding claim, wherein the imaging device is a scanner.8. A method of communicating calibration data according to any of claims 1 to 6, wherein the imaging device is a digital camera.9. A computer program product comprising program code means adapted in use to perform the method of communicating calibration data according to any of claims 1 to 8, when the computer program code is executed upon a computer device. C')10. A system for communicating calibration data relating to a colour (0 calibration article, the article having a number of coloured regions and a 0 15 calibration image containing the calibration data, the system comprising: an imaging device arranged to image the article to thereby produce corresponding device data; and means for automatically performing the steps of obtaining the calibration data from the calibration image and using the calibration data to obtain colour data, wherein the colour data represents the colour of the coloured regions within the said 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 claims to 12, wherein, in use, the calibration image is printed onto the calibration article.14. A system for communicating calibration data according to any of claims to 12, wherein, in use, the calibration image is photo-imaged on the calibration article.15. A system for communicating calibration data according to any of claims 10 to 14, wherein the colour regions are formed from a predetermined biological stain material. C') (0N r