WO2019141200A1

WO2019141200A1 - Image analysis process and system

Info

Publication number: WO2019141200A1
Application number: PCT/CN2019/072073
Authority: WO
Inventors: Ka Wing CHENG; Kin Wing WONG; Kong Chan; Juan CHENG; Wing Chi Tang; Koon Chung Hui
Original assignee: Master Dynamic Limited
Priority date: 2018-01-17
Filing date: 2019-01-17
Publication date: 2019-07-25
Also published as: CN111837024B; CN111837024A

Abstract

An image acquisition system for acquiring an image of an article, said system including an optical image acquisition device for acquiring an image of an article, wherein said article is at least partially transparent; a first light source for transmitting light through said article; a first linear polarizer and a second linear polarizer, wherein said first linear polarizer and said second linear polarizer are disposed between the optical image acquisition device and the first light source, the first linear polarizer being disposed proximal to the first light source and the second linear polarizer being disposed distal to the first light source and adjacent the optical image acquisition device; and an article support member disposed between the first linear polarizer and the second linear polarizer for supporting the article, wherein the article support member is optically transparent; wherein said first linear polarizer and said second linear polarizer are operably oriented in crossed orientations to each other such that light emitted from the first light source is substantially prevented from being received by the optical image acquisition device; and wherein upon the article being supported by the article support member, the article alters the polarization of light polarized by the first linear polarizer such that the article is optically detectable by the image acquisition device, and such that an image is acquirable by the image acquisition device comprising the article which contrasted from background against which an image of the article is acquired.

Description

IMAGE ANALYSIS PROCESS AND SYSTEM

Technical Field

The present invention relates to a system and a process for analyzing the visual characteristics of a gemstone. More particularly, the present invention provides a system and a process for analyzing the colour characteristics of a gemstone article formed from precious stone, semi-precious or ornamental stones.

Background of the Invention

Colour judgement of a colour of an object or article is a very important process in human assessment of objects or articles, and is an optical parameter used for assessment and grading of gemstones is colour, in particular for luxury goods such as gemstones and articles formed from gemstones.

The assessment and grading of optical parameters, in particular colour, of a gemstone is used in the gemstone industry to denote a grading which contributes to the value of a gemstone or article formed from a gemstone material, and, the value can be highly dependent on the cotour. As such, repeatable and reliable colour judgement is required for consistency of colour grading.

However, colour recognition is a complex parameter for humans, and it is common to have misjudgment of colour, both between difference people and as well when a same person is performing an assessment and, and such inconsistency can cause conflict can cause misclassification.

For some types of gemstones, in particular diamonds, there exist industry established and accepted colour grading scales. known as its colourlessness. The more colourless the diamond, the higher. By way of example, the Gemological Institute of America (GIA) has a colour grade from D to Z, and a master set of stones is used for visual comparison in order to grade a diamond by its colour.

In other colour grading systems, standard data sets in the form of colour cards or charts are utilized, for again for visual comparison with a gemstone or an article.

In such cases, repetitive training of colour graders is required, with a view so that different graders can reproduce the same assessment results, with a view to providing uniformity and consistency between colour grading personnel.

However and regardless, when humans are grading a gemstone or an article, different graders can provide a different or varying colour assessment. Further, same graders can also provide inconsistent results for a same gemstone or article, and such inconsistencies can cause incorrect grading which can adversely impact upon the value of a gemstone or article, as well as cause other commercially unacceptable consequences.

Object of the Invention

It is an object of the present invention to provide a system and a process for analyzing the visual characteristics of a gemstone, in particular colour, which overcomes or at least partly ameliorates at least some deficiencies as associated with the prior art.

Summary of the Invention

The present invention relates to a system and a process for analyzing the visual characteristics of a gemstone, in particular the visual characteristics of a gemstone article formed from precious stone, semi-precious or ornamental stones.

As will be understood by those skilled in the art and for avoidance of doubt and as defined as follows, precious stones include diamond, sapphire ruby, emerald, semi-precious stones include topaz amber, tourmaline, garnet, amethyst, agate or ornamental stones including jades including jadeite and nephrite.

In accordance with the present invention, the term “gemstone” refers to an article which is formed from a material which is a gemstone, whereby the article has been prepared such that it is at least semi-transparent, meaning that at least some portion of light can be transmitted though or pass therethrough.

Within the gemstone industry globally, gemstones, stones and minerals may be classified differently in some cases and in some geographic regions. Regardless, the present invention is directed to gemstones which may be used in jewellery such as a diamond, ruby, sapphire, as well as articles formed from gemstones such as jade, when prepared, are in the form of a semi-transparent gemstone article, and no limitation should be applied due to alternate classifications, and the examples given should not be considered an exhaustive listing.

In a first aspect, the present invention provides an image acquisition system for acquiring an image of an article, said system including an optical image acquisition device for acquiring an image of an article, wherein said article is at least partially transparent; a first light source for transmitting light through said article; a first linear polarizer and a second linear polarizer, wherein said first linear polarizer and said second linear polarizer are disposed between the optical image acquisition device and the first light source, the first linear polarizer being disposed proximal to the first light source and the second linear polarizer being disposed distal to the first light source and adjacent the optical image acquisition device; and an article support member disposed between the first linear polarizer and a second linear polarizer for supporting the article, wherein the article support member is optically transparent; wherein said first linear polarizer and said second linear polarizer are operably oriented in crossed orientations to each other such that light emitted from the first light source is substantially prevented from being received by the optical image acquisition device; and wherein upon the article being supported by the article support member, the article alters the polarization of light polarized by the first linear polarizer such that the article is optically detectable by the image acquisition device, and such that an image is acquirable by the image acquisition device comprising the article which contrasted from background against which an image of the article is acquired.

Preferably, the system further comprises a second light source disposed between the first linear polarizer and the second liner polarizer for illuminating the article such that upon removal of the polarizers and optical image of the article is acquirable by the optical acquisition device for indicative of the colour of the article.

Preferably, the first light source and the second light source provide light of a constant colour and intensity so as to provide uniform lighting conditions.

Preferably, the first light source and the second light source are selected from the group including such as LED light source, a Xeon lamp light source, a halogen lamp light source, a deuterium lamp light source, an incandescent light bulb light source, a fluorescent lamp light source, a solar simulator light source or the like.

The system may include an integrating sphere system, comprising two spheres interconnected at an aperture providing communication therebetween, wherein the first light source is in communication with a first sphere and the second light source is in communication with a second sphere, and wherein the optical image acquisition device is directed from the periphery of the second sphere towards the aperture, wherein the first linear polarizer occludes the aperture between the two spheres and the second linear polarizer occludes the optical image acquisition device, and wherein the article support member is disposed adjacent the first linear polarizer.

The first linear polarizer and the second linear polarizer are preferably moveable so as to allow for acquisition of an image of the article in the absence of polarized light.

The article support member may be formed from glass, quartz, or a polymeric material. The article support member may include a filter including a neutral density filter, anti-reflection filter, UV reflecting filter or the like.

The system may further include a further support member for supporting the article support member. The further support member may be formed from glass, quartz, or a polymeric material. The further support member may include a filter including a neutral density filter, anti-reflection filter, UV reflecting filter or the like.

The optical image acquisition device is preferably in communication with a processor for analyzing the colour of the article. The processor may be in communication with a data store and wherein the data store includes standardized reference data indicative of a range of colours, and wherein the processor quantitatively correlates the pixel colour values of an image of the article acquired by the optical image acquisition device with the standardized reference data.

In a second aspect, the present invention provides a process operable using a computerized system for grading the colour of an article, wherein said article is at least partially transparent and wherein the colour of the article is correlated with the colour from a set of standardized reference colour data, the computerized system including an optical image acquisition device, a processor module and an output module operably interconnected together, said process including the steps of (i) acquiring via an optical image acquisition device a background image of the environment in which an image of the article is to be acquired and acquiring an article image of the article in said environment, wherein said environment has a predetermined constant light level, (ii) acquiring via the optical image acquisition device a mask image, wherein the mask image is an optical image of the article including the background against which said image of the article is acquired, wherein said mask image is acquired with the article disposed between a first linear polarizer and a second linear polarizer, wherein said first linear polarizer and said second linear polarizer are disposed between the optical image acquisition device and a first light source, the first linear polarizer being disposed proximal to the first light source and the second linear polarizer being disposed distal to the first light source and adjacent the optical image acquisition device, wherein said first linear polarizer and said second linear polarizer are operably oriented in crossed orientations to each other such that light emitted from the first light source is substantially prevented from being received by the optical image acquisition device; and wherein the article alters the polarization of light polarized by the first linear polarizer such that the article is optically detectable by the image acquisition device, and such that the mask image is acquirable by the image acquisition device comprising the article which contrasted from background against which an image of the article is acquired; (iii) in a processor module (a) removing the background from the article image utilising the acquired mask image and correcting the article image by way of flat-field correction using said background image, and (b) comparing data derived from acquisition of the article image having been corrected with data from the with a set of standardized reference colour data; and from an output module, responsive to a predetermined threshold of correlation between the pixel colour values of a region of the article with data derived from input of the first optical image and assigning a colour to said region of the article, an output signal is provided indicative of the colour of said region of the article.

The article image and the mask image are preferably acquired via a system according to the first aspect.

The pixel colour values of the article may be are referenced to a colour definition include those of the group RGB, HSL, HSV, CIE, CMYK, YIQ and the like.

The article is preferably a gemstone, including precious stones, semi-precious stones and ornamental stones.

Brief Description of the Drawings

In order that a more precise understanding of the above-recited invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. The drawings presented herein may not be drawn to scale and any reference to dimensions in the drawings or the following description is specific to the embodiments disclosed.

Figure 1 shows a schematic representation of the system and process in accordance with the present invention;

Figure 2 shows an embodiment of a system in accordance with the present invention; and

Figures 3a, 3b and 3c show representations of quantitative checking showing the correlations of H, S, L values between acquired images using the system of the present invention and standard Pantone colour values.

Detailed Description of the Drawings

Background on Colour

For a coloured object or article, human judgement and interpretation of colour is affected heavily by the lighting conditions in which the object or article is viewed. One important condition is the colour of the surrounding light. The colour of light can be precisely described by the light spectrum. In practice for simplicity, the colour of light can be described by its colour temperature, which demonstrates the light spectrum emitted by an ideal black-body radiator at a particular temperature.

For white light, the light spectrum is similar to that of the solar light spectrum, with a colour temperature at 6,500 K. Under lighting conditions with different colours, such as yellow light (such as incandescent lamps with light temperature at 3,000 K) and white light (sun light with light temperature at 6,500 K) , the colours of an object as observed by the human eye can be completely different.

Another important light condition is light intensity. Even under light with the same colour, for example white light, light intensity variations can also significantly affect human judgement of colour or colours of an object or article. If the lighting conditions are non-uniform, the testing or observation environment can be much more complex. Accordingly, for the assessment of colour by human, a standard environment is needed for repeatable and reliable results.

Problems with Colour Judgement or Assessment

Further, misconception or misinterpretation may also occur between different people in human colour judgement and interpretation. For example, for a same cloth, some people may classify it as being “blue” , whilst some may classify or consider it as being “green” , which means that the human “reference table” for two people can be different for a colour or colours.

Therefore, human judgement of colour must be done with the same basis and terminology. A reference master set with varying colour may be used for this purpose. The reference master sets are each required to have high accuracy and repeatability between different master sets. Each reference master must be homogenously saturated with the specific standard colour in order that a best comparison may be made between an object or article to be graded and the reference master set.

For colour grading assessment of gemstones, the accuracy and usability of master stones as reference sets are not only applicable to different sets of reference master sets, but also the same sets of reference master at different points in time when assessment is made, as it has been found that the colour of reference stones in master sets may vary with time.

As such, the colour of the reference master sets must be permanent without any changes over time, otherwise it is necessary to provide useable or serviceable lifetimes to the reference master sets.

After the expiration of the usable or serviceable lifespan of a reference master, there is no guarantee that the colour will remain stable, and consequently no guarantee as to the accuracy and repeatability of colour grading assessment.

Furthermore, even under well controlled conditions, such as constant light colour, intensity, uniform illumination and good master reference sets, the use human eyes for determining colour can still be an issue to the correct colour judgement of an object or article.

Because of human′s vision tiredness, different colour judgements of the same object or article at different time may have different results. The person looking at the object after looking on many other different coloured objects or articles, can give a different colour assessment than under a different viewing history of the person.

What increases variation in human judgement on colour is a common psychological effect, for colour perception. Variation in colour perception is particularly serious if the object or article has a matrix of assorted colours, or is multi-coloured. Humans can also easily misjudge a colour under the presence of a background colour.

Therefore, during the process of colour judgement, the object or article under assessment should be placed on a background having uniform colour, such as white colour, so as to minimize variance in colour perception.

However, for an object or article with a matrix of different colours, the colour perception cannot be eliminated. These physio-psychological issues make repeatable and reliable colour judgements by human eyes impossible.

Thus, for a system capable of transforming light to the “colour” information, which includes an image acquisition device such as a camera, needs to take some particular handling of an image and also requires some processing basis on this reason as it not directly output the wavelength of the light.

Identified Disadvantages of the Prior Art

The present inventors have identified shortcomings in the manner in which colour of gemstones is assessed, and upon identification of the problems with the prior art, have provided a system and process which overcomes the problems of the prior art, and provides a system and process which is more consistent and reliable.

Problems identified by the present inventors include:

(i) intrinsic factors as recited above including the requirement for colour to be precise, saturated and consistent between master sets, which may be master stones of reference cards or charts;

(ii) extrinsic factors including the environment in which a colour judgement assessment is made, including lighting conditions and background; and

(iii) extrinsic factors pertaining to human judgement, including judgement error due to environment, human perception, human consistency, tiredness and distraction, as well as inherent human error.

Present Invention

In order to address the above shortcomings including repeatability and reliability difficulties, the present invention provides a process and a system to determine and analyse the colours of an object or article, in particular a gemstone.

The system provides well-controlled lighting conditions throughout the time, and machine or electronic vision without vision without the inherent deficiencies and inconsistencies of human vision tiredness. Such a process and system provide advantages over those of the prior art, and provides high repeatability for the analysis of colours of an object or object.

Referring to Figure 1, there is shown a schematic representation of an image acquisition system 100 according to the present invention, for acquiring an image of an article 110, in particular a gemstone.

The system includes an optical image acquisition device 104 for acquiring an image of an article 110, wherein said article 110 is at least partially transparent;

A first light source 102 is provided for transmitting light through the article 110.

The system further includes a first linear polarizer 106 and a second linear polarizer 108, wherein the first linear polarizer 106 and the second linear polarizer 108 are disposed between the optical image acquisition device 104 and the first light source 102. The first linear polarizer 106 is disposed proximal to the first light source 102 and the second linear polarizer 108 is disposed distal to the first light source 102 and adjacent the optical image acquisition device 104.

An article support member (not shown) is disposed between the first linear polarizer 106 and the second linear polarizer 108 for supporting the article 106, wherein the article support member is optically transparent

The first linear polarizer 106 and the second linear polarizer 108 are operably oriented in crossed orientations to each other such that light emitted from the first light source 102 is substantially prevented from being received by the optical image acquisition device 104.

Upon the article 110 being supported by the article support member, the article alters the polarization 116 of light 114 polarized by the first linear polarizer 106 such that the article is optically detectable by the image acquisition device 104, and such that an image is acquirable by the image acquisition device 104 comprising the article 110 which contrasted from background against which an image of the article is acquired.

Referring to Figure 2, there is shown a schematic representation of an embodiment of a system 200 for acquiring an optical image of an article. The system includes two integrating spheres, an upper sphere 210a and a lower sphere 210b which are interconnected at the common aperture 211.

The internal coating of each integrating

sphere

210a, 210b is diffusive white paint, allowing for homogenous diffusive reflections of light inside the spheres 210a, 21 0b. Accordingly, the dual integrating sphere structure can provide a controllable uniform illumination both above and below an article 260 of which an optical image thereof is to be acquired.

For the upper sphere 210a, there is provided an aperture 212 at the most upper portion, referred to as the north pole, and an aperture 213a at the lateral side of the sphere 210a.

In communication with the aperture 212, there is provided an image acquisition device as a camera 220 with a removable linear polariser 250a distal of the camera 220.

For the lower sphere 210b, there is provided an aperture 213b.

The

apertures

213a and 213b are connected and in communication with

light sources

230a and 230b respectively, allow entry and delivery of light into the system 200.

Baffles 231a and 231b are provided and positioned adjacent the

apertures

213a and 213b respectively so as to prevent direct illumination from the

light sources

230a and 230b to the article 260.

A transparent plate 241 is located at the aperture 211 which provides as a transparent platform for the support and removable of a lower linear polariser 250b.

Polarisers

250a and 250b are operably configured in crossing orientation at 90 degrees to each other, which obstructs passage of light. Extending over the transparent plate 241 and the polariser 250b, there is provided a transparent sample platform 242 for placing the article 260 of which an optical image is to be captured thereof.

In accordance with the process of the invention, when an optical image of the article 260 is to be acquired, the article 260 and

polarisers

250a and 250b will be initially removed.

An empty background image is first acquired via the camera 220, in this case a digital camera, and subsequently a second optical image is acquired with the article 260 placed on the sample platform 242.

The second acquired optical image can be flat-field corrected with the empty background image acquired. Flat-field correction, as is known as a term of art, is a method to remove the non-uniformity amongst different pixels of the image. This difference consists of two sources. One source is the sensitivity variations among different pixels on the detector of the camera, and the other is the contribution of optical distortions, such as the optical distortions of lens. After flat-field correction of the image, the image can achieve a high quality.

In order to contrast the article 260 from the background, the present invention utilizes linear polarisers 250a and 250b inserted above and below the object or article can be captured.

The two

polarisers

250a, 250b, are arranged in crossed orientations to each other at 90 degrees. In the absence of the article, the two crossed

polarisers

250a, 250b, cut out all the light entering the camera 220.

In accordance with the present invention, in the presence of an article 260 with refractive properties, the article 260 changes the polarisation of light passing through it.

Similarly as described with reference to Figure 1, only the light passing through the article 260 can reach the optical acquisition device which is camera 220 while the light passing through the background is mostly cut out.

This process of the present invention, physically enhances the image contrast between the article 260 and the background, and is used to produce a mask to assist the background removal.

The present inventors have identified that by use of the actual physical shape and geometry of an article 260 which is at least partially optically transparent, obviates the necessity for utilisation of complex computational algorithms for background removal.

When using computational methods for background removal, complex image analysis is required, and does not always provide clear distinction between background and an article for which subsequent image analysis is required.

The present inventors have utilized the diffractive properties of the article, which fully correspond to the periphery of the article, so as to create an efficient mask for utilization in background removal.

The present inventors have found that a very precise periphery of the article can be determined based on light and physical effects, without the necessity for complex algorithms, and the present invention has proven particularly useful for optical characteristics of gemstones, in particular colour determination of a gemstone.

For gemstones requiring colour assessment, such as jade whereby the colour of the article varies including at the periphery, and whereby the article may be of a white colour, it is often difficult to delineate between background and the article, and image analysis software which typically utilizes interpolation between optical parameters to determine peripheries, cannot readily distinguish accurately between background and an article when colours are similar.

By contrast, the process of the present invention is immune from such colour differentiation errors, as regardless of even if the article has the same colour at is periphery as the background, the diffractive properties of the article allow the article to be seen via the camera whilst blocking out the background due to the implementation of the two 90-degree offset polarisers.

In order to ascertain whether the system of the present invention can provide the correct colour and provide stable performance without drift, standard colour references are used.

Standard colour references are spectrally calibrated by the issuing institutes so their colour values are truly related to the pixel colour values in the image taken by the system. This is particularly important for consistent and accurate colour determination, as different optical acquisition devices, such as cameras, as different brands of camera can have different imaging performance.

Quantitative checking can be done by determining the correlation between the spectrally determined colour values and pixel colour values as acquired by the system of the present invention.

Referring to Figures 3a, 3b and 3c, there are shown examples of quantitative checking showing the correlations of H, S, L values between camera acquired images using the system, of the present invention and Pantone colour references.

With quantitative checking, the performance of different systems can be compared and calibrated so that repeatable and reliable classification can be done without the use of human judgement.

Claims

An image acquisition system for acquiring an image of an article, said system including:

an optical image acquisition device for acquiring an image of an article, wherein said article is at least partially transparent;

a first light source for transmitting light through said article;

a first linear polarizer and a second linear polarizer, wherein said first linear polarizer and said second linear polarizer are disposed between the optical image acquisition device and the first light source, the first linear polarizer being disposed proximal to the first light source and the second linear polarizer being disposed distal to the first light source and adjacent the optical image acquisition device; and

an article support member disposed between the first linear polarizer and the second linear polarizer for supporting the article, wherein the article support member is optically transparent;

wherein said first linear polarizer and said second linear polarizer are operably oriented in crossed orientations to each other such that light emitted from the first light source is substantially prevented from being received by the optical image acquisition device; and

wherein upon the article being supported by the article support member, the article alters the polarization of light polarized by the first linear polarizer such that the article is optically detectable by the image acquisition device, and such that an image is acquirable by the image acquisition device comprising the article which contrasted from background against which an image of the article is acquired.
A system according to claim 1, wherein the system further comprises a second light source disposed between the first linear polarizer and the second liner polarizer for illuminating the article such that upon removal of the polarizers and optical image of the article is acquirable by the optical acquisition device for indicative of the colour of the article.
A system according to claim 2, wherein the first light source and the second light source provide light of a constant colour and intensity so as to provide uniform lighting conditions.
A system according to claim 2 or claim 3, wherein the first light source and the second light source are selected from the group including such as LED light source, a Xeon lamp light source, a halogen lamp light source, a deuterium lamp light source, an incandescent light bulb light source, a fluorescent lamp light source, a solar simulator light source or the like.
A system according to any one of claims 2 to 4, wherein the system includes an integrating sphere system, comprising two spheres interconnected at an aperture providing communication therebetween, wherein the first light source is in communication with a first sphere and the second light source is in communication with a second sphere, and wherein the optical image acquisition device is directed from the periphery of the second sphere towards the aperture, wherein the first linear polarizer occludes the aperture between the two spheres and the second linear polarizer occludes the optical image acquisition device, and wherein the article support member is disposed adjacent the first linear polarizer.
A system according to any one of the preceding claims, wherein the first linear polarizer and the second linear polarizer are moveable so as to allow for acquisition of an image of the article in the absence of polarized light.
A system according to any one of the preceding claims, wherein the article support member is formed from glass, quartz, or a polymeric material.
A system according to any one of the preceding claims, wherein the article support member includes a filter including a neutral density filter, anti-reflection filter, UV reflecting filter or the like.
A system according to any one of the preceding claims, further including further support member for supporting the article support member.
A system according to claim 9, wherein the further support member is formed from glass, quartz, or a polymeric material.
A system according to claim 9 or claim 10, wherein the further support member includes a filter including a neutral density filter, anti-reflection filter, UV reflecting filter or the like.
A system according to any one of the preceding claims, wherein the optical image acquisition device is in communication with a processor for analyzing the colour of the article.
A system according to claim 12, wherein the processor is in communication with a data store and wherein the data store includes standardized reference data indicative of a range of colours, and wherein the processor quantitatively correlates the pixel colour values of an image of the article acquired by the optical image acquisition device with the standardized reference data.
A process operable using a computerized system for grading the colour of an article, wherein said article is at least partially transparent and wherein the colour of the article is correlated with the colour from a set of standardized reference colour data, the computerized system including an optical image acquisition device, a processor module and an output module operably interconnected together, said process including the steps of:

(i) acquiring via an optical image acquisition device a background image of the environment in which an image of the article is to be acquired and acquiring an article image of the article in said environment, wherein said environment has a predetermined constant light level;

(ii) acquiring via the optical image acquisition device a mask image, wherein the mask image is an optical image of the article including the background against which said image of the article is acquired, wherein said mask image is acquired with the article disposed between a first linear polarizer and a second linear polarizer, wherein said first linear polarizer and said second linear polarizer are disposed between the optical image acquisition device and a first light source, the first linear polarizer being disposed proximal to the first light source and the second linear polarizer being disposed distal to the first light source and adjacent the optical image acquisition device, wherein said first linear polarizer and said second linear polarizer are operably oriented in crossed orientations to each other such that light emitted from the first light source is substantially prevented from being received by the optical image acquisition device; and wherein the article alters the polarization of light polarized by the first linear polarizer such that the article is optically detectable by the image acquisition device, and such that the mask image is acquirable by the image acquisition device comprising the article which contrasted from background against which an image of the article is acquired;

(iii) in a processor module (a) removing the background from the article image utilising the acquired mask image and correcting the article image by way of flat-field correction using said background image, and (b) comparing data derived from acquisition of the article image having been corrected with data from the with a set of standardized reference colour data; and

(iv) from an output module, responsive to a predetermined threshold of correlation between the pixel colour values of a region of the article with data derived from input of the first optical image and assigning a colour to said region of the article, an output signal is provided indicative of the colour of said region of the article.
A process according to claim 14, wherein the background image, the article image and the mask image are acquired via a system according to any one of claims 1 to 13.
A process according to claim 14 or claim 15, wherein pixel colour values of the article are referenced to a colour definition include those of the group RGB, HSL, HSV, CIE, CMYK, YIQ and the like.
A process according to any one of claims 14 to 16, wherein the article is a gemstone, including precious stones, semi-precious stones and ornamental stones.