Patent Publication Number: US-2022228912-A1

Title: Colorimeter colour standards

Description:
FIELD OF THE INVENTION 
     The present invention relates to colour standards used in colorimeters, for example colour standards that may be used in a colour comparator disc. In particular, the present invention relates to methods of making colour standards. 
     BACKGROUND TO THE INVENTION 
     A colorimeter is a device for colour grading where the colour of a sample, and hence the sample itself, is graded relative to a set of known standard (or reference) colours of a colour scale. Examples of one dimensional industry specific colour scales include the ASTM colour scales (used commonly in the grading of petroleum products, e.g. ASTM D 1500, ISO 2049 and IP 196), the EBC scale (European Brewing Scale used for grading beers, malts, caramel solutions and other similarly coloured liquids), the European Pharmacopoeia colour scale, the Gardner Colour scale (used for grading similarly coloured liquids such as resins, varnishes, lacquers, drying oils, fatty acids, lecithins, sunflower oil and linseed oil), the Platinum-Cobalt/Hazen/APHA colour scale (used for grading clear to dark amber liquids, for example in the water industry and also for industries relating to clear oils, chemicals and petro-chemicals such as glycerine, plasticisers, solvents, carbon tetrachloride and petroleum spirits), the Saybolt colour scale (used for grading light coloured petroleum products including aviation fuels, kerosene, napthas, white mineral oils, hydrocarbon solvents and petroleum waxes), the DEHA colour scale (used in boiler water analysis to determine the concentration of DEHA (diethylhexyl adipate) available for corrosion inhibiting in the boiler system,) and white and yellow indices (used to grade whiteness and yellowness, for example in the paint and paper industries). 
     A typical colorimeter is shown in  FIG. 1 . This colorimeter has a holder indicated at A that receives a liquid sample to be graded. The liquid sample, contained in a transparent holder, is viewed through a window (shown at B) alongside a colour standard provided by a comparator disc (shown at C) that presents a series of colour standards from a colour scale. Each colour standard comprises a piece of stained glass of the desired colour from the colour scale. The colour comparator disc is circular and the colour standards are arranged around the edge of the disc. The colour standards are placed in order according to the colour scale. The colour comparator disc is rotated to present in turn each colour standard to the viewing window while the sample may also be viewed through the viewing window. A user may then see which colour in the colour scale is best matched to the sample, thereby grading the sample. 
     Traditionally, colour standards are glass pieces, with each glass piece corresponding to a piece of differently-coloured stained glass. Producing stained glass is an inexact process for obtaining a precise colour. There are many factors influencing the glass making process that may affect, for example, the chemical reactions of the colouring agents added to produce a desired colour of glass. Therefore it is very difficult to achieve a particular colour precisely and to avoid inconsistencies in the colour of the glass in standards of this type. 
     Colour standards made in this way must be measured against master colour standards to determine how much they deviate from the desired colour. Once this deviation is known, one or more other colour standards are compounded (i.e. joined, for example using adhesive) to correct for the deviation. This produces a colour standard that comprises of a stack of differently-coloured glass pieces that together produce the desired colour. 
     Notwithstanding the difficulties in making stained glass colour standards, such colour standards enjoy vast longevity in that they will maintain their colours for tens or even hundreds of years. 
     SUMMARY OF THE INVENTION 
     Against this background, and from a first aspect, the present invention resides in a method of making a colorimeter colour standard. The method comprises determining print parameters required to achieve a desired colour for the colour standard, and printing the desired colour on a colour-receiving face of a first transparent piece using the determined print parameters. 
     The first transparent piece may comprise or consist of glass. Alternatively, the first transparent piece may comprise or consist of a plastic such as acrylic. Preferably, the first transparent piece is clear, i.e. uncoloured or not tinted, prior to the step of printing. The first transparent piece may be a flat piece with flat, opposed faces and with the colour-receiving face corresponding to one of the flat, opposed faces. For example, the first transparent piece may be a flat circular disc. 
     The method may comprise printing the desired colour on the colour-receiving face of the first transparent piece using the determined print parameters to cover all of the colour-receiving face. Alternatively, the method may comprise printing the desired colour on the colour-receiving face of the first transparent piece using the determined print parameters to cover only a portion of the colour-receiving face (such as to provide a spot of the desired colour on the colour-receiving face). 
     It has been found that better colour control may be obtained using the proposed printing method. For example, printing techniques such as using a digital printer or screen printing allows accurate reproduction of colours. This results in colour standards that match the desired colour without requiring further colour correction as was described with respect to the prior art method of using stained glass colour standards. 
     The method further comprises, after printing the desired colour on a colour-receiving face, permanently affixing the first transparent piece to a second transparent piece such that the colour-receiving face of the first transparent piece and a joining face of the second piece are positioned at the interface between the first piece and the second piece. 
     The second transparent piece may comprise or consist of glass. Alternatively, the second transparent piece may comprise or consist of a plastic such as acrylic. Preferably, the second transparent piece is clear, i.e. uncoloured or not tinted. The second transparent piece may be a flat piece with flat, opposed faces and with the joining face corresponding to one of the flat, opposed faces. For example, the second transparent piece may be a flat circular disc. The second transparent piece may match the first transparent piece prior to the printing step. The first and second transparent pieces may match in size and shape such that they may be stacked on top of each other to form the colour standard, wherein the colour standard has the same length and width as each transparent piece, but around twice the thickness of each transparent piece. 
     The first transparent piece is permanently affixed to the second transparent piece to the extent that accidental separation of the first and second transparent pieces is prevented, for example such that the first transparent piece requires forcible separation from the second transparent piece rather than allowing the two pieces to be merely pulled apart by hand. 
     Advantageously, permanently affixing the first transparent piece to the second transparent piece with the colour-receiving face positioned at the interface between the first and second transparent pieces provides protection for the printed colour on the colour-receiving face of the first transparent piece. This helps stop the printed colour from being scratched or scraped as it is protected within the colour standard. 
     It has been realised that longevity of the colorimeter colour standards may be achieved that are adequate for the purposes of most colorimeter applications. As noted above, colour standards made using stained glass will maintain their colour for tens or hundreds of years. However, such longevity is not required for most applications. In fact, most colour standards are lost or damaged (e.g. scratched or broken) long before the colour of the standard starts to fade. It has also been appreciated that while the longevity of the printed colour standard is adequate for short term applications (for example in the order of six months), the longevity may be increased significantly by providing protection against degradation of the printed colour due to exposure to ultraviolet radiation. 
     Accordingly, at least one of the first and second transparent pieces may comprise an ultraviolet coating (or treatment) that protects against ultraviolet degradation. For example, a coating may be applied to a face of the first transparent piece. After coating, the first transparent piece may then be heated or baked such that the coating is absorbed or fuses with the material (e.g. glass) of the underlying surface. Optionally, the colour-receiving face of the first transparent piece comprises the ultraviolet coating. The coating step may be performed prior to the printing step, such that the desired colour is printed onto the colour-receiving face that comprises the ultraviolet coating. The ultraviolet coating may cover all of the colour-receiving face, or at least the entire portion of the colour-receiving face onto which the desired colour is printed or is to be printed. Providing the ultraviolet coating on the colour-receiving face is advantageous when this colour-receiving face is internally located in the finished colour standard: if the ultraviolet coating is provided on a reverse side of the first transparent piece, it may then reside on an external face of the colour standard where it may be scratched or otherwise damaged. 
     Alternatively, the second transparent piece may be treated in the any of the ways described in the preceding paragraph such that it comprises an ultraviolet coating (or treatment). This ultraviolet coating may be provided on the joining face of the second transparent piece. 
     However, it is preferred but optional that both the first and second transparent pieces are treated in any of the ways described in the preceding two paragraphs such that they both comprise ultraviolet coatings (or treatments). Hence, the colour-receiving face of the first transparent piece may comprise an ultraviolet coating that protects against ultraviolet degradation and the desired colour may be printed onto the colour-receiving face comprising the ultraviolet coating. Also, the joining face of the second transparent piece may comprise an ultraviolet coating that protects against ultraviolet degradation. This is advantageous as the printed colour may then be sandwiched between two ultraviolet coatings, such that it is protected from degradation from ultraviolet irradiation from both sides, and such that both coatings are located internally and so protected from damage. It has been found that providing protection against ultraviolet degradation in this way results in colour standards that may last tens of years without fading, which is long enough for most colorimeter applications. 
     Optionally, the step of determining print parameters required to achieve a desired colour for the standard comprises determining the printer settings of a digital printer to achieve the desired colour, and the step of printing the desired colour on the colour-receiving face of the first transparent piece comprises operating the digital printer according to the determined printer settings. As used herein, the term “digital printer” means a printer controlled using a digital file, e.g. a flatbed computer-controlled printer. 
     Determining the printer settings of a digital printer may comprise obtaining colour co-ordinates of the desired colour in a colour scale. For example, the colour co-ordinates may be expressed in RGB or another colour scale such as L*a*b. The colour co-ordinates may be obtained through measurement, for example using a spectrometer to measure a reference colour or by using a look-up table or similar that provides an illustration of a colour alongside the corresponding colour co-ordinates. Where the colour co-ordinates are not expressed as CMYK or other printer co-ordinates, the method may then comprise converting the colour co-ordinates into the equivalent CMYK or other printer colour co-ordinates. The method may then comprise using the CMYK or other printer colour co-ordinates to determine the relative amounts of cyan, magenta, yellow and black or other colour inks to use in the step of printing the desired colour on the colour-receiving face of the first transparent piece. For example, these relative amounts of cyan, magenta, yellow and black or other colour inks may be mixed in the digital printer prior to being dispensed through a print head of the digital printer. 
     Determining the printer settings of a digital printer may also comprise determining one or more, but not limited to, of the following printer settings required to achieve the desired colour: ink drop size; separation distance of the printer head from the colour-receiving face of the first slide; and number of passes of the printer head over the colour-receiving face of the first slide. These printer settings may be obtained from a look-up table that lists the recommended printer settings for each of a range of colours. The look-up table may be populated using empirical measurement (i.e. varying settings to achieve a desired colour and recording the settings that produce the best match to the desired colour). 
     Determining the printer settings of a digital printer may also comprise producing an output file for use by the digital printer that includes the determined printer settings, for example the relative amounts of cyan, magenta, yellow and black or other colour inks; the ink drop size; the separation distance of the printer head from the colour-receiving face of the first transparent piece; and the number of passes of the printer head over the colour-receiving face of the first transparent piece. 
     Optionally, printing the desired colour on the colour-receiving face of the first transparent piece comprises printing an ultraviolet curable ink, and the method further comprises using ultraviolet radiation to cure the printed ink. The ink may be cured after each pass of the printer head or the ink may be cured only after all passes of the printer head are complete. The ultraviolet radiation is best provided from the print receiving face side of the first transparent piece so that it does not get attenuated by the ultraviolet coating (or treatment). 
     Optionally, after the ink has been cured and/or has dried, the method may comprise using the digital printer to print a varnish or other protective coating to the colour-receiving face of the first transparent piece. 
     The step of permanently affixing the first transparent piece to a second transparent piece such that the colour-receiving face and the joining face are positioned at the interface between the first and second transparent pieces may comprise applying a liquid adhesive to the printed colour-receiving face of the first transparent piece, and then joining the colour-receiving face of the first transparent piece to the joining face of the second transparent piece. Using a liquid adhesive may be advantageous where the printing process leaves an uneven print layer on the colour-receiving face of the first transparent piece. For example, the liquid adhesive may be given enough time to flow over or settle over the printed layer so as to form a flat surface for offering up to the joining face of the second transparent piece. In addition or as an alternative, the method may comprise applying liquid adhesive to the joining face of the second transparent piece. Also, the method may comprise pressing the first and second transparent pieces together as the adhesive cures which will assist the adhesive in flowing into thinner sections of the printed layer. 
     Optionally, the adhesive is a clear adhesive, or at least is clear when cured. 
     The liquid adhesive may be an ultraviolet curable adhesive, and the method may further comprise using ultraviolet radiation to cure the adhesive. Preferably, the method comprises illuminating the adhesive with the ultraviolet radiation through the second transparent piece. This is advantageous as it reduces exposure of the printed colour to the ultraviolet radiation. 
     The first transparent piece and/or the second transparent piece may comprise an optical coating. Optionally, the colour-receiving face of the first transparent piece and/or the joining face of the second transparent piece comprise the optical coating such that the optical coating(s) end up in the middle of the colour standard, away from the external surfaces where they may otherwise be exposed to being damaged such as by scratching. 
     Optionally, the method further comprises determining the desired colour for the standard and determining the colour co-ordinates of the desired colour. For example, the method may comprise determining a reference colour scale to be used; determining which colour of the reference colour scale is to be reproduced in the colorimeter colour standard; and determining the colour co-ordinates of that colour as the colour co-ordinates of the desired colour. 
     The present invention also resides in a method of making multiple colorimeter colour standards, comprising the method of making a colorimeter colour standard as described in any of the preceding paragraphs and one or more further steps of determining print settings required to achieve a further desired colour, different to the other desired colour(s), and printing the further desired colour on a face of a transparent piece. This method may be implemented either (a) to produce a single joined part bearing multiple colour standards or (b) to produce multiple individual colour standards. 
     As an example of alternative (a), the method may comprise printing each further desired colour on an unprinted portion of the colour-receiving face of the first transparent piece, and then permanently affixing the printed first piece to the second transparent piece such that the colour-receiving face and the joining face are positioned at the interface between the first and second transparent pieces, thereby providing multiple colour standards on a single joined part. The printed colours may be evenly spaced apart and may have corresponding sizes. For example, the first and second transparent pieces may be circular or annular discs, and the multiple colours may be printed around the edge of the disc. When alternative (a) is followed, the present invention may then extend to a method of making a colorimeter comparator disc comprising this method of making a single joined part having multiple colour standards and mounting the single joined part in a holder. 
     As an example of alternative (b), the method may comprise printing each further desired colour on a colour-receiving face of a different transparent piece, and then permanently affixing each transparent piece after printing to another transparent piece (which has not been subjected to a printing step) such that the colour-receiving face and a joining face of the another transparent piece are positioned at the interface between the different transparent piece and the another transparent piece, thereby providing multiple colour standards. The individual colour standards may have the same size and shape. When alternative (b) is followed, the present invention may then extend to a method of making a colorimeter comparator disc comprising this method of making multiple colour standards and mounting the colour standards in a holder. The colour standards may be mounted to be evenly spaced. 
     Any of the colorimeter comparator discs described above may comprise colour standards having colours of a reference colour scale. 
     The present invention also resides in a colorimeter colour standard comprising a first transparent piece permanently affixed to a second transparent piece such that a colour-receiving face of the first transparent piece and a joining face of the second transparent piece are positioned at the interface between the first transparent piece and the second transparent piece, and wherein the colour-receiving face of the first transparent piece bears a desired colour. For example, a desired colour may have been printed on the colour-receiving face of the first transparent piece. 
     The first transparent piece may comprise or consist of glass. Alternatively, the first transparent piece may comprise or consist of a plastic such as acrylic. Preferably, the first transparent piece is clear, i.e. uncoloured or not tinted, prior to the step of printing. The first transparent piece may be a flat piece with flat, opposed faces and with the colour-receiving face corresponding to one of the flat, opposed faces. For example, the first transparent piece may be a flat circular disc. The desired colour may be printed on the colour-receiving face of the first transparent so as to cover all of the colour-receiving face. Alternatively, the desired colour may be printed on the colour-receiving face of the first transparent piece so as to cover only a portion of the colour-receiving face (such as to provide a spot of the desired colour on the colour-receiving face). 
     The second transparent piece may comprise or consist of glass. Alternatively, the second transparent piece may comprise or consist of a plastic such as acrylic. Preferably, the second transparent piece is clear, i.e. uncoloured or not tinted. The second transparent piece may be a flat piece with flat, opposed faces and with the joining face corresponding to one of the flat, opposed faces. For example, the second transparent piece may be a flat circular disc. The second transparent piece may correspond to an unprinted version of the first transparent piece. The first and second transparent pieces may match in size and shape such that the colour standard has the same length and width as each transparent piece, but around twice the thickness of each transparent piece. 
     The printed first transparent piece is permanently affixed to the second transparent piece to the extent that accidental separation of the first and second transparent pieces is prevented, for example such that the first transparent piece requires forcible separation from the second transparent piece rather than allowing the two pieces to be merely pulled apart by hand. 
     Optionally, at least one of the first and second transparent pieces comprises an ultraviolet coating that protects against ultraviolet degradation. The colour-receiving face of the first transparent piece may comprise the ultraviolet coating and the desired colour is printed onto the colour-receiving face comprising the ultraviolet coating. The ultraviolet coating may be absorbed or fused with the material (e.g. glass) of the underlying surface. 
     Optionally, the second transparent piece comprises an ultraviolet coating that protects against ultraviolet degradation. The joining face of the second transparent piece may comprise the ultraviolet coating. The ultraviolet coating may be absorbed or fused with the material (e.g. glass) of the underlying surface. 
     Optionally, the colour-receiving face of the first transparent piece comprises an ultraviolet coating that protects against ultraviolet degradation and the desired colour is printed on the colour-receiving face comprising the ultraviolet coating, and the joining face of the second transparent piece comprises an ultraviolet coating that protects against ultraviolet degradation. The ultraviolet coatings may be absorbed or fused with the material (e.g. glass) of the underlying surfaces. 
     Optionally, the first transparent piece is permanently affixed to the second transparent piece by an adhesive, optionally an ultraviolet-cured adhesive. 
     Optionally, the first transparent piece and/or the second transparent piece comprise an optical coating. The colour-receiving face of the first transparent piece and/or the joining face of the second transparent piece may comprise the optical coating. 
     Optionally, only a single colour is printed on the colour-receiving face of the first transparent piece. Then, the present invention may also extend to a colorimeter comparator disc comprising a holder; such a colorimeter colour standard mounted within the holder, and at least one further such colorimeter colour standard mounted within the holder. The colours of the colorimeter colour standards may form a reference colour scale. 
     Optionally, multiple desired colours are printed on the colour-receiving face of the first transparent piece thereby providing multiple colorimeter colour standards on a single joined part. Then, the present invention may also extend to a colorimeter comparator disc comprising a holder, and such a single joined part mounted within the holder. The colours of the colorimeter colour standards may form a reference colour scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention can be more readily understood, reference will now be made by way of example only, to the accompanying drawings in which: 
         FIG. 1  is an isometric view of a typical colorimeter; 
         FIG. 2  is a schematic representation of a method of making a colorimeter colour standard; 
         FIG. 3  is a simplified side view of a colorimeter colour standard; 
         FIG. 4  is a schematic representation of a more detailed method of making a colorimeter colour standard; 
         FIG. 5  is a schematic representation showing in greater detail the step of obtaining glass pieces from  FIG. 4 ; 
         FIG. 6  is a simplified side view of a glass piece; 
         FIG. 7  is a schematic representation showing in greater detail the step of determining a desired colour from  FIG. 4 ; 
         FIG. 8  is a schematic representation showing in greater detail the step of determining digital printer settings for the desired colour from  FIG. 4 ; 
         FIG. 9  is a schematic representation showing in greater detail the step of printing the desired colour using the determined digital printer settings on one glass piece from  FIG. 4 ; 
         FIG. 10  is a simplified side view of a glass piece with a printed layer; 
         FIG. 11  is schematic representation showing in greater detail the step of joining the glass pieces to form a colour standard from  FIG. 4 ; 
         FIG. 12  is a simplified side view of the two glass pieces joined to form a colour standard; 
         FIG. 13  is an isometric view of a colorimeter comparator disc comprising multiple colour standards like that of  FIG. 12 ; 
         FIG. 14  is a plan view of a colorimeter comparator disc comprising multiple colour standards provided on a single glass disc; 
         FIG. 15  is an isometric view of a glass piece used to form the glass disc of the comparator disc of  FIG. 14 ; 
         FIG. 16  is a plan view of the single glass disc showing the multiple colour standards provided thereon; 
         FIG. 17  is an isometric view of a half of the holder of the comparator disc of  FIG. 14 ; and 
         FIG. 18  is a side perspective exploded view of the comparator disc of  FIG. 14  that shows the glass disc and the two halves of the holder, with one half rotated about its vertical axis to show its inner face. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method  200  of method of making a colorimeter colour standard  300  is shown in  FIG. 2  and the resulting colorimeter colour standard  300  is shown in  FIG. 3 . The method  200  begins at step  230  where print parameters required to achieve a desired colour for the colour standard  300  are determined. The method continues at step  240  where the desired colour is printed on a colour-receiving face  322  of a first transparent piece  320  using the determined print parameters. Finally, at step  250 , the first transparent piece  320  is permanently affixed to a second transparent piece  340  such that the colour-receiving face  322  of the first transparent piece  320  and a joining face  342  of the second transparent piece  340  are positioned at the interface between the first transparent piece  320  and the second transparent piece  340 . 
     As shown in  FIG. 3 , the colour standard  300  comprises the first transparent piece  320  permanently affixed to the second transparent piece  340  with the colour-receiving face  322  and the joining face  342  facing each other at the interface of the two pieces  320 ,  340 , and wherein a desired colour has been printed on the colour-receiving face  322  of the first transparent piece  322 . The printed colour can be seen as the printed layer  360  in  FIG. 3 . In the exemplary embodiment of  FIG. 3 , the first transparent piece  320  is permanently affixed to the second transparent piece  340  using an adhesive that forms an adhesive layer  380 . 
     A further method  400  of method of making a colorimeter colour standard  1200  is shown in  FIG. 4 , and  FIG. 12  shows the finished colour standard  1200 . 
     The method  400  starts at  410  where two transparent pieces  600  like that shown in  FIG. 6  are obtained. In this embodiment, the transparent pieces comprise a first glass piece  620  and a second glass piece  640 . The first and second glass pieces  620 ,  640  are identical, within manufacturing tolerances. For example, the first and second glass pieces  620 ,  640  may be cut from the same sheet of glass so as to have the same size and dimensions. In this example, the first and second glass pieces  620 ,  640  are flat discs of clear, transparent float glass. 
     Next, at step  420 , the desired colour for the colorimeter colour standard  1200  is determined. Then, at step  430 , the digital printer settings required to print the desired colour on the first glass piece  620  are determined. With the printer settings determined at step  430 , the method may continue at step  440  by printing the desired colour on the print-receiving face  635  of the first glass piece  620  by operating the digital printer according to the determined printer settings. A print layer  660  is formed that covers the print-receiving face  635  of the first glass piece  620 . 
     Finally, the first and second glass pieces  620 ,  640  are joined at step  450 . This step  450  sees adhesive applied to the print layer  660  to form an adhesive layer  680 . The second glass piece  640  is then stacked on the first glass piece  620  with the print layer  660  and adhesive layer  680  sandwiched between the colour-receiving face  622  of the first glass piece  620  and a joining face  642  of the second glass piece  640 . The adhesive of the adhesive layer  680  is then allowed to cure such that the first glass piece  620  becomes permanently affixed to the second glass piece  640 , thereby forming the colorimeter colour standard  1200 . 
       FIG. 4  also shows further steps for making a colorimeter comparator wheel  1300 . As indicated by the loop  455 , the method may comprise repeated iterations of steps  420  to  450  such that series of colorimeter colour standards  1200  are produced. A different desired colour may be selected each time step  420  is performed. For example, the desired colours may correspond to the colours of a colour scale such as an industry standard colour scale (e.g. an ASTM colour scale, the EBC scale, the European Pharmacopoeia colour scale, the Gardner Colour scale, the Platinum-Cobalt/Hazen/APHA colour scale, the Saybolt colour scale, the DEHA colour scale or white and yellow indices). 
     When the series of colorimeter colour standards  1200  are complete, the method may continue to step  460  where the series of colorimeter colour standards  1200  are mounted in a holder  1310  to form a colorimeter comparator wheel  1300 , for example like the one shown in  FIG. 13 . 
       FIG. 5  shows in greater detail step  410  of  FIG. 4  that comprises obtaining the first and second glass pieces  620 ,  640 .  FIG. 5  starts at step  412  where a glass sheet is obtained. The first and second glass pieces  620 ,  640  are cut from this glass sheet at step  414 . For example, a glass sheet having the desired properties may be purchased or a glass sheet having the desired properties may be made. An example of a cross section of a glass piece  600  (either the first glass piece  620  or the second glass piece  640 ) according to one embodiment of the invention is shown in  FIG. 6 , which of course reflects the cross section of the glass sheet from which it is cut. 
     Each glass piece  600  comprises a core  610  of clear, transparent glass which, in this embodiment, is produced using the float glass method. Further coatings are provided for the glass piece  600 . Namely, an ultraviolet (UV) coating  625  is applied to one side of the glass core  610 . Then optical coatings  630  are applied to both sides of the glass piece  600 , namely applied to the side already treated with the UV coating  625  and also the reverse side of the glass core  610 . The UV coating  625  provides protection from UV degradation, i.e. acts as a filter that absorbs UV radiation to reduce the amount of UV radiation passing through the glass piece  600  that may then degrade any material behind or printed on the other side of the glass piece  600 . The optical coatings  630  enhance the optical properties of the glass piece  600 , for example by minimising the reflectance of the glass piece  600 . The coatings  625 ,  630  may be merely applied to the surfaces of the glass core  610  of the glass sheet (either before or after the glass pieces  600  are cut from the glass sheet) or may be further treated to combine with the glass material of the glass core  610 , for example by heating or baking the glass sheet or glass pieces  600  with one or more coatings  625 ,  630  applied. The boundaries between the glass core  610  and the coatings  625 ,  630  are shown as dotted lines in  FIG. 6  to indicate that there may not be a sharp boundary, for example because baking the glass sheet or glass pieces  600  may cause the coating material to diffuse into the glass core  610  or into an adjacent coating. An example of a suitable glass sheet is Museum Glass® produced by Tru Vue Inc.® of McCook, Ill. 60525, USA. 
     As noted above, step  414  comprises cutting the glass pieces  600  from the glass sheet, and the glass pieces  600  are cut to be the same size. The glass pieces  600  may be cut from the glass sheet using any known technique, such as by cutting with a hollow drill bit, water jet cutting or laser cutting. Persons skilled in the art will appreciate that step  414  may be omitted in some embodiments, for example where glass pieces  600  of the required size and quality may be purchased. 
       FIG. 5  finishes with step  416  where the glass pieces  600  are cleaned and/or polished. The finished glass pieces  600  are typically 0.5 mm to 5 mm thick, for example 2.5 mm or approximately 2.5 mm thick. As will be appreciated, the thicknesses of the glass core  610  and coatings  625 ,  630  are not shown to scale in  FIG. 6  as the coatings  625 ,  630  will be much thinner relative to the glass core  610 . 
       FIG. 7  shows in more detail step  420  of  FIG. 4  that comprises determining a desired colour for the colour standard  1200 . In this embodiment, step  422  first sees an industrial colour scale identified like those described above. For example, a colour scale may be identified according to the purpose of a colour comparison to be performed, such as to grade a beer. Then, at step  424 , a desired colour is identified from the colour scale. At step  426 , a definition of the desired colour is obtained. For example, step  426  may comprise obtaining colour coordinates of the desired colour. The colour coordinates may be obtained from a look-up table or similar. The colour co-ordinates may define the colour in a colour space, for example in this exemplary embodiment the colour coordinates are expressed in L*a*b colour space. Generally, colours will be defined using an additive colour space. 
     Step  420  need not require using an industry standard colour scale. For example, a colour scale may be derived from samples. Samples may be prepared with controlled variations of dilution or age or exposure to light (and other radiation). The colour of each sample may then be measured, for example using a spectrometer to obtain colour coordinates for that sample. 
       FIG. 8  shows in more detail step  430  of  FIG. 4  that comprises determining digital printer settings to print the desired colour. Many different types of digital printer may be used, for example a Mimaki® UJF-3042 MO or Mimaki® UJF-6042 Mkll LED UV digital printer available from Mimaki Europe B.V., Stammerdijk 7E,1112 AA Diemen, The Netherlands.  FIG. 8  starts at  432  where the L*a*b* colour coordinates of the desired colour produced at step  426  are translated into CMYK values. Other subtractive colour schemes may be used. Generally, step  432  will comprise a translation of the colour coordinates of the desired colour from an additive colour space to a subtractive colour space so as to be suitable for a printer. Software packages that perform such translations are readily available, for example Adobe&#39;s Photoshop®. The CMYK values set the relative amounts of cyan, magenta, yellow and black inks to be mixed by the digital printer prior to delivery through a print head of the printer. 
     At step  434 , the CMYK values produced at step  432  are checked and adjusted if necessary. In this embodiment, a threshold is applied to each CMYK value and, if the value is below the threshold, that value is reset to zero. This prevents the digital printer from attempting to print colours where colour control becomes difficult due to very low fractions of a particular ink colour being required. The threshold values may be set through trial and error for example, or may be defined by the manufacturer of the digital printer. Step  434  may be omitted. 
     Correct setting of the CMYK values will help in achieving the desired colour for the colour standard  1200 , although further digital printer settings may be required as will now be explained. 
     At step  436 , the ink drop size to be used is determined based on the CMYK values. This is the size of the ink drop delivered by the print head after the individual cyan, magenta, yellow and black inks have mixed. Variations in drop size will affect the smoothness and hence uniformity of the print layer  660  produced. 
     At step  437 , the number of printing passes the print head will make over the first glass piece  620  is determined based on the CMYK values. Variations in the number of passes made will affect the smoothness and hence uniformity of the print layer  660  produced. In addition, an offset may be determined. That is, digital printers often print as a series of spots, each spot corresponding to delivery of an ink drop at a particular location on the substrate (the first glass piece  620  in this embodiment). Printing in the same locations during each pass tends to exaggerate lack of uniformity as the centre point of each spot will be thickest and the common centre points become additive. However, creating an offset in the locations between passes will increase surface uniformity, for example by using an offset of half the separation between spots. Also, the number of passes will affect the density of the colour produced, with more passes producing more dense colours. 
     The number of passes determined at step  437  will be influenced by the ink drop size determined at step  436 . Namely, the larger the ink drop size, the fewer the number of passes required. Hence, steps  436  and  437  may be performed at the same time such that the ink drop size and number of passes are set together. 
     At step  438 , the separation of the printer head from the first glass piece  620  is determined. The digital printer tends to spray the ink drop from the printer head such that a cone of ink is produced. This means that the greater the separation of the printer head from the first glass piece  620 , the larger the spot size produced on the first glass piece  620 . 
     The separation determined at step  438  will be influenced by the ink drop size determined at step  436  and the number of passes determined at step  437 . For example, a greater separation will lead to a less dense colour suggesting a compensatory increase in the number of passes or in the size of the ink drop. 
     The optimum combination of ink drop size determined at step  436 , number of passes determined at step  437  and separation determined at step  438  for a given desired colour and its CMYK values can be found as a matter of trial and error. The results may be stored in a look-up table that relates the CMYK values to ink drop size, number of passes (and offset), and separation of the printer head from the first glass piece  620 . Then, a suitably programmed computer may be used to perform step  430 . For example, the computer may receive the definition of the desired colour in L*a*b coordinates and may translate this into CMYK values by performing step  432  using a look-up table. Having obtained the CMYK values for the desired colour, the computer may perform steps  436 ,  437  and  438  by identifying the CMYK values in another (or the same) look-up table and retrieving the stored ink drop size, number of passes (and offset), and separation against those CMYK values. The computer may then produce an output file specifying these digital printer settings at the end of step  430 . This output file may then be used to operate the digital printer, for example by passing the output file to a printer driver associated with the digital printer. 
       FIG. 9  shows in more detail step  440  of  FIG. 4  that comprises printing the desired colour using the determined digital print settings on the first glass piece  620 . At step  442 , the first glass piece  620  is loaded into the digital printer with the print-receiving face  635  facing the printer head. Then, at step  444 , the printer settings are made available to the printer such that the printer can print on the first glass piece  620 . Optionally, the printer may be used to apply a primer to the print-receiving face  635  before the ink is printed at step  446 . 
     At step  446 , the printer uses the printer settings to perform the specified number of passes over the first glass piece  620  and print the desired colour onto the print-receiving face  635  of the first glass piece  620  to cover the print-receiving face  635 . The print-receiving face  635  of the first glass piece  620  is the optical coating  630  adjacent the ultraviolet coating  625 .  FIG. 10  shows a cross sectional view of the first glass piece  620  to show the layers corresponding to the glass core  610 , the ultraviolet coating  625 , the optical coatings  630  and the print layer  660 . 
     The digital printer delivers an ultraviolet curable ink, such that step  448  sees the printer illuminate the print layer  660  with ultraviolet radiation to cure the ink. A suitable choice of ink is the LH100 range available from Mimaki Europe B.V., Stammerdijk 7E, 1112 AA Diemen, The Netherlands, which offer good scratch and chemical resistance. 
       FIG. 9  also shows a final, optional step  449  where the digital printer may be used to print a varnish or protective coating onto the print layer  660 . 
       FIG. 11  shows in more detail step  450  of  FIG. 4  that comprises joining the first and second glass pieces  620 ,  640  to form the colorimeter colour standard  1200 . Step  451  sees the second (unprinted) glass piece  640  taken and checked for cleanliness (and cleaned again, if necessary). Step  452  sees the first (printed) glass piece  620  taken and cleaned to ensure that the print layer  660  is clean and free from dust and other contaminants. Step  453  sees a liquid adhesive applied to the print layer  660 . The liquid adhesive will flow to form an adhesive layer  680  above the print layer  660 . Furthermore, as noted above, the print layer  660  may not be completely smooth due to the digital printer printing as a series of spots. The liquid adhesive will flow into the thinner sections of the printed layer  660  such that the upper surface of the adhesive layer is flat  680 . If needed, a scraper or similar may be used to spread the adhesive over the printed layer  660  and to ensure a smooth finish is achieved. 
     At step  454 , the second glass piece  640  is offered up to the first glass piece  620  such the joining face  637  is pushed against the adhesive layer  680 . The joining face  637  is the face on the side of the second glass piece  640  having the ultraviolet coating  625 . The resulting sequence of layers of the first and second glass pieces  620 ,  640  is shown in  FIG. 12 . 
     An ultraviolet curable adhesive is used, so that step  455  sees the joined glass pieces  620 ,  640  illuminated with ultraviolet radiation. The adhesive layer  680  is illuminated from the side having the second glass piece  640  such that the ultraviolet radiation does not need to pass through the print layer  660  to get to the adhesive layer  680 , thereby reducing exposure of the print layer  660  to the ultraviolet radiation. When the adhesive layer  680  is dried, the colorimeter colour standard  1200  having the desired colour is formed. The assembled colorimeter colour standard  1200  may be cleaned prior to being used in a colorimeter. 
     The colorimeter colour standard  1200  may be included in a colorimeter comparator disc  1300 . Such a comparator disc  1300  is shown in  FIG. 13  although, for the purposes of clarity, the comparator disc  1300  is shown without the colour standards  1200  installed. The colorimeter comparator disc  1300  may comprise a circular annular disc-like holder  1305  through which a relatively large central circular aperture  1310  extends. A series of relatively small circular apertures  1320  extend through the holder  1305 , and are equally spaced around the holder  1305  and positioned approximately midway between the edge of the central circular aperture  1310  and the outer edge of the holder  1305 . Each aperture  1310 ,  1320  comprises a flange  1330  at the base of the aperture  1310 ,  1320 . 
     The central aperture  1310  receives a correspondingly-sized clear circular glass disc (not shown) that sits on the flange  1330  and is held in place by a circular spring (not shown). The clear glass disc provides a viewing window through which the sample being tested may be viewed in the colorimeter. The clear glass disc may be made from float glass and may be provided with one or more optical coatings and one or more ultraviolet coatings. 
     The series of a relatively small circular apertures  1320  receives a series of colour standards  1200  like the one shown in  FIG. 12 , each of the colour standards  1200  having one of the colours of a colour scale. Each colour standard  1200  is the same size as the apertures  1320  such that each colour standard  1200  sits on a flange  1330  and is held in place by a circular spring (not shown). 
       FIGS. 14 to 18  show an alternative embodiment of a colorimeter comparator disc  1400 . The colorimeter comparator disc  1400  may comprise a circular disc-like holder  1405  comprising a halves  1406  which join together to sandwich a single circular glass disc  1407  therebetween, with the join of the halves  1406  extending around the periphery  1404  of the glass disc  1407 . A series of colour standards  1409  are equally spaced around the comparator disc  1400 . The colour standards  1409  are printed onto the single circular glass disc  1407  and are visible through circular apertures  1420  provided in each half  1406  of the holder  1405 . There is also a relatively large central circular aperture  1410  that opens onto a clear central portion  1411  of the single circular glass disc  1407  through which the sample being tested may be viewed in the colorimeter. 
     The glass disc  1407  bearing multiple colour standards  1409  is made broadly as described above with respect to the colour standards  1200 . Referring back to  FIG. 4 , step  410 , two glass pieces  1431  like those described previously are obtained, although the glass pieces  1431  will be of a larger size. The larger size of the glass pieces  1431  may alter the choice of how to cut the pieces  1431  from the glass sheet. For example, cutting with a hollow drill bit may be less suitable although water jet cutting and laser cutting are likely to remain as good options.  FIG. 15  shows one of a pair of glass pieces  1431  from which the single circular glass disc  1407  is formed. The pair of glass pieces  1431  each include three cut-outs  1410  that extend inwardly from the periphery of the disc  1407 . The purpose of these cut-outs  1410  is described below. 
     Determining a desired colour at step  420  may remain the same other than repeating step  420  via loop  455  will be performed in respect of a single first glass piece  1431 . Determining digital printer settings for each desired colour at step  430  may also remain the same. Print co-ordinates will be required to specify the location, size and shape of the area to be printed. Where individual colour standards  1200  are being printed, the first glass pieces  620  are likely to be mounted in the same position on the printer such that the areas to be printed will have the same location, size and shape for each printing step  440 . In contrast, for multiple colour standards  1409  on a single glass disc  1407 , each colour standard  1409  may have the same size and shape, but will have a different location. 
     The order of steps  420 ,  430  and  440  may be varied as the multiple colour standards  1409  are processed. For example, each of steps  420  and  430  may be completed for all colour standards before the next of steps  430  and  440  are started (i.e. all desired colours are determined at step  420  before any printer settings are determined). Alternatively, all of steps  420 ,  430  and  440  may be performed for a desired colour before steps  420 ,  430  and  440  are repeated for the next desired colour. Other combinations are possible. For example, steps  420  and  430  may be performed for a desired colour before being repeated for each subsequent desired colour and, when all desired colours have been processed in this way, step  440  may be performed to print each of the desired colour. 
       FIG. 16  shows a glass disc  1407  bearing the colour standards  1409 . A further set of circles  1430  are also shown in  FIG. 16 . These circles may contain printed information, for example identifying a colour of one of the standards  1409 . The three cut-outs  1410  can also be seen. 
     Once all colour standards  1409  have been printed, the two glass pieces  1431  may be joined at step  450  in the same way as was described before. Liquid adhesive may be applied to all of one side of a glass piece  1431  or may be applied to only part of a glass piece  1431 . For example, liquid adhesive may be applied to at least all of the printed colours to gain the advantage of the adhesive layer  680  filling thin sections in the print layer  660 . 
     Step  460  sees the glass disc  1407  bearing the colour standards  1409  assembled into the comparator disc  1400  by joining the two halves  1406  of the holder  1405  together with the glass disc  1407  held therebetween, as best seen in  FIG. 18 . 
       FIG. 17  shows one half  1406  of the holder  1405 . The other half  1406  has a complementary configuration.  FIG. 17  shows the circular apertures  1420  through which the colour standards  1409  are viewed, and also the central circular aperture  1410 . Each half  1406  is also provided with three snap fittings  1408  that co-operate to allow the two halves  1406  to be joined, as best seen in  FIG. 18 . The three cut-outs  1410  provided in the glass disc  1407  are sized and positioned to accommodate the snap fittings  1408 . The snap fittings  1408  and cut-outs  1410  are not equally spaced around the holder  1405  and disc  1407  such that the disc  1407  is held in the holder  1405  in a unique orientation. When joined, a peripheral rim  1419  of one half  1406  meet to define an outer edge of the comparator disc  1400 . The rim  1419  and snap fittings  1408  retain and align the glass disc  1407  within the comparator disc  1400 . 
     When assembled, the glass disc  1407  is held snugly between the two halves  1406 , and also retained in place by the snap fittings  1408  and the rim  1419 . The series of relatively small circular apertures  1420  provided in the halves  1406  align with the colour standards  1409  provided on the glass disc  1407 . A further set of apertures  1432  provided in the halves  1406  of the holder  1405  align with the circles  1430  shown in  FIG. 16  that may contain printed labels, as described above. 
     A person skilled in the art will appreciate that the above embodiments may be varied in many different respects without departing from the scope of the appended claims. 
     For example, a preferred order of layers is described above and illustrated in  FIG. 12 . This order is preferred primarily as it ensures the UV coating  625  and the ink layer  660  are protected within the colour standard  1200  and less susceptible to damage from scratching. However, other sequences of the layers are possible. For example, the UV coatings  625  may be placed on the outer sides of the colour standards  1200 . 
     In addition, more or fewer layers may be provided. For example, both the first glass piece  620  and the second glass piece  640  may be printed to provide a pair of print layers  660 . Then either or both the first and second glass pieces  620 ,  640  may receive the liquid adhesive. The UV and optical coatings  625 ,  630  are optional and may be omitted. Also, only a single optical coating per glass piece  600  may be provided, or two UV coatings  625  may be provided with one on each side of a glass piece  600 . 
     Although glass pieces  600  are described above, other forms of transparent pieces  320 ,  340  may be used. For example, clear plastic pieces such as acrylic pieces may be used. 
     Colour values other than CMYK may be used for the printing steps (e.g. steps  430  and  440 . For example, modified CMYK values might be used. The Minnaki® UJF-3042 Mkll printer mentioned above also prints with white ink, and the Minnaki® UJF-6042 MO printer mentioned above prints with cyan, light cyan, magenta, light magenta, yellow, black and white inks. Hence, other relative fractions of the ink to be used other than just CMYK may be required.