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Optics & Laser Technology 38 (2006) 399–404

www.elsevier.com/locate/optlastec

Practical colour management

Susan Williams

Global Colour Solutions, DigiEye plc, Leicester, UK

Available online 24 August 2005

Abstract

Spectrophotometers have been successfully used for colour measurement. This paper addresses digital imaging as a

complementary and alternative method of colour measurement and appearance and an effective communication tool as part of a

practical colour management programme within the supply chain of a textile retailer. The specific needs—to measure and

communicate textured dyed material and printed fabric—are discussed, as well as the colour specification and quality control (QC)

of currently un-measurable fabrics and accessories. A unique method of using digital imaging for the assessment of colour fastness

will also be discussed.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Colour measurement; Digital imaging; Colour management

1. The challenges

The colour of an object is one of the most importantentities when a consumer purchases an item. Colour isoften the hardest to manage as its appearance issubjective and, now that most manufacturing ofcoloured goods is carried out overseas, it becomesincreasingly harder to control. The colour of an objectvaries with the observer and the quality of light.Between 7 and 10 million colours can be seen but wehave very few names to describe them. The human beingalso has a poor colour memory so there has to be amechanism to help to communicate colour. Colourorder systems have been used extensively, where thespecifier and the supplier each have a book of coloursand each shade has a unique reference. Unfortunatelythe shades are often non-colour constant, that is theychange drastically in colour under different lights, andvary from book to book.

The practice of sending spectral data electronically tosuppliers is well established within the textile industry. Aseason’s palette created as solid shades on specificfabrics, such as cotton or polyester, works well and

e front matter r 2005 Elsevier Ltd. All rights reserved.

tlastec.2005.06.001

ess: s.williams@digieyeplc.com.

presents few problems provided that established prac-tices are followed. The real challenge is to set electronicstandards for yarns, lace, knitwear, carpets, marls andprints. It is just as important for suppliers to produceobjective Quality Control (QC) results on the finishedproduct so that the goods are accepted first time. Thesecond challenge is to measure accessories such asjewellery, buttons, bows, trims, bags and shoes. Thefinal challenge is to objectively grade colour fastnesswith respect to staining and change of shade.

2. Current solutions

2.1. Spectrophotometers

Retailers extensively use spectrophotometers to specifytheir standards and then send the spectral data togetherwith the required tolerances to their suppliers. They in turnmeasure the lab dips or production run and compute thecolour differences for all specified light sources, to anagreed tolerance, in order to control metamerism. (Note:metamerism occurs when two samples match in one set ofconditions but no longer match under another. Illuminantmetamerism is the most common form of metamerism.)

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The CMC (Colour Measurement Committee of Society ofDyers and Colorists) equation is most commonly used,where the total colour difference (DE) of 1.0 unit is used asa commercially acceptable tolerance. The savings made areconsiderable in terms of time and shipping costs. However,highly textured or printed samples and accessoriespreviously mentioned cannot be reliably measured to therequired tolerances using these instruments.

2.2. Accurate imaging and colour displays

Colour display and communication systems are usedsuccessfully to complement the spectrophotometersystems using input devices such as optical flat bedscanners and digital cameras. Images can be displayedon a calibrated monitor and coloured either usingreflectance data from a spectrophotometer or byinputting colour values. The advantage of such systemsis that it allows the specifier and the supplier to see thesame appearance of the product as well as having thenumeric information, thereby allowing quick andeffective decisions to be made.

There are two types of imaging inputs: scanners anddigital cameras. Scanners can take images of flat andsmooth samples; however, they cannot capture the trueappearance of more textured surfaces such as knitwear,lace, towelling and carpets or 3D objects such as shoes,zips and buttons.

The use of digital cameras is an obvious solution as animage is produced without contact being made with theobject. However, current imaging systems do notprovide standard and even illumination meeting Com-mission Internationale de l’eclairage (CIE) specifica-tions, or use characterised digital cameras.

3. Spectrophotometers

Colour measuring instruments, such as spectrophot-ometers, are currently being used to produce a‘fingerprint’ of the colour by the specifier or retailer.(Fig. 1 shows a red fingerprint or reflectance curve of aFerrari.) This ‘fingerprint’ is produced in the form of

Fig. 1.

spectral reflectance data, a list of numbers, which can bee-mailed or transferred via web-based data managementsystems to approved suppliers. The suppliers can thenmeasure their submissions to an agreed tolerance priorto shipment using standard colour difference equations.

The process of communication has to be managedand the information defined and controlled. There is avery true statement of ‘rubbish in rubbish out’.

To ensure the necessary confidence in the measure-ment results it is imperative that the standards andsubmissions are measured in exactly the same way. Mostof these techniques must also be applied to any visualassessments. For example it is important that thestandard and submissions are conditioned so as to settemperature and humidity prior to measurement. Thesamples have to be measured, as they would be viewed,with the right thickness, background and size. Therepeatability of the instrumentation, preparation andpresentation of samples overall has to be close butpractically no greater than 0.3 DECMC (total colourdifference) units. Even so this tolerance, which is likelyto be one third of the actual acceptable tolerance, can bea challenge from both a dyeing and a presentationaspect. If digital standards are issued, the instrumentshave to agree within 0.1 DECMC. In practice, instru-ments are tested in different locations to within 0.3DECMC units. With the advent of profiling software, thisagreement can be brought into factory specification.

Once the instrumentation and dyeing systems arecontrolled, there is still a presentation requirementwhere small or heavily textured samples are used, asthese are often difficult or impossible to measure to thetolerance required.

Textile samples have a textured surface to a greater orless degree. Textures vary from smooth surfaces, such asa woven poly/cotton fabric, through to highly texturedsamples of fleece, velour, carpet or towelling. Corduroy,yarn or thread have very directional textured surface,which need to be mounted correctly in order to bemeasured in both a realistic and a repeatable way. Lacenot only has a texture but also may have a gloss and hasthe added challenge that the coloured areas are verysmall and difficult to measure.

Not only is it difficult to measure such samplesreliably, the total appearance of the products cannot becommunicated with current measurement techniques.Hence accurate colour display systems were developed.

4. Digital imaging and the DigiEye system

Digital imaging provides rapid electronic communica-tion of standards between manufacturers and customers,a permanent, durable record of appearance and theability to make hard copies without deterioration.Images can also be manipulated digitally to assess the

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effect of variations in product colour without the needfor costly manufacture of actual examples spanning thefull range of quality. Several methods of characterisingcameras have been discussed and a method wasincorporated in an instrument known as DigiEye. Theapplication of this system for textile specification andquality control is now described [1,2].

Fig. 3.

Fig. 4.

4.1. System

Fig. 2 shows the digital imaging system. The systemcomprises a characterised digital camera, which takes animage of the product within a controlled lightingenvironment. Software controls the camera and displaysthe image on a calibrated monitor. The illuminationcabinet has different modes of illumination, whichallows the user to obtain true, representative images.The cabinet has consistent, even illumination equivalentto the international standard for daylight defined at acolour temperature of 6500K (D65 defined by CIE).The illumination can be varied from diffuse to variousangled illumination, from 451 down to 51.

The camera is characterised using a reference chart of240 colours (ColorChecker DC, GretagMacbeth, NewWindsor, NY (Fig. 3)) measured on a referenceinstrument and then converted to an internationalcolour scale (CIE XYZ values for D65). The image ofthe ColorChecker chart is captured, as shown in Fig. 2,in the controlled lighting environment and then thereference colour XYZ values are mapped to the RGB ofthe camera [3]. The agreement of the camera to theinstrument is normally less than median 1.0 DECMC

units.The CRT monitor is characterised with a colorimeter

in a darkened room, as any light from the environmentwill affect the colour displayed on the screen. The whitepoint of the monitor is set to illuminant D65 and a series

Fig. 2.

of colours with known colour values are displayed andmeasured. There are several methods available forcarrying out this characterization [4]. Fig. 4 shows amonitor with the colorimeter attached to the screen withone of several colours displayed.

The typical accuracy is an average of less than 0.5CIElab units.

It is also possible to profile printers so that accuratehard copies of the images can be printed. To generatethe profile, a range of colours are printed and measuredto build a lookup table or other analytical model.Separate profiles must be generated for each combina-tion of printer, paper and ink. In Fig. 5 a typicalprofiling device is shown, in this case a GretagMacbethSpectrolino spectrophotometer and a Spectroscan. Theprofile can then be used in a number of differentpackages such as Adobe Photoshop.

By using these methods, a system has been developedwhich enables products to be seen accurately on screen

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Fig. 5.

Fig. 6.

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and true colour values to be obtained. Profiled printerscan print the images that low cost printouts can beproduced on demand.

The accuracy of the printer profile is normally quotedas less than an average of 2 CIElab units but most ofthe variation is in high chroma colours where CIElabis deficient. Visually the samples are close to theoutputs.

The colours displayed on the output devices arelimited to colours within the visible spectrum, sofluorescent colours are not possible. Around 80–85%of the colours used commercially can be displayed dueto the gamut or the shades which can be produced eitherfrom the RGB of the monitor or the CMYK of theprinter.

Fig. 7.

5. Features of a commercial digital imaging system

5.1. Setting the standard

It is important to obtain images that are representa-tive of the true appearance of the product and so theillumination is changed from diffuse to angled usingappropriately set mirrors. The angled illuminationallows even the finest surface texture to be seen (e.g.Pilling on textiles). For example diffuse illumination isused for knitted fabrics whilst 451 illumination is bestfor buttons and 201 for satin polyesters. In Fig. 6 atypical knit has been imaged and the true texture can beseen.

Captured images are stored in standard file formats:Windows Bitmap, JPEG or TIFF allowing retrievedimages to be exported to other imaging packages such asAdobe Photoshop, or sent electronically to otherDigiEye users where the colour values can be auto-matically read and displayed.

5.2. Colour measurement

The digital imaging system measures down to a singlepixel giving much more representative colour data for asingle yarn, lace, marls or a small area of a print than atraditional spectrophotometer can achieve. Fig. 7 shows

an image of a sample of lace. The system can measurethe pink area of the petal in the flower.

5.3. Texture profiling

Texture profiles may be created from any image. Theprofiled grey image can be re-coloured with any colourfrom the palette range. The buyer can then evaluate therange of colours on exactly the same textured image,thereby removing any appearance variation that mayprejudice the colour evaluation.

In Fig. 8 there are three different textures shown. Theimages are taken and a grey scale produced whilstkeeping the texture, i.e. a texture map. Once the texturemaps are created they can be re-coloured with anypalette shade.

5.4. Re-colouring

As the system measures down to a single pixel, it ispossible to select small areas of print and re-colour. It is

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Fig. 8.

Fig. 9.

Fig. 10.

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therefore possible to select just the petals of a flower in aprint, as shown in Fig. 9, and change it from a whiteshade to a blue shade using standard colours stored in alibrary. The library can be created from measured databy either a spectrophotometer, the digital imagingsystem or by manual input.

5.5. Approvals

Once the standard is created, identical side-by-sideimages are made, as shown in Fig. 10. One of the copiescan be re-coloured with batch measurements, obtainedfrom a spectrophotometer or the imaging system. This ispresented as an identical image next to the standardallowing on screen colour approval. In addition, thecomparative numeric values are shown using standardcolour difference equations. These pass/fail images canbe sent electronically to the retailer, who will merelyopen the image, to present both visual and numericalinformation of the submission for approval, thereforemaking what would otherwise be a time consuming andcostly process very efficient.

5.6. Colour fastness

Colour fastness testing for staining and change ofshade is critical to obtaining the right quality of goods.Although instruments have been specified, this testing islargely carried out by visual means. This is due to a lackof confidence in the results obtained for change of shadeand because the process is not quick enough. The digitalimaging system together with modified equations gives abetter correlation to visual data than instrumentalmethods currently specified in standard ISO methods.The system also gives more consistent and reliableresults than visual graders. The digital imaging systemhas been developed to grade multiple tests, thereforespeeding up the grading process by a factor of nearlythirty times. Papers referred in Refs. [5–8] show thedevelopment of the measuring methods, the equationand the inter-laboratory trials.

Most importantly, many systems have shown severelimitations due to the test specimen characteristics—highly textured or embroidered material, narrow stripes,small printed patterns and accessories such as buttons

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Fig. 11.

S. Williams / Optics & Laser Technology 38 (2006) 399–404404

cannot be presented uniformly, and therefore visual orinstrumental assessment is impossible. To a non-contactmeasuring system, however, this task is straightforward.Fig. 11 shows the staining of a button on a multi-fibrestrip. The user can select the worst stained area byclicking in the area and the system will average thenearest, neighbouring pixels to this colour.

6. Conclusions

Quick fashion changes are required in-store andglobal supply chains have forced the market to use adigital means of communicating colour and appearance.With different textured fabrics, prints and accessories itis imperative to use digital imaging as a complementaryproduct to existing spectrophotometric techniques. Thispaper discusses how a digital imaging system can beincorporated into a practical colour managementprocedure within a global supply chain.

References

[1] Li C, Luo MR. Owner of British Patent (Application No.

0123810.4), entitled Method of predicting reflectance functions,

DigiEye Plc, 4 October 2001.

[2] Luo MR, Cui GH, Li C. Owner of British Patent (Application No.

0124683.4), entitled Apparatus and method for measuring colour

(DigiEye System), DigiEye Plc, 4 October 2001.

[3] Johnson A. Methods of characterizing colour scanners and digital

cameras. Displays 1996;16(4).

[4] Berns RS. Methods of characterizing CRT displays. Displays

1996;16(4).

[5] Cui GH, Luo MR, Rhodes PA, Rigg B, Dakin J. Grading textile

fastness part 1. Colouration Technol, May 2003;212(5):218.

[6] Cui GH, Luo MR, Rhodes PA, Rigg B, Dakin J. Grading textile

fastness part 2. Colouration Technol, May 2003;219(5):224.

[7] Cui GH, Luo MR, Rigg B, Butterworth M, Dakin J. Grading

textile fastness part 3. Colouration Technol 2004;120(5):226.

[8] Cui GH, LuoMR, Rigg B, Butterworth M, Maplesden N, Dakin J.

Grading textile fastness part 4. Colouration Technol 2003;

120(5):226.