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Rapportserie FSCN - ISSN 1650-5387 2007:51FSCN-rapport R-07-79

Örnsköldsvik 2007

Evaluation of Colour Reproduction of Inkjet Prints

FSCNFibre Science and Communication Network

- ett skogsindustriellt forskningsprogram vid Mittuniversitetet

Nils Pauler

EVALUATION OF COLOUR REPRODUCTION OF INKJET PRINTS Nils Pauler DPC‐Digital Printing Centre, Department of Information Technology and Media, Mid Sweden University, SE‐851 70 Sundsvall, Sweden ISSN 1650‐5387 2007:51; FSCN rapport R‐07‐79:, Mid Sweden Universtity

SUMMARY New evaluation method makes it easier to understand colours of images of inkjet prints. RGB defined colour patches defining the shell and inner parts of colour gamut were printed, measured and compared to correspondent standardised calculated colours. The colours for light, mid and dark tones were evaluated. The method can be used independently of printer settings and if any colour correction is active or not. It is thus possible to analyse how colour adjustments are performed. To test the method, colours and colour corrections of new HP, Canon and Epson inkjet printers were evaluated. An adjustment of dark tones towards higher lightness was essential to get reasonable colours of inkjet printed on plain paper for the Canon and Epson printers. The HP printer gave good print quality already without any colour correction of plain paper printing. An adjustment to higher lightness and correction of colour hue was however critical for HP photo printing. The method gave thus insight in ICC profiles and printer driver profiles for the tested printers. A preliminary PLS analyse showed how the perceived print quality of images could be expressed in terms of measured colour variables. The result is promising since it shows that it is possible to get a grip of the very divergent colour reproduction of desk top printers by this kind of evaluation. This report is based on knowledge created at DPC. Financial support from DPC made it possible to complete the work.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 2(31) 1. BACKGROUND The high diversity of colour reproduction of ink jet prints is a challenge for paper producers who want to improve the performance of their products. There is a demand for methods that can evaluate colour reproduction independently of printer, printer settings and colour adjustment. It is likely that such methods must include colours distributed in the whole colour gamut. It would also be a help if colour reproduction is evaluated in relation to an international standard. The goal is to have a method that could be used together with traditional print tests so that print quality can be evaluated in a more complete way than is possible today (1,2). A former investigation has shown how colours of the shell and the inner parts of printer gamut can be investigated. RGB based tests charts were printed and the L*a*b* values where measured. Correspondent values were also calculated as L*a*b* values using the coding of sRGB (3). Transferring RGB values to L*a*b* is a key stone within colour management and the sRGB coding is described in (4). In this report the colours in form of L*a*b* values calculated with the coding of sRGB are called sRGB colours. The idea is to use RGB test charts that include colours from centre to the shell of colour gamut. The test chart will comprise light, mid and dark tones ranging from mid of the grey axes to highest possible colour strengths in the directions of cyan, magenta, yellow, red, green, blue, black and white. By comparing measured and calculated values (sRGB colours) it will be possible to evaluate each printed colour patches. The goal is to be able to forecast colour strength, colour lightness and colour hue of printed images.

2. METHODS

2.1. The test charts RGB-shell and RGB-star Two RGB based test chart were used; the RGB‐shell and the RGB –star (3). The RGB‐shell comprises 950 colour patches evenly distributed so that it defines the shell of colour gamut see Appendix 1. The RGB‐star has 200 RGB based colour patches distributed as a star in the colour gamut. It starts from the centre of gamut and ends via 10 equally distributed steps at the shell in the direction of blue, green, red yellow magenta and cyan. This is repeated for light, medium and dark colours. The printed patches of RGB‐star are shown in Figure 1.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 3(31)

. Figure 1. The printed RGB -star test chart. The printed patches were evaluated by measuring spectra values with a Spectrolino and calculate L* a* b* D50/2º. The L* a* b* D50/2º of the two RGB test charts (sRGB colours) were calculated using algorithm of Opt Prop (5). In Appendix 2 printed and measured colours and sRGB colours of RGB‐shell and RGB‐star are shown. RGB‐star is visualised inside respectively colour gamut.

2.2. Four types of graphs used in the investigation. It is challenge to visualize printed colours and sRGB colours of the RGB star in the same 3‐D plot. A 3‐D plot requires that you can see it in different viewing angle, otherwise it can be confusing. Four types of 2 D plots will be used in this report. The plots show different properties as the colour strength increases for the tone steps (from mid of grey axis to the shell of the printer gamut) for the light, mid and dark tones. Measured values are compared to sRGB values in the plots. i) plots of L* vs c* to show how lightness develops as the colour strength increases. ii) plots of a* vs b* to show colour hue and colour area for the tones steps. iii) plots of colour lightness versus colour area. The lightness and colour area are calculated as the colour strength increases for the ten tone steps. iv) plots of colour area versus tone steps. This plot show how the gradation of colour area develops for the ten tone steps. See Appendix 3 were the different types of plots are shown.

Light tones

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Grey tones

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 4(31) 2.3. The calculations colour area and colour differences between sRGB colours and the printed colours Greenʹs formula was used for the colour area calculation:

( )( ) ( )( )∑

=

++++ −+−−+=

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1

1111

221i

iiiiiiii aabbbbaaA , ( 1 )

where ai and bi are the a* and b* values respectively, for the colours yellow, red, magenta, blue, cyan and green. The differences between measured L*a*b* values and calculated L*a*b* (sRGB colours) were calculated as (eq 2‐9):

sRGBerpr LLDL −= int (2)

sRGBerpr aaDa −= int (3)

sRGBerpr bbDb −= int (4)

erprerprerpr baC intintint −= (5)

sRGBsRGBsRGB baC −= (6)

sRGBerpr CCDC −= int (7)

222 )()()( DbDaDLDE ++= (8) 222 )()()( DCDLDEDH −−= (9)

2.4. The three ink jet printers and the settings used in the investigation Three new ink jet printers, two setting and two colour corrections were included in the investigation, see Table 1. Note that the HP printer was colour corrected with an ICC profile (relative intend) created at TC Övik. Separate ICC profiles were created for plain paper and photo papers. The Canon and Epson printer were colour corrected by respectively printer driver by choosing the default setting you find when printing from Photoshop 7. Table 1. Inkjet printers and settings used in the investigation. Setting 1 Setting 2 Colour

correction1 Colour correction2

HP 3310 Plain paper Photo paper No ICC profiles Canon IP 5300 Plain paper Photo paper No Printer drivers Epson stylus DX 550

Plain paper Photo paper No Printer drivers

All printing was performed in Photoshop 7, with the settings shown in Table 1. An image of fruits and flowers were printed with the twelve different printing conditions (two substrates, three printers, two colour adjustment)). These prints were evaluated in a simple perception study were print quality were range from 1 to 7. The perceptions were performed by two person and the scores were established as individual judge and thereafter discussed to get a common result. (No big difference was noted by the two judgers)

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 5(31) 2.5 The definition of colours based on RGB Ink jet and laser desktop units normally use RGB as input data. These RGB values are transferred to the process colours CMY in the printer driver. If no colour adjustment is active a very simple transformation can be executed to perform a CMY print, see Table 2. Table 2. The simple transformation of RGB values to CMY for a printer. RGB test chart Printer Colours R G B C M Y Red 1 0 0 0 1 1 Green 0 1 0 1 0 1 Blue 0 0 1 1 1 0 Yellow 1 1 0 0 0 1 Magenta 1 0 1 0 1 0 Cyan 0 1 1 1 0 0

Digital cameras, scanners and computer screen use RGB as primaries. This explains why pictures often are RGB based. In this report the colours denoted Red, Green, Blue, Cyan, Magenta and Yellow are based on RGB, and defined as in RGB test chart of Table 2. Colour corrections normally mean that cyan, magenta and yellow are printed with the also other process inks involved. A way to obtain “clean process inks” can be to deactivate the colour corrections of Photoshop and of the printer driver. Microscope is needed to verify that no other process inks are added in the patches of cyan, magenta and yellow.

3. RESULTS For sake of simplicity a range of plots are shown in Appendices 4‐7. To understand the high degree of diversity of colour reproduction of the three studied printers please look in the Appendices. Appendix 8 comprises a multivariate analyse of measured data related to perception data of printed images.

3.1. Perception of images The result of perception is shown in Table 3. HP showed relative good print quality for plain paper with a slightly better impression without than with colour correction. The plain paper prints of Canon and Epson were very grey and dull, but the impression improved with colour correction. Table 3. Perception of images of plain paper prints and photo prints. .Range 1-7 were 7 is the best. ICC =

Colour correction with ICC profile. Printer HP Canon Epson Corrections No ICC No Printer

Driver No Printer

Driver Plain paper prints

5 4 1 3 2 3

Photo prints

2 6 5 7 3 6

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 6(31) The photo prints were perceived better than plain paper with one exception; colour failure due to not activated colour correction resulted in very bad impression for the prints of HP.

3.2 Colour gamut for plain paper prints There is a difference in shape of colour gamut between the three printers from HP, Canon and Epson see Figure 2. Epson and especial Canon have a distinct down peak not seen for the HP printer. The graphs marked with grid in Figure 4 are colour corrected with ICC profile for HP and with the printer driver for Canon and Epson. It is clear from the figures that colour corrections reduce colour gamut. The typical aggregation between the upper half sphere and the lower half sphere of the gamut for colour corrected prints is also seen (2). (The upper half sphere is composed of as most two process inks, se Appendix 1. In the lower half sphere all three process inks and sometimes also key black are involved). High content of composite and maybe key black can explain the peaks of Canon and Epson.

a) HP printer b) Canon printer

c) Epson printer d) sRGB and HP printer plain paper

Figure 2 a-c. Colour gamut of prints without (no grid) and with colour corrections (grid). Figur 2 d show colour gamut of sRGB, and prints of HP inkjet .

3.3. Colour strength and lightness for plain paper prints The three printers are compared to each other for lightness and colour strength (in form of colour area) in Figure 3. The evaluation comprises light, mid and dark tones from mid of the grey axes to max colour fullness. The unfilled markers represents not colour corrected prints, the filled are printed with colour corrections active. The black mark is sRGB colour values.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 7(31) It is obvious that the colour correction reduced the lightness for HP printer, whereas lightness was increased for Epson and Canon printer. Observe the very low colour strength for the dark tones of not colour corrected Canon and Epson prints. a) HP printer b) Canon printer

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Figure 3. Lightness vs. colour area .Measured values of plain paper prints and sRGB colours based on RGB- star test strip.

These observations agree with the impression of printed pictures. All pictures were dull with low colour fullness as is typical for plain paper print. The prints of Epson and Canon were very dark and colourless without colour correction but the images improved when the colour corrections were activated. The HP print with no colour correction was fairly good and the colour correction did not improve the impression of these images, see also the perception values in tabel1. See Appendix 4 for plots of L* vs. c* of the individual colours. The Appendix 4 comprises values of not colour corrected (Appendix 4(1)) and colour corrected (Appendix 4(2)) prints. More details in how colour adjustment influences colour strengths can be clarified in a*,b* plots were colours of the printed colour patches and sRGB colours are compared, see, Figure 4 a‐e on next page. Following observation can be made from these figures which show the measurements of dark tones.

• The ICC correction of the HP print meant a reduction of colour strength. The hues of all colours were adjusted along and close to sRGB colours. The mid point was biased towards blue, maybe an effect of FWA.

Light sRGBDark sRGBMid sRGBLight NoCor.Dark NoCor.Mid NoCorLight Col.Cor.Mid Col.Cor.Dark Col.Cor.


• The very low colour strength of the Canon printer was improved to high degree by the colour correction. The increased strength for the cyan and the blue is remarkable since colour correction often reduce cyan and blue due to low colour strength of sRGB for these colours. The hues were adjusted towards but not close to sRGB.

• The Epson printer performed as the Canon printer, but the colour strength increase was not as

big as it was for the Canon printer. Figure 4 shows the results for the dark tones. In Appendix 5 the a* b* plots of all three tones series light, mid an dark are presented.

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Figure 4. a * vs. b* for dark tone prints of not colour corrected and colour corrected prints compared to sRGB

colour values (not filled market). Printing on plain paper with a HP ,Canon and Epson printer without and with colour corrections.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 9(31) 3.4. Colour gamut for photo prints The colour gamut of photo prints showed the same type of shape for all three printers, Figure 5 a‐c. The gamut size of Epson was however significant smaller compared to HP and Canon. The graphs with grid are colour adjusted prints. Note how ICC profile compresses the gamut of the printer to be totally inside the gamut of sRGB, Figure 5 d and e.

a) HP printer b) Canon printer

c) Epson printer d) sRGB and HP printer with no correction

e) sRGB and HP printer with ICC correction

Figure 5 a-c.Colour gamut of photo prints without (no grid) and with colour adjustment (grid). d-e. Colour gamut of sRGB, and photo print of HP without and with ICC correction.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 10(31) 3.5. Colour strength and lightness for photo print The colour strength of photo printers is as expected much higher compared to plain paper prints, see Figure 6 and Figure 3. The corrections had an overall increased lightness effect for all printers. The ICC correction made lightness close to the target of sRGB colours for the HP. a) HP printer b) Canon printer

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Figure 6. Lightness vs. colour area. Measured values of photo prints and calculated sRGB values. Lightness vs. colour strength for all tested colours of photo prints, see Appendix 6 (1‐2). The a* vs. b* plots of mid tones are shown in Figure 7. The ICC profile of the HP printer generated almost equal hue as the sRGB colours. For Canon and Epson the colour correction decreased the difference to sRGB colours but a good match was not obtained. The typical bending behaviour of the a* vs. b* relation for cyan was removed. The plots of the three tone series are in Appendix 7.

Light sRGBDark sRGBMid sRGBLight NoCor.Dark NoCor.Mid NoCorLight Col.Cor.Mid Col.Cor.Dark Col.Cor.


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Figure 7. a * vs. b* for mid tones prints of not colour corrected and colour corrected prints compared to sRGB

values(not filled market). Printing on photo paper.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 12(31) 3.6. Colour gradation for plain paper The response of colour area for the ten tone step from centre of the grey axis to full colour strength of light, mid and dark tones are shown in Figure 9 a‐c. It is obvious that colour correction has positive effect on colour and gradation at low colour strength of the Canon and the Epson printer, but the gradation declines for higher colour strength. For comparison the colour values of sRGB are shown in Figure 9 d. a) HP, plain paper b) Canon, plain paper

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Figure 9. Colour area vs. tone steps from the centre of grey axis to full colour strength of light, mid and dark tones. Prints on plain paper in Figure 12 a-c and sRGB in Figure 12 d.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 13(31) 3.7 Colour gradation for photo paper The gradation of colour area of photo prints declined as the colour correction was introduced. This phenomenon was most pronounced for the HP printer which used ICC profile as colour correction, Figure 10.

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Figure 10. Colour area vs. tone steps from the centre of grey axis to full colour strength of light, grey and dark tone for photo prints. Prints on photo paper in Figure 10 a-c and sRGB in Figure 10 d.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 14(31) 3.8 The gradation of black The gradation of black is shown as density values of the grey axis, in Figure 11. The colour correction has only minor influence on the grey scale. HP and Canon have higher density of photo prints then plain paper prints for the darker tones. The Epson printer showed only small differences between plain paper prints and photo prints. The lower print density of the HP plain paper print is obvious in Figure 11 d which shows print density of full tone for colour corrected prints.

a) HP, Plain paper and photo prints b) Canon, Plain paper and photo prints

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Figure 11.a-c. Print density vs. tone steps for plain paper and photo paper. d. Print density of colour corrected black full tone , black bar photo prints, grey bar plain paper prints.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 15(31) 3.9 The Grey Balance for Plain Paper and Photo Paper The grey balance is evaluated by measuring a*,b* vs. L* for the grey tone patches of the RGB‐ star strip, see Figure 12, which show the result of plain paper and photo paper prints of HP, Canon and Epson printers without and with colour corrections. Clear improvements by colour corrections were only observed for photo prints of HP and Epson. Hp Canon Epson

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Figure 12.Grey balance as a*(red), b*(blue) vs. L* , PP=plain paper, PH=Photo

3.10. The relation measured values to perception of Images The colour reproduction data were summarized by calculating mean values of the light, mid and dark tones of the RGB –star. Two data series based on colour differences respectively colour area were created. Extensive statistic analysis’s with PLS models have been performed by letting perception values be the Y variable and measured values the x variables. It was possible to express perception as a linear combination of the measured values see. Both data series added to one series or used separately worked fairly good, see Appendix 8.

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 16(31) One example of multivariate analyse is given in Figure 13. The PLS model gave three significant components and 76% of the perception data was explained by the model and the prediction capacity Q2 cum was 0,73.

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Figure 13 PLS modle. Measured perceived quality vs. calculated print quality ( a linear combination of

measured values). In the tested models lower difference between prints and sRGB colours, higher colour strength of mid and dark tones, and higher lightness of the light tones favoured the calculated values. The result confirms that the perceived values of the images were related to the measured colour variables.

4. CONCLUSION

Following conclusions can be made:

• Colour adjustment is crucial for desktop printing. • The strategy to separately evaluate light, mid and dark tones was successful. • The comparison between the colours of the printer and sRGB colours was a help when

evaluating the result. • It is important to analyse and understand how the shape of colour gamut influence the

colours • Colours of the dark tones should be more closely evaluated • The own created ICC profiles showed lower colour differences (sRGB colours to the colours of

the printer) than colour adjustment performed by the printer driver. • The colours of standard gamut (sRGB in this case) are not always the best choice. The images

of one of the printers (HP on plain paper) with no colour correction activated were perceived better than with ICC profile active. The measured higher general lightness for this printer setting can be the reason for this effect.

• The calculated summary values were enough to explain the perceived print quality, which indicate that it is possible to find key numbers of colour reproduction

5. DISCUSSION The proposed method means that two test charts the RGB –shell and RGB‐star are printed and evaluated by measuring with a Spectrolino. All the quantities and graphs shown in this report can be generated in Matlab and Excell macro, making it fairly easy to perform these analyses. The separate evaluation of light, mid and dark tones was successful. It clarified for example how an increase of lightness of the dark colours of Epson and Canon was essential to gain better colour

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 17(31) strength. Determination of the differences between printed colours and sRGB was also a good way to evaluate the result. The constrained colours of cyan and blue of sRGB can however (as was the case in this investigation) disturb the evaluation. It might be better to use a standard gamut like Adobe RGB that for all colours are larger than the gamut of the printer. The printed images used for perception of print quality must then of course also be in Adobe RGB mode The colour gamut for not colour corrected prints is the base for colour adjustments. A sound strategy for paper producer is therefore to increase colour gamut as much as possible. This investigation has put focused to the shape of gamut especial in the lower part of the gamut. There is a need to understand how colour strength of dark tones can be increased, Can it be influenced by the substrate or is it only controlled by the printer settings and colour adjustments? Another question is how phenomena like bleeding (or dot gain) influence colour reproduction and especially colour gradation in the inner part of colour gamut. The idea with colour profiles is to make the colour of the prints as close as possible to a standard, which means that different properties of substrates will be concealed. If colour management runs perfect there is therefore no need to evaluate the colour reproduction. It is however not likely that there will be profiles for every paper/printer/printer settings/ combinations. This investigation has further shown that ICC profiles and the profile of the printer driver have different properties. It must be if interest for paper producer to increase the knowledge in this field since colour reproduction is so crucial for desktop ink jet. There are for example indications that a printer setting for matt photo substrate of an Epson printer gives better image quality for Data Copy than plain paper setting. The proposed method would probably give an explanation to this observation. A total print quality analyse should include following parts;

• Traditional measurements of mottle, line sharpness, print sharpness, dot gain, print through and colour gamut for the actual printer settings but with all colour corrections deactivated

• An evaluation of colour reproduction (the herein proposed method) by printing RGB‐shell and sRGB star with actual printer setting and with colour adjustment activated

• A test print of well defined images, maybe those recommended by ISO In future these analyses are preferably made by colour calibrated scanners to make the investigation easy to perform (6).

6. LITTERATURE 1. Norberg, O. “The importance in of paper properties in digital printing” Doctoral work Linköping

(2006)

2. Intern communications TC Örnsköldsvik M‐Real

3. Pauler, N. “ICC Profiles for RGB printer” FSCN Report R‐07‐73 Örnsköldsvik (2007)

4. http://www.colo.org, IEC 61966‐2‐1

5. Wågberg, J. “OptProp a Maltlab toolbox for calculation of color related optical properties “ FSCN Report R‐07‐77 Örnsköldsvik (2007)

6. Andersson, Mattias. “Colour calibration of trichromatic capturing devices”. Doctoral work Linköping (2006)

FSCN – Fibre Science and Communication Network ISSN 1650-5387 2007:79 Internet: www.miun.se/fscn FSCN rapport R-07-79 Page 18(31) APPENDIX 1

RGB test chartR G B C M Y255 255 255 0 0 0

255 0 0 0 100 100

0 0 0 100 100 100

3‐4 process inks (CMY+key black)

1‐2 process inks


RGB-shell and RGB-star of printer and of sRGB

Figure 1. RGB –star. Figure 2. RGB-shell and RGB-star of a Canon printer.

Not colour corrected prints on plain paper.

Figure 3. RGB-shell and RGB-star of sRGB. Figure 4. RGB-shell of sRGB and a Canon printer printed on plain paper with no colour corrections.

Light

Dark

Medium


The four types of plots

40

60

80

100

0 20 40 60c*

L*

HP Mid No Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

Figure 1. Lightness L* vs. colour strengths c* for light,

mid and dark tones for cyan. Not colour corrected plain paper prints with HP printer. The black marks are sRGB colours.

Figure 2. a* vs. b* for mid tones for the ten tone steps from grey axis to shell of gamut. Prints with no colour correction with HP printer on plain paper. The open marks are sRGB colours.

34

44

54

64

74

0 5000 10000 15000 20000Colourarea

Ligh

tnes

s L*

HP

0

2000

4000

6000

8000

0 2 4 6 8 10Tone step

Col

our a

rea


Figure 3. Lightness vs. colour area for not corrected (open red marks) and colour corrected prints (red marks). The black marks are sRGB. Prints on plain paper with HP printer.

Figure 4. Colour area vs. tone steps from the grey axis to the shell of gamut for light, mid and dark tones. Colour corrected and not corrected prints on plain paper with HP printer.


(1) L* vs. c* for not colour corrected plain paper prints

HP 3310 NoCorr Plain paper Canon NoCorr Plain paper Epson No Corr Plain paper

40

60

80

100

0 20 40 60c*

L*

40

60

80

100

0 20 40 60c*

L*

40

60

80

100

0 20 40 60c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

20406080

100

0 50 100c*

L*

020406080

0 50 100 150c*

L*

10305070

0 50 100 150c*

L*

10305070

0 50 100 150c*

L*


(2) L* vs. c* for colour corrected plain paper prints HP ICC plain paper Canon Printer Driver plain paper Epson printer Driver plain paper

40

60

80

100

0 20 40 60c*

L*

40

60

80

100

0 20 40 60c*

L*

40

60

80

100

0 20 40 60c*

L*

20

40

60

80

0 50 100

c*

L*

20

40

60

80

0 50 100

c*

L*

20

40

60

80

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

40

60

80

100

0 50 100c*

L*

406080

100

0 50 100c*L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100c*

L*

40

60

80

100

0 50 100

c*

L*

40

60

80

100

0 50 100c*

L*

20406080

100

0 50 100c*

L*

0

20

40

60

80

0 50 100 150

c*

L*

10

30

50

70

0 50 100 150c*

L*

0

20

40

60

80

0 50 100 150c*

L*


R_07_79 NPauler 23 (1)

(1) a* vs b* for plain paper prints Light tones Mid tones Dark tones HP No

HP Light No Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

HP Mid No Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

HP Dark No profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60

a*

b*

HP ICC

HP Light ICC Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

HP Mid ICC Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

HP Dark ICC Profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60 80

a*

b*

Can No

Can Light No Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

Can Mid No Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

Can Dark No profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60

a*

b*

Can Cor.

Can Light printer driver Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

Can Mid printer driver Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

Can Dark printer driver Profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60 80

a*

b*

Eps No

Eps Light No Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

Eps Mid No Profile

-120

-70

-20

30

80

-90 -40 10 60 110

a*

b*

Eps Dark No profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60

a*

b*

Eps Cor:

Eps Light printer driver Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

Eps Mid printer driver Profile

-120-70-203080

-90 -40 10 60 110

a*

b*

Eps Dark printer driver Profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60 80

a*

b*


(1) c* vs.L* for not colour corrected photo prints

HP Canon Epson

40

60

80

100

0 20 40 60 80

c*

L*

40

60

80

100

0 20 40 60 80

c*L*

40

60

80

100

0 20 40 60

c*

L*

20

40

60

80

0 50 100c*

L*

20

40

60

80

0 50 100

c*

L*

20

40

60

80

0 50 100

c*

L*

40

60

80

100

0 50 100 150c*

L*

40

60

80

100

0 50 100 150c*

L*

40

60

80

100

0 50 100 150c*

L*

20

40

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80

0 50 100c*

L*

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40

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80

0 50 100c*

L*

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40

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80

0 50 100c*

L*

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100

0 50 100c*

L*

40

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0 50 100c*

L*

40

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100

0 50 100c*

L*

020406080

0 50 100 150c*

L*

020406080

0 50 100 150c*

L*

020406080

0 50 100 150c*

L*


(2) c* vs.L* for colour corrected photo prints

HP Canon Epson

40

60

80

100

0 20 40 60

c*

L*

40

60

80

100

0 20 40 60c*

L*

40

60

80

100

0 20 40 60c*

L*

20

40

60

80

0 50 100

c*

L*

20

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80

0 50 100

c*

L*

20

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80

0 50 100

c*

L*

40

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100

0 50 100c*

L*

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0 50 100c*

L*

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0 50 100c*

L*

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80

0 50 100c*

L*

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80

0 50 100c*

L*

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0 50 100c*

L*

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0 50 100

c*

L*

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0 50 100

c*

L*

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c*

L*

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c*

L*

020

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6080

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c*

L*

020406080

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c*

L*


R_07_79 NPauler 26 (1)

(1) a* vs b* photo prints Light tones Mid Toned Dark tones

HP No

HP Light No Profile

-70

-50

-30

-10

10

30

50

70

-70 -50 -30 -10 10 30 50 70

a*

b*

Hp Mid No Profile

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-90 -40 10 60 110

a*

b*

Hp Dark No profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60

a*

b*

HP ICC

HP Light ICC Profile

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10

30

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70

-70 -50 -30 -10 10 30 50 70

a*

b*

HP Mid ICC Profile

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a*

b*

HP Dark ICC Profile

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-60 -40 -20 0 20 40 60 80

a*

b*

Ca No

Can Light No Profile

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70

-70 -50 -30 -10 10 30 50 70

a*

b*

Can Mid No Profile

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-20

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80

-90 -40 10 60 110

a*

b*

Can Dark No profile

-70

-50

-30

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10

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50

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-60 -40 -20 0 20 40 60

a*

b*

Can Cor.

Can Light ICC Profile

-70

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10

30

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70

-70 -50 -30 -10 10 30 50 70

a*

b*

Can Mid ICC Profile

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80

-90 -40 10 60 110

a*

b*

Can Dark ICC Profile

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-50

-30

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10

30

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70

-60 -40 -20 0 20 40 60 80

a*

b*

Eps No

Eps Light No Profile

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-50

-30

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10

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70

-70 -50 -30 -10 10 30 50 70

a*

b*

Eps Mid No Profile

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-20

30

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-90 -40 10 60 110

a*

b*

Eps Dark No profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60

a*

b*

Eps Cor:

Eps Light ICC Profile

-70

-50

-30

-10

10

30

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70

-70 -50 -30 -10 10 30 50 70

a*

b*

Eps Mid ICC Profile

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-20

30

80

-90 -40 10 60 110

a*

b*

Eps Dark ICC Profile

-70

-50

-30

-10

10

30

50

70

-60 -40 -20 0 20 40 60 80

a*

b*


Multivariate analyses Extensive multivariate analysis was performed with different combination of measured variables set and samples set. Variable sets used in the multivariate analyse:

1. Mean colour area of the light, mid and dark tones. The values of the nine tone of the respectively tone series were separated into quantities for the first five tones (low colour strength) and the other four tones (high colour strength).

2. Colour differences values (De, DL, Dc and Dh) between sRGB and the printed tone patches of

light, mid and dark tones Sample sets used in the multivariate analyse:

1. six plain paper prints

2. six photo prints The measured values are shown as barcode graphs in page 2‐4 in this Appendix. The perception values were expressed as a linear combination of the measured values with PLS models. The different combination of the variable sets and sample sets gave high regression coefficient (R) and good prediction capacity Q2 cum, Table 1. The X data set 2 based on colour differences generated an improved model, but the model generated with colour area values gave also high regression coefficient. It was interesting to find that also for the six plain paper respectively the six photo papers were possible to find good PLS modles. Table 1. Result for different X‐data sets of the PLS multivariate analyses. The perception data was the Y variable.

X‐Variables Samples PLS Data 1 2 1 2

Colour Area

Tot.Gamut

De (DL,Dc,Dh

)

Plain Paper

Photo print

Significant Components

Regr. Coef.R Q2 Cum

x x x X 3 0,94 0,77 x x x 2 0,89 0,61 x x x 3 0,96 0,83 x x x 3 0,99 0,85 x x x 1 0,93 0,73

The modelling power of the mid and dark tones were high in all models.

Colour differences values for plain paper of light, mid and dark tones Low colour strength (negative values of DC, see equation 7) for mid and dark tones and colour failure (Dh see equation 9) are the dominate parts of colour differences between printer and sRGB colours, Figure 1. The colour correction reduced the differences in colour strength for mid and dark tones of the Canon and the Epson printer. .


HPNo Correction

-40,00

-20,00

0,00

20,00

40,00

De DL Dc Dh

LightmidDark

HP Correction

-40,0

-20,0

0,0

20,0

40,0

De DL Dc Dh

LightmidDark

Canon No Correction

-40,00

-20,00

0,00

20,00

40,00

De DL Dc Dh

LightMidDark

Canon Correction

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-20,00

0,00

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40,00

De DL Dc Dh

LightMidDark

Epson No Correction

-40

-20

0

20

40

De DL Dc Dh

LightMidDark

Epson Correction

-40

-20

0

20

40

De DL Dc Dh

LightMidDark

Figure 1. Colour differences De, DL, Dc and Dh between sRGB and the printers for light, mid and dark tones without (No Correction) and with colour correction.

Colour area for plain paper of light, mid and dark tones

The impact of colour correction on colour area of the light, mid and dark tones of the three printers is shown in Figure 2a‐c. Mean colour area of the first 4 tone step (low colour strength) and the following steps (5‐9 high colour strength) are indicated. For Epson and Canon the colour area of the first four tone steps (light blue marked) is increased whereas colours area is reduced for the higher colour strength when colour correction was activated. See also figure 9 in the main report where the gradation is shown.


The total colour gamut of not corrected and colour corrected printer are shown in Figure 2 d. a) HP, colour area for light, mid and dark tones not

corrected and corrected b) Canon, colour area for light, mid and dark tones not

corrected and corrected HP

-2000

0

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8000

L M D LC MC DC

Col

.Are

a

HighColStrLowColStr

Canon

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8000

L M D LC MC DC

Col

.Are

a

HighColStLowColStr

c) Epson, colour area for light, mid and dark tones not corrected and corrected

d) Total colour gamut of the three printers

Epson

-2000

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L M D LC MC DC

Col

.Are

a

HighColStrLowColStr

0

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150

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HP Ca Eps

Col

our G

amut

KD

E

NC

Figure 2 a-c. Colour area for the four tone steps 1-4 marked as LowColStr and the tone steps 5-9 marked as HighColStr. Figure 2d shows total colour gamut for not corrected and colour corrected prints.


Colour differences for photo paper of ligh, mid and dark tones

The corrected (with ICC profile) HP printer gave the lowest colour differences compared to sRGB colours. Observe that the Epson printer show high differences in colour strength (DC) and that the differences were reduced for the darker tones when colour correction was activated.

HPNo Correction

-40

-20

0

20

40

HP DL Dc Dh

LightMidDark

HP Correction

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0

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HP DL Dc Dh

LightMidDark

Canon No Correction

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0

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De DL Dc Dh

LightMidDark

Canon Correction

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0

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De DL Dc Dh

LightMidDark

Epson No Correction

-40,00

-20,00

0,00

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40,00

De DL Dc Dh

LightMidDark

Epson Correction

-40,00

-20,00

0,00

20,00

40,00

De DL Dc Dh

LighMidDark

Figure 3. Colour differences De, DL, Dc and Dh between sRGB and the printers for light, mid and dark tones without (No Correction) and with colour correction.


Colour area photo paper of light, mid and dark tones

The colour correction of HP photo prints with ICC profile had a reducing impact on colour area. Observe also that the colour area of dark tones for Epson printer was very small, but it was increased by the colour correction, Figure 4. a) HP, photo prints b) Canon, photo prints

HP

0

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L M D LC MC DC

Col

.Are

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HColStrLColStr

Canon

0

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L M D LC MC DCC

olou

r ar

ea

HColStrLColStr

c) Epson photo prints d) Total colour gamut all printers

Epson

0

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15000

L M D LC MC DC

Col

.Are

a

HColStrLColStr

0,00

200,00

400,00

600,00

800,00

HP Ca Eps

Col

our G

amut

KD

E

NoCorr

Figure 4 a-c. Colour area for the four tone steps 1-4 marked as LowColStr and the tone steps 5-9 marked as HighColStr. Figure 4d shows colour gamut for not corrected and colour corrected prints. Printing on plain paper.

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