Title:
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Assessment of Colour and Colour Fastness of Textile Materials
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The main aim of this research was to investigate the possible uses of digital image
scanning to assess the colour and colour fastness of textiles. This method has the
potential to be a simpler alternative to camera based colour fastness assessment, for
example'by the DigiEye system..
The scanning device used was an Epson Perfection 2400 Photo scanner, a mid-range
quality device. The scanner was operated in· two different modes, 24 and 48 bits
colour resolution and was calibrated by adjusting its setting, testing its repeatability
for colour measurement and examining the spatial uniformity of response of the
scanner. The results showed that the spatial uniformity of the 'scanner response was
excellent for the majority of the scanning area but less uniform for the areas along its
scanning borders. The repeatability of the scanner values for the colour measurement
purposes was reasonable. The colour and colour fastness determination by the 24 and
the 48 bits scanner modes indicated that there were no practical benefits to the use of
the 48 bits.
The colorimetric response of the scanner was characterised with the XYZ and the
L*a*b* approaches in which the RGB responses of the scanner were transformed to
the XYZ tristimulus values and to the L*a*b* values, respectively. It was shown that
pre-treatment of the data by linearisation against either luminance or mean
reflectance gave very good results with the XYZ approach to scanner.
characterisation and that linearisation against L* gave excellent results for the
L*a*b* approach to scanner characterisation. In the characterisation of the scanner
two different techniques for parameter optimisation were applied, regression and
iteration. The experiments show that the iteration techniques marginally improved
the performance of the XYZ approach but it had no significant effect on the L*a*b*
approach. The experiments conducted to find the best colour chart to use for
colorimetric characterisation of the scanner showed that the TC chart which was
made from an atlas of textile colour patches outperformed the CCDCchart which
was made of 165 patches. of the ColorChecker DC. From the performance results of
the colorimetric characterisation with different polynomials in the regression
techniques it was found that using a second order polynomial to relate RGB to output
colour parameters had the best generalisation and a good memorisation effect.
It was concluded that for colour fastness assessment the scanner should be
characterised with a regression technique fitting the coefficients of a second order
polynomial us'ing the RGB to L*a*b* approach 'of colorimetric characterisation and
theL* method for linearisation of the eeDe chart patches' RGB. The relative
spectral power distribution (SPD) of the scanner lamp was also measured and it was
concluded that part ofresidual inaccuracies of the scanner colour measurement could
have arisen from the spiky shape ofthe SPD.
The characterised scanner was used for instrumental colour fastness assessment of
two sets of panels, one set of 60 staining and 15 colour change panels called the old
set and a set of240 staining and 40 colour change panels made during the research to
increase the range and uniformity ofthe colour distribution ofthe panels. An existing
set of observer results were available for the old set and were obtained from 38
observers from 12 laboratories ofthe UK. The grades for the new set were obtained
from 6 observers from two laboratories in the UK. The instrumental colour fastness
grades of the sets were determined by the ISO, the GRS and the GRC formulae using
a spectrophotometer, DigiEye system and the scanner. On the basis of the results of
these devices in comparison to the average visual grading, it was c,oncluded that the
ISO staining formula as well as the GRS and the GRC predict grades with smaller
disagreement to the average of the visual grades than the inter observer variation.
However, the ISO colour change formula determined grades with a higher level of
disagreement to the average visual grade that was higher than the inter-observer
variation. The scanner grading showed very good agreement to the·
spectrophotometric and DigiEye grading with the ISO staining, the GRS and the
GRC formulae but only moderate agreement with the ISO colour change formulae.
The scanner repeatability for colour fastness assessment was determined and shown
to be very good, however, as the frequency of assessment by the scanner in a
research situation is not comparable to its industrial application, it was suggested to
characterise the scanner once a day. The characterisation of the scanner with the
research TC chart was shown to produce np significant improvement in the results
that justify the difficulty of the preparation and maintemmce ofthis chart.
A comparison was made between visual grades obtained from experienced and from
inexperienced assessors of colour fastness. The results suggest that the grades given
by the inexperienced are comparable to those assessed by the experienced assessors
and the disagreements between the average grades are less than the experienced
assessors inter-observer variation. However, as the experimental conditions for the
set of the inexperienced assessors were the same but .were not the same for the
member of the set of the experienced assessors it was suggested that further study is
needed.
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