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Distinct differences between laser-modified and untreated fabric samples were recorded using the CIELAB (CIE L*a*b*) colour model. Samples dyed with a 0.5% shade and at 1% were assessed for comparison (Figure 102). L*a*b* colour values were measured - L* = lightness – darkness, a* = redness – greenness b* = yellowness – blueness (Figure 103). In the model, a* and b* simulate how the signals are transported to the brain. These values are converted to Hue and Chroma to define colour in ‘L*C*H* space’ (L*/Lightness; C*/Chroma; H* /Hue).

Figure 102: Fabric samples used in reflectance testing (Batch: Green C)

Figure 103: CIELAB (CIE L*a*b*) colour model, SDC, 2013

Delta E assessment based on the L*C*H* colour model, quantified total colour difference between fabrics by the analysis of each sample. The total colour difference between samples in a batch and/or the standard is E. It is a single number that is calculated from the difference in lightness, the difference in chroma and the difference in hue (L*, C* and H*). CMC is the metric used to measure Delta. In this research, a ratio of 2:1 (CMC) was applied, commonly used for acceptability, required by industry standards.

Table 16 documents the L* - Lightness value of each fabric sample per batch including the depth of shade (DoS %) at which samples were dyed. Each batch comprised a range of variable samples including DoS% (Depth of Shade) and tonal density GS% (Greyscale) differences along with standard untreated specimens. Fabrics were defined by five categories in relation to their processing parameters – 1) Standard/Std. - Dyed/Not Treated; 2) GS: 70%’

- Laser-dyed; 3) GS: 80%’ - Laser-dyed; 4) GS: 100%’ - Laser-dyed; 5) Undyed (treated and not treated).

Sample / BATCH DOS

Table 16: CIELAB Lightness (L*) values per sample type

Samples that were ‘dyed/not treated’ produced higher L* values compared to treated fabrics as expected. Similarly, untreated fabrics dyed with 0.5% depth of shade also produced higher L* values than those dyed with 1%. Figure 104 shows that in general, 1% DoS fabrics modified at GS: 70% yielded the smallest % difference in lightness compared to the standard (dyed/not treated) specimen. GS: 100% specimens yielded lower L*% due to increased fibre-laser interaction and so increased dye uptake, therefore a darker shade. Exceptions to this trend can be seen with Sample 6/RED E, whereby GS: 80% produced a higher L* value than GS:

70%. This result was simply attributed to marginal experimental deviation. Overall, results were influenced by tonal difference due to variable energy density applied when laser processing the fabrics. Deeper dyeing as a result of increased laser modification, yielded

reduced L* values. Likewise, less fibre-laser interaction produced greater lightness. A lower L*% was recorded for all laser treated samples compared the standard specimen. This outcome further demonstrates the impact of the laser-dye process in relation to colour. Laser-dyeing enables increased dye uptake capability compared to untreated fabrics due to the interaction between fibres and laser beam energy which changes the fibre structure through the irradiation process. This in turn, induces a deeper dyeing effect to laser modified fabrics.

Figure 104: Lightness (L*) graph showing different values for treated fabric samples Table 17: Samples in numerical order

Hue and Chroma values for dyed and treated fabrics were plotted in L*C*H* lab space by converting a* and b* (L*a*b*) shown in Figure 105 and Table 19. This method identified the quality of a colour, determined by its dominant wavelength (hue), as well as intensity (chroma).

Starting at positive a* as 0°/360° - red, moving anticlockwise 90° - yellow, to 180°- green and 270° - blue. Chroma is the distance from the centre where intensity is weakest/neutral to where the colour is plotted further away, therefore stronger/brighter. Chroma is calculated from a*

and b* coordinates. This assessment provided a definitive reliable language for describing colour change regarding a laser-dye process in a way that is transferrable and relevant to the textile industry.

Table 19 reports ‘a’ and ‘b’ values regarding hue and chroma for each specimen per batch (previously identified in Table 16), Figure 105/Table 18 represents this data. Laser/dye processing laser parameters applied determined a unique position for fabric samples quantified in L*C*H* lab space. Each cluster is relative to a measure of metarmerism associated with dye/shade and dominant wavelength. Batches 5 and 7 yielded high chroma

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and distinct hue seen in the outer circle(s). Undyed samples/batch 10 yielded the lowest hue and chroma data as expected, along with batches 8 and 9 each positioned around the central region.

Figure 105: Fabric samples quantified in L*C*H* lab space

Table 18: Key - CIE Lab Space (left)

Table 19: Hue and Chroma values per batch/specimen (right)

The total colour difference between the batch and the standard (1% DoS; Not treated) is Delta E (∆E). It is a single number calculated from the difference in lightness, the difference in chroma and the difference in hue – L*, C* and H* recorded in Table 20. These results

shows Lightness (∆L*) consistently in the negative (-). This simply indicates a darkening of the colour, due laser treatment which can be understood. No correlation can be seen for how Chroma (∆C*) moved demonstrated in the graph (Figure 107). The Hue (∆H*) appeared to always move the same direction for all samples generating positive (+) ∆E, rather than negative numerical values. This trend confirmed laser-dyeing enhanced coloration which triggered greater dominance in colour appearance, signified by the increased quality of the hue, as discussed previously.

Table 20: Delta E data

Standard (Std) – Dyed/Not Treated

Laser-dyed (Treated)

Figure 106: Delta E Colour difference: Lightness results (∆L*)

Figure 107: Delta E Colour difference: Chroma results (∆C*)

Figure 108: Delta E Colour difference: Hue results (∆H*) -8

Quantitative data obtained using the CIELAB colour model provided further knowledge about the impact of laser modification on dye uptake in relation to the specified laser/dye processing parameters explored. This type of scientific interrogation and analysis of colour concerning a combined laser and dye process does not exist in current textile/laser studies.