1.9 light absorption, reflection and colour

1. TABLE OF CONTENTS 1.9 LIGHT ABSORPTION, REFLECTION AND COLOUR colour technologists in terms of three visual characteristics: Hue and wavelength position of light absorption 1.9.2 Measurement of dye and pigment strength 1.9.3 Dullness and brightness characteristics 1 COMPILED BY TANVEER AHMED

2. 1.9 LIGHT ABSORPTION, REFLECTION AND COLOUR As we have seen, colour arises in dyed or pigmented material as a result of the Selective absorption of radiation within the visible region of the electromagnetic spectrum. It has long been recognised that a relation exists between the hue of a coloured sample and the wavelength regions over which light absorption is strong, although the colour is actually determined (at least under normal conditions of illumination and viewing) mainly by the spectral energy distribution of the radiation reflected from the coloured opaque sample. 2 COMPILED BY TANVEER AHMED

3. colour technologists in terms of three visual characteristics: (a) hue (b) strength or depth (c) brightness or dullness. The most recent edition of Colour terms and definitions, published by the Society of Dyers and Colourists , gives the following definitions 3 COMPILED BY TANVEER AHMED

4. terms and definitions hue: that attribute of colour whereby it is recognised as being predominantly red, green, blue, yellow, violet, brown, etc. 4 COMPILED BY TANVEER AHMED

5. terms and definitions strength (of a dye): the colour yield of a given quantity of dye in relation to an arbitrarily chosen standard; (of a dyeing or print) synonymous with depth 5 COMPILED BY TANVEER AHMED

6. terms and definitions depth: that colour quality an increase in which is associated with an increase in the quantity of colorant present, all other conditions (such as viewing conditions, for instance) remaining the same 6 COMPILED BY TANVEER AHMED

7. terms and definitions dullness (of a colour ): that colour quality an increase of which is comparable to the effect of the addition of a small quantity of neutral grey colorant, whereby a match cannot be made by adjusting the strength brightness: the converse of dullness. 7 COMPILED BY TANVEER AHMED

8. 1.9.1 Hue and wavelength position of light absorption Basing measurements on the Beer–Lambert law as discussed in section 1.8.4, Figure 1.30 shows the variation of the absorption coefficients in solution of three acid dyes of different hue, compared with the corresponding Kubelka–Munk coefficients ( Kd / Sf ) derived from reflectance measurements of wool material dyed with the same three dyes 8 COMPILED BY TANVEER AHMED

9. Hue and wavelength position of light absorption The solution absorption curves are surprisingly similar to the absorption curves derived from the Kubelka–Munk analysis. (Such close similarity may not always be found.) The yellow dye absorbs over the near-UV and blue wavelength regions of the visible region with a maximum absorption λ max near 400 nm, the red dye absorbs in the green region ( λmax about 510 nm) and the blue dye absorbs in the orange-red region with λ max about 610 nm. 9 COMPILED BY TANVEER AHMED

10. Hue and wavelength position of light absorption These absorption curves have half-band widths (range of wavelengths at half the maximum absorption intensity) of about 100 nm, the blue dye showing some absorption over the whole visible spectrum. The general relationship between observed hue and wavelength region in which the maximum value lies is illustrated in Table 1.6. 10 COMPILED BY TANVEER AHMED

11. position of light absorption The precise hue description will depend mainly on the wavelength position of the absorption band and partly on the band width and the overall shape of the absorption curve, but also on the observer’s personal interpretation of the meaning of the hue terms used. Moreover, the wavelength ranges and associated hue descriptions given elsewhere may vary from that given in Table 1.6 (in section 1.2.1 we noted that the ‘true’ hues of blue, green and yellow have been observed to occur with monochromatic lights of wavelengths 436, 517 and 577 nm respectively and strictly these should lie near the middle of the appropriate radiation hue regions). 11 COMPILED BY TANVEER AHMED

12. 1.9.2 Measurement of dye and pigment strength Addition of a dye to an initially undyed or white substrate results in a decrease in reflectance which is greatest in the region in which the dye absorbs light. For the typical red acid dye considered in Figure 1.29, the changes in reflectance with increasing concentration of dye are illustrated in Figure 1.31. These show that for this dye the absorption maximum (reflectance minimum) occurs in the region of 510 nm, with the decrease in reflectance falling off rapidly as the depth increases. The undyed wool has a distinctly yellowish cast, as suggested by the steeply sloping reflectance curve with minimum reflectance at 400 nm. 12 COMPILED BY TANVEER AHMED

13. Measurement of dye and pigment strength The reflectance data at λ max from these curves were used to produce the linear Kubelka–Munk plot shown in Figure 1.29. The actual quoted concentration (expressed as a percentage mass of dye on fibre ) is fixed arbitrarily by the dye manufacturer in terms of a so-called standard depth defined by samples of pigmented card produced by the Society of Dyers and Colourists , or other standardising body, and defined as international standard depths in DIN 53.235 and BS1006 : A01 : 1978. These standard depths are a series of arbitrarily chosen depths, each judged to be equal for all hues, which enable dyeing, fastness or other properties to be compared on a uniform basis 13 COMPILED BY TANVEER AHMED

14. Measurement of dye and pigment strength Once the standard depth of a particular dye (or pigment) has been defined, relative strength measurements on subsequent batches are evaluated by preparing the dyed or pigmented sample under defined dyeing or application conditions, and testing strength variations such as 80, 90, 100, 110 and 120% for the sample being assessed. The resulting strength series of the sample colorant is then compared visually in a colourmatching booth with the standard sample (prepared simultaneously) accepted as being representative of the 100% strength of the colorant. 14 COMPILED BY TANVEER AHMED

15. Measurement of dye and pigment strength In many cases dye manufacturers have gradually replaced the dyeing test for strength determination with solution spectrophotometry based on a simple ratio determination of the absorbance values at λ max, as expressed implicitly by the Beer–Lambert law. The full experimental details of the standard procedures for carrying out such solution strength tests have been published . Such relative strength tests based on optical measurements on the dye solutions are, however, valid only if the chemical composition of the dye can be consistently reproduced, and hence if the dye can be produced with reproducible affinity or uptake characteristics on the appropriate fibres and a reproducible absorption spectrum in solution. 15 COMPILED BY TANVEER AHMED

16. 1.9.3 Dullness and brightness characteristics The dullness/brightness variation is best illustrated in terms of the reflectance curves for two dyes of similar hue and strength which differ mainly in terms of their brightnesses . Thus we may compare CI Acid Red 57 (mentioned above) with a duller metal-complex red dye, Neolan Red BRE, both applied to wool (Figure 1.32). 16 COMPILED BY TANVEER AHMED

17. Dullness and brightness characteristics The spectral reflectance curve of the latter dye shows greater background absorption across the absorption spectrum, an effect which is akin to adding a uniformly absorbing grey dye to the brighter acid dye sample. The absorption peak in bright dyes tends to be sharper or more pronounced than in their duller counterparts. 17 COMPILED BY TANVEER AHMED