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TABLE OF CONTENTS

1. 1.9 LIGHT ABSORPTION, REFLECTION AND COLOUR

2. colour technologists in terms of three visual characteristics:

3. Hue and wavelength position of light absorption

4.

1.9.2 Measurement of dye and pigment strength

5. 1.9.3 Dullness and brightness characteristics

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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 sampleand 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.

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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

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TERMS AND DEFINITIONS

hue: that attribute of colour

whereby it is recognised as being predominantly

red, green, blue, yellow, violet, brown, etc.

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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

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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

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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 strengthbrightness: the converse of dullness.

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1.9.1 HUE AND WAVELENGTH POSITION OF LIGHT ABSORPTIONBasing 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

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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 λmax 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 λmax about 610 nm.

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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.

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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).

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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.

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MEASUREMENT OF DYE AND PIGMENT STRENGTH The reflectance data at λmax 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

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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.

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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 λmax, 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.

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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).

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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.