1. TABLE OF CONTENTS
1. 1.9 LIGHT ABSORPTION, REFLECTION AND
COLOUR
2. colour technologists in terms of three visual
characteristics:
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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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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
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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
2 normal conditions of illumination and viewing)
mainly by the
spectral energy distribution of the
radiation reflected from the coloured opaque sample.

3. COLOUR TECHNOLOGISTS IN TERMS OF THREE
VISUAL CHARACTERISTICS:
 (a) hue
 (b) strength or depth
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 (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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4. TERMS AND DEFINITIONS
 hue:
 that attribute of colour whereby it is
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recognised as being predominantly
 red,
 green,
 blue,
 yellow,
 violet,
 brown, etc.
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5. TERMS AND DEFINITIONS
 strength (of a dye):
 the colour yield of a given quantity
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of dye
 in relation to an
 arbitrarily chosen standard;
 (of a dyeing or print) synonymous
with depth
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6. TERMS AND DEFINITIONS
 depth:
 that colour quality
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 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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7. TERMS AND DEFINITIONS
 dullness (of a colour):
 that colour quality
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 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 7
brightness: the converse of dullness.

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,
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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)
8
derived from reflectance measurements of wool
material dyed with the same three dyes

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 λmax near 400 nm,
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 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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10. HUE AND WAVELENGTH POSITION OF LIGHT
ABSORPTION
 These absorption curves have
half-band widths
 (range of wavelengths at half the
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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

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
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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 11
 near the middle of the appropriate radiation hue regions).

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,
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 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
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 minimum reflectance at 400 nm.

13. 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
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
 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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14. MEASUREMENT OF DYE AND PIGMENT
STRENGTH
 Once the standard depth of a particular dye (or pigment)
has been defined,
 relative strength measurements
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 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 14
 representative of the 100% strength of the colorant.

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 λmax,
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
 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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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
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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

17. DULLNESS AND BRIGHTNESS
CHARACTERISTICS
 The spectral reflectance
curve of the latter dye
shows greater
background absorption
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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