The document discusses light absorption, reflection, and color. It defines key terms used in color technology like hue, strength/depth, and brightness/dullness. It explains that hue is determined by the wavelength region where light absorption is strongest. Strength refers to the color yield of a dye. Depth and brightness are also defined. Examples are given of absorption curves for dyes of different colors and how they relate to hue. Methods for measuring dye and pigment strength are outlined. The factors that influence dullness versus brightness in dyes are explained.

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