microspectrofluorimetry of fluorescent dyes and brighteners on single textile fibres: part 2 —...

Forensic Science International, 51 (1991) 221-237 Elsevier Scientific Publishers Ireland Ltd.

221

MICROSPECTROFLUORIMETRY OF FLUORESCENT DYES AND BRIGHTENERS ON SINGLE TEXTILE FIBRES: PART 2 - COLOUR MEASUREMENTS

ANDREW W. HARTSHORNE and DAVID K. LAING

Central Research and Support Establishment, Home Office Forensic Science Service, Aldermaston, Reading, Berkshire, RG7 ,$PN (U.K.)

(Received January llth, 1991) (Accepted August 23rd, 1991)

Summary

The use of a microspectrofluorimeter to obtain emission spectra for discriminating fluorescent dyes and brighteners on single textile fibres has been described previously. Such spectra also lend themselves to the specification of fluorescence colours by the Commission Internationale de 1’Eclairage system of tristimulus calorimetry. This system of colour specification is demonstrated with a range of fluorescent dyestuffs and brightening agents. The applicability of results to visual inspection and comparison of fluorescence colours in forensic fibre examination is discussed.

Key words: Microspectrofluorimetry; Single fibres; Dyes; Brighteners; Colour measurements

Introduction

Previous papers [1,2] have described how the method of complementary tristimulus calorimetry may be used to specify the colour of individual textile fibres. By combining visible absorbance spectra with standard functions representing illuminant and observer, all colours may be represented by just two parameters called complementary chromaticity coordinates. The system is a nat- ural development of microspectrophotometry, and is now in use to describe and match the colours of fibres in a large data collection [3,4].

A method of obtaining fluorescence emission spectra from single textile fibres has been presented [5]. The utilisation of such spectra for recording fluorescence emission colours in a similar manner to that for visible absorption is of value, as it would enable such colours to be recorded in the database and provide a simple means of colour matching. In view of the successful application of the Commis- sion Internationale de 1’Eclairage (CIE) system to microspectrophotometry, the use of this method of specification for fluorimetry would appear to be a promis- ing approach.

This paper therefore describes the modifications to the CIE system of colour specification necessary to code fluorescence emission colours. This methodology is then applied to fluorescence emission spectra obtained from a range of samples.

Printed and Published in Ireland

222

X’

Fig. 1. The complementary chromaticity diagram.

Fig. 2. The CIE chromaticity diagram.

Theory

Fluorescent species, under the measurement conditions previously described [5], may be regarded as self-luminous sources. The CIE system of tristimulus colorimetry is applicable to such stimuli [6], albeit with certain modifications.

The major change is that no account is taken of any standard illuminant; tristimulus values are calculated by summation of the products of the emission intensity and the colour-matching functions [7] only. Because emission, rather than absorbance spectra are used in calculation, the complementary chromaticity diagram shown in Fig. 1 no longer applies. The arrangement of hues is transpos- ed and so follows that of the normal CIE chromaticity diagram (Fig. 2).

The neutral point is located at (0.3333, 0.3333) rather than (0.3101, 0.3162). This is a direct consequence of the absence of a standard illuminant in the calculation of tristimulus values; summation of each of the three colour-matching functions for a neutral stimulus gives three equal tristimulus values.

The measurement protocol previously developed [5] does not give standardised spectra: relative, and not absolute emission intensities are produced. Because of this, the magnitudes of the calculated tristimulus values are arbitrary. For a given fluorescent species, they should be proportional to the intensity of the ex- citing radiation, which itself may vary. It is therefore usual to employ a normalis- ing factor to each tristimulus value [S]. The ratio of the three tristimulus values is unchanged by normalisation and the chromaticity coordinates are thus unaffected.

Experimental

Selection of samples To examine the suitability of the modified CIE system for the coding of fluorescence emission colours, fibres dyed with a wide range of fluorescent dyestuffs were selected for measurement. Most were obtained from dye manufacturers’ pattern books. All 44 samples, together with their Colour Index [8] notations, are listed in Table 1. Also shown in the table are the fluorescence colours observed for the cube A and H2 [5] illumination systems.

Many of the samples were selected in a range of dye concentrations, so that the effect on chromaticity coordinates could be assessed. Most of the dyeings were on round, delustred polyamide or polyester fibres, as indicated in the table. Certain samples had been previously examined by microspectrophotometry [l] and microspectrofluorimetry [5].

In addition, fibres dyed with different concentrations of various fluorescent brightening agents were examined. The eight samples selected are shown in Table 2, together with their Colour Index [8] details and the fluorescence colours observed.

Fluorescence colour measurement All samples were mounted on slides using XAM neutral medium improved

white (BDH, Poole, Dorset, U.K.) and a coverslip. Ten determinations of the

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TABLE 1

FLUORESCENT DYES EXAMINED BY FLUORESCENCE COLORIMETRY

Foron and Dispersol dyeings on polyester; Terasil, Cibacet and Acid01 dyeings on Nylon 6 6.

Sample (no., name)

Dye cone. 940

Visual appearance under

cube A cube HZ

1 ICI Dispersol Red C-B 0.6 Pink-grey Orange 2 (Disperse Red 91) 3.5 Dull orange-red Bright orange-red 3 (Disperse Red 91) 7.0 Dull red-orange Bright orange

4 ICI Dispersol Red B-2B 0.4 Dull purple Dull orange 5 (Disperse Red 60) 2.3 Dull red-orange Bright orange 6 (Disperse Red 60) 4.5 Dull red-orange Bright orange

7 ICI Dispersol Orange D-G 0.4 Dull lime

8 (Disperse Orange 32) 2.8 Green-yellow 9 (Disperse Orange 32) 5.5 Br. green-yellow

Bright green- yellow Bright yellow Very bright yellow

10 Sandoz Foron Red E-2GL 0.1 Very dull violet Dull olive 11 Sandoz Foron Red E-2GL 1.0 Dull orange-violet Bright orange 12 Sandoz Foron Red E-2GL 3.0 Very dull orange Red-orange

13 Sandoz Foron Brill. Red 0.2 Very dull violet Dull olive 14 E-RL (Disperse Red 53) 2.0 Dull orange-violet Bright orange 15 E-RL (Disperse Red 53) 6.0 Dull red-orange Bright orange

16 Sandoz Foron Brill. Red 0.2 Very dull violet Dull olive 17 E-2BL (Disperse Red 60) 2.0 Dull orange-violet Bright orange 18 EdBL (Disperse Red 60) 5.0 Dull orange-violet Bright orange

19 Sandoz Foron Brill. Pink 0.2 Very dull blue Dull olive 20 E-5BPW (Disperse Red 11) 2.0 Dull orange-violet Olive 21 E8BPW (Disperse Red 11) 4.0 Dull orange-purple Bright orange-red

22 Sandoz Foron Brill. 0.1 Very dull blue Green 23 Yellow SE-6GFL 1.0 Very dull turquoise Bright green 24 (Disperse Yellow 49) 2.5 Dull turquoise Bright green 25 (Disperse Yellow 49) 3.0 Dull turquoise Bright green

26 Ciba Terasil Brill. 27 Pink 3G 28 (Disperse Red 302) 29 (Disperse Red 302) 30 (Disperse Red 302) 31 (Disperse Red 302) 32 (Disperse Red 302)

0.05 0.1 0.2 0.5 1.0 2.0 3.0

0.05 0.1 0.2

Grey Olive Grey Dull yellow Dull violet Orange-yellow Dull purple Yellow-orange Dull pink-purple Orange Dull pink Bright orange Dull orange Orange

33 Ciba Cibacet Violet 2R 34 (Disperse Violet 1) 35 (Disperse Violet 1)

Very dull violet Very dull orange Very dull violet Very dull orange Dull purple Dull orange

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TABLE 1 (Continued)

Sample (no., name)

Dye conx. Visual appearance under %

cube A cube HZ

36 (Disperse Violet 1) 37 (Disperse Violet 1) 38 (Disperse Violet 1) 39 (Disperse Violet 1)

40 BASF Acid01 Brilliant Yellow 8G (Acid Yellow 184)

0.5 Dull purple 1.0 Dull purple 2.0 Purple 3.0 Purple

2.5 Turquoise

Dull orange-red Dull orange-red Orange-red Blue-red

Very bright green

41 Yellow viscose fibres

42 Pink viscose fibres

43 Red acrylic fibres

44 Day-G10 green acrylic sock

*Dye concentration unknown.

Turquoise-green

Bright mauve

Dull red-orange

Turquoise

Bright lime

Olive

Bright orange

Very bright green

TABLE 2

FLUORESCENT BRIGHTENING AGENTS EXAMINED BY FLUORESCENCE COL- ORIMETRY

Fluolite and Leucophor dyeings on polyester; knitting yarn was Nylon 6 6.

Sample (no., name)

Dye cow %

Visual appearance under

cube A cube H2

1 ICI Fluolite XMF 0.4 Bright blue Bright green (Fl. Brightener 179)

2 Sandoz Leucophor EFR 3 (Fl. Brightener 162:l)

4 Sandoz Leucophor EFG 5

6 Sandoz Leucophor EFA 7

0.4 Blue 0.8 Blue

0.2 Blue 0.4 Bright blue

0.4 Bright blue 0.8 Bright blue

Dull green Dull green

Green Green

Green Green

8 Nylon Baby Knitting Yarn

*Dye concentration unknown.

* Blue Very dull green

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TABLE 3

CHROMATICITY VALUES FOR FLUORESCENT DYESTUFFS

Sample” Dye Cube Mean chrmaticity Error ellipse cone. coordinates %

X Y Major Minor Angle to ClXiS oxis x-Axis length Wth

1 Dispersol Red C-B

2 Dispersol Red C-B

3 Dispersol Red C-B

4 Dispersol Red B-2B



7 Dispersol Orange D-G



10 Foron Red E-2GL

11 Foron Red E-2GL

12 Foron Red EdGL

13 Foron Brill. Red E-RL



16 Foron Brill. Red E-2BL

1’7 Foron Brill. Red E-2BL

18 Foron Brill. Red E-2BL

19 Foron Brill. Pink E-5BPW

0.6

3.5

7.0

0.4

2.3

4.5

0.4

2.8

5.5

0.1

1.0

3.0

0.2

2.0

6.0

0.2

2.0

5.0

0.2

A 0.3780 0.2896 0.1024 0.0085 36.3 H2 0.5556 0.4430 0.0116 0.0001 - 43.9 A 0.5424 0.3699 0.0448 0.0136 11.9 H2 0.5913 0.4079 0.0151 0.0001 -44.5 A 0.5926 0.3724 0.0266 0.0078 -20.5 H2 0.6057 0.3936 0.0149 0.0001 - 44.8

A 0.2870 0.2193 0.0762 0.0216 55.8 H2 0.5706 0.4279 0.0091 0.0001 -43.9 A 0.5187 0.3319 0.0381 0.0090 24.6 H2 0.6045 0.3948 0.0149 0.0002 -44.4 A 0.5578 0.3370 0.0669 0.0129 16.2 H2 0.6211 0.3784 0.0166 0.0001 -44.8

A 0.4318 0.4747 0.0412 0.0025 36.6 H2 0.4729 0.5200 0.0050 0.0001 -42.7 A 0.4937 0.4918 0.0184 0.0067 -5.8 H2 0.5001 0.4948 0.0124 0.0002 - 43.0 A 0.5049 0.4815 0.0192 0.0035 - 35.2 H2 0.5114 0.4844 0.0184 0.0002 -43.5

A 0.2525 0.2191 0.0730 0.0086 43.4 H2 0.5246 0.4706 0.0443 0.0003 - 42.8 A 0.3861 0.2623 0.1397 0.0034 33.5 H2 0.5865 0.4124 0.0244 0.0002 -44.2 A 0.5201 0.3315 0.0816 0.0182 20.1 H2 0.6024 0.3967 0.0212 0.0001 -44.5

A 0.2246 0.1529 0.0283 0.0056 43.0 H2 0.5339 0.4629 0.0141 0.0002 - 43.4 A 0.4533 0.3052 0.0638 0.0084 32.4 H2 0.5812 0.4178 0.0126 0.0002 -44.0 A 0.5596 0.3593 0.0421 0.0049 14.1 H2 0.6064 0.3929 0.0205 0.0001 -44.9

A 0.2266 0.1465 0.0959 0.0064 43.0 H2 0.5529 0.4443 0.0278 0.0001 -42.9 A 0.4900 0.3066 0.0943 0.0103 25.4 H2 0.6078 0.3915 0.0349 0.0001 - 44.6 A 0.5730 0.3454 0.0539 0.0125 22.8 H2 0.6191 0.3803 0.0144 0.0001 - 44.7

A 0.1934 0.1334 0.0345 0.0116 55.0 H2 0.4989 0.4932 0.0285 0.0006 -42.9

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TABLE 3 (continued)

Samplea Dye Cube Mean chromaticity Error ellipse cone. coordinates %

X Y Major Minor Angle to axis axis x-Axis length length

20 Foron Brill. 2.0 A 0.3492 0.1988 0.0894 0.0122 31.3 Pink E-5BPW H2 0.6128 0.3854 0.0300 0.0003 -43.7

21 Foron Brill. 4.0 A 0.5181 0.2751 0.0541 0.0056 23.3 Pink E-5BPW H2 0.6438 0.3555 0.0114 0.0002 -44.1

22 Foron Brill. 0.1 A 0.1927 0.1589 0.0315 0.0093 65.0 Yellow SE-6GFL H2 0.3126 0.6614 0.0308 0.0003 - 40.3

23 Foron Brill. 1.0 A 0.1908 0.2542 0.1396 0.0118 81.2 Yellow SE-6GFL H2 0.2850 0.6851 0.0077 0.0003 -40.6

24 Foron Brill. 2.5 A 0.2244 0.3710 0.0996 0.0177 75.5 Yellow SE-6GFL H2 0.3014 0.6711 0.0062 0.0004 - 39.8

25 Foron Brill. 3.0 A 0.2488 0.4357 0.0374 0.0187 82.6 Yellow SE-6GFL H2 0.3219 0.6532 0.0421 0.0003 -40.7

26 Terasil Brill. 0.05 A 0.2154 0.2089 0.0718 0.0153 61.8 Pink 3G H2 0.4958 0.4983 0.0268 0.0005 - 42.4

27 Terasil Brill. 0.1 A 0.2917 0.2575 0.0573 0.0072 53.0 Pink 3G H2 0.5311 0.4660 0.0133 0.0001 -43.2


29 Terasil Brill. 0.5 A 0.4152 0.3188 0.0276 0.0087 37.7 Pink 3G H2 0.5638 0.4349 0.0065 0.0001 - 42.8




33 Cibacet 0.05 A 0.2736 0.2531 0.0737 0.0115 41.4 Violet 2R H2 0.4840 0.5072 0.0171 0.0007 - 43.3

34 Cibacet 0.1 A 0.2163 0.2009 0.0870 0.0138 52.7 Violet 2R H2 0.5193 0.4748 0.0672 0.0003 -43.4


36 Cibacet 0.5 A 0.2605 0.1577 0.0478 0.0123 62.8 Violet 2R H2 0.5758 0.4202 0.0310 0.0004 - 43.9




40 Acidol Brill. 2.5 A 0.1296 0.3846 0.0692 0.0178 88.8 Yellow 8G H2 0.3068 0.6678 0.0486 0.0003 - 40.2

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TABLE 3 (continued)

Samplea Dye Cube Mean chromaticity Error ellipse cone. coordinates %

X Y Major Minor Angle to axis WiS x-Axis length length

41 Yellow viscose * A 0.2069 0.5579 0.0867 0.0328 87.9 fibre sample H2 0.3554 0.6256 0.0151 0.0002 -41.6

42 Pink viscose * A 0.1628 0.0684 0.0247 0.0054 54.8 fibre sample H2 0.4744 0.5190 0.0511 0.0009 - 43.2

43 Red acrylic * A 0.5983 0.3429 0.0962 0.0162 0.6 fibre sample H2 0.6148 0.3845 0.1046 0.0003 -44.5

44 Day-G10 Green * A. acrylic sock H2

%olour Index details in Table 1. *Dye concentration unknown.

0.1091 0.2467 0.0628 0.0040 - 87.0 0.2827 0.6855 0.0142 0.0005 -41.9

fluorescence emission spectrum of each sample were made for illumination by cube A, using the NanoSpec microspectrofluorimeter and following a standard measurement protocol [5]. From these spectra, tristimulus values and chromaticity coordinates were determined. Errors in measurement were assessed by calculation of mean chromaticity coordinates and error ellipses [l] for each sample. The procedure was then repeated using cube H2.

Results and Discussion

Fluorescent dyes - cube A excitation The mean chromaticity coordinates and error ellipse parameters obtained for

cube A excitation of fluorescent dyestuffs are given in Table 3. The error ellipses are plotted on the chromaticity diagram in Fig. 3, with some being omitted for clarity.

Comparison of Fig. 3 and the colour descriptions given in Table 1 shows good correspondence, the ellipses falling in the expected areas of the chromaticity diagram. As many of the ellipses do not overlap, some degree of colour discrimination is possible. Comparison of results obtained by microspectrophotometry [l] with the present work indicates that the error ellipses obtained from replicate fluorescence spectra are generally much larger than those associated with absorbance spectra from similar colorants.

However, the most important feature observable from Fig. 3 is that different concentrations of the same dyestuff do not give identical chromaticity coordinates. In general, the lowest dye concentrations produced error ellipses lying

229

05

0.4

0.3

Y

0.2

0.1

F 1

0.2 0.3 o-4 x 05 0.6

Fig. 3. Chromaticity diagram for cube A excitation of fluorescent dyestuffs (for sample identification, see Table 3).

in the blue/violet area of the chromaticity diagram. The only exception to this was the 0.4% Dispersol Orange D-G dyeing (sample 7) which gave a lime-green fluorescence. Increases in dye concentration caused movement towards either the red (for the majority of samples) or green (Foron Brilliant Yellow SE-GGFL) areas of the chromaticity diagram. A remarkable exception to these general trends was the behaviour of the Cibacet Violet 2R dyeings (samples 33 - 39); increasing dye concentration caused movement first towards the blue, then a reversal in the direction of red and then finally a shift towards the purple area of the chromaticity diagram.

As emission intensity should be proportional to concentration [9], at least at the lower dye concentrations, chromaticity coordinates should be invariant. Even at concentrations where the approximations made in using Beer’s Law do not apply, relative emission at various wavelengths would have to vary greatly

230

to give the gross shifts in coordinates observed. Another process that could occur at high dye concentrations is self-quenching [lo], which involves radiationless de- activation of the excited state. Again, however, the relative emission intensity at various wavelengths would have to change appreciably to account for the large changes in chromaticity coordinates.

The behaviour of these dyeings is highly reminiscent of the results obtained by microspectrophotometry [1,2], where the influence of the undyed fibre colour gave rise to chromaticity changes with varying dye concentration. Complemen- tary chromaticity coordinates of the different concentration dyeings fell on a straight line connecting the coordinates of the undyed fibre with those of the pure dye. In the present study, Fig. 3 appears to show an analogous phenome- non, although the lines are curved in most cases. To produce such lines, more than one fluorescent species must be present, with its additive contribution to the overall emission remaining constant with increasing dye concentration.

For the Dispersol dyeings, this is almost certainly the case, the manufacturer

TABLE 4

CHROMATICITY VALUES FOR FLUORESCENT BRIGHTENING AGENTS

SampW Dye Cube Mean chromaticity Error ellipse COVL coordinates %

X Y Major Minor Angle axis axis to length length X-&S

1 Fluolite XMF 0.4 A 0.1501 0.0383 0.0024 0.0005 -79.1 H2 0.2355 0.7278 0.0626 0.0010 -40.5

2 Leucophor EFR

3 Leucophor EFR

0.4 A 0.1490 0.0507 0.0044 0.0004 -79.2 H2 0.2906 0.6808 0.0157 0.0005 -40.2

0.8 A 0.1490 0.0499 0.0053 0.0005 -80.4 H2 0.2640 0.7053 0.0374 0.0007 -39.7

4 Leucophor EFG

5 Leucophor EFG

0.2 A 0.1459 0.0607 0.0215 0.0026 -84.0 H2 0.3561 0.6238 0.1018 0.0006 -41.2

0.4 A 0.1480 0.0653 0.0170 0.0007 -88.6 H2 0.3488 0.6294 0.0675 0.0003 -40.9

6 Leucophor EFA

7 Leucophor EFA

0.4 A 0.1479 0.0509 0.0109 0.0005 -82.7 H2 0.3282 0.6482 0.1005 0.0012 -41.5

0.8 A 0.1481 0.0548 0.0044 0.0005 88.8 H2 0.3081 0.6651 0.0536 0.0004 -41.2

* A 0.1495 0.0465 0.0086 0.0033 -47.6 H2 0.3842 0.5968 0.1150 0.0004 -41.5

8 Nylon Baby knitting yarn

%olour Index details in Table 2. *Dye concentration unknown.

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stating that the undyed tibre had been treated with a fluorescent brightening agent, Fluolite XMF. Plotting its chromaticity coordinates (Table 4) on Fig. 3 shows that they lie at one end of the Dispersol lines. A similar application of a fluorescent brightening agent to the Foron and Terasil dyeings is thus most probable. Further evidence for this is provided by the fact that most error ellipses lie parallel to the lines. However, the behaviour of the Cibacet Violet 2R dyeings cannot be explained by this hypothesis.

In microspectrophotometry, the effect of the undyed fibre substrate can be removed by subtracting its complementary tristimulus values from those of the dyed fibres [2]. However, as related above, the absolute magnitudes of fluorescence intensities and tristimulus values obtained by the present method of measurement are unknown. The effects of a second fluorescent species cannot therefore be removed by a similar subtraction.

The curving of the chromaticity lines towards the red area of the chromaticity diagram may be caused by self-absorption [lo], which occurs at high dye concentrations. In essence, if a colorant’s visible absorption band overlaps wavelengths of fluorescence emission, then a proportion of the emitted light will be absorbed by the dye. Previous work [5] showed that the red dyes examined had absorption maxima in the region 500 - 560 nm. As dye concentration increases, the emitted

0 70

0.68

Y

0.66

0.64

0.62 C 6 0.28 0.30 0.32 x 034 0.36 0.38

Fig. 4. Chromaticity diagram for cube H2 excitation of yellow and green fluorescent dyestuffs (for sample identification, see Table 3).

232

green wavelengths will be preferentially absorbed, shifting the fluorescence colour towards the red. As most of the dyes examined gave little fluorescence in the green area of the spectrum, only a small shift should be observed. However, the Dispersol Orange D-G dyeings (samples 7 - 9) gave far more fluorescence at the green wavelengths. For this dyestuff, the effects of self-absorption should be more evident. Figure 3 shows that there is a significant shift towards red for this colorant.

The situation may be further complicated by the effects of fluorescence decay. Although the dyes themselves are unlikely to exhibit this property, it is known [5] that a marked decay of emission intensity occurs with fluorescent brightening agents. The contribution of the fluorescent brightener emission to the overall observed colour will thus decrease and a change in chromaticity will take place. This provides a possible explanation for the relatively large sizes of the calculated error ellipses.

Fluorescent dyes - cube H2 excitation The mean chromaticity coordinates and associated error ellipse parameters ob-

tained for the various dyestuffs under cube H2 illumination are given in Table 3. From this, it can be seen that the aspect ratios of the ellipses are very large

055

050

0.45

Y

0.40

0.35

0.65 0.70

Fig. 5. Chromaticity diagram for cube H2 excitation of red and pink fluorescent dyestuffs (for sample identification, see Table 3).

233

indeed, due to the small minor axis lengths. Because of this, in the chromaticity diagrams of Figs. 4 and 5, ellipses have been drawn as lines of length equal to the major axis, with some being omitted for clarity. The ellipse major axis lengths are generally less than those obtained with cube A excitation.

As can be seen in Fig. 4 (yellow and green samples) and Fig. 5 (other colours), all the ellipses are concentrated into a narrow portion of the chromaticity diagram, close to and roughly parallel with the yellow-to-red segment of the spectrum locus. This results from the short wavelength cut-off of cube H2 at 520 nm, only wavelengths between 520- 710 nm being allowed. This means that calculated chromaticity coordinates can only fall in an area bounded by a straight line joining the chromaticity coordinates of the 520 nm and 710 nm spectral lines and that portion of the spectrum locus between these two points. The resulting restricted chromaticity diagram is indicated in Fig. 6.

Emission spectra from cube A excitation are also restricted in wavelength, over the range 430 - 710 nm. However, in this case, the chromaticity point of the 430 nm spectral line lies so close to the violet limit of the spectrum locus that only a very small area of the chromaticity diagram is inaccessible, as shown in Fig. 6.

It was stated above that the neutral point for self-luminous sources lies at (0.3333, 0.3333). However, this is only true if every wavelength can contribute to the emitted fluorescence. As seen above, the short wavelength cut-offs of

Fig. 6. Restricted chromaticity diagram resulting from cubes A and H2 excitation. (E) Neutral point for self-luminous sources; (EA) apparent neutral point using cube A excitation; (EH2) apparent neutral point using cube H2 excitation.

234

o.oe

0.07

Y

0.06

o-05

0.04

O*Of

467nm

\ 0 1

\

\ 461nm

0,13 o-15 0.16

Fig. 7. Chromaticity diagram for cube A excitation of fluorescent brightening agents (for sample identification, see Table 4).

235

cubes A and H2 impose restrictions on the wavelengths which can be included in colour calculations. For these two systems, a neutral stimulus is one which gives equal emitted fluorescence intensity at wavelengths equal to and above the respective cut-off limits of 430 and 520 nm. Recalculation gives an apparent neutral point of (0.3393, 0.3457) for the cube A system and (0.4848, 0.5065) for cube H2. These points are shown in Fig. 6.

In spite of these restrictions, Figs. 4 and 5 show that some degree of colour discrimination is still available with cube H2 excitation. Those emission colours described in Table 1 as green give ellipses located close to the 555 nm spectral line coordinates, whilst those of orange/red appearance occur at longer wavelengths.

In a similar manner to cube A, different concentrations of the same dyestuff do not give identical chromaticity coordinates, due to the influence of a fluorescent brightening agent. Again, values moved towards the red region of the chromaticity diagram with increasing dye concentration. All the dyestuffs for which a range of concentrations were available conform to this pattern, with the exception of the four Foron Brilliant Yellow SE-6GFL dyeings (samples 22 - 25).

0.75

o-70

Y

0.6E

Fig. 8. Chromaticity diagram for cube H2 excitation of fluorescent brightening agents (for sample identification, see Table 4).

236

For this colorant, increasing dye concentration appears to cause movement towards the green area of the chromaticity diagram, followed by a reversal towards the red. The cause of this is unclear; as the visible spectrum of this colorant shows little absorption at the wavelengths of fluorescence emission, self- absorption is unlikely. However, anomalous behaviour due to the effects of decay cannot be discounted.

Fluorescent brightening agents For the fluorescent brightening agents examined, the mean chromaticity coor-

dinates and error ellipse parameters for both cube A and H2 excitation are given in Table 4. The error ellipses are plotted in Figs. 7 and 8, respectively.

Figure 7 shows that cube A illumination provided colour values concentrated in the blue region of the chromaticity diagram. As only one fluorescent species is now present, chromaticity coordinates are unaffected by changes in dye concentration, various colorant levels giving overlapping or closely adjacent error ellipses. This is also reflected in the size of the ellipses, which in general are much smaller than those obtained for the coloured fibres.

Cube H2 illumination produced error ellipses located close to the coordinates of the 550 nm spectral line, as shown in Fig. 8. Again, the ellipses have been represented as lines equal in length to the major axis. In contrast to the fluorescent dyestuffs, the ellipse major axis lengths are far greater than those calculated for cube A excitation. This may be due to the low emission intensity produced by cube H2 excitation of these materials.

Conclusions

The work described shows that an objective method of description of fluorescence emission colours is possible, with errors in measurement being easi- ly described by the use of error ellipses. The results also have a number of conse- quences for the visual inspection and comparison of fibres in casework. Firstly, for the excitation arrangements employed (identical to those used for comparison fluorescence microscopy), cube A allowed an almost complete range of fluorescence colours to be obtained, whereas the cut-off of cube H2 gave rise to a very restricted range of colours. This limitation could be overcome by replace- ment of the suppression filter with one having a short wavelength cut-off around 500 nm. Roughly half of the standard chromaticity diagram would then be acces- sible while retaining the high emission intensities [5] produced by violet-blue excitation. Secondly, the effects of concentration changes, multiple fluorescent species and decay mean that gross changes in the observed colour may occur for fibres from the same source. In consequence, caution must be exercised when judging fluorescence colour matches.

References

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2 A.W. Hartshorne and D.K. Laing. The definition of colour for single textile fibres by microspectrophotometry. Forensic Sci. ht., 34 (1987) 107 - 129.

3 D.K. Laing, A.W. Hartshorne, R. Cook and G. Robinson. A fiber data collection for forensic scientists - collection and examination methods. J. Forensic Sci., 32 (1987) 364 - 369.

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microspectrofluorimetry of fluorescent dyes and brighteners on single textile fibres: part 2 —...

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