A rapid infrared luminescence method for differentiation of ink writing

A rapid infrared luminescence method for differentiation of ink writing

Forensic Science International, 15 (1980) 153 - 159 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 153 A RAPID INFRARED LUMINESCENCE...

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Forensic Science International, 15 (1980) 153 - 159 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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A RAPID INFRARED LUMINESCENCE METHOD FOR DIFFERENTIATION OF INK WRITING

D. R. ROHILLA,

S. K. DAS GUPTA, S. K. GUPTA and H. L. BAMI

Central Forensic Science Laboratory, New Delhi . 110022 (India)

Central Bureau of Investigation, R. K. Puram,

(Received June 4, 1979; in revised form September 20, 1979)

Summary A simple and direct method is described for observing and photographing infrared luminescence of ink writing using an infrared image converter and fibre optics system of the Projectina Comparator Model 8031. This technique has been evaluated by studying 100 inks of Indian origin and 44 reference ink dyes with two types of papers. The merits of the technique and its application are discussed.

Infrared radiation has been exploited to the maximum extent in the field of ink comparison using the technique of reflected infrared photography [ 1 - 31, visual examination with an infrared image converter [ 3, 41 using various filters (700 - 850 nm), and infrared luminescence photography [ 1 - 151. In infrared luminescence, when the document is excited by radiations in the blue-green region (400 - 625 nm), the infrared radiation emitted from ink writing is usually of the longer wavelengths, i.e. 725 nm and above. Some of the ink dyes have the property of infrared luminescence and hence the inks containing these dyes show infrared luminescence. Various types of excitation sources coupled with different filters have been used in the 400 625 nm range and the emitted radiation (usually in the infrared or far-red range) were directly recorded on infrared-sensitive films. Light sources, filters and other detection devices for infrared luminescence have been discussed by a number of authors 11 - 111. Ellen and Creer [7] and Chowdhry et al. [ 31 employed a specially constructed infrared luminescence box for recording the luminescence of various inks on documents using high-speed infrared film and 10% copper sulphate solution as filter of incident light. Television display of infrared luminescence has been achieved by Richards [14] using a video technique, while Hardcastle and Hall [ 151 recently reported intensification of the infrared luminescence effect using liquid nitrogen for cooling the documents. Howes [13] recently observed the infrared luminescence of various ink writings using “Star-tron Night Vision System” Model MK 222 employing eight combinations of glass filters, and compared this method with other conventional photographic methods. As an extension of the available techniques, the authors have directly observed and recorded the infrared luminescence effect using a conventional

Fig. 1. Set-up for recording infrared luminescence of ink writings using an infrared image converter and fiber optics system of the Projectina Comparator.

infrared image converter and the fibre optic system of Projectina Comparator Model 8031 in a darkened room. The technique dispenses with the use of 10% copper sulphate solution cells, infrared film and other cumbersome arrangements. The technique has proved both fast and more reliable for the questioned document examiner who can directly study infrared luminescence in relation to problems of ink comparison. The new method has also been applied to Indian inks and ink dyes to confirm its usefulness, as well as to acquire basic data keeping in mind also the type of paper involved.

Experimental Technique An infrared image converter (Leitz) was used together with a Projectina Comparator Model 8031. The pair of fibre optics of the Projectina Comparator was used whereby a blue-green filter (No. BG38 6031) was placed before each of the two light sources (halogen lamps). Thus each of the two fibre optics emitted a uniform beam of light in the blue-green region (400 625 nm). The document was placed under the infrared image converter and the pair of uniform beams of light was focussed on the ink writing without illuminating the entire document (Fig. 1). The experiment was carried out in a dark room by switching off the lights and drawing the curtains. The Leitz infrared image converter is generally supplied with a set of filters covering the 700 - 900 nm range. It was observed that infrared luminescence of

Fig. 2. Infrared luminescence

photograph

of writing in blue ink from a ball-point pen.

the ink writings could be converted to white lines against a dark background when a 750 nm or 780 nm infrared filter was used. The 750 nm infrared filter was finally selected for the study. Infrared luminescence was not effectively achieved by this technique when the 700 nm filter was used. This filter, however, was used to determine the infrared reflectance using the conventional light source. Similarly, filters beyond 785 nm (i.e. 810, 850 and 900 nm) were also found not to be suitable for infrared luminescence studies by this method. Infrared luminescence, when observed, was recorded photographically using a fast panchromatic film (400 ASA) and an exposure time of about 30 minutes (Fig. 2). Although the fibre optics system of the Projectina Comparator were used in this study, any other similar device could serve the same purpose. The basic approach was to use the two common instruments readily available to the expert to obtain the full data quickly. Examination of writings with Indian inks and dyes As a part of an overall research programme, this technique has been evaluated to differentiate various types of standard inks available on the Indian market on the basis of their infrared luminescence properties. One hundred different fountain pen inks, ball-point pen inks and fibre-tip pen inks of blue, black, red and green colours were selected for infrared luminescence studies. Similarly, 44 synthetic dyes generally used as constituents of Indian inks were also studied. The writings used in this study were made

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with the above inks and dyes on white bond paper cards as well as on ordinary white paper.

Discussion The method A filter of copper sulphate solution (10%) to cut off red and infrared radiations from the excitation source has been used for recording infrared luminescence on infrared films by a number of authors [ 1 - 111. However, Ellen and Creer [7] observed weak infrared luminescence when an infrared image converter was used instead of photography with infrared film under the same conditions. The question of using a copper sulphate solution filter in the Projectina Comparator presented large hurdles and hence was not attempted. In the present study, involving BG 38 (blue-green glass filter), we had an opportunity to evaluate the new technique with respect to some of the ink writings that had been previously studied for their infrared luminescence by the conventional method [ 31. It was observed that nine inks out of 71 previously showing infrared luminescence [ 31, also gave positive results by the present technique. Moreover, nine other inks which did not display infrared luminescence previously, also gave faint but positive infrared luminescence with the present technique. It was therefore evident that the present technique has a larger range and higher sensitivity. Several workers [ 7,10,11,13] have used Wratten filters 87C (800 nm), 87 (750 nm), 88A (725 nm) and 89B (690 nm) for filtering out the emitted radiations in the conventional infrared luminescence technique. The cut-off values for these filters vary between 800 and 690 nm, and they gave various degrees of success under the experimental conditions studied. In the present technique, the Leitz infrared filter with cut-off value at 750 nm was found to be the most suitable. Infrared luminescence using the 700 nm filter was weak since the emissions from 700 nm filter are a mixture of far-red and infrared radiations. The infrared radiation in the presence of far-red radiation resulted in a weak display of infrared luminescence due to interference by the far-red. Use of the 700 nm filter resulted in a weak luminescence even in the case of inks which otherwise gave a very strong luminescence with the 750 nm Leitz infrared filter. This supported the conclusion drawn earlier. Leitz infrared filters beyond 780 nm did not allow the required transmission and were found to be unsuitable for use with the infrared image converter. It is apparent that the 750 nm filter cuts off far-red radiation adequately and that the infrared image converter works most efficiently in eliminating background interference and highlighting the infrared luminescence. Kevern [ 111 also recorded a better performance with the 750 nm infrared filter. In the present technique the fibre optics of the Projectina Comparator offer a very concentrated source of the required light directed onto a small area of the document with the required illumination characteristics. The resultant infrared luminescence is hence equally as strong. It was also observed

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that the intensity of the infrared luminescence is directly related to the intensity of illumination of the document. This condition is difficult to fulfil by other systems of illumination where the illumination is more diffuse and a large amount of heat is generated, making it uncomfortable to use for prolonged periods. The present system of illumination is compact. It can be directed to the required area with the desired intensity and is easy to manipulate for long periods. It must be admitted that the area of examination will be limited under these conditions. With the conventional technique the use of infrared-sensitive films has not been without its problems. Infrared films according to their make and type are sensitive to blue radiations like any other panchromatic black and white films and are also sensitive to far-red and infrared radiations. Their performance varies since these films have a limited life. Obtaining consistent results is therefore a problem. Furthermore, with the conventional technique, one has to wait for the film to be developed to know the result. Non-detection of infrared luminescence in a given case may sometimes be due to fault of the film and/or its processing. Several trials may be necessary to confirm the results. On the other hand, with the present technique the document examiner has a direct view of the results achieved. Photographs, if needed for an expert opinion, can be made. Very often, if the results are not positive, the necessity of prolonged film exposure, etc., can be dispensed with. The expert thus has greater control and confidence in the work which is being carried out. Data for Indian inks and dyes Out of the total of 100 fountain pen, ball-point pen and fibre-tip pen inks studied, 24 inks showed infrared luminescence on both types of paper, while 22 inks absorbed these radiations and the writing appeared as dark strokes. The remaining 54 inks were rendered invisible under the conditions of this test. Infrared luminescence of inks by this method was a good parameter for comparison. The luminescence was found to be significant amongst green fountain pen inks, blue fibre-tip pen inks and blue ball-point pen inks. Within the inks of the same colour of a given type, the differences were often not significant so far as infrared luminescence was concerned. This was primarily due to the fact that often the dyes involved were identical. The type of the paper used did affect the infrared luminescence property of both the dyes and the inks with this technique. For instance, dyes such as Acid Orange GII extra, Auramina 0, Ink Blue and Methylene Blue 2BH showed no infrared luminescence when written on white bond paper. However, on ordinary white paper a faint luminescence could be observed in these dyes. Similarly, writings made with certain blue and green fountain pen inks did not luminesce on white bond paper but showed faint infrared luminescence on ordinary white paper. It was found, however, that some of the inks such as a green fibre-tip pen ink and red ball-point pen inks, although containing infrared luminescent dyes, did not show infrared luminescence possibly due to the quenching effect of solvents or chemical additives present in the inks.

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It appeared that the masking effect of ink adjuvants was significant in certain cases in quenching the infrared luminescence. Additives in superior bond paper, especially fluorescent brighteners, could perhaps be responsible for diminishing the infrared luminescence by the method studied [ 161.

Conclusion The technique presented utilises the fibre optics light sources of the Projectina Comparator for rapid scanning of ink writings for their infrared luminescence using a conventional infrared image converter. As required, direct photography of the results can be achieved without recourse to special infrared films, etc. The whole procedure is simple and rapid for a busy document examiner. Coupled with visual examination under the infrared image converter, the present infrared luminescence technique gives an additional parameter for ink differentiation. The basic data on various inks and dyes keeping in mind the type of paper involved can thus provide several variables as a basis for ink comparison. The nondestructive optical techniques continue to enjoy an advantage over other methods of ink comparison. The present method offers a simple and rapid infrared luminescence examination facility to the expert for achieving better standards of ink comparison.

Acknowledgement Financial support to Mr. D. R. Rohilla as research D., M. H. A., New Delhi, is gratefully acknowledged.

fellow by B. P. R. &

References 1 2

3 4 5 6 7 8

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9 P. Veillon, 0. Rothenbuehler and J. Mathyer, Some remarks on optical examination of inks. ht. Crim. Police Rev., 262 (1972) 238 - 255. 10 J. E. Costain and G. W. Lewis, A practical guide to infrared luminescence applied to questioned document problems. J. Police Sci. Admin., 1 (1973) 209 - 218. 11 R. M. Kevern, Infrared luminescence from thin layer chromatogram of inks. J. Forensic Sci. Sot., 13 (1973) 25 - 28. 12 0. Hilton, Identification and differentiation between coloured pencils. Forensic Sci., 6 (1975) 221 - 228. 13 D. S. Howes, A rapid screening device for infrared luminescence examination of questioned documents. Can. Sot. Forensic Sci. J., 11 (1978) 23 - 40. 14 G. B. Richards, The application of electronic video technique to infrared and UV examination. J. Forensic Sci., 22 (1977) 53 - 60. 15 R. A. Hardcastle and M. G. Hall, A technique for the enhancement of the infrared luminescence of inks. J. Forensic Sci. Sot., 18 (1973) 53 - 55. 16 S. K. Gupta, S. L. Mukhi and H. L. Bami, Differentiation of inks on documents by dequenching of ultraviolet fluorescence: A case report; Forensic Sci. Znt., 12 (1978) 61 - 64.