Forensic Science International, 45 (1996163-71 Elsevier Scientific Publishers Ireland Ltd.

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THE APPLICATION OF HALF-SILVERED MIRROR IN CONJUNCTION WITH CASTING TECHNIQUES TO DOCUMENT EXAMINATION

SC. LEUNG and M.M.H. TANG

Forensic Science Division, Government Laboratory, l/F Chung Nam Building, 1, Lockhart Road, Wanchai, Hong Kong

(Received June 13th. 19891 (Revision received August 18th, 19891 (Accepted August Zlst, 19891

Summary

The Projectina Universal Comparison Projector is well known for the examination of docu- ments. Where sufficient surface details cannot be seen under conventional lighting conditions, the half-silvered mirror accessory will give enhanced contrast and clear edged appearance to the object being studied. A cast of the indented surface can help to eliminate any background inter- ference due to undesirable reflection resulting from surface printing or irregularies. In this paper, enhancement by the application of a casting technique together with the half-silvered mirror accessory of the Projectina Universal Comparison Projector is discussed.

Key words: Document examination; Half-silvered mirror: Casting technique; Comparison projec- tor

Introduction

Since its presentation at the 7th meeting of the International Association of Forensic Sciences in 1975, the Projectina Universal Comparison Projector has become well-known in the field of document examination. It was consid- ered to be useful for comparing typewriting, photocopies and computer- printed and mechanically produced documents [1,2]. Pfefferli [3] maintained that the basic purpose of the Projectina Universal Comparison Projector was the superimposition of documents in the macro range and when comple- mented with the IR-TV system, it became a versatile instrument for a large number of possible applications. With the many accessories, the potential capabilities of the Projectina can be extended; the half-silvered mirror, for example, was considered to be useful for the examination of embossing seals [4] by providing vertical lighting to the object being examined.

Casting techniques are generally useful for such examinations as toolmarks, dental materials and vehicle identification numbers [5,6]. Totty [7] discussed the preparation of casts using fusible alloy spraying equipment

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and the technique was applied to the comparison of defects in plastic labell- ing tape and to the decipherment of impressions of handwriting. Thorpe [8] described the uses of silicone rubber casting in document examination and anticipated that in future, the solution of many and varied problems might be greatly aided by the intelligent use of casting compounds.

In this paper, the application of the casting technique using ‘Mikrosil’ and the half-silvered mirror accessory of the Projectina Universal Comparison Projector to solve image enhancement problems is discussed.

Vertical Illumination and the Half-silvered Mirror

The half-silvered mirror accessory of the Projectina Universal Comparison Projector consists of a condenser which collimates light from the lamp to form parallel beams onto a half-silvered mirror aligned at an angle of 45O to the optical axis of the light source (Fig. 11, thus providing vertical illumina- tion on the object being examined. The optics are such that when a beam of light strikes on a horizontal surface, it is reflected along its own path, part of which reaches the eye and the surface therefore appears to be brightly illu- minated. when the light falls on a slanting surface, such as the edge of an object, it is reflected away and will not reach the eye, causing the edge to appear dark (Fig. 21. The effect of vertical illumination is illustrated in Fig. 3 with a coin. Using the half-silvered mirror, distracting shadows which are inevitably present with side lighting, can be eliminated and the image will have a clear edged appearance due to enhanced contrast at the edges. Fine detail which cannot be seen using other lighting conditions usually becomes evident.

Shaneyfelt [9] described the examination of faint markings on metal sur- faces using direct vertical illumination from a beam splitter placed in front

(a) (b)

Fig. 1. (a) The Projectina Universal Comparison Projector fitted with the half-silvered mirror (arrow). (b) The half-silvered mirror accessory.

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/

I; k//////////,,,,

Object being examined

Fig. 2. Optics of the half-silvered mirror.

Collimated beams of Light

Fig. 3. Appearance of a coin under vertical illumination - note the enhanced contrast at the edges.

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of a camera lens. For questioned document examinations, counterfeit coins, embossing seals and impressed particulars on credit cards are typical exam- ples which can be examined with the half-silvered mirror to yield satisfactory results. In Fig. 4a, the overlapping double embossed impression located on the laminated photograph of a Thailand Passport is evident under vertical illumination. Comparison with the corresponding portion of the seal on the back of the page in Fig. 4b revealed that one of the double embossed impression was not genuine.

While writing impressions can be recovered using the Electrostatic Detec- tion Apparatus (ESDA) [lo], examination results on indentations on glossy ‘polymer coated’ paper are generally poor. This can be attributed to the insu- lating polymer coating on the surface of paper rendering excessively long charging time [ll]. The half-silvered mirror can be used as a supplementary method for examinations of this type (Fig. 51, the advantage of which is espe- cially evident when the indented writing is present on non-reflecting (such as black) surfaces where side lighting cannot bring out the indentation.

Vertical illumination provided by the half-silvered mirror enables a two- dimensional examination of questioned documents which can be used to advantage to bring out invisible or indistinct features. Unfortunately, it has the disadvantage that certain surface irregularities which give rise to dis- tracting reflections and any highly contrasting background of the document interfere with the observation. Casting techniques can be used to remove these undesirable abberations (Fig. 61.

(a) 0-d

Fig. 4. (a) Double embossed impressions on the laminated holder’s photograph of a passport. (b) Corresponding portion of the seal on the back of the page (reverse print).

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Fig. 5. Examination of indented writing on the surface of a piece of polymer coated paper with the half-silvered mirror.

The Casting Technique

Mikrosil (a casting material for forensic use) made in Sweden by Kjell Carlsson Innovation [5] has been formulated to give good rendering of small detail, good contrast for microscopical observation, good releasing ability for the cast and short setting times. The casting material is supplied together with two tubes of hardener (for fast and slow setting). To obtain a mould of the indentation, a thin layer of a thorough mixture of the casting material and hardener is spread over the surface to be cast. Care must be exercised to remove any trapped air as its presence can lead to loss of detail or to the introduction of spurious characteristics. When all the bubbles have been worked out, an extra layer of mixture is added to give a stronger backing. A suitable weight on a thin plastic sheet or a metal plate is then placed over the cast to give it a flat base on setting. The setting time can range from a few minutes to more than an hour, depending on the temperature and the type and amount of hardener used. For the inexperienced, it is always easier to start with a smaller amount of hardener to prevent premature setting. The authors found that an appropriate mixture was obtained with equal lengths from the dissimilar tubes of casting material and slow hardener giv- ing a setting time of about 10 minutes.

The casting technique using Mikrosil has been applied to the examination of embossed impressions such as those present on travelling documents [4]. Figure 7b illustrates the enhanced contrast produced by vertical illumination

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on a double embossed impression on a Thailand Passport. It can be seen that the Mikrosil cast in Fig. ‘7~ provides a better resolution of the same impres- sion.

The casting technique using Mikrosil can also be used in the examination of credit cards. With the introduction of laser holographs as a security feature on many credit cards, it has become more difficult to produce counterfeit credit cards good enough to avoid arousing suspicion. Hence a new trend of credit card forgery in Hong Kong has emerged in which particulars on stolen genuine cards are flattened under controlled heat and pressure and new particlulars, then re-embossed. Very often, this is also accompanied by alteration of information in the magnetic stripes on the credit cards to match with the re-embossed particulars. Such credit cards are often so skilfully forged that conventional observational methods do not yield conclusive results. The half-silvered mirror attachment of the Projec- tina is useful for the examination of this type but the printing on the card often hinders observation. The Mikrosil casting technique which enables the production of an accurate replica of the embossed particulars, can effectively eliminate the background interference problem.

Figure 8 illustrates a comparison of side lighting and vertical illumination on an altered credit card and its Mikrosil cast. The superiority of the cast in rendering small detail decipherable, is best demonstrated with the shallow hot pressed number indicated by an arrow.

Discussion

Vertical illumination provided by the half-silvered mirror can reveal evi- dence that cannot be produced by any other method but there is the disad- vantage that certain surface irregularities give rise to undesirable reflections, and any highly contrasting background can interfere with the observation. In such cases, a cast of the questioned document can be exam- ined in detail with the undesirable reflections and background interference successfully eliminated.

With paper documents, one possible drawback in the use of Mikrosil is that the paper surface may become stained with brown spots, and a decision may need to be made on a case-by-case basis whether the staining can be tolerated, to produce the desired decipherment.

The authors do not experience serious objections in relation to Mikrosil staining in criminal trials. The staining, however, can be reduced or elimi- nated by prior treatment of the document with mould release agents (e.g. Releasil A, manufactured by Dow Corning1 or with hair sprays (e.g. Spray Net, manufactured by Helene Curtis). The former has the disadvantage of changing the opacity of the paper rendering it translucent, while the latter will dissolve ink. It appears, on the other hand, that Mikrosil was not spe- cially developed for document work and the staining problem can be resolved if the casting material was made colourless or transparent.

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Acknowledgements

The authors would like to record here their gratitude to Mr. N.S. Lee, the Government Chemist of Hong Kong for his interest and encouragement in this work and to Dr. B.N. Dailly, Assistant Government Chemist for proof reading the draft. Thanks are also due to Mr. T.C. Poon who helped in the preparation of the manuscript.

References

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D.M. Ellen, The role of the document examiners in offences of criminal deception. J. Foren- sic Sci. Sot., 1’7 (19771 105- 112. J.M. Winchester, Use of the Projectina Universal Comparison Projector in comparing type- writing, photocopies, and computer-printed and mechanically produced documents. J. Foren- sic Sci, 25 (19801390-397. P.W. Pfefferli, The Projectina Universal Comparison Projector. J. Forensic Sci Sot., 22 (19821 217 - 220. S.C. Leung, Methods to detect forged embossing seals. Paper presented in the 8th Interna- tional Australian Forensic Science Symposium, Perth, 1983. K. Carlsson, A new casting material for forensic use. Int. Grim. Police Rev., 346 (19811 74- 78. G.S. Klees, An effective material & method for casting vehicle identification number. J. Forensic Sci Sot., 21 (19811 347-348. R.N. Totty, Casting techniques using metal spraying equipment. J. Forensic Sci. Sot., 17 (1977) 15- 20. W.D. Thorpe, The uses of silicone rubber by the document examiner. J. Forensic Sci., 16 (1971) 530- 534. L.L. Shaneyfelt, Obliterations; alterations and related document problems. J. Forensic Sci, 16 (19711331-342. D.M. Ellen, D.J. Foster and D.J. Morantz, The use of electrostatic imaging in the detection of indented impressions. Forensic Sci. Int., 15 (19801 53-60. L. Wanxiang and C. Xiaoling, A study of the principle of the electrostatic imaging tech- nique. J. Forensic Sci. Sot., 28 (19881 237-242.