The Development of a Wireless Electrostatic Mark Lifting Method and its use at Crime Scenes

Transcription

1 Technical Note The Development of a Wireless Electrostatic Mark Lifting Method and its use at Crime Scenes Robert Milne Forensic Intelligence Metropolitan Police New Scotland Yard, United Kingdom Abstract: This paper outlines the basic principles and practices involved in the technique of electrostatic dust mark lifting (ESL). Details are included about the development of a three-electrode wireless method used in some currently available commercial devices. Introduction The first recorded instance of dust and light objects being lifted by an attractive force was in 630 BC, when Plato recorded in his dialogue Timaeus that Thales of Miletus used a jet of amber rubbed on silk to lift feathers and other light objects. We now know that the amber had become electrically charged by friction. Indeed, the word electricity comes from the Greek word for amber (i.e., Elektron) [1]. Frictional or static electricity was the first form of electrical phenomena investigated by scientists. Charge generated between different substances was called triboelectricity. This electrical charge phenomenon is that which enables the attraction of dust marks to electrostatic film. A quantitative investigation of electric charge was made by Charles Augustine Coulomb ( ). Coulomb devised a torsion balance, which he used to measure the electrostatic Received October 18, 2010; accepted March 3, / 62 (2), 2012

2 attractive or repulsive force between two pith balls. He found that the electrical force between the two bodies followed the inverse square law. See Figure 1 for a version of his equation applied to electrostatic lifting. The dielectric constant for Mylar film is approximately 5. Figure 1 A version of Coulomb s Law for ESL lifting [2]. In this version of Coulomb s Law, d is the thickness of the Mylar film that insulates the charged upper surface of the film from the substrate carrying the dust mark(s). The negative charge from the electrostatic dusting lifting (ESL) device is distributed over the upper surface of the lifting film, which is made of vacuum deposited aluminum a good electrical conductor. The resistance is between the earth plate and the reactance of the substrate from which the mark is lifted. The technique of ESL in crime investigation originated in Japan in 1965 [3]. In 1970, the first device was constructed using thermionic tube technology by a police officer, Kato Masao [4]. Masao used a television high-voltage supply and made his own lifting film by attaching aluminum foil to rolls of vinyl film. The technique was developed because of the extensive use of woven reed and paper products in Japanese f looring. In the early 1980s, the Metropolitan Police Department of Technology produced a few high-voltage supplies with large earth plates, cables, and brass block electrodes for the purpose of supplying crime scene equipment. The cost of devices available at the time was in the 1500 range ($3000 US). Equipment was shared between agencies and had to be collected over large distances. The ESL device used by the Metropolitan Police in London was bulky and had brittle high-voltage wires, which often broke off, resulting in the equipment being sent for repair. The batteries in the equipment were rechargeable, but because the equipment was shared often, the batteries were seldom charged. 62 (2), 2012 \ 155

3 In 1995, I designed the first three-electrode wireless ESL. It had no wires to break, ran on a standard alkaline 9-volt battery, and the cost at the time was only 200 each. In the device, two electrodes in line form continuity contacts when placed on a metal earth plate, and the third electrode delivers a high-voltage negative charge, the positive charge being from the earth plate, thus eliminating the need for wires, clips, and brass blocks as conductors. These wireless ESL devices were purchased in bulk by many police departments, and crime scene investigators were able to benefit from the personal issue of these devices. Electrostatic Mark Lifting The technique of electrostatic lifting enables crime scene examiners to search for and recover footwear dust marks from surfaces (Figure 2) where often it would be impossible by other means. Dust marks can be very faint and difficult to find on textured or patterned surfaces, making direct photography of marks difficult. It is not possible to enhance dust marks by applying powder with brushes because the act of applying powder will damage or destroy the dust marks. In fact, fingerprint powders should not be used until after the floor areas have been searched with ESL film, because metallic powders, such as aluminum, will settle on the search areas and severely compromise the electrostatic lifting of marks. The technique of electrostatic lifting has three main functions. The first function is to lift dust marks found by visible light following photography of the mark, if possible. The second function is to conduct searches for latent dust marks. The third function is to clean up marks following in situ photography and before the use of rubber gelatin lifting sheets. Once the marks have been electrostatically lifted on Mylar film, the film may be attached to a cardboard folder, put in the bottom of an exhibit box, or, in the case of long lifts, carefully rolled up (metallized side outermost) and preserved in a cardboard exhibit box or tube. The preferred method is to put the lift in the bottom of an exhibit box, then photograph it in studio conditions as soon as possible. Quality photography of the lifted marks then presents the marks in a robust form for coding, screening, and comparison by crime scene examiners 156 / 62 (2), 2012

4 Figure 2 An ESL lift from a fabric chair seat cover. Note that the second-level wear detail and manufacturer s name are visible. and forensic scientists. Scientists at the Forensic Science Service Metropolitan Laboratory prefer quality electrostatic lifts to gelatin lifts [5]. Gelatin lifts can suffer from rapid deterioration and shrinkage, especially in warm conditions. Electrostatic lifts can also suffer from deterioration over long periods, but they are not affected by high temperatures in the summer months as is the case with gelatin lifters. The process of electrostatic lifting involves placing a sheet of black Mylar film over a dust mark or floor area, then charging the film with several thousand volts. Mylar film is a polyester film originally manufactured by Du Pont. When manufactured, it is in a clear form. Other contractors coat the film with a black gloss finish on one side and then vacuum deposit aluminum on the other side of the film. Mylar film is supplied in rolls, with the black surface on the outside of the roll enabling long lifts to be made directly from the roll, although individual marks are made by cutting a section from the roll to cover the mark. The aluminum coating enables the electric charge to be evenly distributed across the film. 62 (2), 2012 \ 157

5 The substrates upon which marks are found are often poor conductors of electricity, and several thousand volts are required to make free electrons in the substrate move when attracted to the positive charge on the dust mark lifter s earth plate. Once charged, the film is strongly attracted to the surface, and dust particles forming the mark(s) are lifted perpendicular to the film. Because air is sometimes trapped under the film, an insulated free running roller is required to gently remove any trapped air. In cases where the film does not seem to attract to the substrate, the roller is used to carefully contact the film on the substrate to recover marks. The ESL technique allows the recovery of high-quality, thirdlevel detail marks from smooth conducting or nonconducting surfaces and the recovery of marks from textile surfaces such as carpets, upholstery, and wood-grained surfaces, although heavy texturing will cause lack of detail [5]. The output of the types of voltage multiplier circuits used in ESL devices is not in the form of direct current but is an alternating current field, oscillating, in the case of the prototype design, measured by an oscilloscope at approximately 30 khz. This enables the propagation of the charge from the ESL device through materials, which are commonly viewed as insulators. (One power source that is not covered in this article is the ESL Taser adaptation. In many countries, the possession of a Taser is a criminal offense even if it has been adapted. The high current can cause danger to health, especially those with heart problems, so this abuse of the Taser type of antipersonnel device should be avoided.) The main suppliers of wired and wireless ESL devices use the Cockroft Walton circuit. The only difference with the prototype described in this article is that in more modern devices, integrated circuits are used as an oscillator and amplifier instead of discreet transistors. The transformer in the commercial versions is sometimes an inductor used in mobile phone technology, but its function is the same as the transformer described below. The circuit chosen for the prototype device was the Cockroft Walton voltage multiplier circuit, which is depicted by the ladder of diodes (Figure 3). A form of oscillator is used to convert the direct current from the battery to an alternating current. This is depicted by the two cross-wired transistors TR1 and TR2, called a multivibrator, where the outputs of the transistors are fed back to their inputs, causing oscillations. This alternating current 158 / 62 (2), 2012

6 is fed to two further transistors, TR3 and TR4, here shown in push pull amplifier configuration. The amplified sine wave is further increased in voltage through the transformer, the primary winding of which forms the load for the output of the transistors TR3 and TR4. The secondary windings of the transformer have a high ratio, typically 100:1, producing a high voltage of approximately 800 volts to the Cockroft Walton voltage multiplier circuit, resulting in over 8 kilovolts at its output but at very low current in the micro amps range. Figure 3 Circuit diagram of prototype ESL with three-electrode system. 62 (2), 2012 \ 159

7 It was found that in cases where difficulty was encountered in getting the charged film to attract to certain substrates, earth bonding resulted in a considerable improvement in lifting force. An earth bonding kit was created using static conducting plastic cord of the type used in chassis bonding kits employed by electronics assembly and service workers. Bonding the earth plate of the device to either a mains earth or water pipe creates thousands of high-tension tracks. Ohms Law [6] tells us that the more parallel tracks of electrical current there are, the lower the resistance or, in the case of alternating current, the lower the overall reactance, Z, expressed as: In OHMs Law, if several resistances are connected in parallel, the sum of the reciprocals of their individual resistances is equal to the reciprocal of their total resistance. So with potentially a great number of high tension tracks created by earth bonding, the overall reactance to the alternating current from the ESL device becomes low, resulting in potentially more lifting power being present on the film. Bonding can be made to plaster and concrete walls using a contact plate, even if wall coverings are present, the power transferring by capacitance transfer. Plaster and concrete conduct well possibly because water molecules, which are dipolar, are bound in the materials and can conduct an alternating current at several kilohertz. The effects of the earth bonding kit are reproducible every time an ESL device is earth bonded because there is a visible increase in attraction to the substrate and often a sizzling noise is heard. On extremely conductive surfaces, the film can spark and holes can be burned in the film. 160 / 62 (2), 2012

8 Development of a Commercial Wireless Dust Mark Lifter The main objective of the project (Figure 4) was to make an economically viable, compact device that would be safe to handle and could easily be carried in a crime scene equipment case. Safety concerns were addressed by producing a device that could not become accidentally live in the operator s hands. The device would have to be placed deliberately on its earth plate to function. Once removed from the earth plate, it would switch off. Further, if the device were to be placed on an all metal surface and switched on, the device would in effect shut itself down and not make that surface live. When making lifts from all metal surfaces, a small polycarbonate sheet would have to be placed under the earth plate and power transferred to the substrate by capacitance transfer so lifts could be made. To improve performance on difficult surfaces, an earth bonding kit was designed. Commercial versions of the wireless three-electrode dust mark lifter are available from various manufacturers. Thousands of dust mark lifters of the three-electrode wireless type have been marketed during the past thirteen years. Figure 4 The first electrostatic lift made by the prototype three-electrode wireless ESL device. 62 (2), 2012 \ 161

9 An ESL Technique When searching for dust marks, two techniques are used: oblique lighting and cold searching(using film to cover entire areas to find marks that may not have been found with the oblique lighting technique). Once located, marks can be further lifted with gel. This is a standard search and recovery technique used by the Metropolitan Police Serious Crimes Unit. It should be remembered that not all of the particles composing the mark(s) are lifted by ESL and often marks are cleaned up by making a lift [5]. Obviously, covering entire floor areas with lifting gel to search for marks is not viable because the examiner would not know where the mark boundaries of each faint mark would be, particularly on patterned surfaces. When lifting dust marks with either type, it is best, wherever possible, to make ESL lifts directly from the roll of Mylar film. When a lift is made directly from the roll of film, the problem of trapped air under the film as it is charged is largely eliminated. Lifts from the roll have been made by the author up to several feet long when the conductivity of the surface was good. In some instances, where there is a poor ground, it is better to use individually cut sheets of Mylar. There will be locations where conductivity is really bad and other techniques will have to be used. Damp conditions reduce the effectiveness of ESL lifting and this should be kept in mind before attempting ESL lifting. Each labeled and numbered strip should be carefully rolled up and stored for later examination and mounting of shoe marks in controlled conditions at the office or laboratory. Lifts made from cut panels of Mylar should be packaged in the correct type of folder or box at the time the lifts are made. Mylar should be cut with a scalpel with the black side uppermost. This ensures that shards of the aluminum coating from the upper side of the film caused by using scissors do not hang down making contact with the substrate from which the mark will be lifted. This definitely causes arcing at the cut edge of the film, thus reducing the lifting power. Arcing causes small pockets of ionized gas, which create short circuits to the substrate The use of scissors should definitely be avoided, as should the practice of putting a staple through the film. This will almost certainly cause arcing at the points where the staple perforates the film. In the case of a lift made from a conducting surface, the staple will make a short circuit. It is literally possible to cause burning to Mylar film by excessive charging of the film coupled with arcing caused by irregular cutting of the film. 162 / 62 (2), 2012

10 In most cases, electrostatic dust lifters will lift marks without connection to the mains or electrical earthing points in buildings, such as the main earth bonding via a socket or via a water pipe. If difficulties are experienced on surfaces with high insulation properties, then an earth bonding kit should be used (Figure 5). These kits connect the earth plate of the ESL machine to either the earth connection of an electrical socket via a static conducting plastic cord or to a water or central heating pipe. This creates many high-tension tracks through the building to the ESL film, greatly reducing the electrical resistance to the charge because the multiple high-tension tracks are in parallel. The reduced total resistance applies more lifting power to the film. When dealing with scattered papers on the f loor at crime scenes, remember the crusty, dramatic-looking dirt or dust visual marks on papers will usually be of Level 2 detail. Scan the apparently blank sheets with a charged Mylar film and superb Level 3 detail marks of evidential value will often be found. It is of interest to note that in cases where the film is not attracted well to a surface, the film still has a high voltage charge and the gentle application of a wide insulated roller will cause physical contact with the dust mark, which will lift regardless of whether the suction effect is present. Do not recycle Mylar film by wiping it for re-use; the potential for cross-scene contamination must be avoided. Figure 5 Pathfinder with earth bonding kit. (Photo courtesy of G. Sandling, Crime Scene Products Inc., Port Charlotte, FL.) 62 (2), 2012 \ 163

11 Conclusion Electrostatic dust mark lifting is a most useful technique. It can provide the quality of marks required for screening and comparison, and it can also be used as a technique for cleaning up some marks before gelatin lifting or photography. For further information, please contact: References Robert Milne Crime Scene Investigations Equipment Ltd. Locard House Deethe Farm Estate Cranfield Road Woburn Sands, UK MK 178UR bob@csiequipment.com 1. Weinberg, S. The Discovery of Sub Atomic Particles; Penguin Books Ltd.: Westminster, U.K. 1990; pp 14, Milne, R. The Electrostatic Lifting of Shoe, Tyre and Finger Marks at Crime Scenes. Fingerprint Whorld 1997, 23 (88), Bodziak, W. J. Footwear Impression Evidence, 2 nd ed.; CRC Press: Boca Raton, FL 1999; p Electrostatic Method for Lifting Footprints. Int. Crim. Pol. Revue, 1973, 272, Young, R.; Morantz, D. J.; Davis, R. J.; Hoole, R. Electrostatic Lifting of Dusty Shoe Marks; Metropolitan Police Forensic Science Laboratory, London CRE Technical Note No , pp Scroggie, M. G. Foundations of Wireless and Electronics, 9th ed; Newnes Technical Books: London, 1980; pp 25 27, / 62 (2), 2012