Journal of Forensic Identification

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 (1736 –1806). Coulomb devised
a torsion balance, which he used to measure the electrostatic
Received October 18, 2010; accepted March 3, 2011
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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.
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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 f loor 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
f ilm, the f ilm 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
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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 Laborator y 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 f loor 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.
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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 transfor mer 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
Journal of Forensic Identification
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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.
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It was found that in cases where difficulty was encountered
in getting the charged f ilm to attract to cer tain 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.
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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.
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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 f loor 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.
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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 cr usty, dramatic-looking dir t 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 f ilm 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.)
Journal of Forensic Identification
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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:
Robert Milne
Crime Scene Investigations Equipment Ltd.
Locard House
Deethe Farm Estate
Cranfield Road
Woburn Sands, UK MK 178UR
[email protected]
References
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Marks at Crime Scenes. Fingerprint Whorld 1997, 23 (88),
53–64.
3. Bodziak, W. J. Footwear Impression Evidence, 2 nd ed.; CRC
Press: Boca Raton, FL 1999; p 101. 4. Electrostatic Method for Lifting Footprints. Int. Crim. Pol.
Revue, 1973, 272, 287–292.
5. You ng, R .; Mor a nt z , D. J.; Dav is , R . J.; Ho ole, R .
Electrostatic Lifting of Dusty Shoe Marks; Metropolitan
Police Forensic Science Laboratory, London CRE Technical
Note No. 385. 1983, pp 12–24.
6. Scroggie, M. G. Foundations of Wireless and Electronics,
9th ed; Newnes Technical Books: London, 1980; pp 25–27,
42– 44.
1.
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