WO2010058090A1

WO2010058090A1 - Novel method and apparatus for forensic applications

Info

Publication number: WO2010058090A1
Application number: PCT/FI2009/050952
Authority: WO
Inventors: Ivan Kassamakov; Juha Aaltonen; Heimo Saarikko; Edward HÆGGSTRÖM; Kalle HANHIJÄRVI
Original assignee: Helsingin Yliopisto
Priority date: 2008-11-24
Filing date: 2009-11-24
Publication date: 2010-05-27
Also published as: FI20086117A0

Abstract

The invention relates to a method and apparatus for analysing a forensic sample. The method comprises illuminating the sample with broadband light in an interferometric configuration through an interferometric objective, varying the distance between the interferometric objective and the sample, measuring a plurality of interference patterns of light reflected from the sample at a plurality of distances between the interferometric objective and the sample, and determining from the interference patterns information on the surface topology of the forensic sample. The invention also provides a new use for Scanning White Light Interferometry (SWLI) in forensic applications. By means of the invention, forensic samples can be analysed more reliably.

Description

Novel Method and Apparatus for Forensic Applications

Field of the Invention The invention relates to forensic analyses. In particular, the invention relates to a novel method and apparatus for analysing bullets and cartridges for example to aid crime investigations and the like studies. In addition, the invention concerns other kinds of forensic studies requiring detailed analysing of surface properties.

Background of the Invention

It is generally known that each firearm forms a unique pattern of deformations, that is, grooves, scratches and indentations, to a bullet or cartridge shot with that firearm. This is mainly because of riflings and random manufacturing marks in gun barrels and cartridge cases, which leave a "ballistic fingerprint" on the bullet or on the shell of the cartridge. This fact is often utilised when investigating crime scenes in order to sort out which weapon was used to fire bullets or cartridges found at the crime scene or from a victim. The investigator can, for example, fire a reference bullet with a gun of a suspect and optically study the surfaces of the reference bullet and the bullet of interest and compare the scratches of the bullets. Experiments of this kind are also referred to as ballistic comparison tests.

The above-mentioned ballistic comparison tests are traditionally carried out by imaging the surfaces using a conventional optical microscope or confocal laser optical microscope. Local intensity variations of the images obtained are indicative of surface deformations in the bullets. Example of a microscopy-based forensic analysis methods and apparatuses are disclosed in US 6018394 and US 2003/0128891. The apparatus of US 6018394 comprises a microscope having an optical axis, means for selectively aligning the optical axis with the sample, means for selectively focusing the microscope to image a surface area of the sample, and illumination means mounted to project light onto the imaged surface area. In the method of US 2003/0128891, a plurality or panoramic views from bullets are optically imaged and a whole picture of the surface of the bullet is created using the plurality of views. The formed pictures are then compared using a computer. The publications also refers to a number of other prior art patent publications relating to conventional optical methods and apparatuses used in ballistic comparison tests. However, present optical ballistic comparison tests based on conventional microscopy are in many cases not as accurate as desired. On the other hand, the confocal imaging technique is in principle a single point measurement, which significantly increases the time required for the measurement. Methods necessitating the use of laser light have the disadvantage that high local light intensities are produced, which limit the scope of samples which can be imaged. For example, biological molecules and tissue may be damaged under laser exposure. This is not desired in the case where the sample imaged contains DNA or other evidence crucial to the investigation, only to mention one example. However, the same applies to other light-sensitive compounds

Unique markings, which are intended to aid forensic studies, can also be created to bullets, cartridges or firearms on purpose. Examples of such markings and methods of production and detection thereof are disclosed in US 2006/0174531 and US 2004/0200108.

Summary of the Invention

It is an object of the invention to achieve a more reliable method for forensic sample analyses. In particular it is an object to achieve a method by means of which more information from the samples can be obtained in order to increase the performance of the analyses. A particular aim of the invention is to achieve a method for which the quality requirements of the sample are lower than in previous analysis methods.

It is also an aim of the invention to achieve a novel apparatus for carrying out the present method.

The invention is based on the idea of using interferometric optical imaging for imaging the forensic samples for obtaining direct information on the depth profile of the samples. In particular, the invention utilizes Scanning White Light Interferometry (SWLI) for imaging the forensic samples.

SWLI is a method which is sensitive directly to surface topology, that is, local depth variations of a surface to be measured, provided that the wavelength band used is suitable for the material to be imaged. In the method according to the invention, a forensic sample is analysed by illuminating the sample with broadband light in an interferometric configuration through an interferometric objective, - varying the distance between the interferometric objective and the sample, and measuring a plurality of interference patterns of light reflected from the sample at a plurality of distances between the interferometric objective and the sample, and determining from the interference patterns information on the surface topology of the forensic sample.

The apparatus according to the invention is designed for analysing bullets or bullet cartridges and comprises a holder for the sample, a broadband light source for illuminating the sample, - interferometric means provided on the optical path of said light and being responsive to surface topology of the sample, detector for recording interferograms affected by the sample, and means for deriving the surface topology of the sample from the interferograms recorded. The holder is capable of holding a cylindrical sample such that the surface topology of its entire cylinder surface can be analysed.

More specifically, the invention is characterized by what is stated in the independent claims.

According to a preferred embodiment, the interferometric configuration is an SWLI configuration. In SWLI, the sample is generally illuminated with low-coherence (broadband) light in order to produce a spatially well- localized interference, which is measured. The interferometric means typically comprises a beam splitter, and a moveable interferometric objective such as a Michelson or Mirau interferometer, adapted to focus a portion of light passing the interferometer to the surface of the sample. By varying the distance between the objective and the sample, the sample surface can be piecewise brought into focus of the objective. According to an embodiment, the light source is a broadband light source, such as a broadband light emitting diode (LED), halogen bulb, arc lamp or supercontinuum light source. The light produced by the light source is guided to the sample through an interferometric objective, whose distance from the sample can be electrically controlled, for example, using a piezoelectric scanner. The interference pattern is measured using a detector, such as a CCD (charge-coupled device) or CMOS (complementary metal oxide semiconductor) camera or the like, which comprises a one- or two-dimensional array of detector subunits (pixels). A plurality of interferograms are recorded by the detector at different distances of the interferometric objective. From the interferograms, a depth profile on one or two dimensions of the sample is reconstructed.

The wavelength band used in the measurement lies preferably in the visible (380 - 750 nm) or infrared (IR, 750 nm - 1000 μm) range. According to a particularly preferred embodiment, the measurement is carried out at near infrared (NIR, 750 nm - 1.4 μm) range.

The sample can be, for example, a projectile, typically a bullet, cartridge, cartridge shell, cartridge chamber, barrel, firing pin of a firearm or any part of these. The firearm can be a gun, in particular a handgun (e.g. pistol or revolver), rifle, assault rifle. That is, in addition to ammunition or part thereof, the sample can be a weapon itself or part thereof.

The present method is suitable not only for analyses related to shooting, but also to other kinds of forensic analyses, such as unveiling theft, forgery, smuggling or the like crime investigations. For example, the sample can be a fingerprint or signature on a substrate, an anti-counterfeit marking or a micro- or nano-fabricated authenticity product. In general, the sample can be any three-dimensional micro- or nano-structured formation, which includes or is intended to include forensically important information on for example the origin, past use, proprietor or manufacturer of the sample. In a typical forensic analysis the sample is further compared with a reference sample or a reference model and, optionally, a decision on the similarity of the sample with the reference is made.

The invention provides substantial advantages. Most importantly, it allows the addition of direct 3-dimensional information to automated ballistic identification tools. This is beneficial at least for two reasons. First, it provides a macroscopic level of detail for visualizing and analyzing the general shape of samples. This typically requires a depth resolution in the order of tens to hundreds of microns. Second, it provides a microscopic level for the depth measurements of markings and striae, to allow an optimal digital comparison and the analysis of fine surface details. This requires a depth resolution at the sub-micron level for bullets and a resolution at the micron level for cartridge cases. SWLI offers the desired optical resolution and dynamical range for these applications.

Both the in-plane resolution (along the surface of the sample) and depth resolution (perpendicular to the surface of the sample) of modern SWLI has been found to be sufficient for forensic studies. Thus, accurate information on the topology of the sample is obtained. An advantage of the invention as compared with e.g. conventional microscope imaging is that SWLI is directly indicative of the depth profile of the sample and is not dependent on lighting conditions as conventional optical imaging. The present method is also not as sensitive to colour non-uniformity of the sample as conventional optical imaging, as the depth profile of the samples is not based directly on optical image intensity, but reconstructed using the interferograms recorded. By selecting the imaging wavelength range suitably, one can also "see through" undesired foreign bodies (e.g. organic matter) on the surface of the sample (e.g. metallic bullet), and thus reduce the steps and time required for sample preparation. In particular, IR and NIR wavelengths have shown potential in this respect when analysing metallic objects.

An interferometric 3D measurement, in which data from a large portion of the sample surface are recorded simultaneously, is considerably faster than traditional single point measurements, such as confocal imaging. Thus, measurement times can be reduced.

One significant advantage of the invention is that using the same instrumentation, both the 3D and 2D images of the sample surface can be recorded without changing the position of the sample. When recording the 3D depth information (topology), the instrument is driven in scanning interferometric mode according to the SWLI protocol, whereas the 2D image (texture) can be recorded without scanning, typically by blocking the light path in the reference arm of the interferometric objective. Thus, in the 2D mode the device resembles a conventional digital camera. This provides, in addition to the accurate depth information provided by the SWLI, all the information of conventional microscopic study. Further advantages and embodiments of the invention are discussed in the following detailed description with reference to attached drawings.

Brief Description of the Drawings

Figure 1 shows a schematic illustration of the instrumentation required for interferometric forensic imaging.

Figures 2A and 2B show 2D (top) 3D (bottom) images of two sides of a wire cut with the same tool.

Detailed Description of Embodiments

The term "white light" (as in Scanning White Light Interferometry) as used herein is equivalent to term "broadband light", in contrast to monochromatic light as is lasers. Thus, the term covers broadly any such multichromatic range of optical radiation for which the reflectance of the sample is substantially non-zero. Typically, the bandwidth of light is at least 100 nm, in particular at least 300 nm.

The term SWLI refers to the interferometric technique known per se, which includes illuminating the sample with white (i.e. broadband) light in an interferometric setup using various distances of an interferometric objective and the sample and detecting the optical interference patterns affected by the sample. The image reconstruction is based on the fact that maximum interference contrast is obtained from a particular location of the sample when that location is in focus of the interferometric objective. One example of carrying out a SWLI measurement, as well as a preferred image reconstruction technique for use within the present invention is disclosed in the licentiate thesis of Aaltonen, Juha (Envelope peak detection in scanning white light interferometry, Helsinki , 2002), which is incorporated herein by reference. The principles behind SWLI are discussed, for example, in P. De Groot and L. Deck, "Interferograms in the spatial frequency domain, " J. Mod. Opt. 42(2), 389 (1995).

The term "interferogram" refers to a recording of an optical interference pattern caused from a light divided into a beam reflected from the sample and a reference beam not hitting the sample using an interferometer. The term "micro" refers to structures having dimensions in the range of 1 - 1000 μm. The term "nano" or "sub-micron" refers to structures having dimensions in the range of 1 - 1000 nm.

With reference to Fig. 1 , the measurement apparatus according to one embodiment comprises a light source 10, which is directed to an interferometric beam-splitter 12 after being collimated in a collimator 11. The light source 10 may be any light source capable of producing wideband light, such as a white light LED, as illustrated in the Figure. The beam-splitter 12 guides the incident light to an interferometric objective 13 and further to a sample 14. The interferometric objective 13 comprises means for dividing the incident light into two different arms, one of which is directed to the sample (measurement arm) and one of which acts as a constant reference arm, the objective 13 typically being of Michelson or Mirau type. The objective 13 is designed to focus light at the wavelength band chosen for the measurement. The sample 14 is placed on a sample holder 15 such that its upper surface is located approximately at the focal distance of the objective 13. The objective 13 is mounted on moveable support 13' such that its distance from the sample 14 can be varied in order to alter the optical path length of the measurement arm of the objective 13. The moveable support 13' can be a piezoactively moveable support, which is connected to a piezoactive actuator 18. From the sample 14, the light is reflected back to the interferometric objective, interferes with the light of the reference arm, and is further guided back to the beam-splitter 12. The beam-splitter is adapted to guide the light through focusing optics 16 to a detector 17, such as a CCD or CMOS camera. Thus, the detector 17 sees an interference image caused by the interferometric objective 13. The imaging process is repeated at several distances between the objective 13 and the sample 14, whereby the depth profile of the sample 14 surface can be reconstructed from the measurement results according to the SWLI principle.

According to the SWLI principle, the contrast of interference, i.e. intensity difference between neighbouring interference fringes, is at maximum when the sample surface is in the focus of the interferometric objective. In practice, an envelope function is typically calculated based on the intensities of the interference fringes for achieving the accurate location of the surface. As the detector comprises a plurality of pixels in a row or array, each pixel recording an interference signal corresponding to a specific location on the sample, a two- or three-dimensional depth profile of the sample surface can be reconstructed.

Also other suitable moving means than piezoactive moving means can be provided for achieving controlled movement of the interferometric objective. Further, in an alternative configuration, the sample can be moved with respect to a static objective, whereby the optical path length of the arm is changed. Also the optical path length of the reference arm can be varied.

To allow measurement of a whole cylinder surface of the sample, such as a bullet or cartridge, the sample can be rotatably held by to sample holder. For example, there may be provided two holding members (not shown) with bearings provided at the two longitudinal ends sample such that the sample can be rotated about its longitudinal middle axis. At least one of the holding members is connected to a stepper motor for rotating the sample in a controlled manner. The sample holder or the interferometric objective can be arranged to be laterally (i.e. in in-plane direction of the sample surface) moveable in one or two dimensions. During the imaging sequence, the sample is rotated at suitable steps, and optionally translated laterally with respect to the objective, such that its whole mantle surface, or at least a considerable portion thereof can be imaged. The partial measurement data can be combined in the image reconstruction stage for obtaining a topographical image of a larger surface area.

When the surface topology of the sample of interest and the reference sample have been measured, the topologies of the samples are compared in order to address their similarity. This can be done using image analysis and/or statistical data analysis methods known per se.

According to a typical embodiment, the measurement is carried out at the visible wavelength range. This has the advantage that a number of suitable light sources are commercially available and a conventional interferometric objective and optical detector can be used.

According to an alternative embodiment, the measurement is carried out at the IR wavelength range, in particular NIR range. In some cases (e.g. some metallic samples, such as bullets) this has the advantage that the imaging contrast is higher than when measuring at the visible wavelength range. Thus, the location of the surface can be more accurately determined. In addition, the transmittance of e.g. organic matter for IR light is typically high whereby undesired material comprising such material on the surface to be measured has less influence on the measurement result than when using visible light.

The imaging sequence, i.e., controlling of the light source, adjusting the distance of the interferometric objective from the sample, and storing of data from the detector is controlled by one or more computers.

As appreciated by a person skilled in the art, the measuring apparatus may comprise also other parts than specifically described herein. In particular, there may be provided one or more optical instruments such as lenses, collimators, apertures, and filters on the optical path.

As shortly mentioned above, the invention can be used for analysing also other forensic samples than bullets, cartridges and firearms. Examples of other uses of the invention are discussed briefly below.

Cutting tools, such as knives and axes, and striking weapons, such as hammers, produce a unique patterns of markings on the object the tool is used on. By means of the present invention, these "fingerprints" can be analysed and compared with each other or the tool itself.

Figs. 2 A and 2B show a comparison between two sides of a wire, respectively, ripped with the same tool. The topmost images show 2D microscopic photographs and the undermost images the 3D SWLI images (tone corresponds with the depth) obtained using the present forensic imaging apparatus and method. It is seen that the slashes on the both sides are very similar and it can be concluded that they have been made with the same tool. In addition, it can be seen that for example the effect caused by the dust particle clearly seen in the photographic image is less annoying in the SWLI image.

The authenticity of hand- written signatures can be analysed not only based on the in-plane trace of pen, but also by analysing the depth profile of the substrate, such as paper. Paper, among other substrates, is an irreversibly compressible material, which maintains the information on the pen pressure subjected thereon. The depth differences are in the scale of micrometers and thus well detectable by optical interference measurements. The pressure variations are much more difficult to counterfeit than the simple in-plane shape of the signature, whereby the invention provides an effective tool for detecting forgery of documents.

Anti-counterfeit markings of several kinds have been designed for preventing forgery of goods. Many of these utilize microstructures produced on the good itself or on its package. The invention can be used for rapidly and robustly detecting the microstructures.

Claims

1. A method of analysing a forensic sample, comprising illuminating the sample with broadband light in an interferometric configuration through an interferometric objective, varying the distance between the interferometric objective and the sample, measuring a plurality of interference patterns of light reflected from the sample at a plurality of distances between the interferometric objective and the sample, and determining from the interference patterns information on the surface topology of the forensic sample.

2. The method according to claim 1, wherein the measurement is carried out using the Scanning White Light Interferometry (SWLI) process.

3. The method according to any of the preceding claims, wherein the light source is a broadband light source, such as a broadband LED, halogen bulb, arc lamp, or supercontinuum light source, the light is guided to the sample through an interferometric objective of Michelson or Mirau type, whose distance from the sample can be electrically controlled, for example, using a piezoelectric scanner, and the interference pattern is measured using a detector, such as a CCD or CMOS camera or the like.

4. The method according to any of the preceding claims, wherein the light at infrared (IR) range, preferably near infrared (NIR) range is used.

5. The method according to any of the preceding claims, wherein in addition to measuring the surface topology of the sample using the interferometric configuration, a two- dimensional image of the sample is recorded using the same instrumentation.

6. The method according to any of the preceding claims, wherein the forensic sample is a bullet or cartridge.

7. The method according to claim 6, wherein the bullet or cartridge is rotated during the measurement.

8. The method according to any of the preceding claims, wherein the measurement is carried out for at least two different samples and the surface topologies of said samples are compared.

9. Use of Scanning White Light Interferometry for analysing surface topology of a forensic sample.

10. The use according to claim 9, wherein the forensic sample is a bullet or cartridge.

11. The use according to claim 9, wherein the forensic sample is an object containing a slash caused by a cutting or striking tool.

12. The use according to claim 9, wherein the forensic sample is a signature, a fingerprint or an anti-counterfeit marking.

13. An apparatus for analysing a forensic sample, comprising - a holder for the sample, a broadband light source for illuminating the sample, interferometric means provided on the optical path of said light and being responsive to surface topology of the sample, detector for recording interferograms affected by the sample, and - means for deriving the surface topology of the sample from the interferograms recorded. wherein the holder is capable of holding a cylindrical sample, such as a bullet or cartridge, such that the surface topology of its entire cylinder surface can be analysed.

14. The apparatus according to claim 13, wherein the holder is capable of rotating the cylindrical sample along its longitudinal axis and, optionally, translating the cylindrical sample in at least one lateral dimension.

15. The apparatus according to claim 13 or 14, wherein the interferometric means comprise a moveable interferometric objective, preferably of Michelson or Mirau type, adapted to focus a portion of light passing the interferometer to the surface of the sample.

16. The apparatus according to any of claims 13 - 15, comprising means for recording, in addition to interferograms, a non-interference optical image of the sample when placed in said sample holder.