CHAPTER 1: Light for Crime Scene Examination

Forensic science is a very powerful investigative tool, irreplaceable in many instances for the elucidation of complex cases and for an objective understanding of the dynamics of criminal acts. Looking back at the history of forensic science, every time a new technique became available for acquiring data on the crime scene, a disruptive step forward was introduced in the ability of police forces to identify and prosecute criminals and eventually to fight crime. This was especially true at the end of the 19th century when the first studies on fingerprints as means for the identification of individuals were published by Faulds and Galton.^1,2  Just a few years had elapsed from these seminal works when, in Argentine in 1892, Juan Vucetich was the first to solve a criminal case using fingerprints for the identification of a felon. This started the era of modern forensic science. An equally revolutionary advancement came with the development of DNA typing, in 1985.³  Since then, more and more sensitive techniques have been devised, decreasing the minimum sample size for obtaining a reliable DNA profile. Less than 30 years later, it is almost impossible to imagine investigation without DNA. Technology and science are shaping and enhancing the ability of forensic science to achieve its purpose, i.e. the study of traces related to crimes.⁴  Traces can be defined as the remnants of an activity and forensic science endeavours to deduce from the traces left on the crime scene as much information as possible on the crime itself. This concept is very well synthesised by the well known Locard’s principle, which is often defined as ‘every contact leaves a trace’ even though Locard himself never formulated such an expression. Locard’s words are very effective in stating this basic concept:

it is impossible for a criminal to act, and especially to act with the intensity that a crime requires, without leaving traces of his presence.⁵  This was later elaborated introducing the notion that traces can be evidence left by the felon on the crime scene, but also, for a reverse action, they can be items collected from the crime scene and transferred to the felon.⁶ 

Kirk very fittingly formulated the definition of traces as mute witnesses:

wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value.⁷ 

From this short historical introduction, it should clearly emerge that traces and the Locard’s principle are foundation stones without which forensic science would not exist. Acknowledging that the purpose of forensic science is interrogating material remnants of a criminal activity provides a theoretical and philosophical framework for implementing science into the administration of justice in the most effective way. Differently to what appears in fiction, the role of the forensic scientist, in fact, is not to determine if the suspect is guilty or not, but it is to reconstruct as precisely as possible the chain of events associated to a crime, giving to the Court, to the investigators or to the lawyers reliable information to properly do their job.

In such a context, if contact is always accompanied by the transfer of some material, then the analysis and characterisation of such material can allow the forensic scientist to describe, prove or confirm the contact that originated it. Of course, such a logical path will have a successful outcome depending on a number of non-negligible factors. Transfer, persistence and recovery are the three main processes that, if successful, allow the trace, and especially the information associated with it, to reach the laboratory and eventually the Courtroom. In other words, traces must be transferred onto the crime scene or to the felon, they must remain on the crime scene or on the felon, and they must be found and retrieved from the crime scene or from the felon. This latter step is a considerable bottleneck in the process. Transfer and persistence depend on the dynamics of the crime, they are not related to the training or ability of investigators or scientists. However, if suitable procedures are not applied in the search and recovery of the items from the crime scene, there is a severe risk that some important information is lost or that contamination is introduced. In both cases, the work of investigators would be hindered rather than aided by forensic science. The fragility, the lability, the latency and the corruptibility of traces calls for highly qualified personnel operating on the crime scene, because any mistake made in this phase will jeopardise all the subsequent analyses and interpretation.

Crime scene investigation started as the set of procedures aimed at crystallising the crime scene and at describing it. Ottolenghi, a pioneer in forensic sciences in Italy, in the early 1900’s, extended the concept of Bertillon’s portrait parlé, which was used for giving an objective description of individuals, to the crime scene.⁸  Just as a detailed description of the physical features of a person can bring to a non-ambiguous identification, a careful depiction of the crime scene can give investigators and all those involved in the judicial process a solid foundation on which the verification of crimes and the search for the perpetrators can be developed.

If on one hand crime scene examination started as a mere descriptive activity, the modern implementation of such a critical step of forensic science includes the proactive search for items, the screening of the traces and the application of field tests.

Even though the number of texts dedicated to crime scene examination is much lower than forensic science books, most police forces and supernational bodies, such as European Network of Forensic Science Institutes, publish guidelines or best practice manuals, sometimes available on their websites.^9–11  It is not the purpose of this chapter to describe the technicalities of crime scene investigation, such as how to approach the crime scene, how to move around it and how to collect and store evidence. For such details, the interested reader is referred to the specialised literature.^12,13  and to the aforementioned guidelines and best practice manuals.

As anticipated above, crime scene examination is a set of analytical activities aimed at searching, collecting and preserving all the elements which, either per se or, even more importantly, due to their spatial location, can be considered evidence useful for the reconstruction for the dynamics of a crime and for identification of the perpetrators. The contextualisation of the items is therefore a fundamental element for attributing evidential value to a trace. The operator is not a mere gatherer of items, but is rather a specialist with a strong forensic background which can guide him towards an educated evaluation of traces and of their interrelationship. This is especially relevant in equivocal cases of death, in crimes perpetrated in a domestic context, or in cases of staged or simulated crime scenes, in which the significance of each trace is not due to the nature of the trace itself, but on its coherence with the possible hypotheses which can be set forth on the dynamics of the event.

Differently from what TV shows and fictional literature suggests, crime scene investigators are humans, and as such rely on their senses for searching and examining traces. Our eyes, though, have a limited sensitivity and much information would be lost both because it is too small to be detected in the chaos of a crime scene and because it is latent and invisible to the naked eye. The purpose of this chapter is to review the technical approaches which can be followed for widening the human senses and thus make the search for traces on a crime scene more effective and more productive.

As will emerge more clearly later in this chapter, exploiting the interactions between light and matter is a very effective method to detect latent traces and to find information on a crime scene.

Light is electromagnetic radiation, i.e. it is radiant energy which propagates as a wave. The features which define a wave are the wavelength, λ, i.e. the distance between adjacent crests or troughs, or the frequency, ν, i.e. the number of cycles passing by a fixed point per unit time (Figure 1.1).

Frequency is also a very important parameter because it is related to the energy of the electromagnetic radiation by the well known relationship E = hν, where E is the energy of the photon and h is the Planck constant, 6.62 × 10⁻³⁴ J s. In other words, the higher the frequency, the greater the energy of the radiation. For practical and historical reasons, the electromagnetic spectrum, i.e. the set of all the possible radiations, has been broken down in several regions according to wavelength (Figure 1.2), even though the physical behaviour of electromagnetic radiation does not change as a function of frequency or energy.

The human eye is only sensitive to the visible range, a quite small portion of the whole electromagnetic spectrum with wavelengths comprised between 380 and 780 nm, which is a significant limit in the field of crime scene examination, where a number of traces remain latent when examined with this light. It should also be kept in mind that the sensitivity of the human eye is not equal for all the wavelengths of the visible range, but has a maximum for green, and decreases significantly towards red and blue/violet. The use of detectors rather than the naked eye, when working with these wavelengths, can significantly improve the chances of success in the search for tiny traces. Other useful portions of the electromagnetic spectrum, inaccessible to the human eye, but easily detectable with suitable technologies, are the near ultraviolet radiation, which is more energetic than visible light and has wavelengths from 200 to 380 nm, and the near infrared region, with a lower energy than the visible light and wavelengths comprised between 0.78 and 2.5 µm.

Before proceeding with the various observation techniques which may be useful during the examination of a crime scene, it may be useful to summarise the different phenomena which may happen when light interacts with matter. The discussion will focus on a reflection geometry of illumination, because in most of the practical instances the traces observed are opaque, and not transparent.

When light containing all the visible wavelengths (white light) impinges an object, the object will absorb only certain wavelengths, whereas unabsorbed wavelengths will be reflected. These latter wavelengths will be perceived as a colour (Figure 1.3).

The colour seen will be the complementary colours to the absorbed colours. Figure 1.4 shows the “colour wheel”, colours that are opposite on the wheel are complementary.

In other words, the object will absorb some of the colours contained in white light, allowing only the unabsorbed hues to be reflected back and to reach the eye of the observer. In our daily life we are very familiar with the absorption of visible light and the vision of colours, but the same physical phenomenon happens when the impinging light is from other regions of the electromagnetic spectrum, e.g. ultraviolet or infrared light.

Scattering is another phenomenon which may happen when an object is illuminated with light. When the wavelength of incoming light is comparable with the size of the illuminated objects, these start vibrating with the same frequency of light and they themselves become spherical sources of radiation. When this happens, some of the light is diffused at 360° around the object, blurring the purely geometrical propagation of light. In fact, when light interacts with smooth and shiny surfaces the radiation is reflected, with respect to the perpendicular to the surface, with an angle which equals the angle of incidence (Figure 1.5a). This mode is called specular reflection. In contrast, if light encounters a rough surface, scattering occurs, and it is reflected in all directions of space, diffusing the radiation in the surrounding space (Figure 1.5b). In such case, the phenomenon is called diffuse reflection.

Fluorescence is a further phenomenon which is very useful for detecting traces. When a molecule absorbs light, it absorbs energy which is used to promote electrons to an excited state. Disexcitation may happen by the re-emission of light of the same wavelength, with a concurrent return of the electrons to their ground state. However, an alternative mechanism, called fluorescence, exists, in which some non-radiative decays, i.e. processes in which energy is lost without emission of radiation, accompany the return to the ground state by emission of light. In other words, when fluorescence happens, a compound absorbs light of a particular wavelength and energy, and re-emits radiation with a longer wavelength and lower energy. Many biological traces display fluorescence, which therefore is a very efficient method for their detection.

A crime scene is a very complex environment, in which traces with an evidential value coexist with a large number of items which do not carry any significant information of the event. Moreover, traces are often tiny, sometimes the perpetrator tries to erase them, and thus their detection is not easy. Finally, many kinds of traces are not directly visible to the naked eye, because they are not coloured and/or because they have a similar colour to the background surface where they lay. The purpose of the crime scene investigator is therefore to enhance, as much as possible, the contrast between the trace and the background, in order to clearly visualise it, aiding both its detection, documentation and retrieval.¹⁴ 

As mentioned above, the human eye has a sensitivity limited to the visible range, and within this wavelength region it is more effective in detecting green rather than red or violet.

A white light source is certainly suitable for a first survey of the scene, for detecting the most evident traces. However, the diversity of materials, in different colours and with different surface roughness, which can be encountered on crime scenes, calls for the application of more sophisticated approaches, if one wants to avoid missing important pieces of information.

In the first instance, absorption can be exploited to improve contrast. Using an illumination source of the same colour as the surface, the background will appear much lighter, and stains or traces on it will be dark features. An alternative to this could be illuminating with a colour complementary to that of the surface, which will appear dark, whereas traces should stand out as lighter features.

The choice of the illuminating wavelength also depends on the particular type of trace which the investigator is looking for. For example, the UV-visible spectrum of blood shows a prominent absorption peak at 415 nm.¹⁵  If a blood spattered surface is observed with monochromatic light with this wavelength, the blood spots will appear as dark regions, because they will absorb all the radiation impinging upon them. If the surface does not absorb at 415 nm, or even if it reflects a portion of it, the substrate will lighten up, improving the contrast of the image. This can be practically performed with commercial tunable-wavelength light sources if working in dark conditions, or by illuminating with white light and observing through a filter at 415 nm.

Illuminating the sample with UV light and observing it with a UV sensitive camera, i.e. performing reflective ultraviolet photography, can be an effective way to improve contrast.¹⁶  In some cases, traces which reflect UV light well stand on a UV opaque surface. Alternatively, it can happen that the substrate is reflective, whereas the trace is not. In both cases, reflective UV photography will catch a very good contrast between the reflective and non-reflective particulars of the object observed. Generally, if some pattern of a substance on a surface can be sensed, there are high chances that the pattern will stand out much more clearly if imaged in the near-UV. Short wavelength UV illumination is usually even more efficient, but at the same time it is a more aggressive kind of radiation, which could damage the most degradable and light sensitive materials.¹⁷  Great care must be exercised when working with short wavelength UV light, because it can also be harmful for the operator: suitable protection for eyes and skin must be worn. Observation in the UV range with a reflection geometry is particularly suitable for observing bite marks and bruises on victims of aggression, because the penetration depth of UV light into the skin is deeper than that of visible light. In any case, good results can be obtained illuminating with the visible wavelengths of 415, 455 or 535 nm, associated to yellow, orange and red observation filters, respectively.

Observation with oblique or grazing illumination will help detecting traces on smooth surfaces, or particles, grooves or other 3D features which lay on a surface, such as footprints or blood prints.

If on one hand these are quite practical approaches, on the other hand they are not very efficient in detecting fingerprints or other kinds of important biological fluids, such as semen, vaginal fluids, or saliva. In these cases, fluorescence is a very powerful tool. Semen, for example, is known to fluoresce in different conditions.¹⁸ 

Figure 1.6 shows, for example, that if semen is excited with light at 450 nm, it will emit fluorescent radiation with a maximum at about 520 nm. Practically, this operation is carried out using a tunable-wavelength light source set at 450 nm, and observing through a yellow/orange filter (e.g. 590 nm).¹⁹  If the surface is not very reflective at this wavelength, the semen stains should appear as bright yellow/orange spots on a dark background. If the substrate were luminescent in these illumination conditions, the contrast between the stain and the background would not be ideal. In such cases, other wavelengths should be tried, because semen, as many other biological fluids, is fluorescent in a wide range of conditions. It should be born in mind that as the wavelength of illuminating radiation is increased, the emission maximum is subsequently increased. Therefore, if for semen an illumination wavelength of 500 or 550 nm is used, the observation filter must have a colour more shifted to the red, orange and red, respectively.

Fluorescence can be used also to detect gunshot residues on the hands or clothes of a suspected shooter.^20–22  A simple set up can be illumination with 455 nm light and observation with an orange filter.

In field work, commercial light sources are employed, which allow the tuning of the wavelength of the lamp used for illuminating the scene. These are coupled to filters for the camera objectives or goggles to be worn by the operator with differing colours, in order to enhance the detection of the fluorescence luminescence. Each manufacturer will provide a handy table which indicates the combinations of illumination wavelength and observation filter most suitable for each class of trace, e.g. semen, vaginal fluid, saliva, blood, sweat, etc.

So far, only fluorescence excited by visible radiation and emitted in the visible range has been discussed. However, expanding the investigation beyond the visible limit, especially in the ultraviolet (UV) region, widens the possibility of detecting traces on a crime scene. Several substances fluoresce when excited by UV light, emitting either in the visible or in the UV. As mentioned before, this phenomenon, during investigation, is exploited by illuminating the crime scene by UV lamps, which allows one to choose the desired wavelength, and then observe it through a filter which selects the wavelength of maximum emission expected from the type of trace which is searched for. UV detectors should be used in the case of UV-UV fluorescence observations. In any case, since the visible emission due to UV excitation is usually located in the blue-violet region of the visible spectrum, where our eye is not very sensitive, the use of cameras or visors will help enhancing weaker traces. UV irradiation can help because many substrates are more reflective in the visible than in the UV, so contrast between fluorescent traces, which will appear bright, and the non-UV-reflective surface, which will appear dark, is greatly improved. This approach would be in principle very useful for biological fluids, which have a significant fluorescence when excited by UV. However, since DNA is known to be severely degraded by UV irradiation^23,24  and since the UV lamps used in field activity have a high intensity, the use of UV light, especially with a short wavelength, should be avoided for the search for biological traces.

On the other hand, UV irradiation can be very useful for the detection of many other traces, fingerprints in particular.²⁵ 

Visible fingerprints are left either by fingers touching malleable materials, such as clay or mud, or by fingers stained by coloured substances, like paint or blood. They can be detected by illuminating them with just visible light, or by taking pictures in the absorption mode for enhancing contrast with respect to the substrate. These are rather rare cases, though, because the majority of fingerprints are latent and invisible to the naked eye. Fingerprints are composed of a dilute aqueous solution with about 2% proteins and lipids exuded by the sudoriparous pores in the fingertips or palms of the hand. Therefore, these traces are very hard to detect. When illuminated with UV light, fingerprints emit at 330 and a 440 nm, the emission at 330 nm being the strongest. It has been reported that 280 nm is the optimal excitation wavelength for maximising fluorescence.²⁶  It is therefore feasible to detect fingerprints by illuminating them with UV light and observing their blue visible emission. However, problems may arise when it is necessary to examine surfaces with a strong luminescence in the visible, due to optical brighteners. This is the case of some architectural paints, paper, or textiles.

Figure 1.7 shows some examples of untreated fingerprints on white paper. Since this substrate strongly fluoresces between 400 and 500 nm, in a spectral range which overlaps with the blue emission of fingerprints at 440 nm, it is necessary to perform imaging using the emission peak at 330 nm, which does not overlap with the fluorescence of paper. Being the light with a wavelength of 330 nm in the UV region, it will be necessary to use cameras with suitable detectors which, however, are quite common and available at affordable prices. For imaging a 360 nm band-pass will be suitable. Figure 1.7a–c show fluorescence images with excitation at 230, 280 and 300 nm, respectively. It may be seen that the clearest fluorescence image of a fingerprint was obtained with excitation at 280 nm.

As a further complication, it must be added that fluorescence emission from fingerprints tends to fade and/or change colour as the fingermark ages.²⁷ 

Of course, it is not always possible to obtain an image of a fingerprint which is clear enough for a dactyloscopic assessment to be carried out, especially in difficult cases, such as weak prints or prints on surfaces that produce highly luminescent backgrounds. For this purpose, several chemical development methods have been devised for improving the level of detail and contrast, in order to obtain the largest amount of information on this trace.²⁸  This topic will be further discussed and detailed in another chapter of this book.

The blue fluorescence emission due to optical brighteners can be very useful for detecting trace evidence such as textile fibres or paint chips or smears. Moreover, the lack of blue fluorescence emission can also yield useful information, such as in the case of counterfeit documents, where erasures or abrasions can alter the optical response of the paper substrate. In such cases, the paper illuminated by UV light will fluoresce emitting blue light, and the alterations will appear as dark areas.

Near-infrared is a suitable type of light as well,²¹  for example for the detection of bone fragments on the ground or on complex surfaces. A near-infrared hyperspectral camera, associated to a data treatment based on chemometry, proved capable of distinguishing a wide variety of bones, including aged and weathered bone from stone, sand, leaves, moss, and wood.²⁹  Alsberg et al.³⁰  developed a specific instrument, PryJector, for highlighting in situ details which are otherwise invisible or difficult for human eyes to detect. By a combination of hyperspectral imaging, chemometric modelling, and back-projection by a computer projector, it is possible to emphasize the presence of particular items by creating a false coloured chemical image. Figure 1.8 shows the potential of this technique. A hyperspectral camera working in the 930–1670 nm range records a full spectrum in each pixel of a scene. Chemical information is extracted by multivariate data treatment, and each pixel is associated to a particular class, for example bone or non-bone in the current example. This chemical image is then projected back on the surface of interest, colouring the items of possible interest, in this case the bones in a heterogeneous mixture of bones, leaves, and soil.

The observation in different light conditions has been also proposed for distinguishing between fresh and old bruises, because it allows the detection of the yellow chromophores of bilirubin. Recent bruises in fact contain mainly blood, whereas as time goes by bilirubin progressively accumulates in the wounded region.³¹ 

Near IR radiation is particularly useful in the questioned document field. Many inks, with similar colours in the visible range, can be differentiated according to how they absorb or reflect IR. Figure 1.9 shows an application of this concept to obliterated writing. As evident, illumination by IR light succeeds in revealing the writing hidden beneath an obliteration only if there is a wavelength where the obliteration itself becomes transparent to IR, whereas the text remains opaque. If the situation is the opposite, i.e. the obliteration is opaque to IR and the text is transparent, it will not be possible to reconstruct what was originally written in the document, before the cancellation.³² 

IR illumination is very well suited also for detecting document counterfeiting by the addition of text. Figure 1.10 reports how, exploiting the different response of different inks to IR light, it is possible to clearly highlight that some alteration to the original text happened.

A last approach worth mentioning is the exploitation of chemiluminescence. Different from fluorescence, which is an intrinsic characteristic of the target molecule (or of some of the molecules contained in the target trace), chemiluminescence consists of the emission of light triggered by a chemical reaction. In other words, if fluorescence can be activated just by illumination with light of the proper wavelength, for chemiluminescence interaction with a suitable reactant is needed. Luminol (5-Amino-2,3-dihydrophthalazine-1,4-dione) is the most renowned reagent for the detection of blood on the crime scene. Luminol and hydrogen peroxide are sprayed on the surface where blood traces are being searched for. If blood is present, the iron present in haemoglobin will act as a catalyst in Scheme 1.1. The first product is in the excited state, and then it loses the extra energy by emission of blue light.

From the mechanism of action, it is clear that any iron-containing substrate will not be suitable for treatment with Luminol, because it will give false positive results.

Latent blood traces, when treated with Luminol, will glow blue light which can be photographed. It should be noted that the luminescence lasts for about 30 seconds and then fades. Moreover, especially if the blood traces are small, the emitted intensity is quite low, and so the use of Luminol requires that the camera is operated in a darkened room, and with long exposure.

Despite Luminol being a reagent involved in a chemical reaction with blood components, its use does not jeopardise further treatments of the stains which are found. In particular, DNA typing remains feasible, because treatment with Luminol does not degrade it.

In the previous section, a number of approaches have been described for detecting traces at a crime scene. Documentation is as important as finding traces, because, as introduced in the first paragraph of this chapter, most often the information related to the items depends on their location or on their relationship with other objects on the crime scene.

The modes of light-matter interaction described in paragraph 1.2 can be exploited to photograph traces on the crime scene.³³ 

A coloured object or stain can be visualised emphasising it against the substrate where it stands. If there is a chromatic difference between the trace and its substrate, then it is possible to enhance it with photographs in the absorption mode. The best approach is illuminating the surface with monochromatic light, obtained by a suitable filter. Working in the dark is suggested for the best results. The colour of light to shine on the surface must be complementary to the colour of the trace, and as close as possible to that of the substrate. For example, to enhance an orange stain on a blue substrate, blue light will be used. In this case, the light will be reflected by the surface, whereas it will be absorbed by the stain. Therefore, just the blue light reflected by the surface will reach the detector of the camera, and as a result the stain will stand out as dark against a light background. The technique can be applied using a filter to monochromatise illuminating light and observing with an unfiltered camera or by the naked eye (Figure 1.11a). As an alternative, a filter can be applied to the objective of the camera, or the object can be observed through a filter, while illuminating it with white light (Figure 1.11b). The former method is more efficient, but must be performed in the dark. The latter has the advantage that can be applied in sunlight or in conditions of artificial lighting.

Figure 1.12 shows an example of the potential of absorption photography in enhancing the contrast of stains or traces.

As an alternative, the trace can be imaged as a light feature on a dark background illuminating with light which will be absorbed by the substrate and reflected by the trace. For the example examined before of an orange stain on a blue surface, orange light will work well: this radiation will be reflected by the stain, but it will be absorbed by the substrate. Orange is in fact a complementary colour to blue, being opposite to it in the colour wheel of Figure 1.4.

Imaging in the absorption mode shows its limits when the colour of the trace is very similar to that of the substrate.

Imaging techniques which exploit the diffusion of light can be very useful for observing traces which lie on a smooth or very reflecting surface, or for enhancing their 3D structure. Examples include greasy fingerprints on a shiny glass surface or latent grooves on a piece of paper. This technique can be applied either with monochromatic or with white light. In the case where monochromatisation was available, a colour as similar as possible to the colour of the trace should be chosen, or at least a complementary colour to that of the surface. The method works best, in fact, on smooth and dark surfaces which will absorb most of the radiation impinging on them, reflecting the rest. The rationale of this technique can be understood if the two main mechanisms of reflection of light from a surface are acknowledged, i.e. specular reflection and diffuse reflection. When light impinges the surface, it will be specularly reflected by its smooth regions, and it will be diffused by the rough regions. Illumination is oriented at an angle such that specular reflection does not reach the camera, whereas diffused light, which is directed in all the directions of space, can be acquired by the detector. This way, the trace appears as a light feature on a dark surface. The technique yields even better results if some of the radiation is absorbed from the substrate, making the background of the image even darker and thus the contrast even higher. In this case also, it is suggested that one should work in the dark, for better results.

As evident from Figure 1.13, illumination should be angled, especially if it is of interest to enhance the 3D structure of the observed object. Oblique or incident light should be used in this case, and there is no particular need for monochromatisation: white light can be used. The physical phenomenon which is exploited is the same as above: when light almost parallel to the surface impinges some protruding feature, it is scattered towards the camera, whereas the rest of light does not reach the detector.

Figure 1.14 shows how the diffuse reflectance mode allows to image a fingerprint left on a dark and smooth plastic surface.

An alternative set up for taking pictures in reflection mode is by an episcopic coaxial illumination set up. In this case, through a semitransparent mirror, incoming light is directed towards the object perpendicularly to its surface. Reflected light follows the same path and reaches the camera. In this case, the smooth part of the sample will be more efficient in reflecting light, whereas the rough trace will diffuse some of the radiation away from the camera. As a consequence, with this geometry the trace will appear dark against a light background.

In this chapter, a number of approaches have been described for a thorough examination of the crime scene. They are all aimed at the detection of traces, which are subsequently documented, collected, and brought to the laboratory. In other words, traditionally the laboratory is the place where the analysis, identification and interpretation of evidence is carried out. There is a growing trend to bring to the crime scene some of the instrumentations and techniques applied in the laboratory. Under this perspective, the investigator can start screening the items, in order to pick those which are more significant and to reduce the overload of the laboratory and the amount of collected material.

Among the techniques which fit the scope of this book, i.e. the relationship between light and the crime scene, two can be cited: miniaturised spectroscopy and presumptive tests.

Given the small size that traces often have, forensic science exerted a strong driving force towards miniaturisation of analytical instruments. This is true not only for the miniaturisation aimed at analysing very small items, but also aimed at making instruments portable for allowing their use in the field. Raman, UV-visible, IR, mass spectrometers are currently available in portable, and also hand-held versions, by which first instance analyses can be performed directly on the crime scene to identify a material and decide whether or not it is worth collecting it. The performance of such instruments is obviously lower than their benchtop counterparts in the laboratory, but they still can give useful data in some cases. Needless to say, it is of the utmost importance that the operator is fully trained not just on the practical use of the instrument, but on the interpretation of the data. Otherwise, the risk is that decisions are taken on the strategy of crime scene examination based on wrongly interpreted data, jeopardising the whole investigation.

Presumptive tests have been in the tool box of the crime scene investigator for quite a long time. These are mainly colorimetric tests, which can give a quick and easy general idea on the nature of an unknown material. Table 1.1 summarises some examples in several fields of forensic science.

Presumptive tests must be used with care. Their main limitation is a lack of selectivity. For example, amylase is found in a variety of body fluids, such as saliva, blood, urine, sweat, tears, semen, breast milk, feces, and vaginal secretions. A positive test to the amylase test may support the hypothesis that the tested sample is indeed saliva, but chances are that it could be another kind of biological fluid. False positives are always an option. For instance, aspirin gives a positive reaction to the Marquis test, even though it is not an illicit drug. Many tests for blood involve a redox step catalysed by haemoglobin present in blood, which however can be replicated also by the peroxidase enzymes which can be found in many kinds of fruit. Any presumptive test must be followed by a confirmatory analysis, carried out with established protocols in a proper laboratory.

In the case of tests of the antigen-antibody type, the selectivity is extremely high, and the risk of false positives is negligible. However, laboratory tests are needed to extract the most useful information from the traces, i.e. DNA.

Another drawback of presumptive colorimetric tests is that they are destructive. In fact, they require a small aliquot of the unknown sample to be treated with the reagent, which after the test will no longer be suitable for further analyses.

In this chapter, a number of approaches has been described for a thorough examination of the crime scene. Focus has been mainly on the physical bases of each approach. There is currently a large number of commercial instruments allowing the investigation of the crime scene with many different types and geometries of illumination. Hyperspectral instruments are being manufactured, and the fast advancement of software and hardware allowed the introduction to the market spherical cameras or 3D scanners which automatically acquire images from their surroundings and which can merge this data array to obtain a 3D picture of the crime scene. Software allows also one to take measurements within the crime scene, without actually entering it. This is an interesting technology which can greatly aid the assessment of the crime scene. Systems like these are useful on one hand, but they should be used with great care. They can be employed as an effective way for documenting the location and relative position of all the items in the crime scene, but they cannot substitute an assessment in person. Even though it is an exquisitely technical activity, crime scene investigation retains a strong dependence on the ability of a trained examiner to catch the subtle details which lead to the discovery of traces and of patterns. Tools such as these avoid the need to enter the crime scene for operations such as planimetry drawing or measuring steps. However, an excessive virtualisation of the crime scene should be avoided, because there is the risk of focusing too much on the most evident traces, losing the tiny particulars which only a hands-on investigation of the crime scene can give.

As in other branches of forensic science, there is not, among the tools which have been described, a silver bullet which solves every situation. The variability encountered in crime scenes just allows for the drafting of guidelines and best practices, no rigid rule can be established. The most fitting approach for each different circumstance must be identified, and this is dependent on the training and capabilities of the personnel operating on the crime scene. Choices must be made on every crime scene, about which items should be collected and which traces should be considered useful. In the opinion of the authors, these are best left to an expert investigator, rather than to an instrument. Competence, more than tools or technologies, is the most powerful weapon that can be used for solving crimes.