Light for Crime Scene Examination

Valerio Causin and Giuseppe Guzzini

Table of Contents
1.1. Introduction 3
1.2. A Brief Summary on the Theory of Light 5
1.3. Imaging on the Crime Scene: Finding Traces 9
1.3.1. Observation in the Absorption/Reflection Mode 9
1.3.2. Fluorescence 10
1.3.3. Chemiluminescence 15
1.4. Photographic Techniques: Documenting Traces 15
1.4.1. Absorption Mode 15
1.4.2. Diffuse Reflection Mode 19
1.5. The Lab-on-the-scene Approach 22
1.6. Conclusion 23
References 24

1.1. Introduction

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. 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. 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. 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.

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.

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...