Laser

Laser technologies are used for a wide range of purposes in laser-based products, including CD players, DNA screening machines, forensic tools, missile guiding devices, mapping and topographic instruments, and surgical devices. A laser is basically an intense beam of light. Ordinary light is scattered in variable wavelengths and frequencies, whereas laser beams are highly organized light with all photons traveling in the same frequency and wavelength. Laser (or light amplification by stimulated emission radiation) is a technology that allows controlled photonic release from atoms in specific wavelengths, thus producing a directional monochromatic (singlecolor) light beam of high coherence (e.g., tightly organized photons with synchronized wave fronts of the same frequency). Forensic science applications of laser technologies include a wide range of devices and techniques, such as laser spectroscopy, interferometric measurements ( laser mapping systems), laser scanning, bullet trajectory projections, and laser photography.

Laser technologies are a growing market in forensics and crime investigation, with new tools designed specifically for this field. Crime scene investigation and reconstruction ballistics can be a time-consuming task, with crucial evidence such as trace fingerprints sometimes overlooked by the human eye. Bullet trajectory calculations with tapes and traditional reconstruction methods may take several hours in complex crime scenes. The use of bullet trajectory laser rods improves precision and saves time. Laser rods are used to determine the exact point of origin and distance from which a gun was fired, or, when more than one person was shooting, the exact original location and trajectory angle of each bullet fired. Laser rods are placed in each bullet hole found in the scene and activated to emit light. Laser beams flowing from each hole will reproduce the complete trajectory pattern of all bullets fired, making visible the entire exchange in a manner that can be photographed. Therefore, forensic technicians are able to track the trajectory of each bullet back to its point of origin, as well as to identify bullets that ricocheted from objects and changed direction.

Another useful application of lasers in forensic science is spectroscopy. Spectroscopy involves the analysis of materials by studying the reflection and absorption of light for the identification of traces of substance residues such as accelerants, illegal drugs, or poisons. Laser spectroscopy determine the molecular structure of materials and chemical compounds. Infrared laser spectroscopy can determine molecular structures of polymers on surfaces and gas phase ions, and is used to detect explosive components or illegal drugs in samples. Some portable spectrometers can analyze evidence at the crime scene, inside plastic bags or glass bottles, water solutions, as well as residual particles on surfaces.

Laser fluorescence is another method of analysis that can be used at the crime scene. One practical example is the small portable lasers in the shape of narrow flashlights, which are used to scan surfaces of a scene in search of fingerprints. As the beam travels on the surface of objects, furniture, walls, or doors, fingerprints become visible due to the rapid absorption and release of light by atoms present in the printed substance. This time-saving scan allows the location of fingerprints in places where they would otherwise be hard to find, as well as the quick location of fingerprints in an entire area. Once located and mapped, fingerprints can be dusted with fluorescent powder to be photographed.

Often more than one method is used to detect toxic industrial components present in the environment, such as coupling plasma mass spectrometry with laser spectroscopy. These techniques are used by the Federal Bureau of Investigation (FBI) to identify security dyes and gas residues in stolen cash. Laser desorption mass spectrometry (LDMS) is a technique used to identify substances in fabric, dyes, and security inks. Ink security systems are used to protect cash in ATM machines and bank safe contents. The ink is pressurized to release a concentrated red dye spray from the ATM cassettes when triggered by an anti-tampering electronic sensor, spraying an indelible stain on the currency. The skin and clothes of criminals are also marked, thus creating evidence. Other security systems use tear gas and red staining or smoke and dye for similar purposes. Smoke and dye in bank vaults release a hot cloud of red smoke that marks valuables and criminals, whereas tear gas and dye systems intend to stain evidence and temporarily disrupt the robbery, gaining time for the police arrival at the scene. LDMS is used to identify these markers in currencies and other items of evidence, and also facilitates tracking stolen currency in circulation.

Laser radars are law enforcement devices that measure the speed of vehicles. Laser speed guns are portable and can be pointed by police officers directly to a vehicle. A pulse infrared light is emitted towards the targeted vehicle, reflecting on its surface and returning to the gun where a sensor calculates the nanoseconds elapsed between emission and reflection, determining the distance to the car. As the car is in movement and the laser gun pulses laser light thousands of times per second, repeating calculations and comparing the many results, it can accurately determine the speed of the vehicle. Some laser speed devices are mounted on poles in strategic places by the roadside, in connection with high-speed photograph cameras that take a picture of the car and license number when triggered by the laser radar.

Other laser-based measurement tools, such as 3-D laser stations, are used to reconstruct the events underlying road accidents involving several vehicles or mass crime scenes such as nightclub or supermarket bombings. The scene is first photographed from all angles, and then 3-D mapping laser equipment is used to scan the entire area, registering several point positions. Some laser scanners have the capacity to capture 5,000 measurements per second, such as the one that was used in forensic analysis of the terrorist nightclub bombing in Bali in 2003. Photographs and mapping data are then downloaded into software that calculates point-to point distances and angles, automatically reconstructing three-dimensional images of the scene.

Some DNA typing machines also use laser fluorescence to identify certain molecules during the automated DNA sequencing process of certain DNA segments known as short tandem repeats (STR). STR sequences and lengths are so specific to individuals that they led to the expression "DNA fingerprinting." Another DNA technique is laser micro-dissection, used for sperm identification in semen samples. This method has high sensitivity, and permits the isolation of individual sperm cells from other cell types present in the sample. STR profiling can be accomplished from minute DNA samples with this technique, after DNA purification using high-sensitivity kits.