Finger prints
...ve been taken from a person using ink and paper. Classifying the general pattern of fingerprints allows them to be stored in a filing system and subsequently retrieved for comparison. Methods have also been devised for developing fingerprint impressions left by criminals at the scene of a crime. The most common uses a brush and powder to mark the fingerprint, which is then photographed and lifted from the surface using tape. The reliability of fingerprints for criminal identification is complicated by the need to use crime scene prints that may be partial or distorted and by the technical competency of the person identifying the print (computer identification is often used as an aid). Modern technology has allowed this process to transfer to computers with the arrival of automated fingerprint identification system (AFIS). Fingerprint detection: The most widely used method of detecting latent prints is to dust using a fine powder that adheres to the traces of oil and sweat. There any many different kinds of powder in use today for example aluminum dust, which is gray and highly visible on dark and mirrored surfaces, carbon black for white surfaces, and luminescent powders, which fluoresce under ultraviolet light. (4) Iodine fuming: This is the oldest of the chemical methods for detecting fingerprints. Solid iodine sublimes (becomes a vapor without becoming a liquid) at room temperature. If an object, say a piece of paper, is placed in a chamber that has had some crystals of iodine placed in it, any fingerprints on the object will appear as brownish prints. What is thought to be happening is that the iodine vapor is dissolving in the skin oils (or may be the traces of water from the perspiration) that make up the print. Whatever is happening the print is temporary and will soon fade. It must therefore either be photographed immediately or "fixed" by spraying with starch solution to give a blue print, which will last longer. Ninhydrin: Iodine fuming has to a great extent replaced by other methods including spraying with the chemical, Ninhydrin which reacts with amino acids present in the skin secretions to give purple prints. Superglue: The active ingredient in superglues is a cyanoacrylate ester. When vapors of these compounds come into contact with fingerprints the molecules of the cyanoacrylate attach to the print and polymerize. The visible prints produced are white; to improve their detectability they are often treated with a fluorescent dye, such as Rhodamine 6G, before being photographed under a special light source or laser. Researchers discovered some years ago that some molecules/components of sweat fluoresce when irradiated with certain types of light or lasers. This phenomenon is called luminescence. Preservation: Once it has been detected, the permanent preservation of the fingerprint, independent of its nature, is important for future comparison and use as potential evidence. Photographing the image must be carried out before any lifting or attempts at preservation are made. Photography is carried out on a 1:1 scale with the cameras lens flush against the surface and usually also alongside a measuring scale. The print is the lifted by the use of Scotch tape and placed on a labeled card providing maximum contrast with the powder formulation. If the object upon which the print is impinged is small, the print is covered with cellophane and the whole object is removed from the scene. Search and compare: The fingerprint examiner will then compare these crime scene photographs with photographs of suspects’ fingerprints using a comparison microscope when comparing a print from a crime scene to a known print of a suspect the examiner is looking for minutiae in the same place on each print. This is complicated as it is rare to get a whole fingerprint at a crime scene so it is often only a section of a print that is being compared. It is also common for the print to be distorted as it is pressed or rolled onto a surface, so that two prints from the same finger of the same person often don't look the same to the naked eye. Digital Imaging for Fingerprint Enhancement (DIFE) involves the conversion of a picture or image of a print into a digital file, usually by the use of scanners and digital cameras. The scientific process for this is by use of a grid of detectors known as a charge coupled device (CCD) that detect light patterns and convert them to an electrical signal. The detectors each correspond to the pixels on a digital image; each pixel takes on a particular shade of gray according to the quantity of light the sensor it is representing detects. The more sensors used in the CCD, the more pixels can be created and therefore the more accurate and detailed is the representing image. The digital file created can then be enhanced in several ways; an interfering background can be removed from the image, the contrast between the print and the background can be increased and the intensity difference between each pixel can be increased in order to sharpen up the image. (5) The use of this sy...