Impression Evidence

Puzzle

• Detective Logan surveys the scene. A woman is shot to death and is lying face down in the parking lot. A light snow covers the ground. Leading to the body is a set of footprints which run between two sets of parallel tracks. Leading away from the dead woman are just two sets of parallel tracks. What person should Detective Logan be looking for?

Bigfoot Buttocks Imprint

Strongest Evidence YetBy Kurt Kleiner

http://www.newscientist.com/news/news.jsp?id=ns22701512-22-00

IMpression Evidence

• Impression evidence physical evidence that results from the contact between two objects or surfaces

• One of the largest classes of evidence– Falls into three types:

• Imprint• Indentation• Striation

Imprint

Two dimensional product formed as one object transfers material onto a surface

- Can be patent (visible) or latent (invisible)

Latent Impressions

Impressions hidden to the eye but can be visualized through the use of special dusting and electrostatic techniques or chemical developers– Oils, fine soil, and other debris can be carried

onto clean floors– Clean shoes or feet can transfer materials to

a newly waxed or polished floor

Indentations

Three dimensional impressions which are left when one object comes in contact with the surface of a softer material, such as snow, mud, soil, or soap

Striations

Occurs when an object comes in contact with a softer surface, creates an indentation, and then the object moves

Toolmark Identification

A tool mark is any impression, scratch, or abrasion made when contact occurs between a tool and an object

• The impressions made by the tools could link the tool to a crime scene and ultimately to the tool’s owner

Collecting Impression Evidence

• Can be broken into three different steps:

1. Photographing the impressions

2. Lifting latent impressions

3. Casting plastic impressions

Photographing Impression Evidence

• Photographs should be taken before anyone alters or touches the impression

• Impression should fill field of view– A picture of the impression by itself– An identifying label and a ruler in position should be placed in the

second and subsequent pictures• Provides scale

• Picture should be taken perpendicular to impression and at a couple different angles

• If left on a dark surface, criminalist might do magnesium smoking – this leaves a white residue (magnesium oxide) on the surface allowing details to show up

Lifting Latent Impressions

• Several different methods to make latent impressions visible:– Luminol – causes blood to glow in the dark

• Footprints become visible and can be photographed

– Dusting – like with fingerprints• Reveals impression and makes possible for

photographs and lifting

Electrostatic Dusting

• Works by applying a small electrostatic charge on a piece of lifting film, which is then placed over the latent print– The film picks up and holds the dust of the

latent print

• Can be used to lift impressions from paper, flooring, carpeting, pavement

Gel LiftingGel Lifting• A gel lifter is a layer of thick gel sandwiched between paper backing and a plastic cover sheet• Best used on oily or moist impressions• Print is first dusted with powder• Protective cover is peeled off lifter and gel is

pressed over the print• Gel lifter is removed and the cover is replaced• Gel lifter is photographed

Casting Plastic Impressions

• A three dimensional impression is called a cast and may be made to preserve physical evidence

• Methods vary depending on what materials casts are left in– Plaster of Paris – sand or dirt– Dental stone – snow

• Hardens faster than Plaster of Paris

– Silicone – typically used in tool marks