Impressions

impression evidence

a marking produced when an object comes into contact with another surface, leaving behind and indentation or print

-pattern is a mirror image orientation

types of impression evidence

-footwear impressions

-tire tracks

-fabric impressions

-toolmarks

-impressions of weapons

2D impression

-have length, width, and no depth

-on a hard surface

3D impression

-have length, width, and depth

-in a softer surface (mud, snow, etc)

dry origin impressions

-shoe and substrate are dry when impression is made

-material transferred is generally dust

wet origin impressions

shoe and/or substrate is wet when impression is made

positive impressions

residue visually represents the areas of the sole that came in contact with the surface

negative impressions

contact areas of the sole REMOVE residue from the surface

recovery methods for impressions

-cast (3D impressions)

-lift (2D impressions)

examination quality photographs

-a scale and a label must be used

-tripod should be used

-the film plane should be parallel to the impression

-proper lighting is critical

-use several exposures

FUSS

Fill the frame

Use a scale

Side lighting

Several shots

steps for recovering dust print impressions

-visualize dust print with oblique (side) lighting

-photograph

-document

-lift the impression

when an impression is lifted or casted, the orientation....

switches back to the original orientation

electrostatic lifting

device applies electrical charge to lifting film placed over dust print, film is drawn to the surface by static electricity, the dust particles are attracted to the lifting film

-usually first technique used

-can be used on almost any dry surface

gelatin lifter

high quality elastic sheets of rubber w/ low adhesive gelatin layer on one side

fingerprint powder

what can be used if lifting techniques don't work?

crystal violet or amido black

how are bloody impressions enhanced?

wear

to make, cause, or produce changes in a tread by friction from rubbing, scraping, or other contact with a substrate

-NOT individualizing

tire tracks gives us information such as...

-possible make/model

-crime scene reconstruction

-approximation of mileage on tire

-comparisons to known tires

tread pattern

includes multiple ribs, blocks, lugs, or a combo of all 3

rib

continuous strip of tread rubber around a tires circumference, multiple side by side ribs are typically molded across a tires tread

block/element

freestanding, independent tread blocks around the tires circumference, multiple side by side rows of blocks are typically molded across a tires tread

lug

extra large, freestanding, independent lugs positioned around tires circumference

circumferential grooves

major grooves molded around the tire

lateral grooves

major grooves molded across the tire

tie bars

short rubber lines molded across lateral grooves between adjacent tread blocks

sipes

typically found in all-season, all-terrain, and winter tires, they are thin slits molded into an otherwise smooth tread surface to improve traction

noise treatment

the varied arrangement of different pitch lengths of the tire tread elements around the circumference of a tire

tread wear indicators (TWIs)

areas raised about 1/16'' above the groove in specific locations around the tire used to determine when to get new tires, markings on the sidewall indicate their locations

tread wear

progressive wearing away of a tire due to abrasion on a road surface, general wear pattern is predictable for each type of tire

test impression for tire comparison

usually 6-8 ft long for an entire rotation, tire is coated w/ vaseline or vegetable oil, then powder and rolled across surface, can be sectioned according to TWIs

noise treatments helps to...

locate the specific area around the circumference that could have made the impression

what is a firearm?

a device for accelerating a projectile to high speed and sending it towards a target

what is a tool?

a device that performs for facilitates manual or mechanical work

what is a toolmark?

a pattern resulting from a harder marking device being forced against a softer object

track

a mark or series of marks left by a person, animal, or thing