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Preparation Of Fingerprint Charts For Court Testimony

In testifying to fingerprint identification, the expert often prepares

charts to visually aid the court and jury in understanding the nature

of his testimony. Many times it is undoubtedly difficult for the

layman to perceive, from a vocal explanation alone, the full import of

an expert's testimony, due to its technical nature; consequently, some

graphic representation of the facts presented is amply justified and

ed. The preparation of the charts is ultimately the sole

responsibility of the expert using them. As a matter of interest to

law enforcement personnel engaged in fingerprint work, a brief

explanation of the preparation of such charts follows, along with

suggestions and remarks based on long experience in these matters.

To do the work conveniently, it will be necessary to have available,

in addition to the ordinary photographic developing and printing

materials, a projection enlarger which will enlarge preferably to at

least ten diameters. In the projection method of enlargement, the

image is printed directly from the original negative, and the

preparation of an enlarged negative is unnecessary.

Aside from the photographic equipment, the needed materials are: a

roll of scotch photographic tape 1 inch wide to outline the areas of

the fingerprints on the negatives to be used; some stiff cardboard

approximately 1/32 inch thick on which to mount the prepared charts; a

tube of rubber cement; and a bottle of translucent ink, other than

black or white.

A light-box on which to view the negatives while blocking, and a

lettering set to draw the lines and numbers uniformly on the charts,

while not absolutely essential, are helpful conveniences. A light-box

is basically a frosted pane of glass with a light beneath it to

produce soft, even, non-glaring illumination. If no light-box is

available, a clear window may be utilized in blocking the negatives.

If the expert finds it necessary to have an outside source prepare his

photographs, he should retain personal custody of the evidence during

the operation.

The original latent print and inked print with which it is identical

should be photographed actual size. This procedure eliminates

guesswork in enlarging both to the same degree. Whatever areas of the

two prints are deemed requisite to illustrate the method of

identification are then outlined (blocked) on the negatives with the

masking tape, so that only those areas will show in the subsequent

enlargements. Generally, if the legible area of the latent print is

small, it is well to show the complete print. If the area is large,

however, as in a palm print, an area which will not make the chart too

bulky or unwieldy may be selected.

In blocking, the negative is affixed to the window pane or light-box

by means of strips of photographic tape across the corners, with the

side to be blocked up. This prevents constant shifting of the negative

while it is being prepared. The latent print should be blocked first.

Corners of the blocked areas should be square. Care should be

exercised to have as nearly as possible the same ridge formations

shown and the ridge formations in the same upright or horizontal

positions. This may be facilitated by fixing a negative, bearing ruled

squares, between the negative being blocked and the glass to which it

is attached.

If the latent print was developed or photographed as a light print on

a dark background, a reverse-color negative should be prepared and

blocked in order that both prints may appear as black ridges on light

backgrounds. This is done by placing the original negative adjacent to

a new sheet of film and exposing it. The resultant negative contains

the same image as the original except that the color of the image has

been reversed.

If the negative is a photograph of an opaque lift, the print appears

in reverse position; that is, as a mirror image, and the negative will

accordingly have to be blocked from the dull or emulsion side in order

for it to appear in a position comparable to that of the inked print.

Failure to present the prints in question in the same color and

position may possibly confuse the observer and nullify the purpose for

which the chart is made.

The degree of enlargement is not important in itself, so long as the

ridges of the latent print are readily distinguishable by the eye. Ten

diameters have been found adequate, although any enlargement from 5 to

30 will serve. It should be remembered, however, that small

enlargements are difficult to see a few feet away and that large ones

lose some of the contrast between ridges and background. A white

border of at least 1-1/2 inches or a width equal to about one-third

the enlarged area should be left for charting purposes.

Any chart prepared must be technically correct; that is, the

corresponding ridge characteristics in the two prints must be

similarly numbered and indicated.

Several ways of pointing out the similar ridge formations have been

observed, but the one which appears soundest is also simplest and

consists of merely marking the characteristics with lines and numbers.

All of the ridge characteristics in the prints need not be charted.

Twelve characteristics are ample to illustrate an identification, but

it is neither claimed nor implied that this number is required.

All fingerprint identifications are made by observing that two

impressions have ridge characteristics of similar shapes which occupy

the same relative positions in the patterns.

Methods involving superimposition of the prints are not recommended

because such a procedure is possible only in a very few instances, due

to the distortion of ridges in most prints through pressure and

twisting. Such a procedure is not necessarily a test of identity.

Likewise, presenting charts with the shapes of the characteristics

drawn in the margin is not recommended. Individual ridge

characteristics may vary slightly in actual shape or physical position

due to twisting, pressure, incomplete inking, condition of latent

print when developed, powder adhering to background, etc.

Identifications are based on a number of characteristics viewed in a

unit relationship and not on the microscopic appearances of single


Since the enlarged photographs appear in black and white, an ink other

than black or white should be used to line the chart. Such an ink

should be preferably translucent so that it will be possible to see

the ridges which it traverses. A translucent carmine drawing ink

serves well. In placing the lines on the chart, they should be

arranged so that they do not cross or touch.

The chart will present a clearer, neater, and more pleasing appearance

if it is numbered clockwise and the numbers are evenly spaced (fig.

429). It is not necessary, however, to place the numbers evenly around

the photograph.

[Illustration: 429. Chart illustrating method of fingerprint


Ordinarily, the numbers are placed on three sides and the type of

print (latent or ink) noted at the bottom. In any case, the manner of

numbering should be subservient to an explanation of the

characteristics in an orderly sequence; and, if the situation warrants

it, all of the points may be illustrated on a single side of the


A single line should be drawn from each characteristic to a numbered

point on the margin. Care should be taken to draw the line exactly to

the characteristic point, not short of it, beyond it, or obscuring it.

Erasures should be avoided. If the ink runs or blots, it is sometimes

possible to remove it with a cloth dampened in denatured alcohol,

without damaging the photograph.

If the enlargement is great, that is, 25 or 30 diameters, it might be

well to draw a small circle around each characteristic and then draw

the line from the circle to the number, since the ridge will be much

thicker than the illustrating line. All lines and numbers should be

checked for absolute accuracy. The expert should also study the

enlargements for apparent discrepancies in the prints, which he

might be called upon to explain.

The charted enlargements are readily mounted on stiff cardboard with

rubber cement, which may be purchased in small tubes. After cementing

the photograph to the cardboard, it should be placed under a heavy

flat object which will cover the entire surface until dry to prevent

warping and wrinkling. After drying, trim the two enlargements to the

same square size with heavy scissors, a pen knife or scalpel, and

fasten them together, book-fashion, with strips of the photographic

tape used in blocking the negatives. Of course, if charts are large,

20 to 36 inches square, mounting is unnecessary and they will have to

be supported in the courtroom with thumbtacks or metal rings.

Some courts do not permit numbering or lining of the photographs and

the enlargements alone in these cases will have to suffice. If there

is some question about admissibility of the charted enlargements, it

is well to prepare an extra uncharted set.