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 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
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
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
[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.