Chapter 17

_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 rewarded. 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 characteristics.

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.

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 photograph.

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.