The Science of Fingerprints Part 29

The latent print must not be inside the open end of the camera, nor must it be beyond; otherwise, the picture will be blurred.

[Ill.u.s.tration: 426. When object being photographed does not cover camera opening, outside light is excluded with piece of cloth.]

Since the camera has its own light source, any leakage of outside light will cause overexposure of the film. Consequently, if the surface of the object bearing the latent print which is to be photographed is uneven or does not cover the entire front of the camera opening, it will be necessary to use some opaque material such as a focusing cloth or heavy dark material to cover the front of the camera so as to exclude all outside light (fig. 426). If a latent print on a pane of gla.s.s or an automobile window is being photographed, it will be necessary to back up the latent so that there will be no light leakage. Material showing a pattern or grain should not be used for this purpose as any such pattern will photograph in the background and possibly obliterate the ridges of the latent print.

To insure an equal distribution of the light over the latents being photographed, the impressions should be centered in the opening of the camera. This is accomplished by opening the angular front section of the camera after the metal plate covering the front has been opened, and setting the aperture over the latent impressions so that they will be as near the center as possible. Then, holding the camera firmly in place, it is carefully closed (fig. 427).

During exposure the camera must be held perfectly still. Any movement of the camera or object will result in a fuzzy or double image.

In photographing a small, movable object such as a bottle or tumbler, the camera should not be placed on its end and an attempt made to balance the object across the opening. Instead, the camera should be placed on its side and the bottle or tumbler built up to the opening so that there is no necessity for holding the object (fig. 428). There will be, of course, instances where the camera will have to be held, such as to the side of a wall, cabinet or automobile. Here an extreme effort should be made to avoid moving the camera or permitting it to slip during exposure.

[Ill.u.s.tration: 427. Centering of latent in aperture insures equal distribution of light over print.]

Antic.i.p.ating the possible use of the photographic negatives in a court proceeding, it becomes of paramount importance to be able to identify them. This is done by using what is called an identification tag. The tag consists of a small piece of paper bearing the date, initials of the examiner, and possibly a case number, and it should be hand-written. The tag is placed near the latent prints being photographed so that it will appear in the picture. It should be borne in mind that concentration should be on the latent impressions, which must be centered, and the identification tag should be to one side and not covering any of the latent prints. Another method of identification, if the surface permits, is to write the above-mentioned data on the surface of the object bearing the latents so that the information set out will also be a part of the picture.

Too much emphasis cannot be placed on the importance of the identification tag. The lack of such data, by discretion of the court, may exclude the latents as evidence, in the absence of the original specimen bearing the latents.

[Ill.u.s.tration: 428. In photographing objects with curved or irregular surfaces, camera should be laid flat and latent-bearing surface built up to opening.]

The following are suggested exposure times for Tri-X film (available in 2-1/4" x 3-1/4" film packs) with battery-operated cameras having lenses without diaphragms:

Black powder print on white or light background--snapshot Gray or white powder print on black or dark background--1 second

These same exposure times can be used on some cameras having lenses with diaphragms, provided the lens opening is set at f6.3.

The above exposure times are for cameras with batteries in average condition. If batteries become weak the exposures may be increased slightly.

In making snapshots the shutter mechanism should be manipulated as rapidly as possible since slow motion will appreciably lengthen the exposure. In making time exposures the camera shutter must be held open for the desired time. Personnel with photographic experience may desire to use cut film with the fingerprint camera. A few tests will determine the optimum exposure times for any particular type of film.

Briefly, the procedure for taking photographs of latents is as follows: The film pack is placed into the film pack adapter with the safety paper side of the film pack to the slide side of the adapter, care being taken to see that all of the paper tabs are outside of the adapter. The adapter is placed on the camera in its proper position by opening a slide clamp attached to the camera, fitting the side of the adapter into the slot away from the clamp and pushing it down flat into the opening. Don't try to slide the adapter into the opening from the top. The adapter is locked in position by closing the clamp. Next the slide is removed and the tab marked "Safety Cover" pulled out as far as it will come and torn off. The camera is placed in position and the first exposure made; then the tab marked "1" is pulled and the next film is in position for exposure. This procedure is followed with each succeeding film until all have been exposed.

When the last tab has been pulled out, the pack can be removed from the adapter in daylight. If all of the films have not been exposed the slide is replaced into the adapter and the film pack removed from it in a darkroom, as previously stated.

As a matter of regular policy, it is recommended that more than one exposure be made of each latent, varying the normal exposure time to insure satisfactory results, especially when the contrast is not a good black on white or gray on black.

Before starting to photograph, note the following:

- Check shutter action.

- Check bulbs, batteries, and lights.

- Center latents in opening of camera.

- Latent being photographed must be flush with opening of camera.

- Outside light must be excluded.

- Include identification tag in photograph.

- Remove slide and pull safety tab of film pack before making exposure.

- Hold camera still while making exposure.

- Pull correct number tab after each exposure (be careful not to pull more than one).

- Do not use grained or uneven material as a backing when photographing latents on transparent gla.s.s.

- Mirrors, polished chrome, and nickel plate will photograph black in the fingerprint camera.

The foregoing has dealt with the standard use of the fingerprint camera when the direct light afforded by the camera gives suitable results. There will be cases, however, where the results from the use of the direct light may not be adequate. Such cases may involve molded or embedded prints, such as prints in putty, wax, soap, etc. Should direct light give poor results, side lighting may prove effective.

This can be done by loosening two of the bulbs on one side so that they will not light. The light given by the other two bulbs is directed so as to pa.s.s at right angles, as much as possible, across the ridges of the embedded latent print. Adjustment of the exposure time must be made when this is done.

CHAPTER XVI

_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 gla.s.s 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 ill.u.s.trate 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 gla.s.s 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 ill.u.s.trate an identification, but it is neither claimed nor implied that this number is required.