Technique of photographing fingerprint impressions on a variety of objects and surfaces; widely used in police work to establish the identity of suspects and to obtain and present evidence.
The basis of the use of fingerprints in crime detection is the fact that the ridge pattern of the skin on the fingers of a person is unique and permanent throughout his life.
Like many other branches of photography, fingerprint recording is a specialist job and requires a knowledge of the science of fingerprints by the operator to enable him to produce the result the expert needs to reach his findings.
There are four types of finger impression which it is sometimes necessary to photograph. They are:-
1) Latent impressions- sometimes invisible to the naked eye.
2) Moulded impressions- left behind after the fingers have been coated with some substance such as blood.
3) "Pick-up" impressions- where the ridges of the fingers have picked up dust or powder and left behind the pattern in the dust.
4) Inked impressions- taken on a form or card for record purposes.
The satisfactory recording, photographically, of a latent fingerprint found at the scene of a crime depends to a large extent on the correct choice of powder used to develop the impression when it is found. For the other types it is usually only a question of choosing the correct film and illumination.
The equipment used varies according to the location and article on which the impressions are found. Many are in difficult positions requiring some ingenuity on the part of the operator to get his camera into position.
When conditions were suitable, a technical double extension camera (stand camera) enables the impression to be recorded in actual size. Special fingerprint cameras have the lens set at a fixed distance from the film and the impression, again to give an actual size image. Occasionally 35 mm cameras were used but are not generally favoured. Nowadays digital cameras are used.
Illumination can be by daylight, photoflood, other tungsten light or by flash. The fingerprint camera were normally battery-operated using four built-in bulbs. Some of the specially made instruments used electronic flash.
When dealing with light impressions on a dark background, a camera fitted with a prism or optical mirror can be used. This eliminates some of the intermediate stages in the preparation of the final print. Filters can be of great assistance in rendering the background darker or lighter.
Inked impressions on cards or forms can be copied satisfactorily on a process camera or by means of xerography in the case of good clear impressions.
As the aim is, in most cases to increase contrast, process film is widely used but panchromatic and orthochromatic films are as popular. For a very weak low contrast mark, a "lith" type material as used in graphic arts may occasionally be used coupled with extreme contrast development. The choice depends to a large extent on the strength of the impression and the colour of the background.
With a developed (e.g. powder-dusted) latent impression or a moulded or pick-up one, the choice of camera depends very much on where the impression is and what it is on. If a non-portable surface is in the range of the technical camera or fingerprint camera, either can be used. If there is a sequence of impressions or part of a palm, the larger format of the technical camera may be required, to record the whole area on one negative.
The technical camera is set up with the bellows extended to give an image size of 1:1. When focusing the camera, the surface on which the impression appears, the lens panel and the focusing screen should, wherever possible, be parallel to avoid distortion. Accurate focusing is essential in view of the shallow depth of field at this extension. On curved surfaces or surfaces which slant away from the camera, a small aperture is necessary to render the whole of the impression sharp.
With daylight, a white card used as a reflector may assist in evening up the illumination when the light is coming from one side only. Portable artificial light sources should be moved into such a position that the image is shown at its best on the focusing screen. On strongly-patterned, but shiny surfaces, the impression may be made visible by deliberately lighting the surface so that the latter reflects glare into the camera lens. The impression then stands out against the glare. Oblique lighting may be necessary to show up certain moulded impressions (such as those found in soft putty) or pick-ups. Flash bulbs should not be too powerful and can be used to advantage at an oblique angle. They are to be preferred when dealing with anything like soft putty, as the heat from the photoflood or other lamp may melt the subject.
If it is not possible to record the impression in actual size, some sort of a marker photographed with it provides a guide for future enlarging.
The fingerprint camera is normally hand held, with or without a template in position over the impression to be photographed. At the front of the camera, legs or an enclosed mouth rest against the surface being photographed. Some cameras require a time exposure as their source of illumination is low. In this case the camera must be held perfectly still during the exposure. Modern fingerprint cameras with more powerful light sources permit exposures of l/30th second or faster at small enough apertures to bring impressions on a curved surface within the available depth of field. Usually both aperture and shutter speed are fixed to yield a correctly exposed picture with a specified light source and film speed.
Impressions revealed by the use of a light powder can be photographed with a camera fitted with a prism or an optical mirror direct on to bromide paper. This gives a print which is right way round but with the tones reversed, i.e. the impression appears dark in colour on a light-coloured background. If several prints are required, then several exposures have to be made. This is simplified by using a special multiple sliding back holding a long narrow piece of bromide paper. This method saves time and material in the production of the prints.
Portable articles on which fingerprints are found are taken from the scene to the studio where better facilities are generally available to deal with them.
The copying of inked impressions from forms or cards is still best done by orthodox photography using any stand, fingerprint or process camera.
Special treatment is required when dealing with mirrors. An impression can be seen on the surface of the glass and another on the silvered back of the mirror and one appears superimposed on the other. A dark powder is used to reveal the impression and a piece of ground glass is placed ground side down on the impression. The mark on the surface of the mirror is clearly visible but the one on the silvering of the mirror cannot be seen. The photograph is then taken in the same way as any other print.
Weak marks on glass can sometimes be dealt with better by illuminating them from behind and photographing them in silhouette against the light.
Impression on a multi-coloured surface are dealt with by using a powder which flouresces under an ultra-violet lamp. Only the flourescing ridges of the pattern are recorded and the colours of the background completely eliminated.
No matter whether the fingerprint has been revealed with a light-coloured powder or a dark powder the resulting print must show the impression as a dark pattern on a light background. This is to permit comparison of both impressions in the same form (i.e. the fingerprint form with its black inked impression and a photograph showing black impressions on a light base).
Where the fingerprint has been developed by the use of a black powder, all that is required is a straightforward print from the negative on a suitable grade of paper. If a light-coloured powder has been used, a transparency made from the negative serves to produce subsequent prints. The transparency is made by contact printing the negative on to a piece of slow process film with the negative placed upside down in the carrier. A thin-based film (e.g. 3/1000 inch) is used for the negative to minimize any loss of definition due to the two emulsions not being in close contact. The use of a prism or mirror does away with the need to make a transparency or even a negative.
Photographing the impression in actual size in the first instance simplifies subsequent processing, as prints or transparencies can all be made by contact printing.
Contact prints can be made on bromide or contact papers. Many operators prefer the former in the belief that it records more of the range of tones sometimes found in the impression or that it provides a better balance between the weak and the strong areas.