DCS 5 APPLICATIONS //
Contrast Imaging for Fingermark Photography
Images of fingermarks can be crucial in forensic investigations for biometric identification of an individual, but achieving high quality images can be challenging due to the huge variability of evidence and fingermark treatment type. Often, fingerprints may have been deposited on coloured surfaces which can affect the contrast when white light is used for photography.
When white light is directed onto a surface, certain wavelengths (or colours) are absorbed, and others are reflected – this is what makes surfaces appear coloured. When coloured light is used to illuminate a surface, it is the differences in absorption and reflection which allow the contrast to be maximised between the fingermark and the background. This can be a very simple technique which can be utilised to enhance the contrast before software enhancements are applied to further improve the quality of the fingermark image.
Coloured light will lighten itself and darken the opposite colour. This means that if a surface is illuminated with its opposite (or complementary) colour, then it will strongly absorb this light and darken, whereas light of the same colour would be reflected, lightening the appearance of the surface. The effect of colour contrast imaging is best observed when using black and white photography and this type of fingermark image should always be viewed in greyscale.
Fig.1 A visual representation of the colour wheel
This can be helpful when selecting a colour to increase the contrast between the fingermark and the background.
There are always two things to consider when photographing fingermarks – the fingermark and the background. Both the fingermark and the exhibit substrate it has been developed on will affect which colour of light will be most effective at enhancing the contrast.
A comprehensive fingermark photography workstation for the detection, capture, and enhancement of almost any type of fingerprint on any surface or background – DCS5 consists of a range of light sources, including the FLS with access to over 600 different wavelengths of light to allow the user to access the exact colour needed to increase the contrast in their captured images. The system is built around a Nikon D6 camera and specifically-designed fingermark enhancement software to give the user the tools required to capture and enhance high quality fingermark images for identification. The DCS5 features an innovative capture preview setting, which allows users to alter camera settings and lighting to achieve the best image prior to capturing the final, audited image. Users can capture a number of images under different conditions, before choosing the image with the best contrast for final capture. This feature can be particularly useful for the application of colour contrast imaging.
The FLS is a filtered light source which grants access to 600 different wavelengths of light, from 400-1000nm. This gives us access to fine-tune not only the colour, but the actual wavelength of light that is being used to illuminate the sample, an ability which can be particularly useful and effective for contrast imaging.
Examples of Colour Contrast Imaging
When white light is directed onto ninhydrin on a white substrate, the white background reflects all of the wavelengths of light, whereas the purple fingerprint ridges absorb the green wavelengths which causes the human eye to see them as purple.
If green light is used to illuminate the same sample, the background will reflect all the green light directed onto it into the camera, whereas the purple ridges will strongly absorb this light and darken, as less of this light from the ridges will reach the camera. This means when the image is viewed in greyscale, the contrast between the ridges and background is increased.
Ninhydrin is a commonly encountered treatment when the developed fingermarks can benefit from the use colour contrast imaging.
When the amino acids in fingerprints react with ninhydrin, they develop as Ruhemann’s Purple – a dark purple colour. If a green light is used to photograph these images, the dark purple ridges will absorb the green light and darken, as shown in Fig.2
NINHYDRIN ON PAPER
Fig 2. Images of ninhydrin on a white surface, captured using white light and green light.
When green light is used to capture the image, the developed purple ridges absorb the light and darken. The effect of the increased contrast is then best displayed in greyscale.
It is not just the fingermark which needs to be considered when selecting a colour of light to use when photographing a fingermark – the background is of equal importance.
Where there is more than one possible colour to use for illumination, it cannot be known which would be the most effective – therefore multiple options may need to be tried to find the colour which will give the best contrast.
In the case of the print shown in Fig 3., there are several different colours of light which could be used to enhance the contrast in different ways, as there are several factors to consider. This gives a range of options, allowing the best image to be chosen without having to alter the lighting conditions, using the capture previews setting on the DCS-5.
NINHYDRIN ON CARD
White light image (a), greyscaled (b)
The colour of the card in the background and the red writing are interfering with the purple ridges
Image captured using yellow light (a), greyscaled (b) to lighten the background and reduce the interference from fibres within the cardboard
Image captured using green light (a), greyscaled (b)
This will darken the purple ridges as they’ll absorb the green light. However, it has also darkened the red text, meaning ridge detail may lose contrast in this area.
Image captured using red light (a), greyscaled (b) so that the interfering text will lighten and even out the background
Fig.3 Images captured using a range of coloured light to decrease the affect of the background on the ridges in the image.
When the fingermark on the exhibit appears dark or has undergone a treatment so that the fingermark appears dark in colour, the aim is to lighten the background to maximise the contrast. To do this, the same-coloured light is used as the interfering background to increase the contrast between the fingermark and the background.
ON COLOURED BACKGROUND
White light image (a), greyscaled (b) the blue printing in the background is affecting the contrast that can be achieved between the fingermark and the background
Image captured using blue light (a), greyscaled (b) – using blue light lightens the interfering background and therefore increases the contrast visible between the black fingermarkridges and the background
Fig.4 Images of a black powdered fingerprint on a blue background
When fingerprints are treated with cyanoacrylate, any fingermarks which are present will develop as white ridges. Coloured light can also be used if the substrate has a coloured background, in order to increase the contrast in the images. When the fingermark is white, the opposite colour of light to the interfering background can be used to darken the background to enhance the contrast.
ON DRINKS CAN
White light image (a), greyscaled and inverted (b). Where the fingermark passes over the dark blue label, contrast is lost. It is difficult to distinguish the white ridges from the blue background.
Yellow light image (a), greyscaled and inverted (b). As the fingermark is white and the label is blue, using a yellow light can darken the blue label and increase the contrast against the fingermark, allowing it to be more clearly visualised on the label.
Fig.5 A fingermark developed on a drinks can using cyanoacrylate
Blood is a contaminant that could often contain fingermarkss. As It can appear as a red-brown stain on a surface, often blue light can be employed in place of white light to increase contrast. The blood can strongly absorb blue light and cause it darken on the substrate.
White light image (a), greyscaled (b)
Blue light image (a), greyscaled (b). Using the blue light the fingermark darkens as expected due to the blue light being absorbed by the blood. However, there are fibres in the background which also absorb this colour of light and darken. This enhances the interference that arises due to the background
Image captured using yellow light (a), greyscaled (b). A yellow light can be used to “flatten out” the texture of the cardboard, and decrease its interference with the fingermark upon it, a problem often associated with this substrate type.
Fig.6 A bloody fingermark deposited on a cardboard surface