Medical Uses Of Infrared Photography Cultural Studies Essay

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Tattoos have been an invaluable method for the identification of non - skeletonised remains. The design and/or location of the tattoo can be substantially exclusive to provide adequate identification, and especially when combined with other distinguishing features they can definitively identify and individual.

During an autopsy the pathologist will take note of tattoos just as they would any other distinguishing marks, such as birth marks, defects or scars. In cases such as mass disasters, it can sometimes be one of only few methods available for the initial examination of the remains. In 2001, Kingsholm et al studied several unidentified bodies and remains found in Danish waters, some of which had tattoos. Discovery and detailing of any tattoos can also aid in tracing unidentified bodies back to their cultural backgrounds, thus in some cases tattoos have important historical aspects.

Tattoos can also indicate a history of incarceration (Mallon et al 1999) and in such cases the individual sporting this unique design may not wish to be easily identified. Repeat offenders for example, may choose to remove or alter their defining features and members of gangs or organised crime groups may wish to no longer be associated with that particular faction. Some tattoos may serve to connect one individual with another, such as 'love' tattoos, which can also aid in identification of the individual sporting the design.

Different light sources and filter techniques have been used routinely in criminal investigations; the following uses of infrared (IR) photography presented in this introduction are all relevant to this study as they go some length towards explaining the nature of infrared and therefore what might be expected, allowing for deduction of a hypothesis which is presented later in this report.

Infrared describes the part of the spectrum just beyond the visible red wavelengths (700 - 1200 nm)

There are many used of infrared photography spanning many different fields.

For art photography purposes, Infrared can be used to photograph objects in the distance, or in foggy conditions due to the infrareds ability to penetrate the haze (Milsom 2001). For portrait photography, infrared is sometimes favoured to give the appearance of a clear complexion

Infrared's (IR) ability to penetrate the superficial layers of the epidermis of skin is exploited in medical photography as a method of photographing venous patterns underneath the skin and of documenting healing under lesions in the skin. They found that imaging in the near-infrared range provided relatively good contrast of subcutaneous veins. This works due to the fact that haemoglobin is a chromophore that absorbs near infrared, and the skin absorbs very little IR relative to the absorption of infrared demonstrated by the veins (Haxthausen, 1933), or transmits or reflects most of the near infrared spectrum, therefore it appears lighter by contrast to the darkened veins.

Zharov et al (2004) identified the potential of this technique as a diagnostic method for varicose veins at a depth of 1-3mm into the skin.

The use of infrared photography for the detection of varicose veins or any other subcutaneous abnormalities is demonstrated by Marshall (1981). This research uses infrared reflectance (as well as ultraviolet) to measure the densities across pigmented lesions of the skin and found it to be a useful method.

The study by Haxthausen (1933) found that for documenting superficial afflictions, such as psoriasis, ordinary photography was far superior to infrared photography, as under the infrared conditions, the imperfections were removed. Afflictions that occurred in the deeper layers of the skin were captured best using infrared photography.

In cases of burn injury is can be difficult to assess the damage or the thermal burn depth. Anselmo et al (1976) found that infrared photography could be used as a valuable and non invasive method of assess burn depth. Their experiment used Wratten 89A infrared filter Infrared photography allowed for the differentiation between viable and necrotic dermis.

Infrared photography of bloodstains and Gunshot residue

Forensic applications of infrared photography include detecting gunshot residue on clothing (Bailey et al 2007) and less commonly, for detecting bloodstains on dark clothing or at crime scenes. A report by Raymond and Hall in 1986 illustrated a dark coloured sofa, showcased in the report as a black and white photograph for the visual spectrum comparison. In this photograph there is no obvious bloodstain, it cannot be distinguished from the rest of the sofa due to the dark colour of the sofa. The infrared photograph was taken using the Wratten #88a filter. In this photograph the sofa had lightened and now by comparison, the area of bloodstained sofa (now darkened by contrast) could easily be distinguished.

When using infrared photography to detect and document traces of blood on dark clothing, the infrared will make the blood appear darker and the surrounding clothing appear lighter by contrast. This is all due to the absorbing capabilities of the clothing and of the blood. Blood absorbs throughout the visible spectrum and the near infrared spectrum (typically absorbing most wavelengths of near infrared 700-900nm) and so its appearance in infrared records will be darkened, in contrast the clothing might only absorb through the visible light range of the spectrum, and so in the infrared records will appear lighter, or transparent.

Dark clothing can hinder the successful visualisation of blood spatter patterns in much the same way that darkened mummified skin can hinder the detection of tattoos, or that charring on a fire damaged document can affect the successful visualisation of the writing. A study by Perkins (2005) used digital infrared photography, Wratten #89B filter to photograph blood spatter on several different materials. The dark clothing appeared to reflect the infrared, thus enhancing the contrast between the clothing and the blood patterns.

Bailey et al (2007) used digital infrared photography to better visualise gunshot residue on dark clothing. It is very similar in both methodology and results to the use of this technique in bloodstain analysis. The camera ISO was set at 200 and the filter used was the Wratten #87. The GSR, undetected under visible light conditions, appeared dark against a lightened cloth under IR.

The use of infrared in analysis of obliterated writings and questioned documents

Infrared photography also has many applications in the field of forensic science.

It is a common method for detecting obliterated writing (Creer 1976) for detecting forged, or altered documents, such as cheques and to aid in the examination of writing obscured by charring on fire damaged documents (Bartha. 1973)

McCaul et al (2007) discuss the problems facing forensic scientists when traditional photography techniques fall short at documenting certain evidence. In the examination of documents, IR can be used to detect forgeries or alterations, relying on the fact that the visually similar inks may reflect or absorb infrared at varying levels and wavelengths.

Parallels can be drawn between the uses of infrared for examining obliterated writing and this study into examining original tattoos from underneath cover tattoos. The use of correction fluid or other inks to cover writing serves to render the underlying text illegible, in the same way that the cover tattoo serves to distort, hide or completely cover the original underlying tattoo. If infrared photography can allow for the underlying writing to be visualised, than it is entirely possible that underlying original tattoo could be recovered. The successful recovery of obliterated writings is dependent on the different inks used and their infrared absorbing capabilities. Some ink, when irradiated with infrared, will absorb it. This is due to the presence of different Chromophores in the different inks (Ellen, 2006)

A chromophore is the chemical group of a molecule that is responsible for the molecules colour, and they absorb, reflect and transmit different wavelengths. Other examples include chlorophyll, melanin and amethyst.

Infrared photography can also be used to examine/restore writing on charred documents (Bartha 1973). The success of the visualisation is dependent on the degree of charring. The charred paper is darkened due to partly converted resinous material before being completely degraded to elementary carbon. The carbon in the pen ink absorbs the infrared and so appears dark under IR, by contrast the charred paper looks lighter. The Video Spectral Comparator (VSC) is often used in the examination of obliterated writing (G M Mokrzycki 1999). The VSC uses Infrared radiant energy and filters to see through inks and other obliterations, and reveal obscured objects. The use of infrared photography is not an uncommon method for visualising and recording obliterated writing, S. Sugawara (2004) discusses the use of both near and middle infrared in deciphering obliterated writings by looking at writing made by 101 different pens.

Erasures describe inks that have been made invisible by removal of the colour components of the inks. Sometimes when these components are removed, remnants are left behind. Sometimes whatever remains on, or just below, the surface can be detected using infrared. This is the same principle encountered in a paper by McKechnie et al (2008) in which infrared was used to detect remnants of ink left behind in the skin post-laser removal treatment. The findings of this research will be analysed in greater detail later on in this research paper.

Other uses of infrared photography

The uses of Infrared photography do not begin and end at medical and forensic uses however, for example, Bridgeman and Gibson (1963) used infrared to examine paintings.

A paper by J R J Van Aperen De Boer (1969) successfully applies Infrared Reflectography to view the under-drawings of carbon pencil, with varying degrees of success of medieval paintings.

Pencil lead (which is primarily graphite) absorbs throughout the infrared range of the spectrum, and the visible range. Because of this, under both near and far infrared conditions the graphite will remain as readily viewable as to the naked eye.

Thus far this project has looked at both medical and forensic uses of infrared photography, many of which have in common the idea that infrared can be used to recover, or detect traces of one material (for example; ink) from underneath another material (such as correction fluid). These underlying or otherwise 'camouflaged' materials are not readily viewable with the naked eye, or easily recorded using ordinary photographic methods. This is the same principle as is to be employed in this research.

The detection of latent residue tattoo ink pigments

The use of infrared photography to record tattoos is by no means a recent phenomenon. As long ago as 1938 Jörg used infrared photography to detect tattoos that were otherwise undetectable with the naked eye.

Although infrared photography has been utilized to study tattoos, there is not a great deal of literature available on the topic. Below some specific examples are given. The following are 2 examples found provide useful validation of infrared penetrating capabilities coupled with its specific use for detecting tattoo designs that have been affected by environmental conditions, or purposefully made difficult to visualise:

Mckechnie et al use infrared photography to attempt to visualise latent tattoo ink residue from laser removed tattoos.

This research article only used 2 participants, and with a success rate of 50% and so a follow up experiment would be required using a larger sample size to validate the findings. Both participants had their tattoos removed by laser treatment to the extent that they were no longer visible to the naked human eye. One of the participants' tattoos was professionally done; the other was an amateur tattoo. Although the authors explain the difference between amateur and professional tattoos as a possible factor (that is that professional tattoos use more ink and are injected deeper into the dermis than amateur tattoos) and cross reference it with infrared's ability to penetrate the skin, they do not expand on the colour or pigment as being factors resulting in the success or failure of the trial.

Visualising tattoos on mummified remains using infrared photography

One of the original journal articles of interest that could be said to have initiated the thought behind this project idea, or at the very least inspire a belief of the success of the project, uses infrared Reflectography to examine tattoos on mummified remains, the mummified tissue normally being darkened to the extent that visualisation under normal photography conditions is near impossible (Alvrus et al 2001) It showcases the usefulness of infrared for lightening certain aspects of a subject in order to see others. In this case, the darkened mummified tissue made it difficult to visualise the tattoo under normal photographic conditions (visible light). Under Infrared conditions however, the contrast between the tattoo and the surrounding skin was enhanced; the darkened mummified skin appeared lighter, and by contrast the tattoo (which appeared darker) was readily visible. This is due to the differing absorption/reflection of infrared. The substances in the tattoo absorb the majority of the infrared; the surrounding skin reflects the infrared.

A similar technique is used in the identification of tattooing on a 1600 year old mummified body found in Alaska (Smith and Zimmerman 1975) Tattooing was identified on the hands and forearms of the Eskimo female remains using infrared photography. The darkened skin obscured the tattoos to the extent that they could not be viewed under visible light spectral range photography.

Skin thickness as a factor

One of the other variables accounted for in this project is the area on the body of that tattoo, the thought behind this being that the thickness of the skin might come into play. According to E J Wood (1985) the thickness of the epidermis ranges from 0.06 - 0.1mm (from eyelids, to back and callused areas, respectively). The dermis ranges from 2 - 4mm thick, and accounts for the bulk of the skin. Although any differences in skin depth tend to be minute, they may still account for variable success rates due to the migrating nature of tattoo ink through the dermis and the penetrating capabilities of infrared. A skilled tattooist will not allow the needle to penetrate the skin no deeper than 2mm, the reasons for which are discussed below in 'The tattoo process'. The ink must be deposited deeper than the epidermis, or else the ink will fade as the outer layers of the skin shed, therefore the thickness of the epidermis will affect how deep the ink must be deposited. It is also assumed that the thickness of the dermis will correlate with the thickness of the epidermis. This is why this research paper has chosen to look at the area on the body of the tattoo as a possible factor.

The tattoo process

The process of tattooing involves injecting pigment, suspended in a carrier, through the epidermis and into the dermis of the skin, no more than 2mm or else the tattooist risk the ink bleeding, creating a smudged effect, regardless of the tattoo technique employed (the settings of the modern tattoo machine do not allow for the needle to penetrate any deeper than 2mm). There is no exact science as to the pressure exerted on the needle by the individual tattooist, as it is a matter of personal judgement (direct quotation from Mark - tattooist from Danny's tattoo Studio in Sneinton, Nottingham) depending on the area of the body on which the tattoo is being performed. Tattooing on an individual's back or upper arm will require greater pressure because the skin is much thicker, the setting of the tattoo machine will also have to be altered for deeper penetration through the epidermis and into the dermis.

Once injected into the upper layer of the dermis, the pigment is suspended in the fibroblasts (Sperry 1991). When looking at a tattoo, the tattoo is being viewed through the epidermis.

Over time the deposited tattoo ink will disperse deeper into the dermis, and so it is possible that time, both between original and cover, and time since cover, could be a feature in the investigation.

Tattoo Ink pigments

More often than not, the tattooist will use a tattoo ink containing pigments which will be manufactured outside of the tattooist's own premises, the exact content and purity of which is in most cases, unknown. There are certain guidelines in place for manufacturers of tattoo inks to abide by when producing the inks, however manufacturers of tattoo ink are not forced by law to disclose the ingredients used in the inks. A study by Timko et al (2001) found that, of 30 inks studied, the most commonly identified elements were aluminium, oxygen, titanium and carbon at 87, 73, and 67 percent respectively.

Professional tattoo artists have access to over 100 different colours (Kirby et al 2005), many of these are mixtures of colours, for example red and white to make pink, thus making it difficult to classify pigment-wise. The research by Kirby et al (2005) found significant variability in pigment cluster sizes in professional tattoos, compared to amateur tattoos. They also found colour pigment granules to be larger than black pigment granules.

This was initially an area of interest, but not one that this research paper will focus on due to the difficulty encountered in obtaining the relevant information.

This study uses infrared photography in an attempt to visualise an original tattoo from underneath a cover tattoo, the success of which could be due to a number of different factors. A summary of these factors are as follows:

To look at the colours used in both the original tattoo and the cover tattoo and how this affects the successful visualization of the original tattoo using infrared.

To look for any correlation linking time since cover tattoo (up to date of photograph) and success of the experiment. This essentially will be looking at the effects of the migration of tattoo ink.

To look at any trends involving time elapsed between original tattoo and cover tattoo, and the success of the infrared photographs obtained. As above, this will be examining the effects of ink particle migration.

To look at the area on the body of the tattoo and discover if there is a relationship between this and the success of a particular photograph. Essentially, this is investigation skin density as a factor.

Taking into account all the research discussed previously and the results obtained by the various studies in the field of infrared photography a few hypotheses have been deduced:

The first is that the penetrative nature of infrared suggests it is possible to recover the underlying pigment of the original tattoo from beneath the cover tattoo. The reasons behind this are thought at this stage to be due to the colours present, or perhaps even the concentration of black used in the colour mixtures.

The way in which an object appears when photographed using infrared is a direct consequence of their absorbing capabilities.

For the purposes of this particular study inspiration was drawn for the research into questioned documents, or more specifically the detection of different inks using infrared. Most inks have different absorbing capabilities, some will only absorb through the visible light spectrum, and some throughout the visible light and into the infrared range. The reason behind this, as mentioned earlier, is due to the chromophores in the dye molecules.

In the same way that infrared might be used to detect the underwritings of obliterated text, IR could be used to detect the underlying tattoo pigment, depending on the chromophores, or colours used.

The second hypothesis is that any variation in the depth/thickness of the skin will be such a minute difference that it is unlikely to affect the overall success of the project.

The factors of time (time between original and cover, and time since cover) are likely to have minimal effect, if any, on the successful visual recovery of the underlying tattoo. It is thought that any trends are more likely to be observed in the extreme time periods, for example, between tattoos that are under a year old and tattoos that are over 30 years old. This is because the most recently done tattoos will not have been affected by migration, whereas very aged tattoos will have undergone decades of migration of the ink particles.

Method and materials

A sample number of 33 (n33) individuals were used in this research, all of whom volunteered their time and consented for the presentation of their tattoos anonymously in this research paper. All that was required of potential participants was that they had a cover tattoo, there was no other criteria selected for, for example age or gender. The participants were then required to fill out a consent form, as no photographs could be taken without. The participants were asked to fill out a short questionnaire to aid in investigation of the various factors discussed in the introduction, an example of which can be found in the appendix.

This project utilizes the near infrared part of the spectrum (the far infrared part of the spectrum is commonly used for thermal imaging). The infrared filters allow infrared light to pass through the camera and blocks most of the visible light spectrum.

The Fujifilm IS pro digital SLR camera is sensitive to both ultraviolet and infrared light between 380nm and 1000nm.

Optimum settings for the camera were assessed during several pre-experiments. Manual setting, ISO 200 and aperture f/6.3, which provided adequate depth of field for the infrared photographs

Filters 87 and 88a were used, which are described as deep infrared filters; allow no visible light to pass through.

The Wratten 87 filter has a sharp cut-off at ~800nm, and so transmits freely throughout the infrared region.

The 88A has a slightly narrower window with an effective cut-off at ~750nm.

The control photographs were taken using a UV/IR barrier filter.

All photographs are taken at a 90° angle from the subject (as close to as possible without actually measuring the exact angle) this was to eliminate any false positives due to raised tattoos (which can happen in hot conditions).

A colour scale, a grey scale and a sizing scale are held in place around the tattoo.

The first photo taken is the control photograph, using the IR-UV cut lens. This allows visible light to be captured and so essentially, is just a normal (visible light) flash photography picture.

The second photograph removes the UV-IR cut lens, replacing it with an IR filter. The first used is the Wratten 87, as many photographs as necessary are then taken using this filter. The third set of photographs is taken with the 88a IR filter.

The Metz 45CL4 electronic flash gun was the light meter of choice, the setting of which differs between photos depending on the light source available at the location.

A number of photographs were taken of each tattoo; one control photograph, and 2 photographs using each filter. The first infrared photograph was to capture the entire tattoo, including the scales. A second photograph was taken if deemed necessary, which was a close up shot, focussing on any areas of interest highlighted in the first infrared photograph in order to capture better detail.

Once all of the photograph records were obtained they were categorized into 'successful' and 'unsuccessful'. Of the successful results, due to the varying degrees of success encountered from record to record, it was felt necessary to categorize further still with the introduction of a grading system.

The grading system is as follows:

0 - None of the original tattoo could be visualised

1 - Partial original tattoo visualisation

2 - Most of original tattoo can be seen

3 - All of the original tattoo can be seen from underneath the cover tattoo.

Please note that some of the images presented in this report have only undergone basic alterations post production. The changes made to the infrared images were limited to minor brightness and contrast changes to accentuate any detail in the pictures that may have been lost when the images were resized and in order to retain detail in the printed hard copy. These changes are as follows:

Brightness was altered to a maximum of around -10% for the infrared images on Microsoft Office Word 2007.

Contrast was altered to a maximum of +30% on Microsoft Office Word 2007.

These values are approximate, and dependant on the detail captured in the original, unaltered photographs. The images were not altered in any other way

The underlying tattoo can be somewhat visualised under visible light photography conditions however visualisation of the original tattoo is accentuated using the Infrared as it removes the camouflage or interference provided by purple shading of the cover tattoo. This is a grade 2 example.

This was graded as 2 because a substantial amount of the original tattoo can be recovered. In this case, the tattoo, under visible light conditions, is not an obvious cover tattoo as no detail of the underlying tattoo can be detected due to the use of shading and the faded nature of the original tattoo. With the shading of the cover tattoo removed using the IR filters the detail in the original tattoo can be visualised. The black shading of the cover tattoo on the eagle's head and wings is not visually removed by the infrared, and so part of the cover tattoo is still camouflaged.

Note - The dark green and blue of the original tattoo appear darker in the infrared than the other colours of the cover tattoo. The purple in the background has completely disappeared in the infrared photograph.

This is a particularly interesting record as it demonstrates the absorbing capabilities of black ink. Although some of the original design can been seen in the control photograph it is somewhat polluted by the dark colours of the cover tattoo. The blue and particularly by the purple of the cover design are visually removed in the infrared photograph, highlighting the contrast and allowing for better determination of the actual design.

It would be expected that if there were a trend amongst these results that as the time since the cover tattoo increases, so would either the success or failure rate. At the same time, the remaining (factor - either success or failure) would decrease as the other increases to show that there is a clear correlation. This does not occur, and so suggests that there is no link between time since the cover tattoo, and the overall success rate.

Also there does not appear to be any detectable trend in the observed graph patterns between fig. 9 and fig.10

A summary table of records, including information on colours used, the grade allocated to each record, and a brief description of what can be seen with each record, is available in the appendices (Appendix 4) the findings of which are discussed in the discussion section of this research paper

It should be noted that this graph only presents the number of cover tattoos with black ink in the design and does not take into account whether or not the black ink is directly responsible for the obscuring of the original tattoo design. It does, however, present a slight trend, which is to be expected.

In cases where the cover tattoo is colourful, the Wratten 87 filter appeared to be the filter of choice as it removed all colour of the cover design to reveal the underlying original tattoo.

In cases where the cover tattoo consisted of cut black or grey shading, the Wratten 88a appeared to be the ideal filter to better accentuate the original design from the cover tattoo design traffic.

Of the unsuccessful examples:

The following results are characterised according to the observed reason why the original tattoo could not be visualised. Please note that in 100% of the unsuccessful records the cover tattoo design contained black ink, however in some cases it is not deemed to be the reason for unsuccessful visualisation (In some cases for example, the black of the cover does not cover the original design)

Discussion

This research has proved definitively that infrared photography can be employed to detect an original tattoo design from underneath a cover tattoo.

Of the several factors investigated in this paper that were thought to affect the success of the recovery of the original tattoo using the IR filters, the only factor with any real merit were the colours used in the original and cover tattoos.

The time since the cover tattoo was investigated due to the behaviour of the ink once deposited in the skin. As explained in the tattoo process section, once the ink is deposited, over time some of the particles will disperse throughout the dermis. It was thought that the effect of this, if any, would be to hinder any successful recovery of the underlying tattoo.

No obvious correlation was discovered between the time since the cover tattoo and the successful recovery of the original tattoo from underneath the cover tattoo.

Time between original tattoo and cover tattoo also yielded no apparent association to the success rate. The graphs and a brief description of the findings can be seen in the results section of this paper (fig. 18 and Fig. 19)

When looking at the area on the body as a factor (essentially that is looking at the depth or thickness of the skin) it seemed necessary to take two examples from areas of differing skin thickness and compare the success rates from each.

The skin thickness of the wrist will be significantly less than the thickness of the skin on the back, which along with the soles of the hands and feet, is one of the areas on the body of the largest skin density. Of the results for the wrist tattoo, 50% were successful. This was the same as with the results from the examples photographed on the back (Fig.17) suggests that the area on the body of the tattoo had little or no bearing on the results.

As predicted it is the colours used in both the original underlying tattoo, and the cover tattoo, that has the most bearing on the successful visualisation of the original tattoo.

Visualisation of the underlying tattoo was most striking and effective with the examples with purple, red, light blue and white cover tattoos - this is likely to be due to the different chromophores (the functional group of the pigment molecule that gives it its colour) in the different colours used, as they will absorb or reflect the infrared wavelengths at varying levels. Red and purple cover designs yielded the best results because these colours were completely visually removed by the infrared filters, leaving only the underlying original tattoo visible.

Referring to Fig.1 and 2 of record 26, the purple of the cover design is completely visually removed under the infrared photographic conditions, allowing the black outline of the original underlying tattoo to be completely visualised.

Records 22, 26 and 26b are perhaps the best examples, each receiving grade 3 in the grading system for complete visualisation of the original tattoos. The original designs can be easily distinguished as there is no interference from any fragments of the cover tattoo left behind under the infrared conditions.

Record 22 (Fig.12 and 13) is an example of a darker original tattoo design with a black outline covered with red and purple ink design. The original tattoo design in record 22 could be only somewhat detected using visible light photography and electronic flash, however the detail of the designs could not be visualised due to deliberate shading of the cover tattoo. Under IR photographic conditions, the red of the cover tattoo appeared transparent, and the underlying design could now be viewed with no interference of the cover tattoo.

In Record 4b, the original design was significantly faded black writing. Referring back to the work of McKechnie et al (2008) it is already established that infrared will detect and accentuate faded or purposefully removed tattoo residue in the skin.

Record 4b also illustrates this, as the detail barely detectable by the naked eye becomes significantly more legible using infrared filters.

Of the 20 successful results, 100% of the records had black ink in the underlying original tattoos, be it block colour, shading or just outline. In such cases as records 26 and 22 it was the black outline of the original underlying design that was most striking in the infrared image. As mentioned previously, this is because the black ink absorbs throughout the near infrared range of the spectrum, and so any infrared that penetrated the skin and was not absorbed by the various ink pigments used in the cover tattoo, would be absorbed by the black, and this would give a very strong successful visualisation. Black ink used in the cover tattoo would prevent the infrared from detecting anything beneath it (fig. 25)

Of the results graded 2 on the success scale, approximately 72.7% had black in the cover tattoo. In those tattoos in which all of the original tattoo could be visualised, 0% had black in the cover tattoo. Of the results graded 1 on the success scale, 100% of the cover tattoos designs had an element of black ink, be it block colour, shading or just outline. Of the unsuccessful records (graded 0), the majority (approximately 66.6%) were black cover tattoos, although 100% of the results had black elements in the cover tattoo however, in cases such as where the original tattoo is integrated into the cover tattoo, it is thought that the presence of black is not the primary reason for the unsuccessful visualisation of the original tattoo. Please refer to the appendix 5 (the complete record collection)

Of the successful results, the variable levels of success were more often than not attributed to presence of black in the cover tattoo, which unlike purple or red, remained strong in the infrared photographs.

Grade 2 records, in the majority of cases, stopped short of complete visualisation (grade 3) due to the presence of black ink in the cover design that still obscured part of the original design. This is because the black pigment absorbs the infrared, and works in much the same way as using infrared to decipher obliterated writings. The infrared penetrates the correction fluid and is absorbed by the underlying black ink (Ellen, 2006).

Second in the running for reasons as to why the experiments in these cases were unsuccessful was that the original tattoo design was integrated into that of the cover tattoo, which meant that regardless of any success in removal of colour under IR conditions, the design of the original tattoo could not be readily distinguished from that of the cover design. This accounts for 4 out of the 18 unsuccessful results. For the remaining 2 examples in the 'other' category, the reasons for the lack of success was attributed to the use of similar ink colours in both the original and the cover tattoo designs and so it was difficult to differentiate between elements of the two.

The other of the examples had had the original tattoo treated with 2 laser removal treatments prior to the cover tattoo.

The grey used in the cover tattoos is either a black and white mix or a cut black (diluted with distilled water) thus diluting the amount of black pigment, which could provide a rationale as to why in some cases, under IR conditions, the grey cover lightens allowing the original tattoo, providing the ink of which is darker, to be visualised more readily.

In some cases, particularly where the original tattoo used dark or black ink covered by a lighter tattoo or shading, the flash conditions alone were sufficient to detect all or part of the original design.

There were a number of instances where it was difficult to categorize the findings:

Photograph 4b was a particularly interesting example of this. The cover tattoo did not attempt to camouflage the entire original tattoo, it was not necessary as the original tattoo was sufficiently faded so as to make it difficult, although not impossible, to detect with the naked human eye. Although the original tattoo could not be visualised from underneath the black cover design, it did serve to better visualise the original tattoo from in between the cover design.

Difficulty was encountered when attempting to categorize the results in the grading system.

Record 16 was an interesting case in point for the complexity of categorizing the results. In this particular record, using infrared photography a rectangular shape, or pattern was detected that was not visible in the control record, however, the original underlying tattoo design was that of a love heart, and so it was assumed that the rectangular pattern was a part of the cover design and as no love heart design could be detected, the record was graded 0.

Record 29 was another confounding example, as the cover tattoo was incomplete, meaning that the original tattoo could be somewhat viewed under visible light conditions. The cover tattoo was a series of outlines with little shading, all in black ink. This meant that the original design was visible in both the control and infrared photographs, and so difficulty was encountered in categorising this particular record.

A CD of the individual records is included in this research paper (appendix 5)

Conclusion

There are a number of reasons why an individual might want to cover a tattoo, such as a desire to keep a hidden past, to disguise affiliation with a particular gang or culture, to symbolically remove a past loved ones mark, or perhaps more generally just to disguise any distinguishing feature that might make them readily identifiable.

This research has proved that in certain situations it can be possible to detect and visually recover an original tattoo design once the tattoo has been permanently covered, using infrared photography.

Out of all the variables investigated in this paper, the only one with any merit appeared to be the colours used in both the original and cover tattoo.

Successful visualisation was dependent on the chromophores in the ink, which are responsible for the pigment molecule's colour. These reflect and absorb infrared at varying levels.

Black ink is well documented to absorb infrared, and this is validated by the findings of this project. In all of the records, control and infrared alike, the black remained the most prominent element in terms of colour.

The colours most readily visually removed in all of the infrared records were purple and red, as they transmit throughout the visible range of the spectrum, but reflect all infrared radiation and so although to the naked human eye the purple appears as solid block colour, and the design cannot be seen underneath (records 26 and 26b) however, under infrared conditions the colours appear transparent, revealing the hidden design.

Greens and blues, although removed by the Wratten #87 filter, were still present in the infrared records taken using the Wratten #88a filter.

This research validates the hypothesis that infrared photography can be used to visually recover underlying tattoos from cover tattoo designs done purposefully to obstruct the visualisation of said underlying tattoo. The success of this technology is dependent on the colours used, namely if the tattoo is covered with black ink the infrared will not detect anything beneath it. This is an invaluable technique that could be used to identify individuals with a history of incarceration or gang affiliation that may have had their identifiable symbol deliberately covered.

Future prospects

Further study could be done to assess the composition of inks. This was a difficult task as manufacturers are not forced to disclose the ingredients of the inks.

Other future prospects include the possibility of looking specifically at laser treated original tattoos underneath cover tattoos using infrared photography.

There are opportunities to further manipulate the images on Photoshop software. These include pinpointing areas of extreme colour changes, that is the areas of the records where the infrared visually removes the colour, and finding the differences in the RGB values to provide more data for statistical analysis.

Appendix 1

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