3 February 1998
By the Select Committee appointed
to consider Science and Technology.
DIGITAL IMAGES AS EVIDENCE
A Picture is Worth
a Thousand Words
1.1 The tabloid at
the supermarket check-out may have another "Elvis is Alive"
story with a picture of him holding the last month's issue: inside
there might be a story about a World War II Lancaster bomber landing
on the moon, with photographs to prove it. But by and large people
do not believe these photographs however real they look, and it
is doubtful if the authors expect them to. So why should people
doubt the photographs in a broadsheet of the Chancellor of the
Exchequer holding up his red box on Budget Day in front of No.
11 (The Guardian 3 July 1997)?
Even those who approach what is written with a reasonable degree
of scepticism have a general presumption that a photograph is
what it purports to be. Further, if a jury in a criminal case
is shown by the prosecution a clear and unambiguous photograph
of the defendant caught in the act, in the absence of any other
evidence, will the members regard it with just the same degree
of scepticism as they had for the picture of the Lancaster on
the moon? And should they?
1.2 Questions such
as these led the Committee to embark on its study of the use of
digital images as evidence (see Box 1 for background information
on images). What initially seemed likely to be a short enquiry
into an apparently straightforward and well contained topic was
found to be far less constrained. The ubiquity of digital technology
meant the subject broadened from images, typically photographs,
to closed circuit television (CCTV) on the one hand and databanks
on the other. Digital technology has a pervasive influence on
many apparently unrelated aspects of society, which was one of
the reasons why in our previous report "The Information Society:
Agenda for Action in the UK"
we recommended that the Government take a holistic approach by
setting up an Information Society Task Force to identify the barriers
to the development of the Information Society in the United Kingdom
and to recommend appropriate remedies.
|Box 1: What is an image? Some definitions
|The traditional image captured by photography is an analogue image. The images are recorded as a variation in some physical property, for example frequency, amplitude, or activation state of a photochemical emulsion (of colour dyes and silver halides, for example, as a transparency or a negative). This varies with the brightness, colour and contrast of the scene being recorded.
|A digital image is a numerical representation recorded simply as a series of binary digits (bits): either one or zero with no values in between. The image is captured by being focused onto an electronic sensor (a charge coupled device, CCD) which is made up of individual light-sensitive elements called pixels (picture elements). These act as switches modifying an electrical current on or off and the information is processed by a computer. Images may be either moving or still, the data can be stored on a variety of media (eg compact disc, computer hard disk, or digital tape), and the image may be displayed either on an electronic display (computer monitor, TV screen or computer projection) or as a hard-copy print.
|The quality of a digital image depends on the number of sensor pixels and the size of the image as it is displayed. High resolution digital cameras on general sale at the moment have 'million pixel' sensors 1,152 x 864 pixels, (eg the Kodak DC210 for around £650) which will produce a standard five inch by three and three quarter inch photo with a resolution of 115 dots per inch. Compared to an analogue print of the same size, this sort of resolution is very poor but is suitable for displaying on a computer screen. Higher resolution sensors are available and it is likely that they will become more widely available in the near future. Much higher resolution is also available from computer scanners which can be used to digitise paper documents and analogue photographs. Epson, for example, sell a 2,400 dots per inch scanner for just over £500. If linked to their 1,440 dots per inch colour laser printer (for around £200), this can produce a photo-quality hard copy.
1.3 Although there
is a common technological link between the many different issues
raised by digital imaging, their apparent diversity makes it difficult
to draw a common theme for this report. For example, the discussion
of CCTV and city centre surveillance would not be complete without
reference to the issues raised for individual privacy. But while
an examination of the principles behind the moves towards a law
of privacy is clearly not part of the remit we have set ourselves,
an analysis of the implications for the individual created by
the application of new technology, and hence the willingness of
the public to accept it, is. Similarly when considering the implications
of documentary data we found ourselves looking at the Data Protection
Act. Again, with proposed legislation underway it is not our intent
to comment on the Data Protection Act.
Nonetheless, digital imaging technology has broad implications
here for the way we run our society which we cannot ignore. We
have taken the initial focus of our study-the digital image, typically
in the form of a photograph-and looked at the two adjacent topics:
moving images (videos and CCTV) and images of text, either written
or as 'pictures' that exist only as data on a computer. All forms
of evidence can raise questions of authenticity: digital images
are not unique in this respect. For example, questions of forgery
arise with more conventional technology. But digital technology
raises its own sort of issues and these are the subject of this
Report. First we describe some of the ways in which imaging technologies
are being used today.
1.4 Most roadside traffic
enforcement cameras monitoring speeding offences and red light
violations record the image of the offending vehicle onto photographic
film which has to be stored at the roadside. This presents a series
of problems: the film needs to be kept secure, the film needs
to be changed often, the number of incidents recorded is limited
by the film capacity, each film needs to be collected and taken
for processing, and then has to be analysed before a 'Notice of
Intention to Prosecute' is issued. If a digital camera is used
instead, or the image from a traditional camera can be converted
into digital data, then there are many advantages: incidents can
be recorded almost indefinitely as these data are down-loaded
to a computer, the data store for many cameras can be at one central
and secure location (eg in a police station) rather than by the
road, processing is done by computer and, if number plate recognition
software is used, the registered owner of the vehicle can be identified
immediately. The ultimate in automation is for the computer then
to print a Notice of Intention to Prosecute and send it out to
the offender without human intervention- although we emphasise
that this is not current practice.
1.5 Home Office requirements
for digital enforcement systems specify the camera resolution
to be used (approximately 1000 x 1500 pixels), image compression
standards for when the image is transmitted to the computer, transmission
encryption protocols and standards for storing and archiving images.
An authentication code is also required to be added to the image.
The level of security during transmission of the image data is
similar to the encryption used by banks to secure the electronic
transmission of data which occurs when a cash machine (automatic
teller) is used.
1.6 The Abbey National
told us that they had revolutionised their document management
system by using optical scanners linked to computers with character
recognition software. Original paper versions of mortgage applications,
for example, were now scanned electronically to produce a digital
image; this was stored in computer memory from which it could
be retrieved and viewed at any time. Alterations could not be
made to this digital copy, but annotations could be added in the
form of overlays to the image (QQ 334-8). The Abbey National told
us that the digitisation of documents reduced the risk of loss,
damage or alteration, inherent in paper-based processing. The
scanning stage was seen as the critical step and this was under
secure management; the risk of files being deleted or substituted
after this initial processing was controlled through the implementation
of an audit trail which recorded all accesses.
1.7 A surveillance
system inside a large shop or a town centre will usually consist
of a number of cameras with pan, tilt and zoom controls which
feed live colour pictures back to monitors in a control room.
The camera output is also recorded and this is usually on to high
quality videotape. To extend the period between tape changes,
the recordings are often only made at two frames per second. CCTV
systems depend to a large extent on the quality of the camera
and the lens used. For example a system which might be used in
a shopping centre would have a 96x optical and digital zoom, autofocus,
370 degree per second pan and 120 degree per second tilt (to move
the camera rapidly between pre-set positions), and an in-built
image processing computer.
1.8 Some cameras also
have infra red capabilities for surveillance during the hours
of darkness, and motion detection sensors to warn operators of
unusual activity and start recording more detailed images. For
example, the City of London Police use motion detectors to alert
operators when a vehicle has transgressed an exit point of the
traffic control zone in the City. Other camera features available
include: bullet-proof and explosion-proof casings; automatic defence
mechanisms, to ensure that if a camera comes under attack it is
filmed by its neighbours; systems which automatically switch from
full colour to black and white to maximise image resolution if
the lighting level drops; and directional microphones.
1.9 The quality of
image available from digital CCTV cameras is still poor compared
to the high resolution available with analogue cameras, but one
area where they have a great advantage is in covert surveillance.
Tiny digital cameras a few centimetres square can now be bought
which offer 300,000 pixel resolution and low-light sensitivity.
At least one company in the United Kingdom is also offering these
cameras disguised in anything from mirrors and pictures to smoke
detectors and clocks.
1.10 The membership
of our Committee is listed in Appendix 1 and the Call for Evidence
we issued is set out in Appendix 2. The Enquiry was based on the
assistance of a wide range of individuals and organisations who
responded to the Call for Evidence: these are listed in Appendix
3. We are most grateful to them all for their time and effort.
We also wish to express our thanks to those whom we visited, who
made presentations or provided briefing: in particular the City
of London Police together with the Symonds Group Ltd; the Greater
Manchester Police; the Forensic Science Service, and Epsom and
Ewell Borough Council. We have been greatly assisted by our Special
Adviser, Mr Chris Reed, of Queen Mary and Westfield College. We
very much appreciated the help of all those who have contributed
to this Enquiry.
1 Picture editors at The Guardian used Photoshop
software and Apple Macintosh computers to remove an individual
standing behind the Chancellor on the steps of No. 11 Downing
Street on Budget Day, in order to `improve' the image. Back
5th Report, 1995-96, The Information Society: Agenda for Action
in the UK (HL Paper 77). Back
The Government introduced the new Data Protection Bill on 16 January
1998 as this report was being finalised. Back