Admissibility of digital images as evidence

August 2005 CCTV, Surveillance & Remote Monitoring

A common misconception in the surveillance and monitoring industry is that digital images are inadmissible as evidence in court. This white paper examines the current law in the UK on the admissibility of digital images and concludes that such images are indeed admissible. In so doing, this white paper also briefly examines the techniques used in video compression and highlights current 'best practice' in digital image authentication. This white paper is particularly timely in that the Police Scientific Development Branch has just published up-to-date guidance on the use of digital images as evidence1.


Digital technology is ubiquitous. For years we have listened to digital recordings of music on CDs and generally accepted that the quality is superior to that contained on a vinyl record or tape cassette. More recently we have begun to listen to digital radio and have benefited from improved sound quality and increased content. Many of us now watch digital television and take our holiday snaps with a digital camera. The trend is clear; namely, that as computer processing power increases and becomes cheaper, old analog technology is being replaced by new improved digital systems.

This trend is also now evident in the surveillance and monitoring industry. Increasing numbers of CCTV equipment manufacturers are producing digital products (such as digital video recorders (DVRs) and networked video cameras)2. These CCTV equipment manufacturers recognise the significant advantages that digital technology has over analog technology3.

However, some in the surveillance and monitoring industry still regard digital CCTV equipment with suspicion. This is perfectly understandable in that the surveillance and monitoring industry has only recently started to migrate across to digital technology. As with all new technologies there is a certain 'fear of the unknown'. One such perceived 'unknown' is whether or not CCTV images captured digitally will be admissible as evidence in court. This white paper summarises the law in the UK on this particular point4.

What is a digital image?

Before we examine the law on the admissibility of digital images as evidence in UK courts, we should clarify what is meant in this white paper by the phrase 'digital image'5. For present purposes a 'digital image' is either a digital still photograph or a sequence of digital video which can be viewed on a suitable display medium (including, for example, a computer monitor, a television, a CCTV monitor or a print out). A photograph or sequence of video is digital if it is stored in binary format in memory (including, for example, DVDs, CD-Roms, diskettes, RAM, hard drives, RAID Arrays and tape streamer systems). Digital images can be contrasted with more familiar analog images which are typically captured as a variation in some physical property of the recording medium (including, for example, VHS cassettes or photographic film).

The binary nature of digital images gives them certain advantages over analog images. For example, digital images are generally easier to copy perfectly than analog images. Moreover, because a digital image is simply recorded as a set of data in computer memory, it is generally easier to process a digital image than an analog one.

Such processing could, for example, include image enhancement or image analysis. It could also include image manipulation. In addition, before a digital image is even stored in memory it is likely to have undergone some processing. Often such processing will involve the compression of the digital image in order to save memory or to make the digital image easier to transmit across a network onto a storage/viewing device. A common example of this compression processing is the DVD. This shows that digital image compression does not necessarily impair image quality.

It is, however, exactly this ease with which digital images can be processed, or are processed before being recorded in memory, which has caused some in the surveillance and monitoring industry to doubt whether or not digital images are admissible as evidence in court. There are two principal concerns here. The first is that often a digital image will have undergone some compression processing before being stored in memory. The second is that digital images are relatively easy to manipulate.

What is compression processing?

In order to assess how real this first concern is, a high-level understanding of video compression processing is required.

The twin objectives of video compression processing are to reduce the amount of data required to store/send a digital image whilst also maintaining the quality of that digital image. International standards bodies such as the ITU-T and the ISO6 formulate and publish various compression processing standards. There are two main ISO subdivisions that define and regulate image compression standards: namely, JPEG and MPEG. JPEG - Joint Photographic Experts Group - is a group of scientists and industrialists who collectively define and regulate standards for the compression of still images (including, for example, photographs and individual frames captured from video footage). MPEG - Motion Picture Experts Group - undertakes the same activities in respect of motion video (that is, two or more still images that together form a piece of video footage). These two bodies have set standards such as JPEG (commonly used in digital cameras) and MPEG-2 (the current DVD standard). ITU-T (a telecoms focused standards body) has also published standards, such as H.261 and H.263, which define how to compress motion video. However, whereas MPEG-2 is targeted at the digital broadcast market, H.26x (a collective term for the two ITU-T standards) is intended for the compression of video down to a level that can be transmitted across telephone lines and computer data networks.

To compress an image using the JPEG standard, the following steps are required:

* Image capture.

* Image transformation.

* Quantisation.

* Entropy encoding.

To compress a video sequence in accordance with one of the H.26x standards or one of the MPEG standards (commonly known as motion video compression) requires the addition of one more step - namely, motion compensation - resulting in the following process:

* Image capture.

* Image transform.

* Motion compensation.

* Quantisation.

* Entropy encoding.

Image capture

At this stage in compression processing, the video compression device (typically referred to as an encoder7) receives the image from the lens or network, converts it from an analog signal into digital format and stores it into some kind of memory that the later compression processing stages can then use. This 'incoming' image may be down-sampled to a lower resolution to reduce the work required by the later compression stages, thus drastically reducing the amount of data likely to be produced at the end of the compression process. This down-sampling is one of the most important stages in video compression, and is known as a 'lossy' stage (meaning that the data that is discarded in the down-scaling process cannot be reproduced perfectly).

Image transformation

The next stage in the compression process is for the digital image to be converted into a form that is easily understood by the encoder and by the mathematics that perform the compression. Accordingly, this stage simply involves a translation of the image data into a different format. This stage is lossless (meaning that data can be accurately transformed between the two formats without loss of information).


This is the stage at which a great deal of compression occurs. The data produced by the image transform stage is analysed and certain mathematics are performed on it in order to 'shrink' it. Generally this 'shrinking' process is lossless. There are, however, some parts of this 'shrinking' process which are lossy. Both JPEG and MPEG have performed much research into the sensitivity of the human visual system and have identified that human eyes and brains are more sensitive to low frequency light than to that at higher frequencies. That being so, quantisation actually reduces the signal strength of the higher frequencies much more than it does the lower frequency signals.

In so doing, some of the higher frequency light signals are lost. In practical terms this means that those parts of an image which humans cannot or struggle to see are discarded during quantisation. This part of the quantisation process is lossy.

Entropy encoding

Entropy encoding is basically a method of compression that has been utilised in many information storage and transmission systems including computerised data vaults and even Morse code. To perform true entropy encoding, the message that is to be transmitted or stored is first analysed to detect how often each piece of information (a datum) actually occurs in a message. Each datum is then given a code, the length of which depends on how frequently that datum occurs in the message. For example, if this paper was to be entropy encoded, the letter 'E' would be given the shortest code, as it occurs the most frequently. If the letter 'Z' occurs at all (it does now) then it would be given the longest code. Accordingly, this entire article (and in fact any English language text) could be compressed down to a fraction of its original size. A similar process takes place in the entropy encoding of a digital image. The key point to note is that the entropy encoding of a digital image is lossless.

Motion compensation

This stage is included only in motion video compression standards (such as H.261 and MPEG). Motion compensation is also known as 'motion estimation' or 'motion prediction' - although these terms are perhaps a little misleading in that no estimation or prediction actually takes place. The objective of this step is to examine the image to be compressed (the encoder image) and compare it to the previously compressed image (the reference image). Motion compensators look for: (i) new elements in the encoder image; (ii) moved elements in the encoder image; (iii) changed elements in the encoder image; and (iv) unchanged elements between the encoder and reference images. Once the encoder has analysed the two images, looking for these specific elements, the rest of the encoding process is defined:

* New elements are quantised, then entropy encoded.

* The motion of moved elements is encoded.

* The changes in elements are either encoded, or if the change is too great, the element is re-quantised and then entropy encoded.

* Unchanged elements are refreshed at regular intervals, to prevent 'drift' between the compressed and uncompressed images.

This is a high-level summary of the operation of motion compensators which glosses over many of their complexities. It does however demonstrate that the principal function of motion compensation is one of information management. The degree of lossiness of the motion compensation stage of an encoder will depend on how accurately that encoder is set to 'manage' the digital image information; the less accurate the 'management' of the digital image information, the more lossy the motion compensation stage of encoding will be. Typically, encoders enable the user to set the level of accuracy of information management and accordingly the level of lossiness of the motion compensation stage.

The additional step of motion compensation included in standards such as H.263 and MPEG allows motion video sequences to be compressed far more effectively than by compressing each individual frame in a video sequence into a JPEG image - this latter option is commonly known as MJPEG. The difference in compression ratios between MJPEG and H.263 encoded sequences can be greater than 600%. This is a huge efficiency gain and results in video recordings that maintain the fidelity of motion but reduce the bandwidth and storage requirements of the system by up to six times.

Another way to look at the benefits of motion video compression is to do a like for like comparison, as shown in the following table.

The foregoing summary of image compression processing highlights that image information is only discarded at three stages in the process; namely at image capture, at quantisation and, possibly also at motion compensation. Moreover, the amount of information that is discarded may often be controlled by the user of the encoder. In other words, most encoders enable the user to set the quality of the digital image to be recorded; generally, the better the quality of the digital image, the less information is discarded during compression.

It should now be apparent that the compression of digital images is actually quite a procedurally straightforward (if mathematically complex and intensive) exercise. This was certainly the view reached by the House of Lords Science and Technology Select Committee in its report entitled 'Digital images as evidence'8. To date this remains the most authoritative report on the admissibility of digital images as evidence in the UK courts. The Committee examined the various rules9 on the admissibility of images as evidence and came to the clear conclusion that:

"Digital images, which we initially thought might create difficulties for the courts, do not. But many people think that there will be difficulty in obtaining legal acceptance of digital images."10

In other words, the Committee came to the general conclusion that digital images are admissible as evidence in both civil and criminal cases before the UK courts, provided however that, like other types of evidence, those digital images are appropriately authenticated.

Image manipulation and authentication

The second concern which some in the surveillance and monitoring industry have regarding the admissibility of digital images as evidence in court relates to the ease with which a digital image can be processed. As noted above, digital images are usually stored in binary format in computer memory. This generally means that they can be processed just like any other computer data. Indeed many modern films rely on such processing in order to achieve dramatic cinematic effects and most newspapers now use digital processing techniques to touch-up photographs or to create montages.

The Committee recognised this concern and noted that digital images will, like other types of evidence, require to be appropriately authenticated. What is meant by 'authentication' in this context? For these purposes, authentication is the 'process of convincing the court that a document (which would include a digital image) is what it purports to be: of proving the origin of the image and that it has not subsequently been altered (or, where alteration has occurred, proving the nature of the alteration)'11. Such alteration could include, for example, image enhancement12 or image manipulation.

The Committee examined the ways in which evidence is authenticated before the courts and, in particular, the technical methods by which digital images may be authenticated (including, for example, by watermarking, by encryption or by digital signature). The Committee came to the conclusion that:

"Evidence should not necessarily be inadmissible because it does not conform with some specific technological requirement."13

In other words, the Committee was of the view that no particular authentication technology was required before a digital image would be admissible as evidence in court. Authentication technology could however increase the evidential weight of a digital image14.

How then should digital images be authenticated? The Committee provided the following guidance on this point:

"... the provenance of evidence can be enhanced by procedural measures. Good practice for handling digital documents has been set down by the British Standards Institution. Certainly it is good business practice to conform to such standards in preparing documents that might be used in court."

The guidance approved of by the Committee has since been updated by the British Standards Institution15. The British Standards Institution document provides guidance on, amongst other things, procedures and process for the authentication of digital documents (which includes for present purposes digital images), the creation of audit trails to authenticate digital documents and the technologies used in connection with digital documents.

However, the Committee was not of the view that compliance with the guidance provided by the British Standards Institution was the only way in which procedural measures could authenticate a digital image. Rather the British Standards Institution guidance was an example of best practice. Other procedural measures could be equally effective in authenticating digital images. In the Committee's own words:

"The authentication evidence would normally be in the form of an audit trail connecting the original image with the computer record which is to be adduced in evidence and recording what has happened in the interim. As with paper records, the necessary degree of authentication may be proved through oral and circumstantial evidence, if available, or via technological features of the system or the record."16

The recent report17, published by the Police Scientific Development Branch gives further guidance on the sort of procedural measures which may be used to authenticate digital images.


The existing UK legislation18, House of Lords Committee report19 and recent Police Scientific Development Branch report20 are clear that digital images are admissible as evidence in UK courts, provided that the digital images are appropriately authenticated. This is the same rule as applies to other evidence in UK courts. The House of Lords and Police Scientific Development Branch reports are also clear that no particular technological authentication is required in respect of digital images - albeit that technical authentication may increase the evidential weight of digital images.

Rather digital images may be authenticated by use of suitable procedural measures (such as those set out in the British Standards Institute and Police Scientific Development Branch documents21).


1. Police Scientific Development Branch Report 'Digital Imaging Procedure' Version 1.0, March 2002. (

2. For example, Baxall Limited's Destiny IP range of products, COE Limited's X-Class IP range of products, IndigoVision's VideoBridge range of products, Panasonic's NT204, Ultrak, Inc.'s UltraKey IP range of products and Videology Imaging Solutions Inc.'s Lynx IP camera.

3. Some of those advantages are set out in the IndigoVision white paper entitled 'Remote Monitoring and Management of CCTV (over IP-based networks'.

4. Where appropriate this white paper will highlight differences between Scots law and the law of England and Wales and differences between civil and criminal evidence law. This white paper will not however directly address the impact of the Data Protection Act 1998, the Human Rights Act 1998 or the Regulation of Investigatory Powers Act 2000 on the CCTV industry. There is already extensive guidance available on these Statutes (see ' CCTV Code of Practice' issued during July 2000 by the Information Commissioner (formerly the Data Protection Commissioner) (; 'Implications for Public Space Surveillance in the light of the Data Protection Act 1998' issued by the Home Office (; 'CCTV and the Human Rights Act - Public Space Surveillance in light of the European Convention on Human Rights' issued by the Home Office (; and Moreover, these Statutes apply to both analog and digital CCTV systems and accordingly an examination of their impact falls out of the scope of this white paper.

5. For a more detailed review of digital imaging technology see the IndigoVision Limited white paper entitled 'VideoBridge Concepts Overview'.

6. International Telecommunication Union and International Organization for Standardization.

7. Examples of encoders are digital cameras, video servers, networked video cameras and DVRs.

8. House of Lords, Science and Technology Select Committee, Fifth Report entitled 'Digital images as evidence' dated 3 February 1998 (

9. For the purposes of the law of evidence, an image is generally treated as a document. Moreover, typically a digital image will be a copy of the original image stored in binary format in computer memory. In other words, for the purposes of the law of evidence, a digital image is generally treated as a copy of a document. The law has changed slightly since the date of the Committee's report. The following are the relevant statutory provisions in force as at the date of writing (March 2002). Section 8 of the Civil Evidence Act 1995 provides for the law of England and Wales that: 'Where a statement contained in a document is admissible as evidence in civil proceedings, it may be proved by the production of that document, or, whether or not that document is still in existence, by the production of a copy of that document or of the material part of it, authenticated in such manner as the court may approve'. Section 6 of the Civil Evidence (Scotland) Act 1988 provides, 'For the purposes of any civil proceedings, a copy of a document, purporting to be authenticated by a person responsible for the making of the copy, shall, unless the court otherwise directs, be deemed a true copy and treated for evidential purposes as if it were the document itself'. In England and Wales Section 60 of the Youth Justice and Criminal Evidence Act 1999 repeals the previous provisions of Section 69 of the Police and Criminal Evidence Act 1984. Accordingly computer records (including digital images) are now admissible as evidence in criminal proceedings in England and Wales. Paragraph 1 of Schedule 8 to the Criminal Procedure (Scotland) Act 1995 provides, 'For the purposes of any criminal proceedings a copy of, or of a material part of, a document, purporting to be authenticated in such manner and by such person as may be prescribed, shall unless the court otherwise directs, be deemed a true copy and treated for evidential purposes as if it were the document, or the material part, itself'. The general effect of these provisions is to admit digital images as evidence whilst requiring those digital images to be authenticated in some way.

10. Para 5.1 of Note 8 supra.

11. Para 3.2 of Note 8 supra.

12. A technically enhanced version of a video has been held admissible in New Zealand (R v Taylor [1993] 1 NZLR 647).

13. Para 3.18 of Note 8 supra.

14. Para 3.19 of Note 8 supra.

15. British Standards Institution DISC PD 0008:1999 'Legal Admissibility and Evidential Weight of Information Stored Electronically'.

16. Para 3.3 of Note 8 supra.

17. See Note 1 supra.

18. See Note 9 supra.

19. See Note 8 supra.

20. See Note 1 supra.

21. See Notes 1 and 15 supra.

IndigoVision Limited,

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