Video through the eye of a forensic expert
Today’s modern video monitoring systems are designed to perform multiple tasks at the same time. Just the simple presence of a surveillance camera already works as a preventive element, but of course it offers other features as well, such as real-time monitoring or automatic event assessment. From the forensic point of view, the most important thing is to have a recording which reveals the perpetrator and provides proof of guilt.
Captain Ing. Martin Spurný, is a forensic expert and has been evaluating and providing expert opinions on visual analysis to investigators dealing with serious criminal activities. Below he provides his perspective on the use of video surveillance systems in this context.
Video recording as proof
If an investigator sees a surveillance camera at the crime scene they want to see the recording first. Experience has demonstrated that recordings from security cameras continue to play an ever-increasing importance in the investigation of serious crimes.
Visual information is very important when determining the next investigation steps regardless if it concerns a misdemeanour or criminal activity. However, investigators too often only have recordings, which are many times useless. The most frequent issue is insufficient size or scope of the coverage (view), low resolution and low picture frequency, incorrect position of the view, inadequate lighting conditions and inaccurate time specification.
The size and coverage of the view. Can you tell who stole the car? Copyright: Mr Spurný
The size and coverage of the view
The size of the view for security camera systems is defined differently than for a regular film camera. Therefore, when installing the camera it is necessary to determine for what purpose the recordings will be used. Do we only focus on a certain violation of a perimeter, or is it likely that we will need the recording to provide more precise details? Investigators are often challenged with the fact that persons in the monitored area are simply not big enough to determine any useful information.
Better Resolution Means Better Results
The general recommendation is: Avoid old analog CCTV cameras as the HD and megapixel resolutions offered by IP cameras available today can be up to three times better. Naturally, higher resolution gives you more details and therefore better identification of persons and objects. The advantage of digital systems is that they are virtually unlimited as to the maximum picture resolution. The maximum resolution depends only on the selected camera, recording equipment and the network abilities. Further, the technical development in this area is advancing quickly and the performance continues to increase as well.
Analog PTZ camera with 320×240 resolution. Forget CSI Miami. You cannot even identify a person with the best zoom. Copyright: Mr Spurný
An average network camera with megapixel resolution offers three times better resolution than an old analog CCTV camera.
Frames Per Second: An Important metric for improving image quality and usefulness
Investigators often encounter recordings made by older camera systems that cannot be used due to low picture frequency. Even here it is necessary to consider what scene we are looking at and what is the probability of the occurrence of criminal activities. For example, a system monitoring a public area will not require as high identification accuracy as a vehicle license plate identification system monitoring fast-moving vehicles or a cash counting process. Simply put, if you want to use the recording for identification purposes the picture frequency´ should never fall below 8 FPS, and if you want to identify moving vehicles (license plate identification), the frequency should be at least 15 FPS.
View of a cash register with resolution of 720×576 and 2fps frequency (only 1 fps after exporting). To monitor banknote counting one picture per second is not sufficient. Copyright: Mr Spurný
Capt. Ing. Martin Spurnýis a forensics expert focusing on digital recording and video analyses under the authority of the Forensic Techniques and Expertise Department at the Expert Institute of the Police of the Czech Republic. Mr Spurný has also worked as a private forensics expert since 2016. Copyright: Mr Spurný