Home Office Recommendations for Facial Recognition - Simple
The recommended combination of cameras and lenses is vitally important if your system is going to be usable by the police. The Home Office Operational Requirements Manual for CCTV – page 8 shows clearly what is required. Camera and lens selections need to be chosen and based upon a 1.7 metre person, as recommended by this document.
Detection - 10% Screen Height
This will be achieved by capturing the image of a person, which occupies at least 10% of picture screen height on the monitor and is often used where the CCTV pictures are being monitored live. The operator is not concerned about what the person looks like as security guards will be dispatched to apprehend the intruder.
Observation - 25% Screen Height
This will be achieved by capturing the image of a person, which occupies at least 25% of picture screen height on the monitor. This is often used to observe a group of people such as in a town centre and understand what they are doing. Once this has been established operator will, if necessary,
Recognition of a known person - 50% Screen Height
This will be achieved by capturing the image of a person, which occupies at least 50% of picture screen height on the monitor. On a CCTV system this is the minimum size that the police are likely to use in a court case to recognise a person within the picture.
Identification of an unknown person - 100% Screen Height
This will be achieved by capturing the image of a person, which occupies at least 100% of picture screen height on the monitor. This is most often used at doorways to give an excellent high quality picture of people passing through. It is normally required by the police at the entrances to licensed premises.
Please note that unless a Camera achieves ‘Recognition of a Known Person’ it is unlikely to be used by the police to identify a person for prosecution in a court of law. The police have stated that over 80% of the video evidence that they collect fails to meet the required standard.
The biggest problem with CCTV installations is that the wrong lens is chosen. This normally results in people being too small to recognise and with CCTV pictures only being around 40k each, and not the 10 million you get on your photo camera, you cannot electronically zoom in. To be able to recognise a 1.7m person on a standard* CCTV system they must be at least 50% height of screen (the standards of Detect and Observe are not good enough to recognise person simply to observe what a group is doing or detect a person is present in the picture).
To observe a person at 50% you can only view an area the width of two car park spaces (4.3m). The most popular lens sold to customers is a 1/3” 3.6mm lens. This gives a wide angle view of 70% but can only provide facial recognition of 3.2 meters so if it is located at a height of 5m you are depending on an intruder bringing a ladder and climbing up in front of the Camera in order to be recognised. Generally a 3.6mm lens will only enable you to either observe or detect a person.
The table below gives the angle of view and recognition distance for some popular lens formats.
Work out the distance from the Camera and then choose a lens that will provide Facial Recognition. The table will also show the angle of view you will achieve.
Where possible use a Varifocal lens (i.e. 5-50mm) as this makes it easier to adjust to the view at the time of installation. To find out exactly what lenses you will need use our free Online lens calculator.
For more information see Lenses (article).
(*) the requirement for a person to be 50% height of screen is based on the standard PAL screen resolution. To understand the requirements for other resolution formats see Choosing the Right DVR.
Back to How to Install Your Own CCTV System.