CCTV Cameras: Four Heads Better Than One?
When it comes to video surveillance, more heads not necessarily better than one...
We’re interested in installing a multi-head camera which gives us 360-degree surveillance but we want faces out to 20 metres, hopefully more. Could you advise if this sort of camera is better than multiple cameras in terms of raw performance – resolution? The camera models we are looking at are relatively expensive in hardware cost, though this evens out when you take the reduction in installation costs into account. What do you think?
A: These observations are general and should not be considered as pertinent to every multi-head camera. Bear in mind that a 360-degree camera will comprise 4 camera heads. Typically, each of the system’s 1/3rd-inch camera sensors will have a resolution of 1080p and be fitted with a short focal length lens delivering 90-degree angles of view replete with barrel distortion and pixel spread.
The issue here is not so much performance as expectation. A third-inch 1080p sensor fronted by a lens focal length of 2.8mm will give 90-degree coverage that’s 50 metres wide and 23 metres high when depth of field is 25 metres, delivering 38 pixels per metre. If you have a depth of field of 50 metres, your view width might increase to more than 100 metres, but your pixel spread will blow out to 19 pixels per metre.
For identification of a face in good light the face alone needs to be 20 pixels across and if you want court admissible evidence, you’re going to be wanting the face to be around 40 pixels wide. Given a typical face is only 0.13m wide, you’re going to need more than 200 pixels per metre to get the required resolution if the subject is around 4 metres from a 2.8mm lens on a one-third inch 1080p camera. At 20 metres, you’ll get useful details but not faces.
Multi-head cameras are designed for situational awareness – they allow you to track events across a very large area, but they must be supported by higher resolution cameras – or cameras with longer focal lengths that limit pixel spread at choke points if you need faces. That’s 6-8mm focal lengths at 10-12m at a resolution of 1080p, perhaps a little longer if you need faces at 20m.
Something else to be aware of is that some 360-degree units have 720p camera heads combined with 2.8mm lenses. This combination is useful and it certainly poses the lowest possible network load to the single PoE port supporting the overall camera system but you have to install it sympathetically. Depth of field will be an issue and don’t expect ‘crystal clear’ images on digital zoom because you won’t get them.
The best option is to have a supplier install a 360-degree camera on your client’s site and let the security team work with it for a week or 2. Security officers in the control room of a busy and complex site will gauge the effectiveness of combo solutions like this very quickly. I’d be listening to those opinions if face recognition at 20 metres through 360 degrees is more important to your client than saving money.