Perfect pixel count

Moving into IP

Selecting the appropriate surveillance camera to fulfill operational requirements has always been a challenge.

With the introduction of IP cameras, and especially the development of megapixel and HDTV cameras, the need has emerged for a new way of determining how to meet operational requirements. These six steps describe the Axis model for relating operational requirements to modern video and IP cameras.

When surveillance was all analog, selecting a camera to match an operational requirement was mostly about selecting the appropriate lens, as there weren’t many resolutions to choose from. To monitor people’s behavior, various requirements were established based on the height of a person in the scene. Whilst not an official standard, this nonetheless became quite a common way of setting the requirements for detection, recognition, and identification.

Category Operational requirement Body height as percentage of image (4 CIF)
Identification Sufficient detail to enable the identification of an individual beyond reasonable doubt. 100% from the UK source quoted – 150% also commonly used
Recognition A high degree of certainty as to whether or not an individual has been seen previously. 50%
Detection Determine with a high degree of certainty whether or not persons are present in the scene.  10%


Table 1. (UK Home Office Scientific Development Branch, 2009)

Even if the figures in Table 1 are good for standard analog resolutions, they pose challenges for the resolutions of IP cameras. Although it is possible to translate TV lines to pixels, (as in table 2), this is far too complex for real-life situations. Surely there must be a better way?

Category PAL 1080p 720p WSVGA SVGA VGA 2CIF CIF QCIF
Identify 100 38 56 67 67 84 139 139 139
Recognize 50 19 28 34 34 42 70 70 139
Observe 25 10 14 17 17 21 35 35 70
Detect 10 4 6 7 7 9 14 14 28
Monitor 5 2 3 3 3 5 7 7 14


Table 2. Reading the table, the vertical body height percentage representation for Identify should be 38 % with a 1080p HD camera.
Pixel density