Field note · 8 min
CCTV pixel density and DORI, explained carefully
Pixel density connects image width to the width of the scene represented. It is a better planning language than “4K camera,” but it still describes sampling density—not guaranteed recognition or identification.
From lens geometry to pixels per metre
For a rectilinear thin-lens approximation, horizontal scene width at a perpendicular target plane is distance × active sensor width ÷ focal length. Divide configured horizontal output pixels by that scene width to get pixels per metre. The corresponding horizontal angle is twice the arctangent of sensor width divided by twice the focal length.
Use active horizontal sensor width in millimetres. A nominal optical-format label such as 1/2.8-inch is historical notation and is not a physical width to enter directly. Datasheet field-of-view values may be more reliable when the manufacturer does not publish active dimensions.
DORI is a planning vocabulary
DORI groups density into detection, observation, recognition, and identification tasks. Current Axis guidance discussing IEC 62676-4 presents commonly used density values and, importantly, explains that conditions and system quality matter. A threshold helps define design intent; it is not a promise that any operator or algorithm will achieve the task.
State the task first. Detecting that a person entered an area is different from identifying an unknown person. Designing every camera for the most demanding task is usually wasteful; failing to name the task leads to vague coverage claims.
Why real detail can be worse than the geometry
The target may occupy fewer useful pixels because of tilt, perspective, cropping, digital stabilization, distortion, or position away from the focal plane. Motion blur, defocus, low contrast, glare, backlight, headwear, weather, dirty housings, noise reduction, and compression can remove identifying detail even when the nominal density is high.
A wide-angle image can also distribute detail unevenly after lens correction. Evaluate the exact target zones rather than applying one centre-line result to an entire scene.
Turn the number into an acceptance test
Use the calculation to choose a plausible camera position and focal length. Then test representative targets at the nearest, farthest, and edge positions; daylight and low light; stationary and moving; normal and adverse lighting. Review live view, recorded playback, and exported footage.
Keep test images and settings with the design record. Pixel density makes a requirement measurable, while the site test establishes whether the full system meets the real operational need.
Sources and further reading
The article distinguishes arithmetic from professional design judgment. Primary and technical sources:
Axis pixel density based on IEC 62676-4 ↗
Detailed vendor white paper on DORI-style density and real-world limitations.
IEC 62676-4 overview ↗
Current international standard listing and scope.