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CCTV DORI Optics: Engineering Formulas & Sensor Math for UK Security Systems

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CCTV DORI Optics: Engineering Formulas & Sensor Math for UK Security Systems

By Gary Pearce — Lead Infrastructure Engineer at ADI Communications (Direct: 07860 244 274)

When deploying IP surveillance across commercial premises or UK domestic estates, selecting camera focal length based on guesswork guarantees failure. An 8MP 4K sensor produces useless footage if optical density at the target perimeter fails the legal admissibility standard.

In this guide, we break down the British Standard BS EN 62676-4 (DORI framework), the optical trigonometry behind focal length selection, and how we engineered our free client-side CCTV DORI Lens Calculator.

1. The BS EN 62676-4 Optical Standard

The European and British standard BS EN 62676-4 defines surveillance requirements by pixel density (PPM – Pixels Per Metre) across a target plane:

Tier Purpose Required PPM Visual Outcome
D - Detection Perimeter Awareness 25 PPM Discern whether human or vehicle is present
O - Observation Behavioral Tracking 63 PPM See clothing color and general actions
R - Recognition Identity Confirmation 125 PPM Recognize whether person is known to staff
I - Identification Forensic Admissibility 250 PPM Unambiguous facial evidence admissible in UK courts

2. The Optical Geometry Equation

To calculate maximum identification distance ($D_{\text{ident}}$) for a sensor with horizontal pixel resolution ($R_{\text{horiz}}$), focal length ($f$), and sensor physical width ($w_{\text{sensor}}$):

$$D_{\text{ident}} = \frac{R_{\text{horiz}} \times f}{\text{PPM}{\text{target}} \times w{\text{sensor}}}$$

For standard 1/2.8" CMOS sensors commonly fitted in Hikvision and Dahua 4K IP turrets:

  • Sensor Width ($w$): 5.38 mm
  • Horizontal Resolution ($R$): 3840 pixels (4K UHD)
  • Identification PPM: 250 px/m

Benchmark Field Ranges (4K Sensor):

  • 2.8mm Wide Angle: Identifies up to 8.0 metres (Field of View ~108°)
  • 4.0mm Standard Angle: Identifies up to 11.4 metres (Field of View ~88°)
  • 6.0mm Narrow Telephoto: Identifies up to 17.1 metres (Field of View ~53°)

Fitting a 2.8mm wide-angle turret on an 18-metre driveway gives zero court-admissible identification faces; an intruder's face will only render ~110 PPM, rendering forensic capture impossible.

3. Open-Source Implementation & Live Tools

To assist field engineers and system architects, we open-sourced our calculation engine in both pure JavaScript and Python:

4. Engineering Takeaways for Field Cabling

  1. Cat6 Solid Bare Copper: Always use pure copper WF100 or solid Cat6 (never copper-clad aluminium / CCA) to avoid voltage drops under PoE+ load over 60m runs.
  2. Drip Loops & Weatherproof Grommets: Masonry entries must have downward drip loops and silicone grommets to prevent water tracking along the cable jacket.
  3. Lighting & EXIR: Infrared bounces back off moisture and fog; full-color low-light sensors (F1.0 aperture) provide higher contrast identification without IR glare.

For bespoke commercial security, Starlink installation, or Cat6 network engineering in Yorkshire, consult Gary Pearce at ADI Communications or call 07860 244 274.

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