Specification (legacy numbering)
NR/SP/TEL/30023
Optical Requirements of CCTV Systems for Driver Only Operation (Passenger) (formerly RT/E/S/30023)
Issue 3, April 2006
Specification for driver-viewed DOO(P) platform CCTV: how big a target must appear on the monitor, how much of the train side and platform edge must be covered, choosing monitor size, lens and camera spacing, and the commissioning tests, maintenance checks and records.
Clause map — what each clause deals with
| 1 | Purpose | Sets the picture-size and related optical factors that let a driver spot a hazard on platform CCTV monitors; stands in for an unpublished Group spec |
| 2 | Scope | Applies to design and installation of new DOO(P) CCTV and to judging whether existing systems are adequate |
| 3 | Definitions | Defines contrast sensitivity, discernible image, target object, and landscape versus portrait image orientation |
| 4 | Fundamental Requirements | The core visibility criteria, based on driver eyesight research, that every DOO CCTV image must meet |
| 4.1 | Discernable image: angle subtended at eye | Minimum angle a target's image must make at the driver's eye (about 20 minutes of arc) |
| 4.2 | Discernible image: image size relative to monitor | The target's image must take up more than about a tenth of the picture height so it stands out |
| 4.3 | Target object size | Uses a 1 m tall object as the reference target, which must be visible on at least one monitor anywhere along the covered platform |
| 4.4 | Viewable area of the platform | Cameras must together cover the whole train side up to about 2 m high plus the strip near the platform edge, at every stopping position |
| 5 | Monitor Sizes | Monitor choice follows from the minimum image size needed |
| 5.1 | Image size and screen size for a given distance from the driver | Minimum monitor size depends on how far the monitor is from the driver's eye; points to Appendix A for the working |
| 6 | Camera and Lens positioning and Specification | How lens and camera placement are chosen to meet the image criteria |
| 6.1 | Selection of Lens focal length and camera positioning. | Lens angle sets how much platform each camera covers and its near limit; overlap between cameras must be planned; Appendix B gives the method |
| 7 | Commissioning Testing | Commissioning views are taken from the cab of a typical DOO train on the route |
| 7.1 | Test target for resolution tests | Describes the resolution test board with a contrast pattern on one side and plain black on the other, with day/night variants or a Rotakin alternative |
| 7.2 | Test target for viewing tests | Describes the taller plain black board used to prove coverage, or a Rotakin alternative |
| 7.3 | System Tests. | Tests done at each stopping position: walk the target along to prove full coverage, then check image size and that contrast detail can be resolved at the far limit |
| 8 | Maintenance Checks | Maintenance views may be taken from a typical cab or from the platform edge near the stopping point |
| 8.1 | On adjusting a camera position | Re-aiming a camera means repeating the coverage test for it and for the cameras it overlaps |
| 8.2 | On replacing a camera or lens | Replace only like-for-like camera and lens sizes, then rerun the full system tests on that camera and coverage tests on its neighbours |
| 9 | Records | Keep records of each camera's coverage limits along the platform; poles and platforms may be marked to help maintainers |
| Appendix A | Calculation of Minimum Image Size and Monitor Size. | Worked method and lookup table for minimum image height and monitor diagonal against driver-to-monitor distance, landscape or portrait |
| Appendix B | Determining Camera Positions and Coverage. | Worked method for maximum lens angle, picking a real lens, blind spot under each camera, overlap spacing, depth of field, auto iris and gamma setting |