technical
lens & plate size calculations
The positioning of the ANPR camera is critical to the operation of the system.
The Jet software will handle plates at quite acute angles - but a lot of basic photography principles apply.
The Jet Software will read plates from about 1000 pixels in size up to about 5000 - and to start getting good reads as the vehicle approaches, plates need to be about 1800 pixels and we calculate the sizes as follows:
As a general rule a traffic "lane" is about 3m wide - and by filling the viewfinder (monitor) with the entire lane (or slightly less as plates are rarely found right at the edge of a lane) a good image of the plate is obtained.
This is because we grab an image at 768 x 288 (this fills the screen) - and a European Plate is about 520mm x 115mm (about 5:1).
So - if we fill our viewfinder with a 3m lane - a plate would fill about 1/6th horizontally of the screen = 768 / 6 = 128 pixels. So our 3m lane horizontally gives us a field of view vertically of: (3/4) x 3 = 2.25m divided into 288 lines - so 115 mm vertically is (288/225) x 11.5 = 14.72 pixels. A plate is therefore 128 x 14.72 = 1884 pixels . . . simple!
We now need to calculate what lens to fit to our ANPR camera - useful when ordering a JetCam 3 model - but note that the new JetCam IQ range come with zoom lenses - so you won't need all of this!
CCTV cameras come in various formats: 2/3" - 1/2" - 1/3" and the Jet-Cam 3 is based on a 1/2" CCD - which equates to 6.4mm width.
We use the formula: f = d * w / h where:
Other Assumptions:
CCD Width = 6.4 mm
Image Grab Horizontal pixels: 768
Image Grab Vertical pixels: 288
UK Number Plate Width = 520 mm
UK Number Plate Height = 115 mm
How to use the table
The first column shows the distance of the approaching vehicle - and the H-FOV the width of the total image captured (remember a lane is about 3m) - so taking the 16mm lens as an example - look at a distance of 7m - the H-FOV captures a total width of 2.8m and the plate becomes 2249 pixels in size - which is fine for the Jet Engine which needs 1800 for a good read.
So a 16mm lens is suitable here.
If we were only getting about 1100 pixels - the software would not run at its best and occasionally misread say a "D" for an "O" so we would move up to say a 25mm lens.
Extract from Lens Chart for a Jet-Cam 3 fitted with a 16mm Lens
The Horizontal Field of view computes to: 22.6 degrees.
| Distance (m) | H-FOV (m) | Plate/HFOV | Plate H Pixels | Plate V Pixels | Total Pixels |
| 4 | 1.60 | 0.33 | 250 | 28 | 6,889 |
| 5 | 2.00 | 0.26 | 200 | 22 | 4,409 |
| 6 | 2.40 | 0.22 | 166 | 18 | 3,062 |
| 7 | 2.80 | 0.19 | 143 | 16 | 2,249 |
| 8 | 3.20 | 0.16 | 125 | 14 | 1,722 |
| 9 | 3.60 | 0.14 | 111 | 12 | 1,361 |
| 10 | 4.00 | 0.13 | 100 | 11 | 1,102 |
| 11 | 4.40 | 0.12 | 91 | 10 | 911 |
| 12 | 4.80 | 0.11 | 83 | 9 | 765 |
FULL LENS CHARTS FOR Jet-Cam 3
In the following lens charts, the plate reading range is shown:
IN THIS COLOUR |
