Automatic Number Plate Recognition (ANPR) is probably the next
growth product to take off in the UK,(and the world) in fact, it is
already beginning to be the biggest potential earner for installation
companies. Network systems were all the hype in recent years but have
not really grown as the pundits originally predicted. One problem is
that it crosses technological barriers and the needs of available
bandwidth were conveniently obscured.
On the other hand ANPR technology is completely within the
scope of knowledge of CCTV companies, although there are a few new
terms and technologies to come to grips with. As with previous new
developments there is plenty of scope for the industry to deliver more
hype and misinformation. There are pitfalls for the unwary and this and
future articles will aim to explain some of these for the customer and
installer (or, systems integrator).
We are grateful for assistance in producing this article to
CitySync for the APR system and Derwent CCTV for information on
We will start by considering ANPR for a single lane using a
dedicated camera and go on to discuss some other applications such as
multi-lane systems and Town Centre systems with colour cameras.
If you scan a document into your PC and then open it in a word
processor you cannot edit or alter it in any way. This is because it is
simply one bitmap made up of thousands of individual pixels. However
there is software available, frequently a freebie with scanners that
can convert these groups of pixels into characters. This is Optical
Character Recognition (OCR), which scans each group of pixels and
estimates whether or not it could be a letter and replaces the pixels
with the ASCII* code for the letter. For instance the ASCII code for
the lower case 'a' is 01100001. So, the software scans the whole
document and produces a page of letters exactly the same as though you
had typed them in, which can be edited or manipulated in any way.
(*) American Standard Code for Information Interchange.
OCR is the fundamental technology used in ANPR and provides the
capability to store and sort data. ANPR cameras need to be a special
type and set up within certain important parameters as will be
As a vehicle approaches the camera the software takes a series
of 'snapshots' and stores them in a file. When the number plate is of
sufficient size for the OCR software the frame is scanned and the
registration number is converted to ASCII code and held in a list. This
continues for a series of images according to the speed and position of
the vehicle. The list is scanned for similarities and a 'favourite'
selected to retain. The system would typically scan and compare 10-15
images, with 5 being considered the minimum for high accuracy.
that this is the principle of the software we are describing; some
systems only take one image at a certain position.
This then, is the start of the ANPR capture and is totally
dependant on the correct set up of camera, lens, illumination, angle of
view and configuration. Get one wrong and you have a disappointed
customer who won't pay the bill.
At this stage we are concentrating on the number plate capture
but there are many other aspects to be considered for a completely
integrated system, which will be discussed later. Note that the ANPR
capture considered here is monochrome.
To explain some further technology behind successful ANPR capture we
need to look at the behaviour of light. A basic law of light is that
the angle of incidence equals the angle of reflection. However, number
plates in the UK and most other countries have a special
characteristic; they are known as RETRO-REFLECTIVE. The surface is
covered in hundreds of tiny hemispheres which cause light to be
reflected back to the source. This is the same technology used in
safety clothing and signs. No matter from which direction the light is
directed, it always reflects back and makes them very visible.
If a standard colour or monochrome camera was focused to read number
plates it would have to contend with a huge variety of lighting
conditions, daytime, night-time, sunlight, backlight, headlights, and
so on. One configuration simply would not cope with all conditions, so
there is a need to provide a constant level and direction of
illumination irrespective of any other conditions. And so we come to
the development of special cameras for continuous capture of number
The camera must be sensitive to the infrared part of the
spectrum, to at least 850 nanometres. Then it must be fitted with a
filter to restrict the visible part of the spectrum. The lens would
have a manual iris set fully open and the shutter speed set to 1/1000th
second. Finally an infrared source must be fitted adjacent to the
Therefore, taking advantage of the retro-reflective
characteristics of number plates, the illumination from the illuminator
will be reflected directly back to the camera. Thus only infrared light
will be seen without any visible light or other reflections or
refractions. The picture will of course be black with no detail except
for the number plate. The OCR software then takes care of converting
the image to usable code.
Note that this is the sort of image on the monitor both day and night.
This then is the core of ANPR technology, but there are many other
factors to be considered. The first of these are the selection of lens
and the distance to view the vehicle. The size of a UK license plate on
cars and commercial vehicles is approximately 510mm long x 110mm high.
Motorcycles are different being approximately 255 x 200. However more
significantly, the minimum height of the letters must be 79mm. The
current UK font is Charles Wright, although there are some illegal
formats seen. The size of the number plate and the actual characters
will need to be of a certain size when seen by the camera for the OCR
software to function. One line of thought is that the number plate
should be 18% of the scene width; I prefer to consider the vertical
height of the characters, which from previous research should be 3%
minimum for a 400 line camera. This in fact equates very closely to the
18% screen width but is more logical when considering different shapes
of number plates. (For instance when a car plate is 18% of the screen a
motorcycle plate would only be 7 %.) Also note that motorcycles
currently do not have to carry a front number plate, but this could
change in the future.
This provides the first convenient way to calculate the lens
angle. For 79mm high characters the scene height needs to be 2633mm.
(79 being 3% of 2633). Therefore using a scene height of 2.633M and the
known distance, it is a simple matter to calculate the lens angle and
thus the focal length. At this stage the height of the camera has not
been considered but would not make much difference for normal
combinations of distance and camera heights. (See further notes at the
end).The following table shows the lens angle for various distances and
a scene height of 2.633M
Distance to target 5M 10 15 20 25 30 35 40 45 50
Lens angle 29.5° 15° 10° 7.5° 6° 5° 4.3° 3.8° 3.4° 3°
Focal length 2/3" 13mm 25 38 50 63 76 88 99 111 126
Focal length ½" 9mm 18 27 37 46 55 64 72 81 92
Focal length 1/3" 7mm 14 21 27 34 41 48 54 61 69
Focal length ¼" 5mm 10 15 20 25 30 35 40 45 51
From this table you can find the nearest lens focal length for the
sensor size. Where the focal length falls between two available lenses,
a vari-focal lens can solve the issue.
In instances where the camera height is large compared to the
horizontal distance the number plate can produce the effect of being
rotated vertically. It is important to check with the software provider
if this is acceptable.
Another very important consideration is the speed of the vehicles to
be monitored and the width of the area to be covered. There are again
two important interrelated elements connected with vehicle speed. One
is the rate of the image grabber software the other is the speed of the
processor. With modern processors the latter can no be ignored.
Consider a vehicle travelling at 30 MPH, this equates to 13.4
Metres in one second. A UK camera produces 50 fields per second and the
shutter speed is usually 1/50th second. In 1/50th second the vehicle
would travel 0.27 Metres (268mm). This would cause a blurred image and
problems with the OCR translation. For this reason the shutter speed
should be set to 1/1000th second, in which case the vehicle would
travel only 0.013 Metres (13.4mm). The same reasoning applies when
trying to capture moving images with a 35mm camera. This is slightly
less important when looking head on at a vehicle but becomes
increasingly important when the vehicle is at an angle approaching the
Where the camera is positioned other than directly in the line of
the approaching vehicle the ANPR provider must be consulted. Many
systems will not function with more than more 1 or 2 degrees of
horizontal skew or vertical rotation. The CitySync system will cope
with up to 30° of misalignment,
The positioning of the camera is a most important
consideration for satisfactory operation of an ANPR system. This can
vary the percentage of recognitions to number of vehicles from 30% or
40% to near on 100%. The camera location depends on several factors,
Single camera covering a barrier entrance
Probably the best position is for a camera and illuminator in a 1M high bollard viewing directly at the approaching vehicle.
Single camera covering one lane
This could be a pole mounted unit about from 18M to 30M from the vehicle.
Single or multiple cameras covering multiple lanes
This is a special application requiring input from the ANPR provider.
Town centre cameras already installed
Usually the cameras will not have been installed with ANPR in mind
and so the positioning will not be optimised, they will generally be
colour with no infrared illumination and will operating with the
shutter speed set to 1/50th.
The first thing to address is the shutter speed if it is
adjustable. The best would be if the speed can be set remotely, if not
each camera needs to be visited and the speed set manually. The optimum
setting is to 1/1000th. Alternative settings may be 1/250th for traffic
up to 5 MPH and 1/500th for traffic up to 40 MPH. Note that all these
settings will affect the low-light capability of the cameras and a
compromise may be required.
Another consideration is that the camera positions and heights
would not be at the optimum for ANPR. Particular attention must be paid
to the angles of skew and rotation and a guaranty obtained that an
acceptable percentage of recognitions will be achieved.
Cameras on motorway bridges
Again a special application requiring input from the provider.
Congestion charging cameras
This application requires input from the ANPR provider and local authority before even starting to think of a specification.
Cameras in Police vehicles
These are normally colour cameras mounted on a swivel mount and can
view images to the front or either side of the vehicle. This is another
special application requiring input from the provider.
It is often necessary to have a conventional colour image of the
vehicle especially where prosecution or congestion charging is the
application. This would be a separate colour camera mounted alongside
or just below the ANPR camera. Saving the overview image is triggered
by the ANPR camera registering a number plate. This then adds a colour
image to the same file for future reference. It is generally a false
economy to attempt to combine the number plate recognition and overview
using a single camera for 24/7 operation.
There are thousands overseas vehicles on the roads today, many of
them with symbols and other labels incorporated into the plate. The
ANPR system must be capable of reading all of these. The software
should have a built-in list of such plate styles.
Just capturing number plates and storing them is not much use by
itself. The screenshot below shows an ANPR example review screen. The
associated database should be able to provide much more information.
Searching should possible on several fields:
- Full plate.
- Part plate.
- Notes added to image file.
Further functions could be:
- Counting vehicles in and out of premises, leaving a list of all vehicles on site.
- The length of time a vehicle is on site.
- Vehicle speed (from two cameras).
- Employee names can be associated with number plates and access allowed or denied to certain areas.
The following table shows distances travelled for different speeds.
This will give some indication to the number of images that be
captured. From this table it can be seen that fof a vehicle travelling
at 50MPH at 1/50th exposure it will travel nearly half a metre
resulting in a blurred image. Compared to only 22mm at 1/1000th
These are sometimes known as frame grabbers or field grabbers or stores. In reality they all store images or single fields.
This is a particular area where you really pay for what you get.
The main criterion is the speed at which images can be captured. For
instance Video for Windows can only store about 8 images per second
which is only suitable for very slow moving or stationary traffic.
Whereas the Brooktree JG201 can capture at 50 images per second. Prices
for image grabbers can vary from about £60.00 to £500.00.
This article has given an overview of ANPR technology, in some cases
simplified to provide an understanding of the underlying principles.
Many features will vary between manufacturers so don't assume because
something is mentioned in this article it will be available from
anyone. In some cases it may work to obtain number plate recognition
and an overview from one camera but there will be a trade-off such as
reduced low-light performance of a complete inability to recognise
numbers at night. Beware of all-embracing requirements such as; 'one
camera to read every number plate day and night, provide a colour
overview and identify persons in the vehicle'
The need to enlist the cooperation of the ANPR manufacturer cannot be over-emphasised.