10 Tips For Best Free-Flow ANPR Deployment
Introduction
At Imagsa we are asked about the fundamentals of ANPR cameras on a daily
basis. Thus, the basic requirements of an ANPR solution to deliver great
results in traffic control and enforcement projects.
The technology behind ANPR solutions is evolving quickly, and greater
performance can be expected from new cameras than were obtained by
previous generations of ANPR cameras. Also, the use of ANPR cameras has
extended in the last few years, from the traditional parking access control to
the more advanced electronic free flow tolling.
In the guide “10 Tips For Best Free Flow ANPR Deployment” Imagsa has compiled a series of requirements or
check list that shall be considered before deploying an ANPR-based traffic project.
Among the great variety of traffic applications using ANPR cameras, this requirements guide will focus on the
high-end market. Thus, applications that share the following conditions:

Free flow traffic.

High vehicle speeds up to 250 KmH.

Detect vehicles in auto trigger mode, no need to install external triggering sensor such as invasive
inductive loops or lasers.

Great detection and recognition results are expected.
Some traffic applications that utilize this technology are: lane and red light enforcement, average speed and
travel time, dangerous goods control, free flow tolling or police control, among others.
Tip #1 – Resolution of ANPR camera
OCR providers usually suggest a minimum of 2,7 pixels per Character Stroke to achieve high license plate
recognition rates (above 95%). Considering an European standard car or truck license plate with a character
stroke of 1cm, to fully cover 4 meters of the road (3,5 meters lane width plus 0,5m overlapping) it would be
required an image sensor with minimum resolution of 400cm x 2,7pixel/cm = 1.080 pixel.
On the opposite, if an ANPR camera with lower resolution is used (for instance 752x480), the following takes
place:
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understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.
Marketing: 10 Tips for Best free flow ANPR Deployment
Poor Recognition Rate: To cover 4 meters of the road, the resolution of the license plate image will be:
752pixel / 400cm = 1,88 pixel/cm. With this resolution it will not be possible to reach a decent license
plate recognition rate (OCR software providers suggest to use a minimum resolution of 2,7 pix/cm to
achieve 95% recognition rates).
ANPR on 2 meters: To achieve a 2,7 pix/cm resolution with an image sensor of 752x480 pixels, the lane
width that is covered is: 752pix / 2,7pix/cm = 250 cm. Therefore, only the central part of the lane will be
covered with the target resolution of 2,7 pix/cm. Vehicles travelling on the sides will be missed.
Sensitivity in Non-ideal operating conditions: License plate detection and recognition can
underperform when a 752x480 image sensor is used. Furthermore, ANPR in non ideal operation
conditions can suddenly decline. Non ideal operation conditions include dust in the lens, challenging
meteorological conditions such as rain, snow, fog, etc. These circumstances can be overcome if a better
resolution (megapixel) is used, because there is more information, more pixels per centimetre.
Imagsa’s Atalaya ANPR cameras use sensors of 2048 x 1024 pixels resolution. Hence, one Atalaya camera fully
covers 2048pix / 2,7pix/cm = 7,58 meters of road width attaining the target resolution of 2,7 pix/cm. With one
Atalaya ANPR camera, one installation and one investment is possible to simultaneously detect multiple vehicles
on 2 Lanes of traffic.
Tip #2 – Precise auto-triggered ANPR camera
Capturing the vehicle and an image of the license plate in an exact and deterministic point of the road is essential
to guarantee the target resolution of 2,7 pixels/cm for the license plate, even when a megapixel sensor is used
(see Tip #1).
A great number of auto-triggered ANPR cameras use image sensors at low to medium frame rate speeds:
capturing between 15 to 30 images per second (or FPS, frames per second) in a field of view of around 5 to 10
meters (virtual detection loop). This has at least one important consequence for the ANPR performance: the
resolution of the license plate image decreases as the vehicle travels away from the camera. If the vehicle’s plate
is captured at the shortest distance of the camera’s field of view, then maximum resolution of the license plate
image will be attained. On the contrary, if the vehicle is detected when it is at the limit of the virtual loop
(longest distance from the camera), then the resolution of the plate will be lower (less pixels per centimetre).
An OCR engine will underperform on a plate image with low resolution (as explained in Tip #1).
Furthermore, a non precise auto-triggered ANPR camera will impact the precision of a free-flow travel time or
average speed application. There would be no certainty on where (position, km point) the vehicle is captured and
thus it would create an error on the speed measurement.
Instead of working on a virtual detection loop in a range of 5 to 10 meters, Imagsa's ANPR cameras define a
virtual loop in the lowest part of the overview image (a region of interest of 1,5m - see video). This is the closest
point to the camera. In this region, 250 images per second are captured and analysed in real-time thanks to the
power and intelligence of the Mercury’Traffic ultra-fast smart camera. Thus, the vehicles’ plates are always
detected at the same point, with the same target resolution of 2,7 pix/cm. An optimum plate resolution and a
good illumination of the plate image ensure that the OCR engine always achieves high recognition rates.
Additionally, Atalaya’s highly precise auto-trigger is key to achieve great precision on a free-flow (no externally
triggered) average speed enforcement solution.
The information contained in this document is confidential and proprietary to Imagsa Technologies S.A. Imagsa Technologies S.A. submits this document with the
understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.
Marketing: 10 Tips for Best free flow ANPR Deployment
Tip #3 – Correct Plate Illumination with Multi Exposure
In order to achieve best license plate recognition rate, it is required a high-resolution (Tip #1, Tip #2), contrasted
and optimally illuminated image of the license plate.
Some variables affecting to the illumination of the license plate are:

Global Variables: overall scene illumination, meteorological conditions, day-night time, sunrise, etc.

Local Variables: reflective and non-reflective plates, physical conditions of plate (dirty, broken, ...), local
shadows on the plate, etc.
These illumination challenges in an outdoor application affect the recognition performance of the ANPR. These
challenges cannot be solved with standard cameras, with no smart and dynamic control of the exposure time of
every caption. This is due to the physical fact that silicon
imagers have limitations on their dynamic range.
Thus, it is very important that the camera adapts the exposure
time of the image sensor to these variable illumination
conditions.
The Multi-Exposure technique is an approach to overcome
these global and local illumination variations affecting the
license plate region.
It consists on capturing the image of the license plate several
times with different exposure times. Therefore, it is possible
to get an image of the plate with a good illumination. Then the OCR engine will correctly recognize the number
plate.
Imagsa’s ANPR cameras implement an innovative auto-adaptive multi exposure technique at high-speed (250
FPS), achieving a good illumination of the plate image regardless of local and global conditions. Even nonreflective license plates can be detected with this technique. The clue is having a high-speed sensor and a smart
control of it.
Tip #4 – Achieve top ANPR performance I: Detection
Definition: Vehicle Detection Rate
Overall performance of an ANPR camera is computed by multiplying the Detection Rate by the Recognition
Rate.
Detection rate is measured by the number of vehicles (plates) detected by the ANPR camera over the Total
number of vehicles going through the camera’s field of view.
Detection Rates range is 50% to 99%, depending on the selected ANPR solution.
Motion vs Plate Detection
Standard ANPR cameras with video triggering detect a vehicle by means of motion detection techniques. Motion
detection consists on analysing the differences between video frames. When a difference occurs, this technique
considers that there is a vehicle and it captures an image to read the license plate.
This technique has two main drawbacks:

Illumination: Performance is sensible to illumination variations, and it has false detections caused by
shadows of a vehicle travelling in the adjacent lane, or even by shadows of trees next to a road.
Shadows create false detections and undermine the performance of the ANPR solution.

Miss Detection: In addition to false detections, some ANPR cameras are affected by miss detections.
This is a more critical issue, as it means that not all vehicles will be detected. Miss detections can occur
by a variety of factors. But mainly because the motion detection algorithm, running in the ANPR camera,
The information contained in this document is confidential and proprietary to Imagsa Technologies S.A. Imagsa Technologies S.A. submits this document with the
understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.
Marketing: 10 Tips for Best free flow ANPR Deployment
does not perceive a sufficient change on opacity in a sequence of images. Sometimes this affects the
overall performance of the ANPR solution, to an extent of just detecting 50% to 80% of vehicles.
Atalaya ANPR cameras implement an innovative license plate detection technique (instead of motion detection).
Basically, it consists on continuously capturing and analysing images to detect a license plate pattern. When there
is a license plate pattern it means that a vehicle is in the field.
This approach avoids the false detections that are common in an ANPR camera based on motion detection. And it
achieves vehicle detection rates over 99%. Meaning that nearly every vehicle (full traffic) is detected by
the Atalaya ANPR camera.
Tip #5 – Achieve top ANPR performance II: Recognition
Detection and Recognition Rates
Overall performance of an ANPR camera is computed by multiplying the Detection Rate by the Recognition
Rate. Detection rate has been analyzed in Tip #4, and it is measured by the number of vehicles (plates) detected
by the ANPR camera over the Total number of vehicles going through the camera’s field of view. Detection Rates
range is 50% to 99%, depending on the selected ANPR solution.
Better Recognition with Good Quality images
Recognition rate mainly depends on two factors:
1.
The quality of the license plate image
2.
OCR (Optical Character Recognition) engine.
OCR is a “mature” technology, and top recognition accuracy can be reached if the input image has good quality.
Thus, quality of the license plate - in terms of Resolution (Tip #1, Tip #2) and Illumination (Tip #3)- is a
prerequisite to achieve best performance of the OCR engine.
So, real improvement in ANPR comes from a good quality image of the license plate.
Imagsa's Atalaya ANPR camera ensures a good quality of the license plate image by using a 2 Megapixel image
sensor and an auto-adaptive Multi Exposure technique at high speed (250 FPS). Therefore, top License Plate
Recognition rates can be achieved.
Tip #6 – Flexible ANPR installation
Where to fix the ANPR camera?
Ideal installation conditions for best ANPR performance are: overhead position, 6 or 7 meters height and around
20 meters focal distance to avoid occlusions.
However, sometimes this ideal installation conditions are not possible. Or an alternative installation position is
preferable to reduce installation costs. For instance, an installation in a roadside pole (instead of a gantry) can
heavily reduce costs of a project.
Thus, an ANPR camera should allow a flexible installation on an overhead position (gantry), at the roadside, in a
range of heights and distances.
Setting Up the ANPR camera
Once the installation point has been decided, it is time for setting up the camera. In order to make this process as
less painful as possible, ANPR camera manufacturers should provide suitable tools and interfaces for an easy setup and camera calibration by the field technician.
The information contained in this document is confidential and proprietary to Imagsa Technologies S.A. Imagsa Technologies S.A. submits this document with the
understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.
Marketing: 10 Tips for Best free flow ANPR Deployment
Atalaya ANPR camera can be installed both overlane and in a roadside position. In order to achieve top ANPR
accuracy from a lateral installation, Atalaya ANPR integrates a real-time perspective correction of license plate
images.
Therefore, top detection (Tip #4) and recognition (Tip #5) rates are achieved in both types of installations. On the
installation side, Imagsa provides an useful and intuitive GUI (Graphical User Interface) to make a trouble-free
installation and achieve top ANPR performance from day one.
Tip #7 –ANPR Programmability Helps Integration
Integration Check-ins
When designing any type of system, also an ANPR-based one, it should be analysed the integration and
implementation times and costs.
ANPR cameras are usually an element of an overall traffic control system. As such, it should be integrated with
other equipments and high-level processes. Thus, in addition to assess the performance of ANPR vendors, it is
important to analyze the camera connectivity and interaction with other equipment.
Things like the communication protocol (TCP/IP or proprietary), the existence of an interface (API, SDK) to
interact with the camera, or even the possibility to run specific and custom software inside the camera, are also
relevant.
In order to minimize the installation and integration costs and speed up the time-to-market, Atalaya has been
conceived as a stand-alone and ready to install ANPR camera. This is an all-in-one and autonomous ANPR
camera.
No additional roadside equipment (PC station, sensors, etc. ) is required. Atalaya ANPR camera integrates a
standard ATOM CPU running a Windows XPe O.S. Hence, system integrators can run their software on it, with no
additional equipment costs.
Additionally, Imagsa provides the tools (SDK) to interact with the camera. There is no vendor lock-in, and the
system integrator can freely develop its own traffic control applications with Atalaya ANPR camera.
Tip #8 – Additional features of ANPR: speed, video, ...
ANPR cameras are now Traffic Data Collectors
Traditionally, ANPR cameras were conceived as single purposed devices: getting license plates images and
performing number plate recognition.
However, new ANPR cameras have the capabilities to deliver much more information. Additionally to number
plate recognition, some ANPR cameras can also obtain overview image of the vehicles (Day and Night), deliver a
video stream or even the instant speed of vehicles.
Therefore, the single-purpose ANPR camera is transformed to a traffic data collection station.
Based on the capability to capture and analyse more than 250 images per second, Atalaya ANPR camera can
track the license plate at high-speed in order to determine the vehicle’s spot speed.
Atalaya ANPR camera can also count vehicles, provide traffic statistics, deliver a video stream and other useful
traffic data.
With one single camera investment and installation, is now possible to deliver more traffic data on top of license
plate recognition. And it is now possible to deploy additional applications and services with the same
infrastructure.
The information contained in this document is confidential and proprietary to Imagsa Technologies S.A. Imagsa Technologies S.A. submits this document with the
understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.
Marketing: 10 Tips for Best free flow ANPR Deployment
Tip #9 – Versatility: integrate different road traffic control applications
ANPR cameras are now Traffic Data Collectors
Following the concept introduced in Tip #8, an ANPR camera can be leveraged to deliver much more than a
single data (license plate number of cars). Some ANPR cameras can deliver further information, such as detection
of Dangerous Goods trucks (ADR plates) or instant speed of vehicles.
One ANPR camera, Multi Applications
A single ANPR device in the road can be used for multiple types of traffic control applications, such as: traffic
statistics (counting), black list police control, toll collection, speed statistics and speed enforcement, travel times,
dangerous goods transport control, among others.
Thus, investing in a single ANPR camera can be leveraged to develop a great variety of traffic control
applications.
Tip #10 – ANPR Cost (to Performance ratio)
Value For Money
Let’s conclude this Guide talking about Cost. It is always a key decision point. But price by itself is meaningless if
not related to the performance, the value that the ANPR camera will deliver to the customer. It is about relying
on ROI metrics more than on price by its own.
1st Recommendation - type of application
Determine the type of application to develop. Do you need a bottom-end license plate recognition camera (i.e.
parking access control) or a high-end ANPR camera (i.e. average speed, free-flow electronic tolling, etc.)?
2nd Recommendation - "Check-ins"
Get a good understanding of the available ANPR solutions for your project. Go through this Guide’s check-ins
and state the pros-cons of each option. This usually helps understanding what performance can you expect for
every solution.
- A low resolution ANPR camera (Tip #1)? Do not expect a great plate recognition accuracy.
- No multi-exposure ANPR camera (Tip #3)? A lot of vehicles’ plates will not be recognized; especially at sunrise,
sunset, under raining conditions and dirty plates.
- Can the ANPR solution run in overhead as well as from a roadside pole (Tip #6)? This could greatly reduce
installation and maintenance costs.
3rd Recommendation - Think Big
As introduced in Tip#8 and Tip #9, a high-performance ANPR camera can deliver a great variety of traffic data.
And it can be used to deploy multiple traffic applications with a single device, one investment. This is about
delivering more features, greater value to the Road and Traffic Administration.
To Conclude
Price is always important, but do not lose the focus. It is about delivering excellent traffic results, more value,
greater service. This is the key selling point for repetitive sales and projects.
Contact us to know further details about Imagsa Technologies or in case of any question.
IMAGSA Technologies S.A.
Navas de Tolosa, 270
08027 Barcelona
Spain
Tel: +34 934 869 345
Fax: +34 934 856 776
E-Mail: [email protected]
Web: www.imagsa.com
The information contained in this document is confidential and proprietary to Imagsa Technologies S.A. Imagsa Technologies S.A. submits this document with the
understanding that it will be held in strict confidence and will not be used for any purpose other than the evaluation of this product and Imagsa Technologies S.A.
qualifications. No part of the document may be circulated, quoted, or reproduced for distribution outside the Client organization without prior written approval from Imagsa
Technologies S.A.