Accuracy of Automatic Number Plate Recognition
(ANPR) and Real World UK Number Plate Problems
Mike Rhead, Robert Gurney, Soodamani Ramalingam
School of Engineering and Technology
University of Hertfordshire
College Lane Campus
Hatfield, Herts AL10 9AB, UK
Centre for Applied Science and Technology
St. Albans, Herts AL4 9HQ, UK
Abstract— This paper considers real world UK number plates
and relates these to ANPR. It considers aspects of the relevant
legislation and standards when applying them to real world
number plates. The varied manufacturing techniques and varied
specifications of component parts are also noted. The varied
fixing methodologies and fixing locations are discussed as well as
the impact on image capture.
Key words—ANPR, number plates, optical character
recognition, National ACPO ANPR Standards (NAAS), ANPR
British Standards, Registration Marks Legislation, misreads,
Automatic Number Plate Recognition (ANPR) within the
UK is a powerful tool and its policing purpose is to deny
criminals the use of our roads . It is a major tool that is used
to improve the security and safety of the general public. In
standard use, ANPR is remarkably accurate. However, the
purpose of this work is to look at the occasions when ANPR
fails to capture the vehicle registration mark (VRM) correctly.
By reviewing the mode of failure it is hoped to be able to
improve the performance of ANPR. Some main causes of
ANPR failures area as follows:
Fixing Screws: Depending upon the character of the VRM
and the juxtaposition of a fixing screw misreads can occur.
Such misreads can be consistent or inconsistent and the paper
gives examples of this. An experiment whereby small diameter
anti-tamper screws were used to secure the number plate to the
vehicle is discussed as well as the relative improvement in
correct read rate.
Fixing Adhesives: Depending upon the method of
manufacture of the number plate, fixing adhesives can also
affect the retro-reflective layer and the photographic properties
of the number plate. This is caused by absorption of the
adhesive into the retro-reflective layer and subsequent impact
on its reflective properties.
Visible and near-infrared Spectrum: ANPR systems can
use both the visible and near infra-red spectrum. The
appearance of real world number plates can be significantly
different when comparing monochromatic or black and white
images from the visible spectrum and the near infra-red
The main cause of a misread read has been found to relate
to screws and screw caps where the fixing is within the
lettering of the VRM. The main failure mode for ANPR
cameras has been found to relate to fixings. The implications
for license plate manufacture , fixing methodology,
legislation and standards are discussed in the following
II. LITERATURE REVIEW
The authors of the paper work with Police ANPR systems.
Their work on other aspects of ANPR is well progressed and
further publications are planned. Countermeasures, cloned
plates and vehicles moving without number plates are an
inevitable aspect of this work and progress has already been
made in this area. However, the outcome of this aspect of
work will not be published nor put into the public domain.
This is a pioneering work; a literature review [3-6]
conducted by the authors revealed that there is no other
published real-world research. These papers assume ideal
experimental conditions. The papers that were identified
assumed ideal conditions.
Our initial work began in 2006 through field trials
determining the accuracy of hot lists. As a result of this work
changes were made to a number of hot lists to improve their
accuracy. In 2009, our attention turned to the cause of
misreads (not read correctly by ANPR systems) and by 2010
we were looking at the cause of missed reads (data missed
completely). During 2009 we also began researching the
NAAS Standard in depth. In 2010 we reviewed the British
Standard test for number plates, the details of the NAAS
standard and also the differences in performance of the
different ANPR cameras available and captured a lot of data
relating to these aspects. We are continuing to experiment with
camera settings (including software) and capture rate.
Field tests have been carried out for a range of ANPR
cameras under identical conditions to assess performance of
the camera, adjustment of camera setting and the performance
of the software within the camera . Here, we are
considering fixings within a VRM. The results of this wide
ranging work are expected to help improve the accuracy of
ANPR to an even higher level.
The following two images in Fig.1 are of the same number
plate. Fig. 1(a) was captured in the visible spectrum and Fig.
1(b) captured in the near infrared spectrum. The impact of the
screw cap can be seen between the two images. The screw cap
is much easier to see in the near infrared image.
Other unpublished work indicated that screws and screw
caps close to or within the lettering of the vehicle registration
mark could cause difficulty for optical character recognition
which in turn can lead to misreads or missed captures. The
impact of the screw fixing can be much greater under near
infrared imagery. This aspect is discussed in detail in the paper.
Figure 1 gives an example of the difference.
A. Initial Tests
Some of our initial (unpublished) work into misreads
indicated that ANPR misreads were caused by a number of
factors that could be exhibited by “real world” number plates.
The misreads were assessed and broken down into 5 categories
[Table I]. Our initial work looked at capture rates and misread
rates for vehicles giving a match to hot lists such as no
insurance. As an extension of this work the causes of 700
misreads to be studied in depth.
This initial work on 700 misreads suggested that number
plate fixings (screws and screw caps) accounted for 72.6% of
misreads. Results of analysis indicate that the top 10 characters
that accounted for 55.6% of incorrect reads and these are
related to screws or screw caps [Table II]. These results are
typical of the issue.
(A) VISIBLE SPECTRUM IMAGE (B) NEAR INFRARED IMAGE
FIGURE 1: DIFFERENCE BETWEEN VISIBLE SPECTRUM AND
INFRARED IMAGE CAPTURES
TABLE I: ATTRIBUTABLE CAUSE OF MISREAD
TABLE II: ATTRIBUTABLE CAUSE OF MISREAD
The fixing problem is more prominent at certain positions
on the Number Plate (NP). Fig. 2 indicates the position or
location of the misread character from this work. The post
2001 UK Number Plate regulation introduced a space between
the 4th and 5th characters. For the same 700 NPs, the character
at position 2 is the most commonly misread location (34%) and
position 5 is the second highest misread location (16%). These
two positions account for 50% of misreads caused by screws
and screw caps.
B. Further Field Tests
A number of vehicles in the UK have number plates that
begin with the letters OU [Figure 1]. Ten specific vehicles were
selected from a Police fleet each one having a screw/screw cap
fitted within the centre of the character U. Each had their
movements monitored over a two month period. There were
997 captures of which 713 (71.5%) were correctly read
indicating a misread rate of 28.5% for these plates. Of the 284
misreads the U was interpreted incorrectly as other characters
Five of the sample vehicles then had their fixing screw
replaced with smaller diameter anti-tamper screws in order to
try to reduce the footprint of the fixing. There were 359
captures for the 5 vehicles. Of these, 358 (99.7%) were
correctly read indicating a misread rate of 0.3%. The difference
in screw sizes are indicated in Figure 3.
From this test we infer that a simple change to the diameter
of the screw head used can significantly impact the accuracy of
the ANPR systems. The diameter of the standard fixing was
about 12mm and that of the anti-tamper fixing was 6mm. The
footprint area of the fixings went from 113mm2 to 28mm2. In
this test, our result indicated a 98.9% improvement in
FIGURE 2: POSITION OF MISREAD LETTER
TABLE III: STATISTICS OF INCORRECT READS
FIGURE 3: DIFFERENCE SCREW SIZES UNDER INFRA-RED
FIGURE 4: SCREW POSITIONS AFFECTING ANPR
TABLE IV: RESULTS OF THE 1,238 VEHICLE SAMPLE
Visible screws within the characters of
Outside the characters of VRM
No visible fixings
Table V Examples of misrepresented number plates
Further research identified screw positioning as being a key
factor causing mis-reads and resulting in incorrect
interpretations by ANPR systems as illustrated in Figure .
A) VISIBLE SPECTRUM B) NEAR INFRARED SPECTRUM
FIGURE 5. DIFFERENCE IN VISIBLE SPECTRUM VS IR
C. Confidence Measures
Having determined that there is a significant problem that
arises from screw fixings, further trials were undertaken to try
to determine the extent of the problem within the UK. To this
end, the fixing methodology was analysed for a sample of
1,238 vehicles, the results of which are as shown in Table IV.
D. Fixing Screws and Resulting Images in Visible and Near-
The legislation requires that where the fixing screw goes
through a white background layer a white screw/screw cap is
used. Similarly a yellow fixing is used for the rear and a black
fixing goes through the actual lettering. The purpose of this is
to ensure that the number plate is easily read by the human eye
for identification and prosecution purposes.
The infrared spectrum used in ANPR applications to
capture number plates is just outside human vision [8,9]. To
give an indication of the difference two sets of number plates
captured in the visible spectrum are shown in [Fig. 5A] and the
same plates captured in the near infra-red spectrum are shown
in [Fig. 5B]. The fixing screws at the left and right hand edge
of the number plates appear as black dots in Figure 5B which
can contribute to mis-reads.
The reason for this is that the white plastic caps used as part
of the fixing are infrared absorbers and have no retro-reflective
properties. It would be illegal to use retro-reflective fixings on
a number plate. The difficulty faced as a result of this is that the
fixings within the VRM lettering appear as large black full
spots within the lettering and this can confuse the OCR
software depending upon the relative size and position of the
fixing with respect the character within the VRM.
E. Screw Caps and Spacing
Illegal use of screw caps and spacing in the visible
spectrum contravenes the statutory instrument. Table VI gives
a few examples of the illegal use of screws (as well as illegal
spacing). Such plates are illegal because they “deceive” the
eye as well as breaching number plate spacing regulations.
Such number plates do cause ANPR misreads.
Further work is also planned where we will look at the
impact of variable spacing using the standard UK number plate
Charles Wright font.
F. Fixings under Infrared
For the data considered in Section III B (Further Field
Tests) the reported misread rate of 28.5% was due to a screw in
the letter U and the size of the screw fixing [Fig. 7]. A further
analysis of 1,287 random number plates indicated that a large
number of plates have screws within the characters of the
VRM, typically 63% ± 4%. This does not imply that 63% of
number plates can be misread because of screw fixings; the
proportion of number plates giving misreads will be well below
this. Thus, one can easily conclude that the elimination of
screw fixings from number plates will improve the accuracy of
FIGURE 7: APPEARANCE OF A WHITE SCREW CAP FIXING ON A
WHITE RETRO-REFLECTIVE NUMBER PLATE UNDER NEAR
G. ANPR Infrared Images
The majority of number plate images used for optical
character recognition (OCR) are monochromatic images. The
images are normally captured by standard optical cameras
fitted with a visible spectrum and/or near infrared filters.
The infrared image is based upon the contrast between the
retro-reflective layer and characters of the VRM. Screws and
screw caps attenuate infrared and appear as black marks shown
in Figure 4 and this in turn can confuse OCR.
In summary, having considered the issue of one can see that
screw caps fitted within the type face can have a detrimental
effect on the ability of the human eye to recognise the VRM
correctly. At the same time, the situation can be even worse
under infrared. The legislation in this area requires clarification
as there are a number of ambiguous interpretations.
IV. INTERPRETATION OF THE STATUTORY INSTRUMENT AND
A. Visible versus the near infrared spectrum
Greater clarity is required in the definition of the Act and
the Standard. The Act appears to cover infrared images because
it refers to “camera and film or any other device” and “true
photographic image”. There is no definition or case law with
respect to this and opinion is divided. The British Standard can
also be seen as ambiguous:
According to the British Standard, the luminous intensity in
a given direction of a source that emits monochromatic
radiation of frequency 555nm is defined by candela
abbreviated as cd.
The units of retro-reflectivity measurement of radiance is
ρ = cd/lx/m2 (1)
cd → candela, luminous intensity
lx → lux, luminous flux per unit area
sr → steradian, a unit of solid angle
m2 → area in square meters
lux is defined by:
lx = 1 cd.str⁄m2 (2)
Substituting (2) in (1), the retro-reflectivity unit of measure
ρ = cd/cd.sr/m2/m2
The equation cancels and becomes
ρ = 1/sr
= per steradian
= per unit of solid angle
Thus, ρ is, in fact, a dimensionless unit and cancels to
become a per steradian.
The measurement relates to reflectance and is independent
of the wavelength of the radiation used for the test. That is, the
ratio of the total amount of radiation reflected by a surface to
the total amount of radiation incident on the surface.
The use of candela is taken by some to infer that the visible
spectrum is the required wavelength range because the unit
candela is mentioned. However, the British Standard  refers
to CIE 15  and CIE 54 . To measure the retro-
reflectivity of the characters and backing layer of a UK number
plate the standard requires illuminant A. I Illuminant A is based
on a tungsten filament bulb operating at a defined temperature
of 2,856 oK. Iluminant A has a defined wavelength range
(300nm to 830nm) and spectral power. The wavelength range
covered enters the near infrared spectrum. It is not clear if the
British Standard is required to be carried out at 555nm or over
the spectrum and spectral power defined in CIE 15 (300 to
The key issue here relates to definition and interpretation of
Statutory Instrument No.561 . The screw fixings clearly
interfere with the optical character recognition software. The
key question relating to these fixings is – are they legal?
The requirements of the CIE standards were originally
applied to UK roads signs and markings with respect to driver
safety. They were subsequently incorporated into BS AU 145d
for UK number plates.
Some infrared ANPR systems operate at up to 940nm. As a
consequence the authors would like to see the defined range of
300-830nm amended within the British Standard. A range of
300-1,000nm would be preferred with reflectance measured as
an integral and at 5nm intervals. This would indicate if the
number plate had uniform reflectance properties over the
defined wavelength range.
B. Performance standards for fixings
The National Association of Chief Police Officers
Automatic Number Plate Recognition Standards (NAAS) 
notes minimum performance standards.
As has been demonstrated fixings can cause an ANPR
misread. If fixings do cause a misread they can be regarded as
an illegitimate UK plate. Under such circumstances these plates
can be discounted from the compliance test. This is not
unreasonable if the optimum performance possible (ideal
world) of the system is being considered. We would like to see
an additional measure whereby misreads attributable to fixings
are also declared. We propose that changes to UK number plate
legislation would tackle the main cause of ANPR misreads.
C. Variety in UK and Schengen number plates
Beyond the consideration of screw fixings there is a wide
variability in font, spacing and other marks in the registration
area of number plates within the UK. UK includes Northern
Ireland, Isle of Man and Channel Islands.
Under EU rules countries are, in effect, borderless for
member states. This is discussed more fully in another paper by
the authors . There is an increased variety of number plates
being presented to ANPR systems and ANPR systems should
develop further to take this into account.
1) UK VARIETY
National ACPO ANPR Standard (NAAS)  notes that for
the avoidance of doubt, number plates from Northern Ireland,
Isle of Man and Channel Islands are regarded as UK number
The proportion of Northern Ireland, Isle of Man and
Channel Island number plates is comparatively small when
compared to standard UK plates. It is interesting to explore the
performance requirement for these plates under the NAAS
standard and the possible implications for ANPR
manufacturers and suppliers.
The Crown Dependencies of the Channel Islands and the
Isle of Man are outside the United Kingdom and European
Union, and have registration marks that differ from those used
in the UK.
Examples of the type and style of number plates included
are given in Fig. 6.
A) NORTHERN IRELAND B) JERSEY
C) GUERNSEY D) ISLE OF MAN
FIGURE 6: VARIETY IN NUMBER PLATES
2) ANPR algorithms to handle Schengen Community
The series of images [App. A-B] relate to a sample of
Schengen community number plates and are given to illustrate
the challenge given to OCR readers in ANPR systems. The
difference in font, syntax, symbols and spacing pose a problem
to OCR utilised in the UK.
V. FUTURE WORK
ANPR captures are extremely accurate. This work has
determined that some number plates can be problematic for
ANPR cameras and that the main reason for a misread for a
UK number plate is caused by screw fixings. Screw fixings
within a VRM can confuse an ANPR system and the result
could be a complete failure to capture or a misread.
Future work we will consider includes:
Camera set up
Instantaneous traffic flow rates compared to
manufacturers hourly plate rate capacity
Comparative testing of different systems using a
defined data set
Various counter measure testing
Number plate manufacturing techniques
Dr. Vivienne Lyons - Home Office; Detective
Superintendent Paul Ealham - Hertfordshire Constabulary; Lisa
Gilmore - Department for Transport; Alastair Thomas - Home
Office; Mark Jones - National Policing Improvement Agency;
Frank Whiteley - former Chief Constable Hertfordshire.
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Use of the Roads. Report by the British Association of Chief
Police Officer’s ANPR Steering Group, March 2005.
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Plate Manufaturer’s Association (BNMA), 2011.
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Detection Algorithm, Proc., IEEE SoutheastCon “Innovating for
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APPENDIX A: LIST OF SCHENGEN STATES
APPENDIX B: SOME EXAMPLES OF SCHENGEN
COMMUNITY NUMBER PLATES