Note: Descriptions are shown in the official language in which they were submitted.
-........ WV 92/03808 PCI/GB91/01405
2~g93
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~- r-~iE~;~,~'C.:.i~RA r~!C ?.~r~TI'~'L~RL'. _-~R [~Sr _~ . -
VT~,~G VEHICLr ~.UMBER-~L~TES
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-~.is _-ventlon relates ~o survelilance a~Daratus ~hich
`a ~ar~_^_larl intended ~or use .. v~ewin~ ven cle
-.umDer-?lates in covert night-survelllance operations.
,
T~e viewing or reading of vehicle numDer-plates at night
in a covert manner has long been a problem. Additional
visible il1umination is usuall~i unacce~table or unavail-
able, (exceDt possiblv in .he orm of _treet lights
n some situations) and the traditional inrra-red illumi-
nator whic~. operates by ~iltering the output cf ~n incan-
-descent source, can be seen by many people even when -~
- 830nm short-wavelength cut filters are employed.
Further, in order to image a front number-plate when
the vehicle headlights are switched on, a considerable
- amount of additional illumir~ation is required if the
number plate is to be read clearly or at all.
,
T ~ is an objec- of t~.e 2resent invention to provide
an apparatus for reading vehicle number-plates, particu-
larly in a covert ~,anner ~uring night-survelllance,
in ~lhich the above-mentioned problems are overcome
or at least reduced.
~ccor~ing to the p-esent invention, there is provided
a surveillance a~paratus comprising an illuminator
for emitting a series of high-energy pulses of light,
and a camera oper~ting in synchronism with the illumina-
tor to receive said p- ses of light. In covert night-
surveillance o~erations, as in vie~7ing vehlcle number-
?lates, this light is infra-red light outside the range
visible to the humlan eve; in the infra-red (IR) ranye
of the spect~um, light is invisible to he human eve
at 940nm or S80nm or possibl~ 820nm, and light-radlation
~f t~ese ~avelengths can be ~rovided bv suitable LED
or iaser sources.
W092/03808 PCT/GB91/nl4n~
- 20~35~
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Preferably, saia illuminatGr comprises an array of light-
emitting diodes (LEDs) or a source or laser light, and
said camera comprises a shuttered Tv camera which will
usually be of minlature si~e.
The illuminator and the (miniature) camera may be combined
in a small package, which is attractive to covert sur-
veillance operations, and it is preferred to position
tAe illuminator close to the camera lens so as to minimise
the included angle between incident and reflect~d light.
The use of (invisible) IR light not only avoids detectlon
of the surveillance operation, but also reduces the
counter-effect which may be produced by the headlamps
of the vehicle being observed, and this effect is further
reduced by providing the camera with a bandpass filter
and reducing the 'open' period of the camera.
If the TV camera is of t:~e (usual) kind which receives
visible light, it is desirable to provide for adjustable
back-focus to accommodate the difference between- the
- focal points of the camera lens for visible light and
for IR light.
Embodiments of the present invention will now be
-described, by way of example, with reference to the
accompanying drawings in which: ;
' ' .
Figure 1 is a graph showing ~um) wavelengths of
light emitted from a conventional incandescent
light source having a filament temperature of about
3000K,
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Figure 2 is a similar graph showlng the wavelengths
` of (I.R.) light emitted 'rom a 340nm illuminator;
f and,
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W092/03808 2 ~ ~ 9 3 5 ~ PCT/GB91/01405
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~igure 3 is a diagramatic view showing apparatus according
.o the ?resent invenrion for viewing or reading vehicle
~umber-plates, ana illustrating the operation of the ~-
apparatus.
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Re~errlng firstly to Figures 1 and 2 of the drawlngs,
it will be noted tha. the light energy transmitted from
a source 10 such as the 3000K bulb (Figu-e 1) contains -
a substantial degree of visible light, and some IR light.
On the other hand, when LEDs are employed, the light
energv is conventionall~ in the range o_ about 840 to
1000nm; LEDs suitable for the present invention have
a centre wavelength of 940nm or 880nm, with a spectral
width of, typical-ly, 50nm (Figure 2).
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The apparatus accorcing to the present invention comprises
the illuminator or light-energy source 10 which, suitably,
has an array of standard 940nm LEDs transmitting IR light
which is invisible to the human eye, as indic:.ted above
with reference to Figure 2~ The array may comprise about
40 to 400 LEDs, depending on the required specification
of the apparatus, and one LED which has proved saticrac-
tory is the AEG TSIP5201 having a continuous rat.ing of
'C~OmA.
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A camera 12 is employed in conjunction ~:rth the LED illum-
inator, in viewing a car number-plate 13, and has electri-
cal connection 14 with the illuminator. The camera com-
prises a shuttered TV came~a having a CCD sensor whic~
is sensitive to IR light, behind a lens unit 15, and
records and/or transmits to a possibly- mot~ TV m~ni.tor
(not s~.own) video signals derived f~om he lllumi;;ation
of the number-plate by the LED arrav 11. A 'shuttered'
TV camera has an integratior. time (Whi1^ ?ictur- i;.form-
ation is being accumulatea) which is shorter _han its
~ield period. Shuttering mav be achievea electron ~all~
or mechani lly, or ~y means of an elec~-o-optical shut-
ter, but i~ !S preferable .-~at the shuttering operation
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WO 92/038~8 2 ~ 8 3 3 ~ 6 PCT/CB91/014~5
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' a silent, as occurs -~ith electronic shuttering. The
standard ,0 per second, field periods for a (U.~.) TV
_amera are 20mS, out the camera mav be shuttered to
1mS or each field which Jould result in the camera
-egisterina only 1/20 of the total light-energv rom
a broadband continuous source operating for 20mS.
~owever, when used in conjunction with an LED illuminator
which produces energy in high-peak pulses which are
synchronized with the camera "open" periods, the camera
can receive a comparable amount of energy during each
shuttering period. In the present embodiment of the
invention, the aforementioned LEDs TSIP5201 have a cont-
inuous ratins of lOOmA, but the driving circuits in
the illuminator 10 feed high-energy pulses of about
1000mA, or possibly more, at a rate of 50 pulses per
second ~ith each pulse having a duration of 1mS; as
a result, the camera receives at least 50% of the energy
from broadband illumination without the restriction
oi shuttering. A rate of 60 pulses per second would
be appropriate if, for example, U.S. system TV monitors
~ere to be employed with U.S. system cameras (12).
It is to be noted that the focal point of any lens,
_or IR light, lies behind the focal point for visible
iight. In order to achieve sharp focussing when employ-
lng a TV camera which also receives visible light, it
is desirable to be able to change the back-focus of
the camera. The PEARPOINT P176 camera is particularly
suitable for use in the apparatus of the present invent-
ion, as it incorporates motorised back-focus which facil-
itates initial set up for each operation; this camera
also provides high sensitivity, extended IR sensitivitv
Up tO 11 Odnm, high resolution and a selectable (on/off)
shutter.
,
In operation, therefore, the LEDs emit Aish enersy due
.o high-peak lOOOmA pulses cf ~ duration of about lmS
- W092/03808 2 0 o ~ 3 5 ~ PCT/GB91/01~05
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in synchronism ~ith the cameras "open" periods of 1mS.
This illumination is sufflcient for reading car number-
?iates at night, under normal conditions. The disadvan-
tage -~hich car headlamps can cause is substantially
reaucea as the level or energy received bv the camera
from the steady light produced by the headlamps during
each period of lmS, effectively of chopped waveform,
is much less than the energy level received during those
periods from the high-peak pulses generated by the LEDs.
To reduce even further the effect of car headlamps,
a band-pass filter may be introduced into the optical
path 16 to the camera; this filter is centered on the
peak output wavelength of the LEDs (in the IR band)
and has a bandwidth which passes most of the LED energy
but rejects light of other wavelengths such as the remai-
ning energy emitted from the broadband vehicle headlamps.
As an alternative to the LED illuminator described,
it is possible to employ a source of laser light which,
although more expensive than LEDs, would allow a much
narrower bandpass filter to be used which could reduce
still further the counter-effect of headlamp illumina-
tion. When employing laser light, which usually has
a wavelength of about 820nm, the possibility of visibil-
ity from certain viewpoints cannot be ignored, but the
small chance of detection is regarded as an acceptable
risk. It should be noted that a person with exceptional
eyesight could, in certain circumstances, detect IR
light of 880nm or conceivably longer wavelength, but
again this is regarded as an acceptable risk. -
According to a further feature of the invention, the
light energy fed to the camera by the LED ?ulses can
be changed on a field-by-field basis, to provide images
or the number-plate ~hich 1ill then appear brighter
or dimmer. These images are proauced at very short
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W092/03808 2 ~ ~ 9 3 ~ 6 PCT/GB91~01405
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nter~als or time, anà mav subsequentl~ be selected
~o ~acilitate the reading or a number-plate which may
oe ~nclear _ue tO poor -erlec~ it~ aused by, for e:~am-
?le, dirt or ~he plate. ~he camera ill usuallf nave
a f~ ed !ris, anà the puise enerqy may De changed by
varying amplitude or duration, and it is to be noted
that reduction of duration, say from lmS in stages to
0.3mS, has the added advantage of reducing possible
blurring caused by movement o~ the vehicle. Similarly,
the camera shuttering time may be chanaea on a field-
bv-field basis, to reduce yet further the counter-effect
of headlamps.
Vehicle number-plates (12) in many countrles including
U.K. are of a type which give very high reflectance,
provided that the llluminator 10 and camera 12 are very
close together. As shown in Figure 3, appropriate arrange-
ment of the illuminator array 11 and the camera lens
can reduce below 5 the included angle ~ between
the incident beam 17 and the reflecteà beam 16. This
narrow angle can result in a reduction in the amount
of illumination required.
The apparatus is useful in viewing vehicle number-plates
during the dark, and also during dawn and dusk, and
may operate continuously to cover daylight hours. It
~ill be appreciated that, for davlight use, and also
in non-covert operations such as traffic surveys, the
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question of visibility may be of little importance and
pulsed illuminators in the spectral bands of wavelengths
shorter than IR may be employed.
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Yet a further advantage of the apparatus of the present
invention is that the illuminator anà TV camera can
be of small size' possiblv packageà 'n the form o_ 2
cube having sides of the order of 100mm. The power
required for the LED il-luminator may be as low as 7
or 8 watts and, operating at 12 volts, permits battery
operation.
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