Note: Descriptions are shown in the official language in which they were submitted.
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X - I~AY L IN 1~ SCII~N SY S TEM
FOR USE~ IN Bl~GGAGE INSP~:CTION
_ P E C I F I C A T I O N_ _ _ _ _ _ _ _ _ _ _
This invention relates to x-ray scan systems for in-
specting the contents of baggage moving on a conveyor belt,
and more particularly to such a system in which the x-rays
are collimated into a narrow linear beam such that succes-
sive portions of the object are scanned as the conveyor
moves such objects past the beam.
The inspection of baggage at airports has now be-
come almost universally mandatory as a safeguard against
the hazards of aircraft highjacking. In order to minimize
the time involved in such inspection and to provide for a
more accurate and efficient inspection without the need for
opening baggage, x-ray scan systems have been developed in
which x-rays are passed through baggage as it is moved on
a conveyor past the x-ray beam and the rays which have
passed through the baggage appropriately processed for dis-
play on a video monitor to provide images of the contents
of the baggage. Such systems have been in use for a num-
ber of years. One problem which has been encountered with
such systems is that even with very low levels of x-ray
radiation, damage can be caused to photographic film and
other articles which are highly sensitive to x-ray radia-
tion. Thus, while the average amount of radiation to which
such radiation sensitive articles are exposed with systems
of the prior art is only of the order of 1 milli-Roentgen,
this still is enough radiation to cause damage thereto~
To avoid this problem, efforts have been made to lower the
radiation. However, with lowered radiation, the signal/
noise ratio of the output signal is lowered which results
in a significant deterioration of the definition of the
video monitor display which is unacceptable for proper bag-
gage inspection. One particular solution to this problem
involves the use of a flying spot scanner in conjunction
with a mechanical chopper which provides a pulsating beam
to which the object is exposed, thereby concentrating the
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beam yet lowering the average radiation on the object. This,
it has been found, lo~ers the radiation without significantly
affecting the quality of the video display, but it has the dis-
advantage of the expense and inherent reliability problems encoun-
S tered with a mechanical chopper. In addition, a large portion of
the output of the x-ray generator is dissipated in the chopper and
not employed in the beam, making for relatively inefficient use
of the available x-ray energy.
The device of the present invention provides means per-
mitting lowering the average radiation to which the object is
exposed to a level of the order of .15 milli-Roentgens at which
damage to photographic film and other x-ray sensitive articles
should be avoided. This end result is achieved without the need
for any mechanical implementation, such as a mechanical chopper
or the like, and with a relatively efficiant utilization of the
available x-ray radiation.
In accordance with the present invention there is provided
a system for inspecting objects including
an x-ray source,
conveyor means for moving said objects through rays in the output of said source,
means for receiving said rays after they have passed through said objects,
means responsive to the output of said receiving means for providing a video
display in accordance with the receiving-means output comprising a light emitting screen
in the form of a strip, an array of photx~diodes opposite said screen for generating
electrical signals in accordance with the light emission of the portions of the screen
thereopposite, separate integrating amplifier means for individually integrating and
amplifying the output of each of said detectors, electronic sampling circuit means for
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.
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time multiplexing the outputs of said integrating amplifier means to provide a series
of sequential pulses representing the outputs of said integratin~ means, means responsive
to said pulses for generating video output signals, and video monitor means responsive
to said video output si~nals for displaying said video output signals,
means for collimating the rays of said source into a narrow fan-shaped beam
prior to their passage throu~h the objects, said beam scannin~ said objects in successive
slices as the conveyor means moves the object therethrou~lS and
means for automatically correcting for diode gain and dark current variations
in each of said diodes whenever there is a time interval between objects passin~ through
said rays which is greater than a predetermined value.
Thus, a specific system of the present invention employs
means for collimating the output of the x-ray source into a linear
beam. This beam is directed towards objects to ~e inspected as
they pass along a conveyor belt. The beam collimation is achieved
by means of collimator plates between which radiation from the
x-ray source is permitted to pass. After the x-ray beam is passed
through the object to be scanned, it strikes against a light-
emitting screen in the form of a strip. The light output of the
screen is received by a linear array of photo-detector diodes,
the individual outputs of which are appropriately integrated and
fed to a sampling circuit. If the time interval between objects
to be scanned passing along the conveyor belt is greater than a
predetermined value of the photo-diode outputs are automatically
corrected for diode gain and dark current variation in each
respective diode to compensate for individual gain and offset
differences.
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O
The sampling circuit effectively time multiplexes the integrated
photo-detector outputs into a single output having successive
pulses representing each of the integrated photo-detector outputs
in sequence. These sequential outputs are converted to digital
form and stored in a memory from where they are fed to video
output circuits for ~isplay on a video monitor. In this manner,
the radiation to which the objects are subjected is reduced to a
very low
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level (of the order of .15 milli-Roent~ens) at which level
damage to photographic film and other x-ray sensitive arti-
cles is ob~iated. This end result is achieved without any
significant depreciation in video monitor picture quality.
It is therefore an object of this invention to
lower the x-ray radiation in a baggage inspection system
without significantly depreciating picture quality.
It is another object of this invention to provide
an improved x-ray inspection system employing a line scan
x-ray beam wherein the radiation to which objects are sub-
jected is substantially decreased, and having improved
picture quality.
Other objects of the invention will become apparent
as the description proceeds in connection with the accompany-
ing drawings of which:
FIG 1 is a functional block diagram of a preferred
embodiment of the invention;
FIG 2 is a schematic perspective drawing illustrat-
ing the system of the invention;
FIG 3 is a functional block diagram illustrating the
video processing and control circuitry of the preferred
embodiment;
FIG 4 is a functional block diagram illustrating
the circuitry of an image corrector unit employed in the
preferred embodiment; and
FIGS 5A and 5B are flow charts defining the opera-
tion of the preferred embodiment.
Referring now to FIG 1, a functional block diagram
of the preferred embodiment of the invention is shown.
X-ray energy generated by x-ray source 11 is collimated
into a linear beam by means of beam collimator 13 which
may, as to be explained in connection with FIG 2, comprise
a slot formed between a pair of metal plates. The beam
which is collimated into a relatively narrow fan-shaped
strip is radiated through an object to be scanned 15 which
is moved normal to the strip by means of a conveyor 17 on
which the object is supported. Thus, successive "slices~
of the object are scanned in sequence by the beam as the
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conveyor belt moves such objects past the beam. After the
x-ray energy has passed through the object, it strikes
against a light emitting (phosphor) screen 18 of the type
normally employed in x-ray displays. Placed opposite
the light emitting screen 18 is a photo-detector array 20,
the photo-detectors being arranged in a line directly oppo-
site the screen. The photo-detectors generate electrical
outputs in accordance with the light energy impinging there-
on, the output of each photo-detector being fed to a corre-
sponding integrating amplifier in integrating amplifierarray 22.
The outputs of the amplifiers of integrating ampli-
fier array 22 are fed to sampling circuit 24 which time
multiplexes the individual signals received from each of
the integrating amplifiers so as to provide a sequential
series of pulses on line 26, each such pulse having an am-
plitude corresponding to that of one of the integrating
amplifier outputs. A conventional memory operating in con-
junction with a sequential readout circuit may be employed.
The signals on line 26 are fed to image corrector 28 which
operates to correct diode gain and dark current variations
for each of the amplifiers in amplifier array 22, as to be
explained further on in the specification. The output of
image corrector 28 is fed to analog/digital converter 30
wherein the signal is converted to digital form and in such
digital form fed from the analog/digital converter to mem-
ory 32. ~emory 32 stores the successive "slices" of the
object being scanned so that they can be properly displayed
on video monitor 37, the output of the memory being fed to
video output circuits 35 and from the output circuits to
the monitor. Power for conveyor m~tor 40, as well as video
monitor 37, is supplied from a common AC power source 42
so that the mechanical scanning afforded by movement of the
conveyor can be appropriately synchronized (through the
video output circuits) with the scan circuits of the video
monitor.
Referring now to FIG 2, a schematic pictorial repre-
sentation of a preferred embodiment of the system of the
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invention is shown. X-ray tube 11 is appropriately mounted
in a suitable housing, to provide x-ray radiation towards
precollimator and collimator plates 4~ and 47, respectively.
Collimator plates 46 and 47 are metal plates constructed of
a material suitable for shielding x-rays, such as steel.
Both the collimator and the precollimator are formed from
pairs of plates 46a and 46b and 47a and 47b, respectively~
these plates being separated from each other by slots 46c
and 47c. The widths of slots 46c and 47c are preadjusted
to produce a fan-shaped x-ray beam 50 having a width or
thickness of 1/8" - 1/16" in the preferred embodiment.
The exact width to which the beam is adjusted is dictated
by the resolution requirements of the system. Beam 50
passes through an object to be scanned 15 and then strikes
against light-emitting screen 18 which is a conventional
x-ray screen of suitable phosphorescent material in the
form of a strip corresponding in width to the width of
beam 50 (i.e., 1/8" - 1/16" in width).
The object 15 is supported on conveyor belt 17
which moves successive portions of the object through
beam 50 such that successive slices of the object are
scanned by the beam. Mounted opposite light emitting
screen 18 is a photo-detector array 20 which may comprise
a linear array of photo-diodes 20a positioned alongside
screen 18. In an operative embodiment of the Lnvention,
screen 18 and the diode array 20 are approximately 30 in-
ches in length with approximately 500 photo-diodes forming
the array. When x-ray photons strike x-ray screen 18,
screen 18 emits light in accordance with the energy in each
such photon and the number of photons which is dependent
upon the characteristics of the portion of the object 15
through which the photon has passed. Thus, the light emitted
by various portions of screen 18 is in accordance with the
characteristics of object 15 being scanned. The photo-diodes
20a receive light from the portions of the screen thereoppo-
site and each photo-diode generates an electrical current
in accordance with the intensity of the light signal received
thereby.
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As already noted in connection with the explanation
of FIG 1, the output of each of photo-diodes 20a is ampli-
fied in a corresponding integrating amplifier and after
time multiplexing and image correction has been accomplished
converted to digital form and~placed in a memory for display
on a video ~nitor.
Referring now to FIG 3, the video processing and
control circuitry of the preferred embodiment is schematic-
ally illustrated. The basic function of this circuitry is
to accept the signal output generated by the photo-detector
array 20 which represents an image of the objects being
scanned, and to store this information in memory so that it
can be appropriately displayed on a video monitor. In the
preferréd embodiment, the circuitry,on command,scans a vert-
ical column of 480 pixels (one pixel for each diode in thedetector array) and then stops, this operation being succes-
sively repeated until all of the vertical columns making up
the desired picture have been scanned. In its operation,
various portions of the circuitry are selectively placed
in one of four separate states by means of state control
circuit 60. This circuit may have four control flip-flops
in its output circuit which are driven to the "1" state in
response to sync control signals fed to the state control
circuit. The four states controlled by the "ZERO", "SCALE~,
"READ" and "PAUSE" outputs of state control circuit 60 repre-
sent: (1) a 1-0-l operation during which the "0" offsets of
the detectors are measured and recorded; (2) a scale opera-
tion during which the full-scale brightness value of the
detectors are measured and recorded; (3) a read operation
during which the brightness values of the outputs of the
detectors (representing the images of the objects being
scanned) are measured and stored in a memory; and (4~ a
pause operation between successive read operations during
which the circuits are reset for a succeeding operation.
It i5 to be noted that the operation of the four states is
independent and all operations can be either simultaneous
or sequential as the command signals may dictate~ Syn-
chronization signals for timing the operating of the cir-
cuits is provided by clock generator 62 which is synchronized
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with the sync control signals.
Upon receiving a command signal for a read operation
from state control circuit 60, pixel address counter 65 is
loaded with a number corresponding to the number of photo-
diodes and proceeds to count down to zero at predeterminedtiming intervals. Pixel address counter provides an address
representing each of the diodes to pixel address translator
66, which translates this address signal into proper format
for driving the diode sampling circuit 24 (FIG 1). Thus,
the output of pixel address translator 66 drives buffer
driver 70 which in turn provides a control signal for the
diode sampling circuit so as to cause the sequential sampling
of the diode outputs in turn in accordance with the output
of the pixel address counter 65~
The signal corresponding to the sample~ output of
each of the photo-diodes is fed to image corrector unit 28
which operates to correct the signal in accordance with the
gain and offset of each selected photo-diode. The detailed
operation of image corrector unit 28 will be described fur-
ther on in the specification in connection with FIG 4~ The
output of image corrector unit 28 is converted from analo~
to digital form by analog/digital converter 3~. The output
of analog/digital converter 3Q is fed to bus, buffer and
interface circuits 78 which provide the input and control
for the ~em~ry units 32 (FIG 1~. The pixel address signal
is fed from pixel address counter to bus, buffer and inter-
face circuits 78 through memory address delay register 80
which operates to delay the address by one sampling cyele
of the diodes, this to match the delay in the sampling oper-
ation.
Column address counter 83 provides a signal to bus,
buffer and interface circuits 78 which in conjunction with
pixel address counter 65 determihe the location in the
memory of any pixel in the displayed image.
When the pixel counter reache~ "0", indicating that
all of the diode outputs have been sampled, the eircuits
are driven to a "pause" state by the output of the "pause"
output of state eontrol circuit 60~ The pixel address
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counter then resets itself to the full count representingthe number of diodes (480 in the illustrative embodiment~
so that it is ready for a succeeding operation.
At the end of each read operation, column address
counter 83 is either incremented or decremented by "1",
depending on the direction in which the column scanning
is golng, i.e., from the left to right or right to left.
Referring now to FIG 4, the operation of the image
corrector unit is illustrated. As has already been noted,
such correction is needed to correct the photo-detector
outputs to compensate for individual gain and offset dif-
ferences. A correction signal for each of the photo-detector
outputs is periodically read into memory circuits for use
to effectively compensate the output of each diode in ac-
cordance with the measured offset and gain thereof. Suchcalibration may be made as often as the situation may demand.
In the preferred embodiment, such calibration is automatic-
ally programmed whenever there is a time interval of two
seconds or more between objects passing through the x-ray
beam. When the output of state control circuit 60 is simul-
taneously commanding "ZERO" and "SCALE" operations, the
image corrector unit functions to generate the desired cor-
rection signal. During this time, the x-ray generator is
automatically turned off by the control circuits for the
measurement of offset.
The signal, S~i), from each of the photo-diodes is
as follows:
S(i~ = ID(i) + G(.i)B(i) (1)
where:
ID(i) is the dar~ current (or offset)
of the photo-diode
G(i~ is the x-ray conversion gain of
the photo-detector
Bti) is the brightness (or x-ray intensity)
at the photo-detector
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To perform its function, the image corrector must correct
S(i) to produce SC(i) where SC(i) is the direct analog of
the brightness B~i). This can be defined as follows:
Sc(i) = S(i) ID(i) (i) (2)
G(il
(see equation 1)
A problem is presented, however, in the measurement of
ID(i) and G(i), as the values of these parameters can
change with temperature and time. To solve this problem,
these parameters are measured periodically and stored in
a RAM. ID(ia~ the offset of the photo-diode, is measured
by reading S~i) with the x-ray source turned off such
that:
S(i) ID(i~ ~ G~ = ID(i) ~31
During zero operations, these offset values are stored in
offset memory 90. Also during this operation, offset
initializex 92 operates to set offset subtractor 93 to "0"~
The ~ain, G~i~, of each diode is measured by read-
ing the signal from each diode S(i~ with the x-ray source
turned on and no object in the path of the x-ray beam. At
this time, all the brightness values, B~i~, correspond to
full white. By subtracting the already measured offset
ID(i) stored in offset memory 90, the gain, G~i~, of each
diode may be measured, as indicated by the following
equation:
(il ID(i~ + B(i~G(i) ~ ID(i~
~4
= B ~ G ~ i ~
In view of the fact that there is no object in the x-ray
beam, BCi~ is egual to "1" and therefore S(i) = G(i)~
The arithmetic inverse of the diode gain G(i) is
calculated by means of the l/X converter 96 which is imple-
mented with an ROM look-up table. The output of l/X
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converter 96 is a gain correction factor, C(i), which is
stored in gain memory 97. During scale operation, ~ain
initializer 98 sets a gain factor of 1/4 into gain mul-
tiplier 100 which multiplies the output of the gain memory
by this factor when it is fed to analog/digital converter
30. The gain setting is so reduced to avoid saturation
of the photo-detectors during the scale operation. Other
gain factors could be used but it has been found in the pre-
ferred embodiment that a factor of 1/4 operates quite
satisfactorily. In this manner, offset errors and gain
differences in the diodes are compensated for so that they
will not affect the imaging.
The operation of the system of the invention is sum-
marized as follows: The array is scanned with the x-ray
beam off. The dark current for each diode is read, 0.0 is
subtracted, the value is multiplied by 1/4, converted to
digital form, and finally stored in the appropriate spot in
the offset memory. When all dark currents have been stored,
the x-ray beam is turned on. Without any parcel in the
x-ray beam, the array is scanned a second time. The full-
scale signal current is read from each diode, the offset
of that diode subtracted, the value multiplied by 1/4,
converted to digital form, and stored in the appropriate
spot in the gain memory.
In operation, the array is continuously scanned.
Each signal value read from the diodes is first corrected
for dark current, and then for diode gain before being con-
verted to a digital number stored in memory.
The flow chart of FIGS 5A and 5B more precisely
defines the operation of the preferred embodiment. The
various terms used in the flow chart are defined as
follows:
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11
Sync - Signal starting each input scan line.
Sync synchroniæes scanning to conveyor
motion.
Zero FF - Flip flop indicating that ~ zero
operation is in progress (when set).
During a zero operation the offset
currents of the photo-diodes are
individually measured and stored in
the offset memory.
Scale FF - Flip flop indicating that a scale
operation is in progress (when set~.
During a scale operation the signal
level representing white (full bright-
ness) of each diode is measured. A
gain correction factor which is the
inverse of the white level is stored
in the gain memory for each diode.
Read FF - Flip flop indicating that a read
operation is in progress (when setl.
During a read operation brightness
values (from the sensor~ are stored
in the Image Memory.
Input Signal - Signal for
the current level of the presently
selected photo-diode. The diode
selected is determined by the Pixel
Counter.
Sl - Signal after offset correction has
been applied.
S2 - Signal after gain correction has been
applied.
Offset - Memory holding 480 offset values cor-
responding to the offset currents of
the photodiodes. Offset (i) repre-
sents the stored offset of the ith
photo-diode.
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12
Galn - rlemory holding 480 gain correction
values corresponding to the inverse
(l/X) of the full brightness levels
of the photo-diodes. Gain (i~ repre-
sents the stored yain correction for
the ith photo-diode.
Image Memory - A memory array storing one complete
television frame. The image is stored
as an array of picture elements
(pixels). Image memory (X,Y~ indicates
the pixel (picture elementl in column
X and line Y where:
0 < X ~ Sll and 0 < Y ~ 479.
Pixel Counter - Counter counting pixels in a column.
Counter determines which photo-diode
is selected.
Column Counter - Counter counting columns in an image.
Used only to address the image memory.
It is to be noted that the system of the invention
can also be used for non-destructive testing and for inspec-
tion of materials and products such as, for example, food,
manufactured products, etc.
While the dnvention has been described and illustrated
in detail, it is to be understood that this is by way of
example only and is not to be taken by way of limitation,
the spirit and scope of the invention being limited only
by the terms of the following claims.
