CA1090482A

CA1090482A - X-ray inspection system for security screening application

Info

Publication number: CA1090482A
Application number: CA283,327A
Authority: CA
Inventors: Robert J.L. Beland; Czeslawa Stefanski; Wlodzimierz K. Stefanski
Original assignee: Picker Canada Ltd
Current assignee: Philips Electronics Ltd Canada
Priority date: 1977-07-22
Filing date: 1977-07-22
Publication date: 1980-11-25

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to an X-ray inspection system for security screening applications of the type which are employed at aircraft terminals to determine if dangerous items are being carried in passenger luggage. The system in accordance with the invention includes a detector station having a linear array of phototransistors, covered with a fluorescent screen, disposed on one side of the conveyor belt which carries the luggage past the detector station. On the other side of the belt, and disposed in parallel arrangement with the array, is an X-ray head, and objects passing between the array and the source will alter the density of the rays emitted by the source so that an X-ray picture can be obtained, as well known in the art. The inventive system uses such an array to eliminate the requirement for optical systems of present screen-ing apparatus. The disclosure also teaches a mode for operating the array which eliminates difficulties due to wide ranging differences in the characteristics of the individual photo-transistors, and it also teaches a mode of operating each of the phototransistors to increase the outputs thereof.

Description

~lV9~

This invention relates to an X-ray detection system and to various elements in the system. A detection station, for use in such a system, includes, in accordance with the invention, an array of detection devices and a novel mode of operation of the array which overcomes difficulties due to variations between individual detection devices making up the array. The invention further relates to a circuit configuration for individual ones of the detection devices.
In a further aspect, the invention also relates to a display system for use with a detection system above mentioned, and to an alarm system, and a method of processing data with the novel alarm system.
It is known in the art to use X-ray heads and associated equipment for security screening applications such as weapon detection systems installed in a large number of airports. The prior art systems have ~et with varying success depending on the following factors~
A equipment resolution and reliability B. operator training and effectiveness C. volume (per unit of time) of objects inspected.
Shortcomings exist in present models of equipment in ; ~-. .
the areas of system resolution, viewed image reproduction and operators control functions. These factors contribute to ; -~
operators fatigue, thereby reducing attention span. ;
In addition, because of the large area of coverage required, the X-ray inspection systems using conventional fluorescent screens are both large and bulky. Typical systems usually employed a complicated and delicate optical system, including mirror, lenses, light amplifiers, and a low level light camera. Further, the systems require elaborate shielding to contain the X-radiation, which, as a norm is quite high because of large X-ray coverage area - typically 66 cm by 80 cm.

:' ' ~ ' " .' .- , . .: , r~

Finally, tube-type storage techniques are used which dictate complex set-up procedures (over 100 adjustments in some cases) and high level maintenance. Automatic detection and alarm functions are difficult to employ ~ecause of the analogue in-formation processing.
It is therefore an object of the invention to provide a system which overcomes the above disadvantages and other dis-advantages as will be readily apparent from the following.
It is a further object to provide a system which is less bulky and smaller than presently available systems.
It is a still further object of the invention to provide a system which is less complex than presently availa~le systems.
It is a more specific o~ject of the invention to provide a system which does not require an optical system that is, mirrors, lenses, light amplifiers and a low-light level camera.
It is a further more specific object of the invention to provide a system which uses digital circuits and digital techniques.
It is a still more specific object of the invention to provide such a system which employs, at the detector station, a -;~
linear array of detection devices.
It is a still more specific object of the invention ~ -to provide a method for operation of the linear array which `
overcomes difficulties due to variations between individual detection devices making up the array.
It is a further o~ject of the invention to provide a novel circuit configuration for individual ones of the ;~
detection devices.
Further objects and advantages of the invention will be apparent from the following description.
In accordance with the invention a detection system comprises: a detection station including a linear array photo-sensitive detector disposed in parallel relationship with a ., .

. , .

f~

source of X-radiation on either side of a path along which are moved items to be inspected, means for processing output data from said detector and means for comparing processed data obtained when an item is passing said detection station with processed data obtained when no item is passing said station whereby to obtain contrast information relating to said items.
The detector preferably comprises a single column equally spaced of phototransistors, the outer surfaces of said phototransistors being covered by a fluorescent screen which is in physical contact with said outer surfaces.
From a different aspect, the invention relates to a linear array photosensitive detector comprising: a single line -of equally spacea photosensitive devices; a fluorescent screen covering and being in physical contact with the free surfaces of said photosensitive devices.
From a still further aspect, the invention relates to - -a circuit configuration for a phototransistor which includes an emitter terminal and a collector terminal, and comprising: a -~
first source of power connected to said collector terminal; and means for connecting said emitter terminal, in one position of said means, to ground through an impedance, and for connecting ; said emitter terminal, in a second position of said means, to a second source of power; whereby, when said means is in its one position said circuit is in a charge mode and when said means is in its second position, said circuit is in a discharge mode.
The invention also relates to a method of conditioning a phototransistor, having a collector terminal connected to a first source of power and an emitter terminal, said method com-prising: connecting said emitter terminal to ground through an impedance for a first period of time; and connecting said emitter terminal to a second source of power for a second period of time, whereby to put said phototransistor in a charge mode during said ~: . ' ,. : ,'' ~V~

first period of time, and in a discharge mode during said second period of time.
'":
The invention still further relates to a method of obtaining contrast information from an array of a plurality of phototransistors whose outer surfaces are covered with a fluo-rescent screen, said method comprising: directing a source of ~
X-radiation at said screen when there is no object in the path ~-between said source and said screen, and measuring the output -~
: -:
of said phototransistors, the intensity of radiation of said source being varied through a plurality of predetermined levels, `
whereby to obtain reference output levels of said photo-, - transistors corresponding with respective ones of said plurality of predetermined intensity levels, directing said source of radiation at said screen when objects may be in said path, the intensity level of said source being adjusted to only one of said predetermined levels, whereby to obtain object output levels, and comparing said object output levels with said reference output levels one at a time; whereby to obtain said ~ `
contrast information, The phototransistors are preferably arranged in a linear array, wherein the output of said phototransistors is , -~
~- measured in the sequence of said phototransistors when there is no object in said path, whereby to obtain respective reference -levels associated with respective ones of said phototransistors, ~ -and wherein the output of said transistors is measured in the ;~ sequence of said transistors when an object may be in said path, the object outpu~ levels each individual phototransistor being compared with the respective reference levels associated with said each individual phototransistor to obtain contrast informa-tion associated with said each individual transistor.
The invention will be better understood by an examina-tion of the following description, together with the accompanying , .

,~... . .. . .

4~3~

drawings, in which:
Figure 1 is a block diagram of the system in . .
accordance with the invention;
Figure 2 is a somewhat schematic diagram of a perspective view of the detection station;
Figure 3 is a side view of the detection station, -Figure 4A is a circuit diagram of a circuit configuration of individual ones of `
the detection devices;
Figure 4B is an equivalent circuit diagram of ~-the circuit in figure 4A in the charge mode;
Figure 4C is an equivalent circuit diagram of -~
the circuit in figure 4A in the discharge . .: -:
mode, Figure 5 is a flowchart of the system operation; ~.
and 20 : Figure 6 is a more detailed drawing of the alarm system components. ~
Referring to figure 1, a system in accordance with ~:
the inventlon comprises a detection section 1, a logic circuit or microprocessor section 3, a video section 5 and an alarm section 7. The detection section includes a linear array photo-sensitive detector 9 of detection devices which will be described in greater detail below. The output of the array is fed to the amplifier sample and hold 11 whose output is, in turn, fed to :~
: the analogue to digital converter 13. Individual ones of the :~ 30 detection devices are selected, for reference calibration and ;- : .
operation purposes, by selector unit 15. X-rays are directed at the array by X-ray head 17, and the intensity or density of ~,. . .

d~

the output of the X-ray head is controlled by X-ray control 19.
The logic or microprocessor circuit 21 consists of a logic circuit unit or a microprocessor. When a microprocessor is used, the program is stored in ROM 23. Reference data con-cerning each of the individual detection devices is stored in RAM(A) 25, and object data from each of the individual detection devices are temporarily stored in a buffer 27 which may comprise a RAM(B). Section 3 may also include a direct memory access device (DMA) 33 which is used for storing information to be displayed on the video display as will be descri~ed below.
The video section consists of a temporary storage 35 ;~
which may comprise the RAM(C). The output of 35 is fed to digital to analogue converter 37 and thence to the TV monitor 39. The control unit 41 and the synchro unit 43 generate the necessary timing signals for memory C and synchronize the output of the memory with the operation of the TV receiver which constitutes the monitor 39.
As can be seen in figures 2 and 3, the linear array of detection devices 9 is disposed on one side of a conveyor belt 49 while the X-ray head 17 is disposed in parallel arrangement with 9, on the other side of the conveyor belt. As best seen ; in figure 3, the linear array 9 comprises a single straight line of photosensitive devices 51. Preferably, the photosensitive devices are phototransistors. In a preferred embodiment, the linear array consists of 256 of such phototransistors. Disposed on the free surface of the phototransistors, and in physical contact therewith, is a screen 53 of such a nature which, when ~ombarded with X-rays on the free surface thereof, will produce light energy on the surface adjacent to the phototransistors.
This screen is preferably a fluorescent screen.
The X-ray head is of the type well known in the art and fitted out with a collimated slot which is parallel and .: .

.'''. . .

opposite to the line of the fluorescent screen. It is desirable that the conversion efficiency of the screen should be high and that the spectro e~citance of the screen should match the spectro response of the phototransistors making up the linear array. It has been found that the RE-228-27 screen and the ZnCdS yellow-green screen provide good spectro matches for the Fairchild FPT610 phototransistors. However, the ZnCdS screen is preferred because of its higher efficiency.
Each of the phototransistors making up the array i9 arranged in circuit configuration as shown in figure 4A. To understand why a novel circuit configuration is required in this use, it is pointed out that current methods for detecting X-rays with semi-conductors include charge couple devices (CCD) sensors, discrete photodiodes and phototransistors or semiconductor detectors of X-ray such as described in application note No. 101, of Reticon Corporation, 910 Benicia Avenue, Sunnyvale, California, The major disadvantage of semiconductor X-ray detectors is the variation of characteristics between elements and the low output signal.
The use of CCD sensors coupled to a fluorescent screen ~-for detection of X-ray provides good resolution of sensitivity.
However, an optical system is required for optimum light transfer and hence there is an increase in cost. Ordinary photodiodes and phototransistors used in the normal mode of operation in a - given special arrangement can eliminate the need for expensive optical systems, but suffer from poor sensitivity especially at low radiation levels. It is possible to improve the overall sensitivity by operating the phototransistors in the special mode in accordance with the instant invention. In figure 4A, 55 comprises a means for changing the mode of the phototransistors from the discharge to a charge mode or vice versa. In figure 4A, this means has been illustrated as a mechanical switch whose ~V~30'~

wiper arm moves to contact 1 to place the phototransistor in a charge mode, or to contact 2 to place it in a discharge mode.
In actual practice, a semiconductor device is used.
In operation, the configuration works as follows re-ferring to figures 4B to 4C:
Assuming that capacitance Cp is discharged initially and switch 55 is placed in position 1 for time Tl, the capacitance charges to a value EC-VBE and, at the end of time Tl, the switch is connected to position 2 for a time T2. The base emitter junction is reversed bias when the switch is in position 2, and the capacitor discharges with a current which is proportional to the photocurrent If and to the dark current ;~ ~;
Io. The value of If depends on the intensity of light incedent on the phototransistor junction. During time T2, the capacitor discharges partially to a value Vc. If the switch is now connected back to a position 1 for time Tl, the capacitor will be charged by a current proportional to the amount by which the phototransistor was discharged on the previous discharge cycle. Hence, the amplitude of the charging current is proportional to the time ~2 and also the currents If and Io.
If If is much greater than Io, the effect f Io can be ignored.
Thus, for a given value of ~2, the amplitude of charging current is proportional to If and hence to the intensity of light. Thus, in order to measure the intensity of light on the phototransistor, it is merely necessary to measure the charging current of the phototransistor at the beginning of a charging cycle.
With the fluorescent screen, the light intensity is proportional to the intensity of the X-rays bombarding the screen. Thus, by having a measure of light intensity, one also has a measure of the intensity or density of the X-rays which bombarded the fluorescent screen.

- 8 _ )48~

With the above circuit, by selecting a proper T1 and T2, it is possible to obtain large signals even for small intensity of X-radiation and hence small If.
Because the characteristics of the 256 phototransistors are not uniform and can be widely diversent, it is necessary to apply a special technique for overcoming the effect of such wide variations. In accordance with the invention, the characteristics of each individual phototransistor is stored in a memory. Object data from individual phototransistors are then compared with the reference data, as above obtained, to determine contrast readings -at the individual phototransistors for a given time.
In a preferred embodiment of the invention, the response of each individual phototransistor, with no object in the field, is measured at 16 different levels of X-ray intensity or density. The steps of the calibration process are illustrated in the top five blocks of figure 5. As can be seen, -~
in accordance with this procedure, the X-ray head is first adjusted to a particular level of the 16 selected levels. In actual practice, the calibration is started at the highest level.
A first phototransistor is selected and its response is loaded into a memory location of RAM(A) 25 of figure 1. The logic -circuits then test to see whether all of the phototransistors have been calibrated at this level. If they have not, then a further phototransistor is selected and calibrated. When all -256 phototransistors are calibrated at this level, then a - second level is chosen. The circuits then test to determine whether all 256 phototransistors have been calibrated on all 16 levels. If they have not, then the X-ray head is adjusted to a different level and all 256 phototransistors are calibrated at this different level. The process continues until all 256 phototransistors have been calibrated on all 16 levels, and ~S

this calibration information is stored in RAM(A) 25.

_ g _ ~,:

'.: . ' ,. ' ~U'~ B~

In operation, it is known that the intensity level of X-rays is reduced by known factors depending on the character-istics of an object to which the X-ray levels pass. Thus, it may be known that X-rays set to the maximum level will be ; ~
reduced to, say, level 8 of the calibrated levels when the X- ~ -rays are passed through a metallic object. Object data from individual phototransistors are then compared with the levels in memory 25, and if a phototransistor has an output which is equal to level 9 or greater of the calibrated levels, then it -is known that this X-ray has not passed through a metallic ~;
object. On the other hand, if the level detected by the photo-transistor is 8 or less, then it is known that the X-ray has passed through a metallic object. In addition to providing the above type of information, comparing the object data with ~ ~ -the 16 preset levels also provides contrast information useful ~
for driving a video display. ~;
The steps of the operational process are illustrated in the lower six blocks of figure 5. As can be seen, the X-ray head is first set to its maximum level. It remains at its maximum level throughout operations. A particular phototranslstor is selected by unit 15 in figure 1. The response of the photo-transistor during operation is then compared with the 16 calibrated levels for that particular phototransistor using a ~-binary search method. In this way, the contrast information is obtained and this contrast information is then loaded into RAM(B) 27 of figure 1.
The phototransistors are selected sequentially and after all 256 phototransistors have been selected, the first phototransistor is once again selected. The contrast information is then transferred to RAM(C) 35 for use in the video display during vertical blanking time of the TV monitor. The information in RAM(C) is read during vertical trace time of the TV monitor after being converted to analogue form, and displayed on a standard TV monitor.
The purpose of the alarm system is to detect dangerous items such as weapons in a travelling bag etc. when the system in accordance with the invention is used for security screening applications at airports or the like. When such a dangerous item is detected by the system, then the system will provide an audible alarm to caution the operators to look more closely at the television screen.
The characteristics by which the dangerous item can be determined are as follows:
1. The factor by which X-radiation is reduced when passing through a material o~ which the object is made, i.e., steel for guns. This characteristic is determined, at each photo- -transistor, by comparing object data with reference data as will be described below.

2. The measurements of the dangerous item. As the dangerous item can be disposed in any attitude within a suitcase, and as only two dimensions are examined with the system under consideration in this description, it is necessary to select the two smallest measurements in two dimensions of the dangerous item. It is also contemplated, in accordance with the invention, to provide a three dimensional picture by disposing a further linear array at right angles to the present linear array, i.e., parallel to the conveyor belt and across the top thereof and extending in a line between the present linear array and the X-ray head with a mating X-ray head disposed under the conveyor belt and in parallel arrangement with the second linear array. If a three dimensional system is produced, then a three dimensional picture will be provided and different standards for alarm initiation would be evolved. However, in the present system, the measurements are taken along the x and y dimensions of figure 2 ~u~

and if an object which reduces the X-ray intensity by the known amount displays measurements in two dimensions being equal to at least the smallest measurement in any one dimension of the dangerous object, then an alarm signal is generated.
To pro~ess and analyse the data, a system as illustrated in figure 6 may be used. In figure 6, 57 is a comparator which compares object data from RAM(B) with reference data from pro-grammed switch in 29. The data is compared to determine whether the object data falls within the class of intensity or density associated with the dangerous object. If it falls within this class, then a "yes" signal is provided at the output of the comparator. The output of the comparator is fed to multiplexor 59 which routes this output to the appropriate address location in memory matrix 61. The size of the memory matrix is chosen so that it can accommodate the number of phototransistors in the linear array as well as the pertinent dimensions of the dangerous weapon In the preferred em~odiment, the memory matrix is a `
256 by 256 matrix, with means 63 for adjusting one dimension of the matrix. This dimension is adjusted by controlling the de- -multiplexor 65 which removes data from the memory matrix in sequence and applies it to counter 67. The output of counter 67 is fed to counters 69 and 71, and the output of these counters is fed to alarm logic 73 which drives alarm device 75 ;;
as will be described below.
To understand what data is stored in the rows and columns, representing the different memory locations, of the memory matrix, we will consider for a moment how objects passing by the detection station are scanned by the detection station.
As above mentioned, the phototransistors are read in sequence starting say, from the top phototransistor and continuing in sequence down to the bottom one. The scan is then continued at the top transistor and worked downwardly again.

- 12 _ - ~lV'~f~

When an object is in the detection station, the infor-mation that is being obtained from the phototransistors is the amount by which X-ray radiation is reduced along a somewhat vertical line on this object. The line is not perfectly vertical in view of the fact that the object is moving while the scanning is going on so that the point at the top of the line is somewhat ahead of the point at the bottom of the line. Thus, the line is really vertically upright but slanted in the direction of motion of the object.
When a second scan is taken, this represents the X-ray reduction due to a second somewhat vertical line along the object, and a third scan results from a third somewhat vertical line along the object, etc., etc.
In the memory matrix, the data in any column represents the output of the phototransistor during a time Tl, i.e., corresponding to a vertical line of the object. As will be appreciated, the time ~1 of row 1 is not identical to the time Tl of row 200. However, the time Tl of row 1 stands in the same relationship to T2 of row 1 as the time Tl of row 200 stands in relationship to the time T2 of row 200. The rows re-present the output of any particular phototransistor at different times, i.e., at different horizontal points along the object.
Thus, the vertical dimensions of any object in the detection station will be traced along the columns of the memory matrix, while the horizontal dimension will be traced along the rows.
As will be appreciated, the multiplexor will insert data into the memory matrix to insert data into the top left-hand corner of the matrix and then to continue downwardly along the first column. After the first column is filled up, the ~0 memory matrix will then insert the next piece of data in the top location of the second column and then continue down the second column. This procedure is continued until the requisite number ~ ~ 9 ~ J

of columns are filled up. At which point, the multiplexor will insert the further piece of data into the top left-hand corner and then continue down on the first column. An alarm condition is considered detected when substantially all of the memory loca-tions in a rectangular block have been filled with `'yes" signals from the comparator 57. A "yes" signal i9 provided from the ~-comparator when the object density is equal to or above/below the reference intensity as determined for each particular photo- ~ ~
transistor for the characteristic 1 above described. ~ -The size of the rectangle is determined as follows: ~
With the selected dimension, it is known that a given ~ -number of phototransistors will be covered by the object when such an object is passing through the detection station. The number of phototransistors is determined simply by dividing the pertinent dimension by the spacing between the phototransistors.
Thus, the pertinent dimension is 75 mm and the spacing between ~
phototransistors is 2.5 mm, then 30 phototransistors would be -covered each time the dangerous item passes through the detector ~ -station. Thus, 30 adjacent memory locations in the same column would have to be filled.
To determine the number of adjacent memory locations which must be filled in a row of the memory matrix in order to consider that a dangerous item has passed, we have to take into ;
account the dimension of the item, again, as well as the speed -with which the conveyor is travelling, as well as the scanning time. Such a mathematical calculation is well within the capabilities of one skilled in the art so that a further dis-cussion will not be had at this point. Instead, for purposes of description, let us assume that 60 adjacent memory locations in any row must be filled in order to consider that a dangerous item has passed.

In view of the fact that momentary fault can occur - 14 _ ... .

in the phototransistors or in the system, it is conceivable that a weapon will pass and only, say 55 of 60 adjacent memory loca-tions in a row will be filled. In order to allow for this type of error, an alarm signal is generated at some number less than the total numbers strictly required. In the present example, we will select 5S adjacent memories in any row as sufficient to give a signal and 27 adjacent memories in any column sufficient to give the alarm signal. With this arrangement, 63 will adjust the demultiplexor and the memory matrix so that only the first 60 columns will be active.
Using the above examples, the system illustrated in figure 6 operates as follows:
When a piece of data is provided from the comparator to the multiplexor, the multiplexor will transfer that data to the ~ -first open row in the column presently being filled. Between the time that the data is being transferred to the first open row ~-and the time when the next piece of data is transferred to the ~-second open row of the column being filled, the data in the first open row, along all columns thereof, is examined and the number of yeses in the open row is counted. If the number of yeses in the first open row is equal to or greater than 55, then counter 67 will provide an output to counter 69. If the number of yeses is less than 55, then counter 67 will provide an output to counter 71. In actual operation, the ~irst yes~output from counter 67 to 69 will only initiate both counters 69 and 67 to be con-ditioned for further counting. Thus, the first yes output to 69 is not recorded as a count, so that only a count of 26 will have to be reached in 69 before an alarm signal is generated. ~-The multiplexor will keep feeding the next piece of data to the next empty row,i-and the empty rows thus filled will continue to be examined, and each time a row is examined, there will be an output from the demultiplexor to the counter 67, and :

an output from the counter 67 to one of counters 69 or 71. If a count of 4 i9 reached on counter 71 before a count of 26 is reached on 69, this indicates that in the 30 previous rows it would not be possible to have 27 '`filled" rows. Thia is because 4 of the rows are "unfilled" so that the greatest number that ~
could be filled in the previous 30 rows is 26. Thus, when the -; -count of 4 is reached on 71, an output signal from 71 is pro-vided to alarm logic 73, and both counters 69 and 71 are reset.
If a count of 26 is reached on counter 69 before a count of ~ is reached on 71, this indicates an alarm condition as, in the pertinent lock of 60 columns by 30 rows of memory location, there are at least 55 times 26 filled memory locations. --In the above description, it will be appreciated that"filled" means a memory location having a "yes" output from comparator 57.
When an alarm condition is detected, the alarm logic will then generate a signal to trigger the alarm 75 which is ~ ;~
preferably an audible signal. It is also contemplated that the `;-alarm signal will generate a box trace around the detected item ~-on the TV screen.
In a preferred embodiment, the linear array has a `
length of 65 cm and a width of approximately 2.5 mm. Thus, ~ ~ ~
the detector strip is also approximately 2 5 mm, and the X- ~ ;
radiation from the X-ray head may be collimated to a slot i app~oximately 65 cm long by 2.5 mm wide.
Each phototransistor in the array is selected every ~ -16.66 mSCC. for a period of 60 usec.
Although a specific embodimènt has been above !
described, this was for the purpose of illustrating, but not limiting, the invention. Various modifications, which will come readily to the mind of one skilled in the art are within the scope of the invention as defined in the appended claims.

- 16 _ .... . .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A detection system comprising:
a detection station including a linear array photo-sensitive detector disposed in parallel relationship with a source of X-radiation on either side of a path along which are moved items to be inspected;
means for processing output data from said detector;
and means for comparing processed data obtained when an item is passing said detection station with processed data obtained when no item is passing said station;
whereby to obtain contrast information relating to said items;
wherein said detector comprises a single column of equally spaced phototransistors, the outer surfaces of said phototransistors being covered by a fluorescent screen which is in physical contact with said outer surfaces.

2. A system as defined in claim 1 wherein said source of X-radiation comprises an X-ray head.

3. A system as defined in claim 1 wherein said means for processing comprises:
a sample and hold amplifier connected to the output of said detector;
an analogue to digital converter connected to the output of said amplifier;
and logic circuits, with associated memory means, connected to the output of said converter.

4. A system as defined in claim 1 and further comprising display means for displaying said contrast information, said display means comprising:
memory means for storing said information in digital form;
digital to analogue converter means connected to the output of said memory means; and a television monitor connected to the output of said converter.

5. A system as defined in claim 1 and further comprising an alarm system for presenting an audible alarm when a dangerous item is detected and comprising:
pattern recognition means for recognizing an alarm condition pattern; and an alarm signal generator for generating an alarm signal when said alarm condition pattern is recognized.