Process and arrangement for reconstituting the thermal flux absorbed by a pyroelectric target

Process and arrangement for reconstituting the signal representing the thermal flux absorbed by a pyroelectric target, the arrangement being used in infrared camera tubes. It comprises means for correcting the signal Sn for reading a point of co-ordinates x, y of the target, during a frame of order n, comprising means for calculating a first component representing the Laplacian of the signal Sn and means for summating the signal Sn with the first component. The correcting means deliver a corrected signal. Means for generating a complementary signal are connected to the output of the calculating means and comprise means for generating a signal representing the temperature .theta..sub.n-1 of the point of co-ordinates x, y during the reading of that point in the course of a frame of order n-1 and a second component representing the Laplacian of that temperature. Means for summating the complementary signal with the corrected signal deliver the reconstituted signal.

This invention relates to an arrangement for reconstituting the signal 
representing the thermal flux absorbed by a sensitive surface using the 
pyroelectric effect, such as for example that of an infrared camera 
system, and to the camera system incorporating this arrangement. 
In camera systems using sensitive surfaces of the type in question, a 
pyroelectric target or mosaic of pyroelectric detectors, an electrical 
polarisation profile and, hence, an induced electrical charge profile, 
i.e. an electrical potential profile, is created by the incident radiation 
absorbed by the sensitive surface over the extent thereof. At each point 
of the sensitive surface, the potential profile is sequentially analysed 
by destructive reading, corresponding to an injection of compensating 
charges, or by non-destructive reading in the absence of injected charges. 
In the case of destructive reading, the signal collected at a point of the 
sensitive surface, equal to the compensation charge injected, is 
proportional to the variation in temperature at that point between two 
analysis instants. 
In the case of non-destructive reading, the information stored in the form 
of an electrical potential degrades at the thermal equilibrium by 
dielectric relaxation within the pyroelectric material. For each type of 
analysis, the temperature at each point of the sensitive surface has to 
undergo variations as a function of time induced either by a relative 
movement between the subject and the camera system or by a sequential 
modulation of the flux of the incident radiation. The signal collected 
during the reading of each point of the sensitive surface is proportional 
to the temperature variation at that point between two analysis instants. 
This type of analysis is attended by the disadvantage that it does not 
faithfully reconstitute the thermal flux due to the incident radiation. In 
particular, the image of a hot point moving over the sensitive surface is 
followed by a trail due to the cooling of the sensitive surface after 
passage of the image. In addition, camera systems using sensitive surfaces 
such as these are limited in terms of spatial resolution by lateral 
diffusion of the heat within the sensitive surface. 
The present invention enables the disadvantages referred to above to be 
obviated by using a process for reconstituting the signal representing the 
thermal flux absorbed by the sensitive surface of a camera system using 
the pyroelectric effect in which, during a periodic reading by successive 
frames of the sensitive surface and for each point of co-ordinates x, y 
thereof, the reading signal Sn of a point of co-ordinates x, y is 
corrected during the reading of the point of co-ordinates during a frame 
of order n by summation with the signal Sn of a first component 
representing the Laplacian of the reading signal, enabling a corrected 
reading signal to be generated, and a complementary signal is added to the 
corrected reading signal, said complementary signal being formed by a 
signal representing the temperature of the point of co-ordinates x, y at 
the reading instant during the frame of order k.ltoreq.n and by a second 
component representing the Laplacian of that temperature. 
The invention also relates to an arrangement for reconstituting the signal 
representing the thermal flux absorbed by the sensitive surface of a 
camera system using the pyroelectric effect, comprising means which 
periodically read said sensitive surface by successive frames and which 
deliver a reading signal Sn of a point of co-ordinates x, y during the 
reading of this point during a frame of order n, the arrangement 
additionally including means for correcting the signal Sn comprising means 
for calculating a first component representing the Laplacian of the signal 
Sn and means for summating the signal Sn with said first component, said 
correcting means delivering a corrected signal at the output of the 
summation means, means for generating a comlementary signal connected to 
the output of said calculating means and comprising means for generating, 
on the one hand, a signal representing the temperature .theta..sub.n-1 of 
the point of co-ordinates x, y during the reading of the point of 
co-ordinates during a frame of order n-1 and, on the other hand, a second 
component representing the Laplacian of said temperature, and means for 
summating the complementary signal and the corrected signal and delivering 
the reconstituted signal. 
The arrangement and process according to the invention are applicable to 
thermal television camera tubes comprising a sensitive surface formed by a 
pyroelectric target from which the information is sequentially extracted 
in the form of current or voltage, and in particular to camera systems of 
which the sensitive surface is formed by a mosaic of pyroelectric 
detectors.

According to the invention, the instantaneous signal Sn for a point of 
co-ordinates x, y issuing from the reading during a frame of order n is 
compared and corrected with the signals issuing from the reading of this 
same point during preceding frames. The signal Sn read at a point of 
co-ordinates x, y during a frame of order n is associated with the thermal 
flux actually absorbed by the sensitive surface at this point between two 
reading instants during two consecutive frames, for example a frame of 
order n and a frame of order n-1, by the following relation (I): 
##EQU1## 
wherein: t represents the variable time, 
T represents the period of a frame between two analysis instants, 
.tau. represents the thermal time constant of the pyroelectric target, 
D represents the lateral thermal diffusion constant of the target Laplacian 
operator 
##EQU2## 
H (x,y,t) represents the power density absorbed at the point (x,y) at the 
instant(t), 
C represents the specific heat per unit volume of the pyroelectric 
material, 
e represents the thickness of the sensitive surface, 
.theta..sub.n-1 =.theta. (x,y, (n-1) T), the temperature of the surface at 
the point of co-ordinates x, y at the instant (n-1) T. 
The above relation (I) is obtained assuming a linear variation in the 
temperature .theta. (x,y,t) of the point x, y between the instants (n-1) T 
and nT separating two reading instants of the point of co-ordinates x, y 
during two successive frames. 
The expression 
##EQU3## 
which is the integral of the flux absorbed between two successive analysis 
instants, represents the useful information and the term Sn proportional 
to .theta..sub.n -.theta..sub.n-1 represents the read signal. 
The assumption of a linear variation in the temperature .theta. (x,y,t) 
between the instants (n-1) T and nT is only strictly verified in the case 
of low spatial frequencies, i.e. frequencies below two pairs of lines per 
millimetre for a target of triglycine sulphate or TGS for the usual 
analysis standards. The characteristic variation times of this 
temperature, which are inversely proportional to the square of this 
spatial frequency, are long in relation to the frame period T of 
approximately 20 milliseconds for a tube with a pyroelectric target. 
In the opposite case of high spatial frequencies, the error introduced by 
the non-linearity of the variation in the temperature .theta. (x,y,t) is 
compensated on average by the cooling phase of the point of co-ordinates 
x, y following its preceding heating phase. 
The useful information or signal representing a thermal flux absorbed by 
the sensitive surface is reconstituted in two steps. 
A first step consists in making a correction to the signal Sn read during a 
frame of order n by algebraically adding to the read signal Sn a first 
component representing the Laplacian of that signal. This correction is 
analogous to an opening correction effected by a filter favouring the high 
spatial frequencies. 
A second step consists in adding to the corrected signal Sn a complementary 
signal formed by a signal representing the temperature .theta..sub.n-1 of 
the point of co-ordinates x, y during the reading thereof during a frame 
of order n-1 or at the instant (n-1) T and by a second component 
representing the Laplacian of that temperature. 
The process by which the useful information is reconstituted may be carried 
out by reconstituting the signal representing the thermal flux from 
signals Sn-1, Sn-2 . . . Sn-p, the temperature .theta..sub.n-(p+1) of the 
point of co-ordinates x, y at the reading instant during the frame of 
order k=n-(p+1) below n and the Laplacian of these quantities by virtue of 
the fact that the temperature .theta..sub.n-1 is expressed by 
.theta..sub.n-1 =Sn-1+Sn-2 . . . Sn-p+.theta..sub.n- (p+1). 
The process may be carried out by reconstituting the signal representing 
the thermal flux from the read signal Sn and the temperature .theta..sub.n 
of the point of co-ordinates x, y read during a frame of the same order 
because, due to the linear relation between Sn, .theta..sub.n and 
.theta..sub.n-1 so that Sn=.theta..sub.n -.theta..sub.n-1, it is possible 
to express .theta..sub.n-1 as a function of Sn and .theta..sub.n, the 
relation (I) thus becoming: 
##EQU4## 
This definition of the integral of the absorbed flux is obtained by adding 
to the corrected reading signal (1-(T/2)(1/.tau.-D.DELTA.)) Sn a 
complementary signal representing the temperature .theta..sub.n of the 
point of co-ordinates x, y at the reading instant during the frame of the 
same order k=n and by a second component representing the Laplacian of 
that temperature. However, these last two functions, which define the 
integral of the flux absorbed from quantities other than Sn and 
.theta..sub.n-1, do not provide for precise separation of the correction 
steps of the signal, said steps corresponding to the aperture correction 
mentioned above during the definition as a function of Sn and 
.theta..sub.n-1. 
In one preferred embodiment of the invention, the first component 
representing the Laplacian at the point of co-ordinates x, y of the signal 
Sn is obtained by using a signal representing the Laplacian of the signal 
Sn at the point of co-ordinates x, y and obtained by comparison of the 
signal Sn at at least four comparison points of the sensitive surface 
separated by a distance d on either side of the point of co-ordinates x, 
y. These four points are preferably situated in twos symmetrically in 
relation to the point of co-ordinates x, y. The directions joining two 
symmetrical points are respectively parallel to the axes of co-ordinates 
of the pyroelectric target. A larger number of comparison points may also 
be used with a view to obtaining a more precise signal representing the 
Laplacian operator of the signal Sn without departing from the scope of 
the invention. In the preferred embodiment of the invention, the 
separation by a distance d of the comparison points enabling the signal 
representing the Laplacian of the signal Sn to be obtained is obtained by 
using a number of delayed signals Sn, of predetermined delay, 
corresponding to the various comparison points of the sensitive surface. 
Thus, assuming an analysis of the pyroelectric target by lines of the 
standard television type, a vertical translation by a distance d from a 
point of co-ordinates x, y corresponds to a delay time V in the signal Sn 
corresponding to one or more analysis lines. In the same way, a horizontal 
translation by a distance d from the point of co-ordinates x, y 
corresponds to a delay time H in the signal Sn allowing for the speed of 
analysis on a line. The Laplacian of the signal Sn at the point of 
co-ordinates x-d, y-d and for four comparison points for example is 
expressed by the relation (II): 
(II) 
EQU S(x-d,y-d)=(1/d.sup.2)S[(t-H)+S(t-V)+S(t-2H-V)+S(t-2V-H)-4S(t-H-V)] 
Summation of said delayed signals gives a signal representing the Laplacian 
of Sn. The delay times H and V are imposed for a given analysis standard, 
taking into account the distance d which has to be selected. This distance 
d is selected to be of the order of the resolution element of the target, 
i.e. typically 200 micrometers. 
The complementary signal formed by a signal representing the temperature 
.theta..sub.n-1 of the point of co-ordinates x, y and by the second 
component representing the Laplacian of that temperature is obtained by 
summation of the successive signals. Thus, the signal representing the 
temperature .theta..sub.n-1 is obtained by summation of the successive 
signals .theta..sub.n-1 =Sn-1+Sn-2+ . . . +S1+.theta.o, .theta.o being the 
initial temperature of the target which is assumed to be uniform, whilst 
.DELTA..theta..sub.n-1 - the Laplacian of the temperature .theta..sub.n-1 
--is obtained by summation of the Laplacians of the successive signals, 
the Laplacian operator being a linear operator. 
The arrangement according to the invention illustrated in FIG. 1 provides 
for reconstitution of the signal representing the thermal flux absorbed by 
the sensitive surface of a camera system using the pyroelectric effect 
comprising means for periodically reading the sensitive surface by 
successive frames. At one of its terminals, the system delivers a reading 
signal Sn of a point of co-ordinates x, y during the reading thereof in 
the course of a frame or order n. The arrangement comprises means for 
correcting the signal Sn. These correcting means comprise means 1 for 
calculating a first component representing the Laplacian of the signal Sn. 
The input terminals 11 and 12 of the calculating means 1 are connected to 
the terminal of the camera system delivering the signal Sn. The output 13 
of the calculating means 1 delivers said first component representing the 
Laplacian of the signal Sn. The output 13 of the calculating means is 
connected by an amplifier 5 of gain 1/2 to the input 21 of means 2 for 
summating the signal Sn with the first component. The output 23 of the 
summation means 2 delivers a corrected signal. The output 13 of the 
calculating means 1 is also connected to the input 31 of means 3 for 
generating a complementary signal and comprising means for generating, on 
the one hand, a signal representing the temperature .theta..sub.n-1 of the 
point of co-ordinates x, y during the reading thereof in the course of a 
frame of order n-1 and, on the other hand, a second component representing 
the Laplacian of that temperature. The output terminal 32 of the means 3 
for generating the complementary signal is connected to the input 41 of 
summation means 4 of which the input terminal 42 is connected to the 
output 23 of the summation means 2. The output terminal 43 of the 
summation means 4 delivers the reconstituted signal S.sub.r. 
In one particular embodiment of the arrangement according to the invention 
illustrated in FIG. 1, the means 1 for calculating the first component 
representing the Laplacian of the signal Sn comprise a summation amplifier 
14. The summation amplifier 14 has a difference input 141 connected to the 
terminal of the system delivering the signal Sn by the input terminal 11 
through means 15 for calculating the Laplacian of the signal. The sum 
input 142 of the summation amplifier 14 is connected to the terminal of 
the camera system delivering the signal Sn through an amplifier 16 of 
predetermined gain. The gain of the amplifier 16 is equal to T/.tau.. The 
output terminal 143 of the summation amplifier 14 connected to the output 
13 of the calculating means 1 delivers the first component. The means for 
generating the complementary signal comprise a summation amplifier 33. A 
sum input 330 of the summation amplifier 33 is connected by the input 
terminal 31 of the calculating means 3 to the output 13 delivering the 
first component. The output 331 of the summation amplifier 33 is 
regeneratively connected by a delay circuit 34 of delay T and an amplifier 
35 of gain G between 0 and 1, 0&lt;G.ltoreq.1, to a second sum input 332 of 
the amplifier 33. The output terminal 342 of the delay circuit 34 
connected to the output terminal 32 of the means for generating a 
complementary signal 3 delivers the complementary signal. The arrangements 
operates as follows: 
The first component is delivered by the output terminal 143 of the 
summation amplifier 14 to the point C in FIG. 1. The summation of the 
signal Sn with the first component is carried out by the summation means 2 
which, at their output 23, deliver the corrected signal Sn. The corrected 
signal Sn is expressed as follows: 
EQU Sn.sub.c =[1+(T/2)(1/.tau.-D.DELTA.)]Sn 
At their output 32, the means for generating the complementary signal 3 
deliver, on the one hand, the signal representing the temperature 
.theta..sub.n-1 of the point of co-ordinates x, y and, on the other hand, 
the second component representing the Laplacian of that temperature by 
approximation of .theta..sub.n-1 by the quantity 
##EQU5## 
and by progressive reduction of the influence of the signals corresponding 
to the previous frames where G represents the gain of the amplifier 35. In 
this case, the gain G of the amplifier 35 is less than 1, G&lt;1. This avoids 
the disadvantage, when the temperature .theta..sub.n-1 is being obtained 
by summation of the successive signals, of introducing a noise level which 
is greater, the larger the number of frames coming into play, the 
incoherent noise being proportional to the square root of the number of 
frames and the spatial noise to the number of frames used. The delay 
circuit 34 enables the signals to be delayed by a time equal to the frame 
period T. The delay circuit may advantageously be formed by a digital 
memory or by a charge transfer register or by two memory tubes. 
The second component representing the Laplacian of that temperature is also 
obtained at the level of the output terminal 342 of the delay circuit 34 
by virtue of the fact that the Laplacian operator is a linear operator, 
the sum of the Laplacians of the signals Sn representing the Laplacian of 
the sum of those signals and hence of the corresponding temperature. 
According to FIG. 2, the means 15 for calculating the Laplacian of the 
signal Sn comprise a summation amplifier 251 followed by an amplifier 255 
of gain DT/d.sup.2. The inputs 2511, 2512, 2513, 2514 of the amplifier 251 
correspond to the number of comparison points of the reading signal 
separated on the sensitive surface by a distance d from the point of 
co-ordinates x, y. These input terminals are each connected to the output 
terminal of the camera system delivering the signal Sn by means of delay 
lines 252, 253. Each delay line 252, 253 respectively produces in the 
signal Sn a delay H and V respectively corresponding to a translation by d 
along the co-ordinate axis x and the co-ordinate axis y of the sensitive 
surface. 
The arrangement shown in FIG. 2 in fact corresponds to the calculating of 
the Laplacian of the signal Sn at a point of co-ordinates x-d, y-d for 
four comparison points, the input 2515, the difference input, being fed by 
the signal Sn through two cascaded delay lines 253 and 252 and an 
amplifier 254 having a gain equal to 4, the input 2514 being fed by the 
signal Sn through two delay lines 253 and one delay line 252 in cascade, 
the input 2513 through two delay lines 252 and one delay line 253 and the 
input 2511 through one delay line 252. In this case, the connection 
between the points A and B of FIG. 1 has to be modified by the addition 
between these two points of a delay line 252 and a delay line 253 
connected in cascade and enabling the corresponding stagger equal to d to 
be obtained between the co-ordinates x, y of the point in question. The 
point B disconnected from the point A may also be directly connected to a 
point D such as shown in FIG. 2 in the case where means for calculating 
the Laplacian of the signal Sn corresponding to the illustration of FIG. 2 
are used. This solution enables the delay lines 252 and 253 inserted 
between the points A and B, as indicated above, to be saved. Any similar 
arrangement comprising a different number of points at which the reading 
signal is extracted and providing for calculation of the Laplacian of the 
reading signal for a point of the sensitive surface falls within the scope 
of the present invention. 
The particular embodiment of the arrangement according to the invention 
illustrated in FIG. 3 relates to a camera system in which the thermal flux 
is sequentially interrupted, as for example in operation with a shutter. 
By assumption, the useful thermal flux absorbed is zero during the closure 
phases, which enables the temperature .theta..sub.n-1 to be determined 
without integration with the preceding frames, the equation giving the 
thermal flux absorbed during the closure phase being expressed by the 
following relation (III): 
EQU 0=[1+T/2(1/.tau.-D.DELTA.)]Sn+T[(1/.tau.-D.DELTA.)].theta..sub.n-1 (III) 
In FIG. 3, the means for generating the complementary signal representing 
the temperature .theta..sub.n-1 and the second component representing the 
Laplacian of that temperature comprise a summation amplifier 38 in 
addition to the arrangement shown in FIG. 1. A first sum input 381 is 
connected to the output 13 of the means 1 delivering the first component. 
A second sum input 382 is connected to the output terminal of the 
summation amplifier 2 by an amplifier 37 of gain -1. The output terminal 
of the summation amplifier 38 and the output terminal of the summation 
amplifier 33 are connected by a common switch 39 to the input terminal 341 
of the delay circuit 34. In operation, the output terminals of the 
summation amplifiers 33 and 38 are alternately connected to the input 
terminal 341 of the delay circuit 34 by a signal in synchronism with the 
closure sequence applied to a control terminal 391 of the switch 39. The 
positions F and O of the switch correspond to the open and closed 
positions of the shutter. The output 43 of the summation means 4, of which 
the inputs 41 and 42 are respectively connected to the outputs 342 of the 
delay circuit 34 and to the output of the summation means 2, comprises a 
memory 6. 
In operation, the switch 39 is switched to the position F when the shutter 
is closed and to the position O when it is open. Accordingly, resetting to 
zero is obtained each time the shutter closes by the direct determination 
of .theta..sub.n-1 in accordance with the relation III during that instant 
by the complementary circuits of the summation amplifier 38. The memory 6 
enables the signal presented during the opening phases to be restored 
during the closure phases of the shutter. 
The amplifier 35 of gain G in the embodiment shown in FIG. 3 has a gain 
equal to 1 or is merely replaced by a regenerative connection. By virtue 
of the resetting to zero effected by the direct determination of 
.theta..sub.n-1 during the closure phase of the shutter and the use of the 
signal representing .theta..sub.n-1 from the beginning of the following 
opening sequence of the shutter, this modification enables all the 
preceding frames to be taken into account without any disadvantage. The 
precision of the complementary signal is thus improved without 
significantly increasing the noise level. 
Of course, the invention is not limited to embodiment described and shown 
which was given solely by way of example.