Thermal radiation measuring system with a radiation measuring device and a shielded reference device

A bolometer which is adapted for a wide-band of radiation including soft adiation, and in which the sensitivity of the time constant (the heat flow from the absorber layer to the dissipator layer) can be exactly preselected without regard to the wave length of the radiation. The bolometer includes an electrically insulating carrier foil which has mounted thereon an absorber layer on one side thereof and a resistance layer on the opposite side of the foil, the resistance layer being part of a resistance measuring bridge. A thermally conductive layer is placed between the absorber layer and the carrier foil. The thermally conductive layer has portions protruding beyond the absorber layer. A heat dissipator is in thermally conductive contact with the protruding portions of the thermally conductive layer to dissipate the heat of the absorber layer. The laterally protruding portions of the thermally conductive layer in contact with the heat dissipator are shielded against the radiation to be measured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a radiation measuring device (bolometer) 
and also relates to a radiation measuring system designed to use same and 
consisting of a radiation measuring device exposed to the radiation and a 
reference measuring device shielded against the radiation. 
2. Description of the Prior Art 
In the large plasma machines commonly used today, an essential part of the 
heating capacity deposited in the plasma is transported to the wall of the 
vacuum vessel by electromagnetic radiation and neutral particles. In order 
to be able to predict the energy balance of the plasma, it is necessary to 
measure the radiation power absolutely, resolved as to time and space, 
over the spectral range in question. The spectral range to be measured 
extends from the infrared range to the range of soft X-radiation in the 10 
keV area. 
For wide-band radiation measurement, radiation measuring devices known as 
bolometers are used. These are radiation detectors which are sensitive to 
a wide spectral range (infrared to soft X-radiation). They measure the 
incident radiation power integrally. The mode of operation of the 
bolometer is based on the absorption of the radiation to be measured and 
the resulting rise in heat of the bolometer detector. The bolometer signal 
is proportional to the temperature increase of the detector and the 
sensitivity depends on the temperature coefficient of the physical effect 
exploited for measuring the temperature increase. 
The following different types of bolometers are used in plasma physics: 
(1) the semiconductor bolometer (germanium layer); 
(2) the thermistor bolometer (nickel absorber with a thermistor insulated 
therefrom); 
(3) the gold or platinum resistors (freely suspended in a convolute or 
spiral shape); 
(4) the pyroelectric bolometer; 
(5) the IR bolometer; 
(6) the foil bolometer; and 
(7) the Thermopile 
The radiation power density emitted on the plasma and hitting the detector 
is of magnitude of a few m.sup.W /cm.sup.2. The use of foil bolometers is 
recommended to measure such small radiation power densities in 
environments, where strong interferences are present caused by neutrons, 
.gamma.-radiation and/or electromagnetic signals. 
Foil bolometers consist basically of three functionally different elements: 
(a) an electrically insulating carrier foil with high mechanical stability, 
on which 
(b) a high-value resistor layer with low heat capacity consisting of thin 
and narrow strips is arranged in a convoluted shape. On the other side of 
the foil there is 
(c) an absorber layer located precisely over the resistor layer. The 
absorber layer is in contact on all its edges with a dissipator. 
The direct relationship between the time required for the heat to dissipate 
in the direction of the dissipator and the dimensions of the absorber 
leads to relatively large configurations of the bolometer detector with 
the conventional design of foil bolometers. However, detectors with large 
configurations have the following disadvantages: 
(a) screen effects of the geometrical optics, caused by the dimensions of 
the detector surface with respect to the distance of the detector from the 
object to be measured and its dimensions; 
(b) require relatively high electrical capacity which makes it impossible 
to use carrier frequency signal transmission; 
(c) lack of sensitivity of the signal transmission to low-frequency 
pick-up. When high-frequency heating is used for the plasma experiment, 
high pick-up must be expected for the bolometer measurements; and 
(d) when a large number of such measuring devices are used large rooms are 
necessary to accommodate the measuring devices even in the case of very 
average space resolution. 
SUMMARY OF THE INVENTION 
Assuming a radiation measuring device having an electrically insulating 
carrier foil on one side of which there is an absorber layer, to be 
exposed to the radiation to be measured, opposite which a high-value 
resistor layer is provided on the other side of the carrier foil, which is 
part of a resistance measuring bridge, the invention is directed to the 
problem of developing the radiation measuring device in such a way that it 
meets the following criteria: 
(1) small dimensions of the detector and consequently 
(a) the possibility of higher space resolution; 
(b) improved mechanical solidity; 
(c) negligible screen effects of the geometrical optics; 
(2) small electrical capacity; 
(3) predetermined time constant; 
(4) resistance to gamma and neutron radiation; 
(5) use detectors which are thermally independent of each other; 
(6) use detectors which are electrically independent of each other; 
(7) eliminate thermal and electrical interference; 
(8) electrical disruptive strength; and 
(9) must be able to be used in a high vacuum up to 300.degree. C. 
This problem is solved according to the invention by the following 
features. The absorber layer passes into a laterally protruding thermally 
conductive layer which is in thermally conductive contact therewith. The 
thermally conductive layer is shielded against the radiation to be 
measured. The absorber layer and, optionally, the thermally conductive 
layer may be made of a precious metal, preferably gold. It is expedient, 
in a further embodiment of the invention, for the thermally conductive 
layer to consist of a layer which protrudes laterally beyond the absorber 
layer and is much thinner than the absorber layer, optionally of one piece 
therewith and/or made of the same material. 
Due to the inventive solution, energy absorption and thermal conductivity 
are separated, whereby the dimensions of the detector can be substantially 
reduced so that, in a further embodiment of the invention, a plurality of 
thermally conductive surfaces each bearing an absorber layer can be 
arranged in series spaced side by side. 
The high-value resistor layers assigned to the absorber layers can each be 
connected in a separate resistance measuring bridge. A different way of 
measuring consists in at least some of the high-value resistor layers 
assigned to the absorber layers being connected in series and the resistor 
groups formed in this way connected in a separate resistance measuring 
bridge. In this way the resistance value of a measuring system can be 
increased accordingly, thereby increasing the voltage across the bridge 
diagonal accordingly with the same power applied to the resistors and 
improving the sensitivity of the measuring system. 
In a further embodiment of the invention, the fixed resistor opposite the 
high-value resistor layer in the measuring bridge is also designed as a 
high-value resistor layer and arranged together with the high-value 
resistor layer on the side of the carrier foil facing away from the 
absorber side, within a boundary corresponding to the boundary of the 
absorber layer. This measure allows for twice the sensitivity of the 
measuring bridge. This is due to the fact that this fixed resistor, which 
is now designed as a high-value resistor layer, is also exposed to changes 
of temperature, namely to the same ones as the high-value resistor layer 
is exposed to, which was fonmerly exposed to radiation alone. 
When, in a further embodiment of the invention, the thermally conductive 
layers are connected with each other via a dissipator, the extent of the 
entire measuring system can be substantially reduced because this prevents 
mutual influence of the various detectors due to the immediate dissipation 
of heat into the surroundings by the dissipator. 
This is preferably effected by constructing the dissipator as a disk made 
of a material exhibiting good thermal conductivity, which contacts the 
thermally conductive layers, has substantially larger dimensions of 
thickness than those of the absorber and thermally conductive layers, and 
has in the area of each absorber layer a recess whose lateral limits are 
spaced from the lateral limits of the absorber layers and whose top 
surface is provided with a window whose dimensions correspond to those of 
the absorber layer. 
Alternatively, the dissipator may be made of the material of the thermally 
conductive layers and be of one piece construction. For this purpose, a 
coating is expediently applied to the carrier foil, the coating has 
recesses, each with a bottom whose central area forms the absorber layer 
which passes, via a thin edge area forming the thermally conductive layer, 
into the material of the coating which forms the complete dissipator. This 
construction allows for better defined geometrical conditions, especially 
since the thermally cqnductive layer can be obtained much more simply. It 
is generally possible to provide the dissipator layer in the area of the 
thermally conductive layer with an undercut design to form a window 
limiting the radiation being exposed to only the absorber layer. However, 
it is more simple, in a further embodiment of the invention, when a 
masking body provided with windows which guarantee that the radiation only 
sees the absorber layers, is placed upon the coating of the carrier foil 
constituting the dissipator. 
The distance between the limit of each recess and the limit of each 
absorber layer is preferably such that the heat transmission between the 
absorber layer and the high-value resistor layer takes place much more 
quickly than the heat transmission from the limit of the absorber layer to 
the dissipator. 
For the adjustment of the time constant of the lateral heat dissipation 
from the absorber layer to the dissipator, it is necessary that the 
distance between the lateral limit of the absorber layers and the 
dissipator be matched with the thickness of the thermally conductive 
layer. 
It is particularly advantageous when solderable terminal areas are provided 
to facilitate assembly on the side of the carrier foil bearing the 
high-value resistor layers, in an arrangement surrounding the resistor 
layers. 
By additionally connecting a reference measuring device, which should be 
housed as close to the radiation measuring device as possible, electrical 
and thermal interference can be directly compensated. By connecting the 
reference measuring device resistor into the second arm of the Wheatstone 
bridge circuit, effects acting on both detectors lead to a symmetrical 
tuning of the bridge arms and thus to compensation which retains the 
tuning of the bridge. When both detectors have identical properties, the 
measuring bridge remains ideally tuned. 
The construction is selected in such a way that the reference measuring 
device is identically constructed to the radiation measuring device, and 
the reference measuring device is disposed in the same housing as the 
radiation measuring device. 
The radiation measuring device and the reference bolometer are preferably 
arranged one behind the other in the housing. 
This results in a particularly advantageous construction of the measuring 
system because the solderable terminal areas are present twice on the side 
of the two carrier foils bearing the resistors, and may be 
series-connected, which results in inner and outer terminal areas. 
The outer solderable terminal areas are then connected conductively to the 
first contact pins in the case of the radiation measuring device, and the 
inner solderable terminal areas are connected conductively to second 
contact pins in the case of the reference measuring device, while the 
outer terminal areas are bored out. 
The first contact pins reach freely through these bores and the ends of the 
first and second contact pins form the common connecting body of the 
measuring system constituting the measuring bolometer and the reference 
bolometer. 
The radiation measuring device is clamped between two dissipators with the 
corresponding recesses and is provided with mechanically stable 
connections. 
The dissipators are aluminum disks which cool the detectors down to the 
housing temperature of the measuring system. The dissipator located on the 
absorber side is designed in such a way that the thermally conductive 
surfaces are in contact at their edges with the dissipator placed thereon. 
This dissipator is provided with notches which in turn each have a window 
which leaves open only the access to the absorber for exposure to the 
radiation to be measured. 
The radiation measuring device may be mounted flat due to the disk-like 
design of the dissipator. Recesses in the dissipator which lies against 
the resistor side may be used to adjust the radiation measuring device 
optically. 
The overall construction is distinguished by particularly simple 
mountability. It is only necessary to punch the lead-in holes through the 
radiation measuring and reference measuring devices for clamping screws 
and the contact pins. 
The radiation measuring and reference measuring devices can then be mounted 
and soldered up, and finally the measuring system can be simply put 
together. 
After assembly all components are only under pressure in the measuring 
direction. The pressure forces not only hold the construction together but 
also supply the contact pressure between the terminal areas and their 
contact pins, and the contact pressure for the contact pins located in the 
base body. 
A ring nut suffices to secure the various elements when they are put 
together in a housing 
In a modified embodiment of the measuring system, the radiation measuring 
device and the reference measuring device are arranged one beside the 
other on the carrier foil, whereby, of course, only the radiation 
measuring device is exposed to the radiation while the reference measuring 
device must be shielded against the radiation to be measured. 
A design which may be provided with a single mask without any overlapping 
of leads is made possible in a further embodiment of the invention, by 
arranging the measuring system in such a way that the high-value resistor 
layers of the radiation measuring device and the high-value resistor 
layers of the reference measuring device are each designed as a 
convolution, and arranging the convolutes of the high-value resistor 
layers of the radiation measuring device and the reference measuring 
device interwoven on the carrier foil and providing on each of the sides 
facing away from each other of this side-by-side arrangement of the 
radiation measuring and reference measuring devices, two leads opening 
into terminal areas. The outer leads wind around the inner leads as well 
as around the terminal areas of these inner leads, whereby one convolute 
of one of the bolometers is connected between the two outer leads and the 
other convolute of this bolometer is connected between the inner leads, 
while the two convolutes of the other bolometer are each connected between 
an outer and an inner lead. The term "convolute" is meant of course in its 
broadest sense and does not include only comb-like formations. 
Spiral-shaped formations can also be inserted into one another and 
provisions for connections to the outer ends of the spirals can be made 
when they are designed as double spirals running in one direction and back 
in the other. When selecting the convolution, it should be ensured that 
connections are possible on the plane of the convolution, so that 
connections which are at an angle to this plane are not required. 
The stated arrangement can be provided in a further embodiment of the 
invention in repeated patterns in a preferably symmetrical arrangement on 
a carrier foil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a carrier foil 1 made of an electrically insulating material, 
for example mica or the synthetic material known by the trademark 
"Capton." These materials are particularly well-suited due to their high 
radiation resistance and stability with respect to high temperatures. 
Onto the carrier foil 1 a thermally conductive layer 2 is vacuum 
metallized; gold or platinum is preferred. The thickness of this layer is 
0.5 .mu.m or less. 
In the center of this thermally conductive layer 2 an absorber layer 3 made 
of the same material as the thermally conductive layer 2 is vacuum 
metallized. This layer has a thickness of 4 .mu.m. The absorber layer 3 
establishes a measuring surface. The thickness of the absorber layer 3 can 
be selected freely in accordance with the radiation to be measured. The 
thermally conductive layer 2 and absorber layer 3 may also be of one piece 
construction. 
Opposite the absorber layer 3 on the other side of the carrier foil 1 there 
is a high-value resistor layer 4 which may have a meander shape, and may 
be made of vacuum metallized gold or platinum. 
A resistor M formed by the resistor layer 4 is located, as shown in FIG. 2, 
in an arm 5 of a Wheatstone bridge 6, which also has three further 
resistors 7, 8 and 9, of which the resistor 9 may be a resistor layer R of 
a reference measuring device. A radiation measuring device 10 is connected 
across a diagonal of the bridge, while a voltage U is applied to the other 
diagonal. Touching thermally conductive layer 2 near its edges, a 
dissipator 11 is placed upon the arrangement. The dissipator has inwardly 
extending projections 12 which fonm a shield leaving free only the access 
to the absorber layer 3 which as indicated schematically in FIG. 2 is 
exposed to radiation F. The carrier foil 1 forms together with the 
thermally conductive layer 2, the absorber layer 3 and the resistor layer 
4, to become the detector of the radiation measuring device, which may 
also be termed a foil bolometer. 
FIG. 3 shows an arrangement with four detectors located side by side as 
seen from the top, without the shielding dissipator, mounted to the 
carrier foil 21, which is designed to have a circular formation in the 
present arrangement. Across diameter II--II, four absorber layers 23a, 
23b, 23c and 23d, are provided in a row, each placed on thermally 
conductive layers 22a, 22b, 22c, 22d. These thermally conductive layers 
are formed in such a way that their outer periphery matches the circular 
circumference of the carrier foil 21 while maintaining corresponding 
distances 24a, 24b and 24c. 
FIG. 4 shows the bottom view of the carrier foil 21, with high-value 
resistor layers 34a, 34b, 34c and 34d thereon. They are placed exactly 
below the absorber layers 23a, 23b, 23c, 23d which are on the other side 
of the carrier foil 21, so that the absorber layer and the corresponding 
high-value resistor layer, for example absorber layer 23a and high-value 
resistor layer 34a, are located exactly opposite one another on the two 
sides of the carrier foil. In the embodiment shown, the high-value 
resistor layers have a meander shape. The various resistor layers are 
connected via connecting lines 35, 36, which are only provided 
individually with reference numbers for resistor layer 34b, to inner 
terminal areas 37 and 38, which communicate with further outer terminal 
areas 39 and 40. Such a formation as can be found in particular in FIGS. 3 
and 4 is provided not only for the radiation measuring device exposed to 
the radiation but also for a reference measuring device provided in the 
measuring system which is not exposed to any radiation but is instead 
shielded against the radiation to be measured. 
The radiation measuring device exposed to the radiation and the reference 
device shielded against the radiation are arranged one behind the other in 
a single housing embodiment shown in FIG. 7. 
The resistor layer(s) of the reference measuring device are electrically 
switched into the Wheatstone bridge 6 of FIG. 2 and correspond to the 
resistor 9. 
FIG. 5 shows a dissipator 41 which is adapted to be placed on the carrier 
foil shown in FIG. 3 with the thermal layers 22a, 22b, 22c and 22d applied 
and the absorber layers 23a, 23b, 23c and 23d placed thereon. The 
dissipator has recesses 42a, 42b, 42c and 42d which have windows 43a, 43b, 
43c and 43d on their top surfaces. 
The arrangement described above is shown clearly in the cross-sectional 
view of FIG. 6. FIG. 6 shows the dissipator 41 with the recesses 42a, 42b, 
42c and 42d in the top surfaces of which, 54a, 54b, 54c and 54d, there are 
windows 43a, 43b, 43c and 43d whose dimensions are identical to the 
dimensions of absorber layers 23a, 23b, 23c and 23d, as can be clearly 
seen in the view depicted in FIG. 3. 
FIG. 7 shows a perspective exploded view of an embodiment of a measuring 
system having the inventive radiation measuring device and a reference 
measuring device. A cylindrical housing 71 is provided at one end with an 
inner collar 72. The arrangement referred to as 73 is supported on the 
inner collar 72 when the arrangement is mounted into the cylindrical 
housing 71 from the right as viewed in FIG. 7. It is secured with the help 
of a ring nut 74 which is screwed into a corresponding inside thread at 
the other end, which is shown on the right when viewing FIG. 7, of the 
cylindrical housing 71. 
Reference character 75 refers to the dissipator 41 just described in 
connection with FIGS. 5 and 6. The dissipator 41 has a corresponding wall 
thickness, the corresponding recesses as well as the corresponding windows 
recited above. Reference character 76 refers to the view of the radiation 
measuring device exposed to the radiation, consisting of a carrier foil 
and thermally conductive layers and absorber layers placed thereon facing 
in the direction of irradiation (arrow F). A base 77 follows, which is 
made of an insulating material and into which the contact pins 78 are 
mounted. The contact pins 78 protrude through the base 77 end face on the 
side facing the radiation measuring device 76 as contact surfaces which 
can easily be connected to the contact surfaces on the radiation measuring 
device 76. The bottom of the carrier foil, i.e. the side carrying the 
resistor layers, is also in contact with a dissipator, i.e. a cylindrical 
body made of aluminum, which is provided, however, with recesses such that 
it does not short-circuit the various connections and resistor layers. 
This dissipator is referred to in the drawing by reference character 79. 
An uninterrupted dividing disk 80, for example an aluminum disk, is mounted 
next to the base 77. The dividing disk 80 completely shields or isolates 
the measuring portion of the system (the radiation measuring device) 
against the reference portion (the reference measuring device). Reference 
character 81 refers to a dissipator which corresponds to the dissipator 75 
in every respect. Reference character 82 refers to a reference measuring 
device which corresponds to the radiation measuring device 76 in every 
respect. A base 83 then follows, which essentially corresponds to the base 
77 and carries contact pins 84 corresponding to the contact pins 78. These 
contact pins 84 are staggered with respect to the contact pins 78 in such 
a way that they are located centrally between the contact pins 78. A 
dissipator 85 is also provided again here, corresponding essentially to 
the dissipator 79. Both types of contact pins 78 and 84 are insertable 
into apertures provided in a common base 86, which is an insulating body 
similar to bases 77 and 83. 
The terminal areas shown on the reference measuring device 82 are bored out 
on the side of the carrier foil carrying the resistor layers, through 
which the contact pins 78 protrude, so that there is no contact between 
the reference bolometer and the measuring bolometer proper, which is 
exposed to the radiation. Instead, the corresponding row of contacts 
protrudes beyond the base 83 and is expediently inserted in a common end 
base bearing a corresponding number of contacts. 
By arranging the radiation measuring and reference measuring devices in 
such a way that the contacts are staggered with respect to each other, and 
by switching into a Wheatstone bridge, compensation of the magnetic fields 
which differ in time is achieved. The selected incorporation of the 
reference measuring device further allows for complete compensation of 
non-measurable portions of radiation z.B. neutrons, .gamma.-radiation 
which only heat the housing and the parts of the measuring system 
communicating therewith without the measurement being affected. 
In the embodiment as shown in FIG. 8, the radiation measuring and reference 
measuring devices are arranged side by side and several such pairs of 
devices, one behind the other, in one row each are illustrated. The 
radiation measuring devices should be located in the right-hand row M, 
while the reference measuring devices may be found in the left-hand row R. 
FIG. 9 illustrates the construction in cross-section. On the carrier foil 
91 is deposited a gold layer 92 by way of vacuum metallizing to form the 
dissipator. It has a thickness, for example, of 25 .mu.m. In the layer 92 
are formed recesses 93 and 94 which each have a bottom whose central area 
95 and 96, respectively, forms an absorber layer which is, for example, 4 
.mu.m thick and passes directly into the dissipator layer 92 via an edge 
area 97 and 98, respectively (see also FIG. 8), which has a 
correspondingly smaller thickness of only 0.5 .mu.m, for example, and 
forms the thermally conductive layer. The absorber layer, the thermally 
conductive layer and the dissipator are thus fonmed of one piece. The 
production of such a formation by the vacuum metallizing or etching method 
does not involve any difficulties. 
In the embodiment shown in FIGS. 8 and 9 the covering of the thermally 
conductive layer and the dissipator is realized by a cover made of 
appropriate material which limits the irradiation to the absorber layer of 
the radiation measuring device by means of windows. For the reference 
measuring devices, the absorber layer is of course also shielded against 
the impinging radiation by this cover. The cover is not shown in FIG. 9. 
The windows limiting the irradiation to the desired absorber layers may 
also be realized by protruding edges on the corresponding recesses 93, but 
this design is difficult as well as elaborate. 
FIG. 10 shows a pair of high-value resistor layers being a part of a pair 
consisting of a radiation measuring device and a reference measuring 
device. These high-value resistor layers are provided on the side facing 
away from the radiation measuring and reference measuring device side, 
precisely adapted to the positions of the radiation measuring and 
reference measuring devices. Arrangements 101 and 102 do not only include 
the resistor winding belonging to the particular measuring resistor M (see 
resistor 4 as in FIG. 2) and the particular reference resistor R (see 
resistor 9 as in FIG. 2), which are each designed here in the shape of a 
flat convolution, but also each winding located opposite in the measuring 
bridge and also designed as a convolute pattern (see resistors 8 and 7 as 
in FIG. 2). Such convolutions can be intertwined or banked, as can be seen 
in FIG. 10, resulting in a flat formation including both resistors. The 
junctions can be directed out of these flat formations 101 and 102 in such 
a way that there is no overlapping anywhere, which substantially 
facilitates the production of the arrangement. 
From the end 103 of one of the convolute-like resistors of the formation 
101, a connecting line leads to a lead 104 which is directed around a 
terminal area 105 and then to an outer lead 106, connected to an outer 
terminal area 107. From the formation 102 a connecting line 108 is 
directed to a lead 100 which is directed around a terminal area 110 and 
then to another outer lead 111 which is, in turn, connected to the other 
outer terminal area 112. The resistor of the formation 101 located at one 
end at the connecting line is located at the other end at the outer lead 
111. The resistor of the fonmation 102 located on one end at the 
connecting line 108 is located at the other end both at an inner lead 113, 
which leads to the terminal area 110, and at a connecting element 114 
which is also connected to the second resistor in the formation 101, which 
is connected at its other end to the lead 115 which leads to the inner 
terminal area 105. Furthermore, one end of the second resistor is 
connected to the lead 115, this resistor being located at its other end at 
the outer lead 106. If one follows the various connections, the overall 
arrangement results in the bridge circuit as shown in FIG. 2, whereby here 
the measuring resistor and opposite resistor 8, and reference resistor 9 
and opposite resistor 7, are each located within the boundary of a 
predetermined field, opposite which the respective absorber layers of the 
radiation measuring and reference measuring devices are located on the 
other side of the carrier foil. 
FIG. 11 shows the multiple arrangement of such circuits shown in detail in 
FIG. 10. So that adjacent circuits A, B, C . . . N do not interfere with 
each other electrically, the terminal areas 121a and 122a of circuit A are 
applied to the alternating voltage supply of the bridge circuit, while the 
connection of the remaining two terminal areas 123a and 124a forms the 
neutral arm. In the case of the next circuit B, the connections are 
reversed, i.e. the neutral arm is located between terminal areas 121b and 
122b, while the supply anm is located between the terminal areas 123b and 
124b. In the case of circuit C, the connecting conditions are again as in 
circuit A, etc. The neutral arms and supply arms are thus always located 
side by side, respectively, so that the low voltages of the neutral arms, 
and high voltages of the supply arms, can at most always affect each 
other, respectively, and the high voltage applied to a supply arm is not 
picked up in the low voltage applied to a neutral arm. It is thus assured 
that the outer leads 106 and 111, as shown in FIG. 10, of adjacent 
circuits always conduct equal potentials. 
The mode of operation of a device according to the invention shall be 
explained in more detail with reference to FIGS. 12 and 15. 
FIG. 12 shows a three-dimensional temperature pattern from the highest 
temperature prevailing on the absorber layer (at the top) to the 
dissipator (ambient temperature) for a point of time T. 
The absorber layer is exposed to the radiation to be measured. The 
radiation absorbed in the absorber layer leads to the absorber layer being 
heated. The heat dissipates in the direction of the high-value resistor 
layer and to the sides. The high-value resistor layer changes its 
resistance value measurably; the higher its value, the more apparent the 
change. The continuous impinging radiation leads to the absorber layer 
being heated continuously until the heat dissipated is equal to the 
absorbed radiated power. The heat is transported off by the large-mass 
dissipator with a high heat capacity and high thermal conductivity with 
negligible heating effect. The change in the resistance of the resistor 
layer on the side of the carrier foil opposite the absorber layer is a 
measure of the temperature change averaged over the surface of the 
resistor or absorber layer. 
It can be shown that the temperature change of the detector is a function 
of power P.sub.o in an approximate manner by an exponential function of 
the following form: 
EQU &gt;.tau..sub.o (t)&gt;=P.sub.o .tau..sub.eff /c.sub.eff 
(1-e.sup.-t/.sbsp..tau..sub.eff) (1) 
The constant .tau..sub.eff and c.sub.eff are called bolometer constants. 
However, the approximation is valid only when the time for the temperature 
to reach a steady state in the direction of the resistor layer is 
substantially faster than the heat transport towards the sides. The time 
constant .tau..sub.eff.sbsb.xy of the thermal conduction in X and Y 
direction must thus be substantially greater than the time constant 
.tau..sub.eff.sbsb.z holding in the Z direction. The reason for this may 
be made clear upon viewing the two diagrams which are shown in FIGS. 13 
and 14. 
FIG. 13 illustrates graphically the difficult description of the resulting 
thenmal processes when deriving the impinging power from the temperature 
change and the resulting measurable resistance change of the bolometer 
resistor, while FIG. 14 illustrates the radiated power P(t) which may be 
derived as follows: 
##EQU1## 
It is, therefore, essential to design the bolometer detector structurally 
in such a way that the heat flow from the absorber side through the thin 
carrier foil to the resistor layer takes place in a much shorter time than 
the lateral dissipation of heat from the absorber layer to the dissipator. 
It is imperative that the measuring bolometer facing the radiation source 
is completely covered by an electrically conductive and thermally 
conductive layer, in order to avoid charging by secondary electrons and 
achieve defined heat transporting conditions to the dissipator and defined 
characteristic data. 
The dissipation of heat may be affected structurally by enlarging or 
reducing the thickness of the absorber layer, as well as by enlarging or 
reducing its length and/or width. 
With a given thickness, a corresponding enlargement of the length and/or 
width results in an enlargement of the time constant. A reduction of 
thickness would require a reduction of the lateral dimensions with an 
equal time constant. However, the selection of the time constant is not 
infinite since this would lead to greater heating of the detector, on the 
one hand, and not to any exploitable increase in the sensitivity of the 
bolometer, on the other hand. The diagram in FIG. 15 graphically 
represents this state of affairs. 
With reference to FIG. 15, it can be seen that for the short time to the 
temperature difference .theta..sub.o of the absorber layer, for the 
detector with a large time constant (B), to that with a small time 
constant (A) is virtually identical. However, for long time periods t the 
temperature change .theta..sub.A is smaller than in case (B). The higher 
sensitivity can thus only be exploited for longer time periods, although 
it suffices when the signal/noise ratio has approximately the value 1000. 
Furthermore, a further increase in the time constant leads to a 
restriction of the dynamics with respect to the radiated power to be 
measured and the inherent radiation of the absorber layer is no longer 
completely negligible due to the higher detector temperature. 
Reference must be made to a particularly advantageous possibility of using 
the inventive measuring system in connection with FIG. 2. 
If a push-pull alternating voltage is selected as the voltage U, an 
alternating voltage staggered by 180.degree. with respect to the lower 
voltage connection is applied to the upper voltage connection. This means 
that the neutral arm always has a voltage level of zero when the bridge is 
balanced. This results in the possibility of measuring against the zero 
level, so that measuring signals are obtained which can be considerably 
amplified because there is no offset whatsoever. Since the influence of 
stray capacitances of cables etc. is thus minimized, the balancing of the 
bridge is considerably simplified and microfony effects are neglectable. A 
further advantage results from the fact that an evaluation according to 
log-in technology can take place due to the use of alternating voltage 
supply for the Wheatstone bridge.