A radiometer has a concave mirror turnable about its optical axis and photoelectric converters which receive light waves reflected by the mirror and are mechanically coupled with the same. An interrupter disk is arranged in the path of the radiation which falls onto the mirror and has a side facing the mirror which is reflective. The converters are heated and a housing surrounds the mirror, the converters and the heater and is provided with a window located on the optical axis and permitting the admission of light to the mirror. An electronic circuit processes the various signals of the device. According to the invention the light-admitting window is arranged in the center of curvature of the hollow part-spherical mirror, which center of curvature is located behind or inwardly of the interrupter disk as seen in the direction of the incoming light. An arrangement is provided for producing a reference signal which is phase-locked with the rotation of the interrupter disk and a rectifier circuit of the electronic circuitry has a phase detector arrangement which is controlled by this reference signal.

BACKGROUND OF THE INVENTION 
The present invention relates to a radiometer. 
More particularly, the invention relates to a radiometer of the type which 
is already known from German published application DE-OS No. 2,639,539 and 
from pending German application P No. 26 39 539.1. 
Still more specifically, the invention relates to an improvement in a 
radiometer of the type mentioned above. 
Radiometers of the type known from the prior art mentioned above, are 
basically perfectly acceptable instruments. However, instruments of this 
type are subject to reproduction errors resulting from the 
function-dictated location of the light admitting window, and the prior 
art contains no teaching as to how such errors can be avoided. 
The book "Das Photographische Objektiv" by Johannes Fluegge, published by 
Springer-Verlag, Vienna, 1955, discusses on pages 190 to 196 the Schmidt 
camera and the Maksutov system, both of which use the so called Schmidt 
mirror. The book states that aberrations can be avoided if the light 
admitting windows of these devices are located in the center of curvature 
of the mirror. These light admitting windows are aspherically deformed 
windows. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the known radiometer in 
such a manner as to improve the imaging quality and the sensitivity 
(temperature resolution and reproducability of the measured values). 
Another object of the invention is to achieve the above mentioned object in 
a relatively simple and uncomplicated manner. 
In keeping with these objects and still others which will become apparent 
hereafter, one feature of the invention resides in a radiometer for 
measuring heat radiating from a defined spatial area, and this radiometer 
may--briefly stated--comprise a concave mirror shaped to resemble a 
segment of a sphere and mounted for turning movement about its optical 
axis. A plurality of photoelectric converters are provided, being 
mechanically coupled with the mirror and arranged in the path of rays 
reflected by the same. A rotating interrupter disk is interposed ahead of 
the mirror in the path of incident radiation and has a reflecting side 
facing towards the mirror. Means are provided for producing a reference 
signal which is coupled in phase-locked relationship with the rotation of 
the disk. Heating means are provided for heating the converters. A housing 
surrounds the mirror, the converters and the heating means and has a wall 
located ahead of the mirror and provided with an incident-light window 
located on the optical axis inwardly of the interrupter disk and arranged 
on the center of curvature of the mirror. Circuit means are provided, 
comprising amplifiers connected to receive signals from the converters, a 
rectifier circuit including a phase-detector circuit controlled by the 
reference signal and a low-pass filter circuit receiving signals from the 
rectifier circuit. Threshold value comparators and a multiplexer are 
operative to sequentially supply the rectified signals to the threshold 
value comparators. 
With a construction, as just defined in accordance with the present 
invention, all rays of light or the light which enter through the center 
of the admitting window form with their associated tangent a right angle 
at the concave mirror. In other words, the radiation will impinge upon the 
mirror under always identical circumstances, from whatever angle it 
arrives. In a radiometer with a window which is located at the center of 
curvature and which additionally turns about this center of curvature, the 
admission window can be made small and be reproduced by the mirror in a 
ratio of 1:1. If the admission window were to be located at a different 
place, then the tangent angles would constantly change. 
The invention will hereafter be described with reference to an exemplary 
embodiment. It is to be understood, however, that is by way of explanation 
only and is not to be considered limiting, the scope of protection sought 
for the invention being defined exclusively in the appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Illustrated in FIGS. 1-3 is a radiometer 11, which has a concave 
part-spherical mirror 2, the open side of which faces the sky when the 
device is in use. A carrier 8 is provided having a portion which in the 
region of the mirror is concentric thereto; at the inner side of the 
carrier 8, at opposite sides of the optical axis 12 and facing the mirror 
2, there are mounted one or more rows of photoelectric converters 3. The 
carrier 8 is configurated as a housing and carries at its side opposite 
the converters 3 the mirror 2, thus fulfilling at the same time the 
function of a mechanically rigid connector between the converters 3 and 
the mirror 2. 
When the radiometer is in operation, it turns about its optical axis 12 and 
the converter 3 scan the sky area which is subdivided into individual 
reproduction sections. The overlapping of the view fields which would 
otherwise occur, is prevented by a slot-type diaphragm arranged 
immediately ahead of the converters 3 and also arranged on a concentric 
circle. All elements mentioned thus far, as well as the pre-amplifier 50 
associated with the converters 3 (see FIG. 4) are surrounded by a housing 
9 on the outer side of which the cable 7 are mounted, which lead to the 
converters 3, respectively to the heating elements 5 therefore. Reference 
16 identifies diagrammatically the drive unit for the radiometer, which is 
not illustrated in detail because it forms no part of the invention and is 
known per se in the art. 
The housing 9 is provided with a window 1 which is located opposite the 
open side of the mirror 2. In fact, the geometric center of the window 1, 
which is preferably of circular outline, coincides with the center of 
curvature 18 of the concave mirror 2. As considered in the direction of 
incoming radiation, the center of curvature 18 is located immediately 
behind the interrupter disk 4 (see FIG. 3). The radiation entering through 
the window 1 is identified with reference 43 and 44 in FIG. 2; however, 
only the radiation 43 which reaches the mirror 2 is desired. The dispersed 
radiation 44 which impinges laterally of the radiation 43 results in 
uncontrollable refractions and it is therefore desired to prevent its 
access to the mirror 2. This is achieved by means of webs 45 which are 
secured in or on the carrier roll 8' at mutually opposite locations and 
extend normal to the optical axis 18. The edges of these webs which face 
towards the converters 3 are recessed in a steplike manner as considered 
in the direction of movement of the incoming radiation. The side of the 
interrupter disk 4, which faces towards the window is made to be 
reflective, so that in alternation with the radiation 43 which passes 
through the window 1 and is reflected by the mirror 2 onto the converters 
3, a reference radiation is formed which is emitted by the converters 3 
which are kept at a constant temperature with the aid of heater 5, and 
which is reflected back onto the converters 3 by the reflective side of 
the interrupter disk 4. 
The alternating current resulting from the radiation received by the 
converters 3 has an approximately rectangular wave shape. Its basic wave 
amplitude is a known function of the temperature difference of the 
measured radiation and the reference radiation. The frequency of the 
converter signals is thus equal to the number of revolutions of the 
interrupter disk 4 times the number of interrupter disk lobes 4A. 
The basic wave amplitude of the electrical signal at the converters 3 is 
determined with the aid of an additional electrical reference signal of 
constant amplitude and pure sine-wave shape; this signal has a defined 
phase position and a frequency corresponding to that of the interrupter 
disk 4. A reference disk 46 is rigidly connected for rotation with the 
interrupter disk 4 via the shaft 4'. The reference disk 46 is of 
transparent material (e.g. glass or an appropriate synthetic plastic 
material) and provided with a coding in form of opaque areas. To obtain 
the reference signal, the disk 46 is scanned via the receiver 47 and for 
this purpose turns in the cutout 47' which may for example be U-shaped. In 
the present exemplary embodiment, the receiver 47 is constructed as an 
electric eye, i.e. a light beam and a receiver for the same. In other 
possible embodiments, however, the scanning can be affected magnetically 
or in other suitable manner known to those skilled in the art. The 
electrical signal produced in the receiver 47 is amplified in the 
integrated circuit 48 (FIG. 4), and limited, and is subsequently 
demodulated in the integrated circuit 49 which is connected following the 
circuit 48. The limitation assures at the output of the demodulator 49 a 
signal of constant amplitude. The coding on the reference disk 46 is so 
selected that the demodulated signal is of purely sine-wave shape. 
The rigid connection of the reference disk and the interrupter disk 
predetermines the phase position of the reference signal with reference to 
the phase position of the signal produced at the converters. Both signals 
have the same zero passes. To determine the electrical signal value 
equivalent to the radiation difference, the phase detector 51 is used, 
e.g. in form of a multiplier circuit. The signal produced at the converter 
sleeve is, as already mentioned, approximately rectangular in wave shape 
and can be reproduced by the equation 1: 
EQU u.sub.d (t)=A sin .omega.t+B sin 3 .omega.t+C sin 5.omega.t+. . . (1) 
wherein 
u.sub.d (t)=momentary detectorsignal 
.omega.=2 f 
f=interruptor frequency 
A, B, C etc.=amplitudes of the different Frequencies, with A being searched 
for 
The reference signal is determined by the equation 2: 
EQU u.sub.r (t)=R sin (.omega.t+.theta.) (2) 
EQU u.sub.d (t).multidot.u.sub.r (t)=R/2[A Acos.theta.-A cos (2.omega.t 
+.theta.)+B cos (2.omega.t-.theta.) -B cos (4.omega.t+.theta.)+ . . . ](3) 
The amplitude R and the phase angle .theta. are constant. The value .theta. 
is determined by the angular position of the reference disk 46 relative to 
the interrupter disk 4. 
If .theta.=0 and the signal is passed through a low-pass filter having an 
upper frequency limit f.sub.G &lt;2 .omega.t, one obtains a direct current 
which is characterized by the equation 4: 
EQU u.sub.d (t).multidot.u.sub.R (t)-k.multidot.A . . . (4) 
Since k is known, this can be taken into account at the amplification so 
that subsequently the desired final result A is obtained. As already 
mentioned, the reference signal must be purely of sine-wave form so that 
direct voltage is obtained only via the basic wave according to equation 
3. The constant amplitude of the reference signal is necessary in order to 
assure that k in the equation 4 remains absolutely constant. 
The signal-to-noise ratio is determined by the band width of the low-pass 
filter 52 which may be far below one Herz. A shifting of the frequency 
limit by changing of the structural components, temperature values and the 
like will result in only minimum changes of the signal-to-noise ratio, 
whereas the amplitude of the direct current is not influenced. Changes in 
the net frequency i.e. in the frequency of the currents in that used to 
operate the device, have no influence because the reference signal is 
synchronised to the detector signal. 
In dependence upon the particular application, the radiometer is to receive 
radiations of a certain part of the spectrum or of several parts of the 
spectrum and is not to receive radiations from other parts of the 
spectrum. This is possible directly at the converter 3 (see FIG. 2) and/or 
at the inlet window 1 by the use of interference layers 3'. 
FIG. 4 shows that pre-amplifier 50 with adjustable amplification are 
connected in circuit behind the converters 3, shown in FIGS. 1-3. The 
output signal is multiplied in order to determine the electrical signal 
value equivalent to the radiation difference; this multiplication takes 
place in the phase detector 51 with the reference signal and subsequently 
the resulting signal is made to pass through the low-pass filter 52 at the 
output of which a direct current according to equation 4 is obtained. The 
constant k of this equation as well as the differential sensitivity 
(volt/watt) of the converters 3 is set with the aid of the adjustable 
pre-amplifier 50. 
The device 60 shown in FIGS. 1-3 is a stepper motor which is connected with 
the housing 9 and turns the same in steps of uniform length until the 
housing has passed through an angle of rotation of 180.degree., whereupon 
it returns in the opposite direction. In each position of the housing the 
temperature-equivalent direct voltages are scanned at the n outputs of the 
low pass filters 52 by means of the electronic multiplexer 53, and passed 
on to the analog-digital converter 60 which is connected in series with 
it. 
In the embodiment illustrated herein, the converter-signal electronic 50-52 
is provided for each individual converter 3, in order to assure that the 
transient state of the filters 52 and the time constant of the converters 
3 need be awaited only once per stepped position before the measuring 
signal becomes stable at the filter output. In this manner the shortest 
possible readout time is obtained. However, although not illustrated in 
the drawing it is perfectly possible to have longer time periods occur or 
to accept them. In such a case, only one multiplier 51 and one low-pass 
filter 52 are required and the multiplexer 53 is connected in circuit 
ahead of them. 
To return to the present embodiment, it is noted that after each step the 
stepper motor 16 remains for a brief time period in the new position, due 
to the operation of its motor control 57 (FIG. 4). This operating ratio is 
obtained by a counting of the pulses from the clock 54 in that the counter 
55 produces a single advancing-step pulse after it has counted a 
predetermined number of clock pulses. The logic circuit 56 switch is 
connected between the motor control 57 and the counter 55 changes the 
direction of rotation as soon as the predetermined number of steps has 
been carried out. The limits switch 58, which is connected between the 
counter 55 and the logic circuit 56 furnishes command signals to the logic 
circuit 56 in the event the motor 16 should go out of step and thus 
actuate the limit switch. At the same time, as the direction of rotation 
of the stepper motor 16 is reversed, the counter 55 is zeroed, i.e. 
returned to its starting position. 
After each of the steps described hereinbefore, the logic circuit 56 yields 
to the clock 59, the command signal to pass n number of pulses via the 
input 59' to the multiplexer 53. The n values for each position are 
converted in the analog to digital converter 60 into digital values and 
supplied to the threshold comparators 64, 65 and 66 of the circuit shown 
in FIG. 4. In the exemplary embodiment, the threshold value S.sub.3 in the 
comparator 66 corresponds to the coldest temperature of a high cloud. The 
threshold value S.sub.2 in the comparator 65 is lower than the value of 
S.sub.3 and corresponds to the coldest temperature of a medium-high cloud, 
whereas the threshold value S.sub.1 of the comparator 64 is in turn lower 
than the value of S.sub.2 and corresponds to the coldest temperature of a 
low cloud. 
The threshold values of the three aforementioned groups of clouds at 
different heights are dependent upon the zenith angle of the individual 
converters 3 in the optical system. This dependency is compensated by the 
zenith weighting device 63 which sets the threshold values in 
correspondence with the associated zenith angle of the signal present at 
the input 64', 65' and 66', of the comparators 64-66 respectively. The 
control of the device 63 is effected via the counter 61 which repeatedly 
counts the 1 to n clock pulses of the clock 59, which is connected in 
circuit between in the logic circuit 56 and the multiplexer 53, 
respectively the analog to digital converter 60, in order to pass on the 
associated threshold values S.sub.1, S.sub.2 and S.sub.3 to the 
comparaters 64-66, respectively. This comparators have at their outputs 
the logical condition "1" or "0", if the associated threshold value is or 
is not exceeded in downward direction. The common further logic circuit 67 
connected with the outputs has in turn one output each for low clouds 
(TB), medium clouds (MB) and high clouds (HB). 
A logical "1" at one of these outputs indicates that clouds are present at 
the corresponding height range. FIG. 5 is a table representing the logic 
67, i.e. associating the cloud condition with the differnt possibilities 
for the threshold values. 
The cloud surface reproduced at the individual convertors 3 depends upon 
the respective picture angle and the respective zenith angle of these 
convertors 3. Different area segments of the reproduced area of sky are 
accounted for in determination of the cloud cover by the use of the device 
62 which is a surface weighting device. The association is determined by 
the counting value of the counter 61, at the output 61' of which the 
surface weighting device 62 is connected. Its output leads to the clock 71 
which in this manner receives the command to pass a predetermined number 
of clock pulses via the input 68" 69" and 70" to the associated gating 
circuits 68, 69 and 70, respectively. If a logic "1" is present at the 
input 68', 69' and 70', while the clock 71 furnishes pulses to the inputs 
68", 69" and 70", then clock pulses are admitted into the corresponding 
gating circuits and counted in the counter 72 for low cloud cover, 73 for 
medium high cloud cover and 74 for high cloud cover; this counting is in 
eighths. A value of 8/8 indicates full cloud coverage at the corresponding 
height. The common counter 71 which is connected between the gating 
circuits 68-70 and their associated counters 72-74 counts all height 
ranges together and indicates the total cloud cover in eighths. The 
outputs of all counters 72-75 are then also connected with an indicator 
76-79 where the result is optically reproduced, for example in form of a 
segment indication. Of course, other ways of providing an optical 
representation of the result are also feasible. 
While the invention has been described hereinbefore with reference to an 
exemplary embodiment, it is to be understood that the invention is not 
limited to this embodiment and that the scope of the appended claims is 
intended to embrace all such modifications as will offer themselves to 
those having skill in the art.