Primary absolute radiometer

Claim 2

Difference of temperature distributions between radiative heating and electric heating.

Claim 3

Loss of heat by part of the incoming radiation being reflected out, or emitted as infrared radiation, through the aperture 27.

Claim 4

Loss of heat by part of the incoming radiation being reflected out, or emitted as infrared radiation, through the aperture 27.

Claim 5

Thermal resistance (temperature drop) in the black matte coating of the receptor cavity 10.

Claim 6

Thermal resistance (temperature drop) in the black matte coating of the receptor cavity 10.

Claim 7

Emitted infrared radiation into external space due to cavity temperature. FIG. 3 illustrates schematically an electronic circuit used for radiation measurements and calibration. As noted hereinbefore with reference to FIG. 2, two leads carry thermopile e.m.f. to be measured while four leads are connected to the heater windings 34 and 44. The two leads connected to the heating coil 44 are not used. A two-pole three-position switch S1 permits a digital voltmeter 50 to be so connected as to measure at positions 1, 2 and 3 the thermopile e.M.f., the voltage across the heating coil 34 and current through the heating coil 34, respectively. The last two measurements are made to determine power applied to the heating coil 34 via a switch S2 for calibration. During calibration, the cap 30 is placed over the aperture 27 (FIG. 1) to prevent radiation from entering the receptor cavity

Claim 8

Emitted infrared radiation into external space due to cavity temperature. FIG. 3 illustrates schematically an electronic circuit used for radiation measurements and calibration. As noted hereinbefore with reference to FIG. 2, two leads carry thermopile e.m.f. to be measured while four leads are connected to the heater windings 34 and 44. The two leads connected to the heating coil 44 are not used. A two-pole three-position switch S1 permits a digital voltmeter 50 to be so connected as to measure at positions 1, 2 and 3 the thermopile e.M.f., the voltage across the heating coil 34 and current through the heating coil 34, respectively. The last two measurements are made to determine power applied to the heating coil 34 via a switch S2 for calibration. During calibration, the cap 30 is placed over the aperture 27 (FIG. 1) to prevent radiation from entering the receptor cavity

Claim 9

Then the switch S2 is closed and electric power is applied to the heating coil 34. That power is measured very accurately by measuring voltage and current with switch S1 in positions 2 and 3, respectively. Following at, the corresponding thermopile e.m.f. is measured with the switch S1 in position 1. By knowing the e.m.f. from the thermopile for known heating power applied to the receptor cavity 10, a calibration factor K for the radiometer is obtained. Thereafter, upon removing the cap 30 from the aperture 27, values of incident radiation are determined by direct proportion. For example, if a calibration run reveals that 100 milliwatts of heating power causes a thermopile output e of 1 millivolt e.m.f., the calibration factor K would be 100, and a radiation input causing a thermopile e.m.f. of 2 millivolts would indicate that the incident radiation producing that e.m.f. is 200 milliwatts per square centimeter in intensity. Thus, the actual equation for a radiometer measurement is given by the following equation: W CKe+P where C is a thermal correction factor for the nonequivalences of the electric heating of the receptor cavity 10 during calibration, K is the calibration factor, and e is the measured thermopile e.m.f. obtained with the cap 30 removed from the aperture 27 (after power has been removed from the heating coil 34 and sufficient time has been allowed for temperatures of the receptor cavity 10 and the compensating cavity 14 to equalize as indicated by a null meter 51). In actual practice, the calibration operation requires about 2 minutes time, not counting time for computing the calibration factor K, and radiation intensity measurements can be made with 1 minute thereafter. The constant p is preferably added to the computed value W to account for energy lost by radiation through the aperture 27 which for the arrangement illustrated in FIGS. 1 and 2 was computed to be 0.0834 milliwatts for an aperture area of one square centimeter and 5* viewing angle. If accuracy to within 0.3 percent is not required, calculation and application of the correction factor C and the addition of a constant p to account for energy lost through the aperture 27 are not required. FIG. 4 illustrates some modifications which may be incorporated in the apparatus illustrated in FIGS. 1 and 2, either individually or in combination. Only half of a sectional view is shown since the structure is symmetrical. The first modification is removal of any view-limiting tube and muffler for hemispherical (180* ) viewing. The heat sink 11 is then supported directly from the inner housing cover 20. With such a modification, it may be necessary to control the temperature of the heat sink 11, depending upon the application, such as by a Peltier heat exchange unit or a jacket through which water of a constant temperature is circulated. To more accurately define the aperture of the receptor cavity 10 without a view-limiting tube, a short hollow cylindrical section 55 is added with an inside diameter equal to the desired aperture diameter. Then to virtually eliminate the possibility of incident radiation entering the space between the cavity 10 and the heat sink 11, a radiation guard is added in the cover 20 in the form of an annular groove 56 coated with a black matte. Most of the radiation which would tend to enter the space between the cavity 10 and the heat sink 11 is absorbed by sUch a radiation guard, and any radiation which gets by the guard is reflected by the external surfaces of the sections 31 and 32 of the cavity 10 into the heat sink 11 since both of the sections 31 and 32 are made of silver and highly polished on the outside. If the inside wall of the heat sink 11 is also blackened, radiation reflected by the external walls of the sections 31 and no heating of the receptor cavity 10 will result from any radiation entering the space between the cavity and the heat sink. It should be noted that removal of the view limiting tube and muffler to provide 180* viewing will make the instrument subject to wind disturbances, but not to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air. That is because the compensating cavity shown as an annular body 14'' and the annular supports 13 and 15 having substantial resistance to thermal conduction, but of course not such high resistance that, after a calibration or radiation measurement, too much time will be required for the temperatures of receptor cavity and the compensating cavity to equalize at the temperature of the heat sink. Thus, even though the effect of sudden pressure changes is eliminated, the effect of air currents, (wind) is not eliminated. A 180* viewing instrument will require a still air environment which can be provided by a hollow glass dome over the radiometer. The provision of the compensating cavity in the form of an annular mass 14'' conserves space and manufacturing costs. To achieve that modification, it is simply necessary to determine the total thermal capacity of the receptor cavity 10 and its heating coil and shield, and then provide the annular mass 14'' with equal thermal capacity. Another modification which may be used individually or in combination with other modifications is an inverted conical section 33'' with the heating coil 34 now inside the receptor cavity 10. The heating coil 34 is electrically insulated from the conical section 33'' as before by a thin layer of electrical insulating material, such as an epoxy resin, but is otherwise in good thermal contact to transfer radiant heat during a radiation measurement and electrical heat during calibration. In order to receive and transfer radiant heat to the conical section 33'', all exposed areas of the heating coil 34 are coated with a black matte just as all inside surfaces of the cavity are coated. This arrangement of the conical section 33'' has all of the advantages of the conical section 33 shown in FIG. 2, namely the ability to reflect any radiation not absorbed into the inside walls of he sections 31 and 32. In order that the heating coil 34 not detract significantly from that, a very fine wire may be used for he coil such that, upon being coated with a black matte, a substantially smooth surface is presented to incident radiation. Alternatively, the heating coil 34 may be placed outside the cavity 10 as before, but an advantage to having it inside the cavity 10 is that a heater shield is not required such as the conical shield 35 shown in FIG. 2. Moreover, the heating of the cavity 10 with electrical power during calibration involves virtually no losses since heat not transferred directly to the conical section 33'' is radiated to the walls of the sections 31 and 32 where it is absorbed, thereby providing accurate equivalence of electrical heating to radiation heating and hence providing more accurate calibration. Still another modification shown in Fig. 5 is the omission of a compensating cavity in which case the cold junctions of the thermopile are connected directly to the heat sink. While the omission of the compensating cavity will render the instrument sensitive to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air, all of the other advantage of the apparatus illustrated in FIGS. 1 and 2 with any of the other modifiCations described with reference to FIG. 4 will still provide a radiometer which will perform satisfactorily for many purposes. Yet another modification which can be used with the embodiment illustrated in FIGS. 1 and 2, either by itself or in combination with other modifications, is the substitution of resistive thermometers for the thermocouples. For example, a first coil of thin wire made of material having a high thermal coefficient of resistance, such as platinum, may be provided in place of the cold junctions, and a second coil of the same wire may be provided in place of the hot junctions. The two

Claim 10

Then the switch S2 is closed and electric power is applied to the heating coil 34. That power is measured very accurately by measuring voltage and current with switch S1 in positions 2 and 3, respectively. Following at, the corresponding thermopile e.m.f. is measured with the switch S1 in position 1. By knowing the e.m.f. from the thermopile for known heating power applied to the receptor cavity 10, a calibration factor K for the radiometer is obtained. Thereafter, upon removing the cap 30 from the aperture 27, values of incident radiation are determined by direct proportion. For example, if a calibration run reveals that 100 milliwatts of heating power causes a thermopile output e of 1 millivolt e.m.f., the calibration factor K would be 100, and a radiation input causing a thermopile e.m.f. of 2 millivolts would indicate that the incident radiation producing that e.m.f. is 200 milliwatts per square centimeter in intensity. Thus, the actual equation for a radiometer measurement is given by the following equation: W CKe+P where C is a thermal correction factor for the nonequivalences of the electric heating of the receptor cavity 10 during calibration, K is the calibration factor, and e is the measured thermopile e.m.f. obtained with the cap 30 removed from the aperture 27 (after power has been removed from the heating coil 34 and sufficient time has been allowed for temperatures of the receptor cavity 10 and the compensating cavity 14 to equalize as indicated by a null meter 51). In actual practice, the calibration operation requires about 2 minutes time, not counting time for computing the calibration factor K, and radiation intensity measurements can be made with 1 minute thereafter. The constant p is preferably added to the computed value W to account for energy lost by radiation through the aperture 27 which for the arrangement illustrated in FIGS. 1 and 2 was computed to be 0.0834 milliwatts for an aperture area of one square centimeter and 5* viewing angle. If accuracy to within 0.3 percent is not required, calculation and application of the correction factor C and the addition of a constant p to account for energy lost through the aperture 27 are not required. FIG. 4 illustrates some modifications which may be incorporated in the apparatus illustrated in FIGS. 1 and 2, either individually or in combination. Only half of a sectional view is shown since the structure is symmetrical. The first modification is removal of any view-limiting tube and muffler for hemispherical (180* ) viewing. The heat sink 11 is then supported directly from the inner housing cover 20. With such a modification, it may be necessary to control the temperature of the heat sink 11, depending upon the application, such as by a Peltier heat exchange unit or a jacket through which water of a constant temperature is circulated. To more accurately define the aperture of the receptor cavity 10 without a view-limiting tube, a short hollow cylindrical section 55 is added with an inside diameter equal to the desired aperture diameter. Then to virtually eliminate the possibility of incident radiation entering the space between the cavity 10 and the heat sink 11, a radiation guard is added in the cover 20 in the form of an annular groove 56 coated with a black matte. Most of the radiation which would tend to enter the space between the cavity 10 and the heat sink 11 is absorbed by sUch a radiation guard, and any radiation which gets by the guard is reflected by the external surfaces of the sections 31 and 32 of the cavity 10 into the heat sink 11 since both of the sections 31 and 32 are made of silver and highly polished on the outside. If the inside wall of the heat sink 11 is also blackened, radiation reflected by the external walls of the sections 31 and no heating of the receptor cavity 10 will result from any radiation entering the space between the cavity and the heat sink. It should be noted that removal of the view limiting tube and muffler to provide 180* viewing will make the instrument subject to wind disturbances, but not to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air. That is because the compensating cavity shown as an annular body 14'' and the annular supports 13 and 15 having substantial resistance to thermal conduction, but of course not such high resistance that, after a calibration or radiation measurement, too much time will be required for the temperatures of receptor cavity and the compensating cavity to equalize at the temperature of the heat sink. Thus, even though the effect of sudden pressure changes is eliminated, the effect of air currents, (wind) is not eliminated. A 180* viewing instrument will require a still air environment which can be provided by a hollow glass dome over the radiometer. The provision of the compensating cavity in the form of an annular mass 14'' conserves space and manufacturing costs. To achieve that modification, it is simply necessary to determine the total thermal capacity of the receptor cavity 10 and its heating coil and shield, and then provide the annular mass 14'' with equal thermal capacity. Another modification which may be used individually or in combination with other modifications is an inverted conical section 33'' with the heating coil 34 now inside the receptor cavity 10. The heating coil 34 is electrically insulated from the conical section 33'' as before by a thin layer of electrical insulating material, such as an epoxy resin, but is otherwise in good thermal contact to transfer radiant heat during a radiation measurement and electrical heat during calibration. In order to receive and transfer radiant heat to the conical section 33'', all exposed areas of the heating coil 34 are coated with a black matte just as all inside surfaces of the cavity are coated. This arrangement of the conical section 33'' has all of the advantages of the conical section 33 shown in FIG. 2, namely the ability to reflect any radiation not absorbed into the inside walls of he sections 31 and 32. In order that the heating coil 34 not detract significantly from that, a very fine wire may be used for he coil such that, upon being coated with a black matte, a substantially smooth surface is presented to incident radiation. Alternatively, the heating coil 34 may be placed outside the cavity 10 as before, but an advantage to having it inside the cavity 10 is that a heater shield is not required such as the conical shield 35 shown in FIG. 2. Moreover, the heating of the cavity 10 with electrical power during calibration involves virtually no losses since heat not transferred directly to the conical section 33'' is radiated to the walls of the sections 31 and 32 where it is absorbed, thereby providing accurate equivalence of electrical heating to radiation heating and hence providing more accurate calibration. Still another modification shown in Fig. 5 is the omission of a compensating cavity in which case the cold junctions of the thermopile are connected directly to the heat sink. While the omission of the compensating cavity will render the instrument sensitive to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air, all of the other advantage of the apparatus illustrated in FIGS. 1 and 2 with any of the other modifiCations described with reference to FIG. 4 will still provide a radiometer which will perform satisfactorily for many purposes. Yet another modification which can be used with the embodiment illustrated in FIGS. 1 and 2, either by itself or in combination with other modifications, is the substitution of resistive thermometers for the thermocouples. For example, a first coil of thin wire made of material having a high thermal coefficient of resistance, such as platinum, may be provided in place of the cold junctions, and a second coil of the same wire may be provided in place of the hot junctions. The two

Claim 11

Loss of heat by part of the incoming radiation being reflected out, or emitted as infrared radiation, through the aperture 27.

Claim 12

Loss of heat by part of the incoming radiation being reflected out, or emitted as infrared radiation, through the aperture 27.

Claim 13

The combination of claim 1 including a housing surrounding said heat sink to shield said heat sink from radiation, said housing including means for limiting the view of a radiation source by said receptor through said heat sink aperture.

Claim 14

The combination of claim 2 including a muffler comprising a tubular member of a thermal conductive material with annular baffles axially disposed between said view limiting means and said heat sink aperture, said muffler being in thermal contact with said heat sink.

Claim 15

Thermal resistance (temperature drop) in the black matte coating of the receptor cavity 10.

Claim 16

Thermal resistance (temperature drop) in the black matte coating of the receptor cavity 10.

Claim 17

Emitted infrared radiation into external space due to cavity temperature. FIG. 3 illustrates schematically an electronic circuit used for radiation measurements and calibration. As noted hereinbefore with reference to FIG. 2, two leads carry thermopile e.m.f. to be measured while four leads are connected to the heater windings 34 and 44. The two leads connected to the heating coil 44 are not used. A two-pole three-position switch S1 permits a digital voltmeter 50 to be so connected as to measure at positions 1, 2 and 3 the thermopile e.M.f., the voltage across the heating coil 34 and current through the heating coil 34, respectively. The last two measurements are made to determine power applied to the heating coil 34 via a switch S2 for calibration. During calibration, the cap 30 is placed over the aperture 27 (FIG. 1) to prevent radiation from entering the receptor cavity

Claim 18

Emitted infrared radiation into external space due to cavity temperature. FIG. 3 illustrates schematically an electronic circuit used for radiation measurements and calibration. As noted hereinbefore with reference to FIG. 2, two leads carry thermopile e.m.f. to be measured while four leads are connected to the heater windings 34 and 44. The two leads connected to the heating coil 44 are not used. A two-pole three-position switch S1 permits a digital voltmeter 50 to be so connected as to measure at positions 1, 2 and 3 the thermopile e.M.f., the voltage across the heating coil 34 and current through the heating coil 34, respectively. The last two measurements are made to determine power applied to the heating coil 34 via a switch S2 for calibration. During calibration, the cap 30 is placed over the aperture 27 (FIG. 1) to prevent radiation from entering the receptor cavity

Claim 19

The combination of claim 3 wherein said view-limiting means includes a tubular member extending from said housing and axially aligned with said muffler and heat sink aperture.

Claim 20

The combination of claim 3 including a thermal insulating jacket separating said housing from said heat sink and muffler.

Claim 21

In a radiometer, the combination comprising: an internally blackened radiation receptor cavity; a heat sink having very large thermal capacity in comparison to said cavity; a supporting member for spacing said cavity away from said heat sink a given distance, said member having a given resistance to heat flow; a compensating body having a thermal capacity substantially equal to the thermal capacity of said cavity; a supporting member for spacing said body away from said heat sink a given distance, said body-supporting member having a resistance to heat flow substantially equal to said given resistance; means for shielding said heat sink receptor cavity, compensating body and supporting members from external radiation except into said cavity; means for limiting radiation into said cavity through a precisely known area outside of said cavity; means for measuring any difference in temperature between said radiation receptor cavity and said compensating body, said difference being a measure of radiation intensity being received by said recePtor cavity; and means for electrically heating said cavity in a manner substantially equivalent to heating said cavity by radiation to calibrate said measuring means.

Claim 22

In a radiometer, the combination comprising: an internally blackened radiation receptor cavity; a heat sink having very large thermal capacity in comparison to said cavity; a supporting member for spacing said cavity away from said heat sink a given distance, said member having a given resistance to heat flow; a compensating body having a thermal capacity substantially equal to the thermal capacity of said cavity; a supporting member for spacing said body away from said heat sink a given distance, said body-supporting member having a resistance to heat flow substantially equal to said given resistance; means for shielding said heat sink receptor cavity, compensating body and supporting members from external radiation except into said cavity; means for limiting radiation into said cavity through a precisely known area outside of said cavity; means for measuring any difference in temperature between said radiation receptor cavity and said compensating body, said difference being a measure of radiation intensity being received by said recePtor cavity; and means for electrically heating said cavity in a manner substantially equivalent to heating said cavity by radiation to calibrate said measuring means.

Claim 23

The combination of claim 6 wherein said receptor cavity comprises a hollow cone made of material having high thermal conductance positioned to receive radiation from said aperture, said aperture being centered on the axis of said cone, and disposed in a plane perpendicular to said axis, and wherein said electrical heating means comprises a coil in thermal contact with said cone.

Claim 24

The combination of claim 7 wherein said cone is disposed to receive radiation in its conical cavity and said heating coil is in thermal contact with the external surface of said cone, and including a conical shield made of high thermal conductive material spaced over said heating coil, the space between said receptor cavity cone and said conical shield being closed by a radially extending portion of said cone at the base thereof, whereby radiant heat from said heating coil absorbed by said shield is conducted into said receptor cavity.

Claim 25

The combination of claim 7 wherein said compensating body comprises a structure of parts substantially identical to parts from which said receptor cavity is made, including said hollow cone, and a dummy coil in thermal contact with a part of said body corresponding to said hollow cone of said conical cavity, said dummy coil being substantially identical to said heating coil.

Claim 26

The combination of claim 6 wherein said radiation receptor cavity comprises: a hollow cylindrical portion disposed with its axis centered on the axis of said aperture to receive radiation through one end thereof, a hollow conical portion disposed within said cylindrical portion with the axis of said conical portion along the axis of said cylindrical portion, and having its base in thermal contact with said cylindrical portion around its entire circumference at the end thereof opposite said one end, and wherein said heating means comprises a coil in thermal contact with said cone on the surface thereof surrounded by said cylindrical portion.

Claim 27

Then the switch S2 is closed and electric power is applied to the heating coil 34. That power is measured very accurately by measuring voltage and current with switch S1 in positions 2 and 3, respectively. Following at, the corresponding thermopile e.m.f. is measured with the switch S1 in position 1. By knowing the e.m.f. from the thermopile for known heating power applied to the receptor cavity 10, a calibration factor K for the radiometer is obtained. Thereafter, upon removing the cap 30 from the aperture 27, values of incident radiation are determined by direct proportion. For example, if a calibration run reveals that 100 milliwatts of heating power causes a thermopile output e of 1 millivolt e.m.f., the calibration factor K would be 100, and a radiation input causing a thermopile e.m.f. of 2 millivolts would indicate that the incident radiation producing that e.m.f. is 200 milliwatts per square centimeter in intensity. Thus, the actual equation for a radiometer measurement is given by the following equation: W CKe+P where C is a thermal correction factor for the nonequivalences of the electric heating of the receptor cavity 10 during calibration, K is the calibration factor, and e is the measured thermopile e.m.f. obtained with the cap 30 removed from the aperture 27 (after power has been removed from the heating coil 34 and sufficient time has been allowed for temperatures of the receptor cavity 10 and the compensating cavity 14 to equalize as indicated by a null meter 51). In actual practice, the calibration operation requires about 2 minutes time, not counting time for computing the calibration factor K, and radiation intensity measurements can be made with 1 minute thereafter. The constant p is preferably added to the computed value W to account for energy lost by radiation through the aperture 27 which for the arrangement illustrated in FIGS. 1 and 2 was computed to be 0.0834 milliwatts for an aperture area of one square centimeter and 5* viewing angle. If accuracy to within 0.3 percent is not required, calculation and application of the correction factor C and the addition of a constant p to account for energy lost through the aperture 27 are not required. FIG. 4 illustrates some modifications which may be incorporated in the apparatus illustrated in FIGS. 1 and 2, either individually or in combination. Only half of a sectional view is shown since the structure is symmetrical. The first modification is removal of any view-limiting tube and muffler for hemispherical (180* ) viewing. The heat sink 11 is then supported directly from the inner housing cover 20. With such a modification, it may be necessary to control the temperature of the heat sink 11, depending upon the application, such as by a Peltier heat exchange unit or a jacket through which water of a constant temperature is circulated. To more accurately define the aperture of the receptor cavity 10 without a view-limiting tube, a short hollow cylindrical section 55 is added with an inside diameter equal to the desired aperture diameter. Then to virtually eliminate the possibility of incident radiation entering the space between the cavity 10 and the heat sink 11, a radiation guard is added in the cover 20 in the form of an annular groove 56 coated with a black matte. Most of the radiation which would tend to enter the space between the cavity 10 and the heat sink 11 is absorbed by sUch a radiation guard, and any radiation which gets by the guard is reflected by the external surfaces of the sections 31 and 32 of the cavity 10 into the heat sink 11 since both of the sections 31 and 32 are made of silver and highly polished on the outside. If the inside wall of the heat sink 11 is also blackened, radiation reflected by the external walls of the sections 31 and no heating of the receptor cavity 10 will result from any radiation entering the space between the cavity and the heat sink. It should be noted that removal of the view limiting tube and muffler to provide 180* viewing will make the instrument subject to wind disturbances, but not to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air. That is because the compensating cavity shown as an annular body 14'' and the annular supports 13 and 15 having substantial resistance to thermal conduction, but of course not such high resistance that, after a calibration or radiation measurement, too much time will be required for the temperatures of receptor cavity and the compensating cavity to equalize at the temperature of the heat sink. Thus, even though the effect of sudden pressure changes is eliminated, the effect of air currents, (wind) is not eliminated. A 180* viewing instrument will require a still air environment which can be provided by a hollow glass dome over the radiometer. The provision of the compensating cavity in the form of an annular mass 14'' conserves space and manufacturing costs. To achieve that modification, it is simply necessary to determine the total thermal capacity of the receptor cavity 10 and its heating coil and shield, and then provide the annular mass 14'' with equal thermal capacity. Another modification which may be used individually or in combination with other modifications is an inverted conical section 33'' with the heating coil 34 now inside the receptor cavity 10. The heating coil 34 is electrically insulated from the conical section 33'' as before by a thin layer of electrical insulating material, such as an epoxy resin, but is otherwise in good thermal contact to transfer radiant heat during a radiation measurement and electrical heat during calibration. In order to receive and transfer radiant heat to the conical section 33'', all exposed areas of the heating coil 34 are coated with a black matte just as all inside surfaces of the cavity are coated. This arrangement of the conical section 33'' has all of the advantages of the conical section 33 shown in FIG. 2, namely the ability to reflect any radiation not absorbed into the inside walls of he sections 31 and 32. In order that the heating coil 34 not detract significantly from that, a very fine wire may be used for he coil such that, upon being coated with a black matte, a substantially smooth surface is presented to incident radiation. Alternatively, the heating coil 34 may be placed outside the cavity 10 as before, but an advantage to having it inside the cavity 10 is that a heater shield is not required such as the conical shield 35 shown in FIG. 2. Moreover, the heating of the cavity 10 with electrical power during calibration involves virtually no losses since heat not transferred directly to the conical section 33'' is radiated to the walls of the sections 31 and 32 where it is absorbed, thereby providing accurate equivalence of electrical heating to radiation heating and hence providing more accurate calibration. Still another modification shown in Fig. 5 is the omission of a compensating cavity in which case the cold junctions of the thermopile are connected directly to the heat sink. While the omission of the compensating cavity will render the instrument sensitive to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air, all of the other advantage of the apparatus illustrated in FIGS. 1 and 2 with any of the other modifiCations described with reference to FIG. 4 will still provide a radiometer which will perform satisfactorily for many purposes. Yet another modification which can be used with the embodiment illustrated in FIGS. 1 and 2, either by itself or in combination with other modifications, is the substitution of resistive thermometers for the thermocouples. For example, a first coil of thin wire made of material having a high thermal coefficient of resistance, such as platinum, may be provided in place of the cold junctions, and a second coil of the same wire may be provided in place of the hot junctions. The two

Claim 28

Then the switch S2 is closed and electric power is applied to the heating coil 34. That power is measured very accurately by measuring voltage and current with switch S1 in positions 2 and 3, respectively. Following at, the corresponding thermopile e.m.f. is measured with the switch S1 in position 1. By knowing the e.m.f. from the thermopile for known heating power applied to the receptor cavity 10, a calibration factor K for the radiometer is obtained. Thereafter, upon removing the cap 30 from the aperture 27, values of incident radiation are determined by direct proportion. For example, if a calibration run reveals that 100 milliwatts of heating power causes a thermopile output e of 1 millivolt e.m.f., the calibration factor K would be 100, and a radiation input causing a thermopile e.m.f. of 2 millivolts would indicate that the incident radiation producing that e.m.f. is 200 milliwatts per square centimeter in intensity. Thus, the actual equation for a radiometer measurement is given by the following equation: W CKe+P where C is a thermal correction factor for the nonequivalences of the electric heating of the receptor cavity 10 during calibration, K is the calibration factor, and e is the measured thermopile e.m.f. obtained with the cap 30 removed from the aperture 27 (after power has been removed from the heating coil 34 and sufficient time has been allowed for temperatures of the receptor cavity 10 and the compensating cavity 14 to equalize as indicated by a null meter 51). In actual practice, the calibration operation requires about 2 minutes time, not counting time for computing the calibration factor K, and radiation intensity measurements can be made with 1 minute thereafter. The constant p is preferably added to the computed value W to account for energy lost by radiation through the aperture 27 which for the arrangement illustrated in FIGS. 1 and 2 was computed to be 0.0834 milliwatts for an aperture area of one square centimeter and 5* viewing angle. If accuracy to within 0.3 percent is not required, calculation and application of the correction factor C and the addition of a constant p to account for energy lost through the aperture 27 are not required. FIG. 4 illustrates some modifications which may be incorporated in the apparatus illustrated in FIGS. 1 and 2, either individually or in combination. Only half of a sectional view is shown since the structure is symmetrical. The first modification is removal of any view-limiting tube and muffler for hemispherical (180* ) viewing. The heat sink 11 is then supported directly from the inner housing cover 20. With such a modification, it may be necessary to control the temperature of the heat sink 11, depending upon the application, such as by a Peltier heat exchange unit or a jacket through which water of a constant temperature is circulated. To more accurately define the aperture of the receptor cavity 10 without a view-limiting tube, a short hollow cylindrical section 55 is added with an inside diameter equal to the desired aperture diameter. Then to virtually eliminate the possibility of incident radiation entering the space between the cavity 10 and the heat sink 11, a radiation guard is added in the cover 20 in the form of an annular groove 56 coated with a black matte. Most of the radiation which would tend to enter the space between the cavity 10 and the heat sink 11 is absorbed by sUch a radiation guard, and any radiation which gets by the guard is reflected by the external surfaces of the sections 31 and 32 of the cavity 10 into the heat sink 11 since both of the sections 31 and 32 are made of silver and highly polished on the outside. If the inside wall of the heat sink 11 is also blackened, radiation reflected by the external walls of the sections 31 and no heating of the receptor cavity 10 will result from any radiation entering the space between the cavity and the heat sink. It should be noted that removal of the view limiting tube and muffler to provide 180* viewing will make the instrument subject to wind disturbances, but not to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air. That is because the compensating cavity shown as an annular body 14'' and the annular supports 13 and 15 having substantial resistance to thermal conduction, but of course not such high resistance that, after a calibration or radiation measurement, too much time will be required for the temperatures of receptor cavity and the compensating cavity to equalize at the temperature of the heat sink. Thus, even though the effect of sudden pressure changes is eliminated, the effect of air currents, (wind) is not eliminated. A 180* viewing instrument will require a still air environment which can be provided by a hollow glass dome over the radiometer. The provision of the compensating cavity in the form of an annular mass 14'' conserves space and manufacturing costs. To achieve that modification, it is simply necessary to determine the total thermal capacity of the receptor cavity 10 and its heating coil and shield, and then provide the annular mass 14'' with equal thermal capacity. Another modification which may be used individually or in combination with other modifications is an inverted conical section 33'' with the heating coil 34 now inside the receptor cavity 10. The heating coil 34 is electrically insulated from the conical section 33'' as before by a thin layer of electrical insulating material, such as an epoxy resin, but is otherwise in good thermal contact to transfer radiant heat during a radiation measurement and electrical heat during calibration. In order to receive and transfer radiant heat to the conical section 33'', all exposed areas of the heating coil 34 are coated with a black matte just as all inside surfaces of the cavity are coated. This arrangement of the conical section 33'' has all of the advantages of the conical section 33 shown in FIG. 2, namely the ability to reflect any radiation not absorbed into the inside walls of he sections 31 and 32. In order that the heating coil 34 not detract significantly from that, a very fine wire may be used for he coil such that, upon being coated with a black matte, a substantially smooth surface is presented to incident radiation. Alternatively, the heating coil 34 may be placed outside the cavity 10 as before, but an advantage to having it inside the cavity 10 is that a heater shield is not required such as the conical shield 35 shown in FIG. 2. Moreover, the heating of the cavity 10 with electrical power during calibration involves virtually no losses since heat not transferred directly to the conical section 33'' is radiated to the walls of the sections 31 and 32 where it is absorbed, thereby providing accurate equivalence of electrical heating to radiation heating and hence providing more accurate calibration. Still another modification shown in Fig. 5 is the omission of a compensating cavity in which case the cold junctions of the thermopile are connected directly to the heat sink. While the omission of the compensating cavity will render the instrument sensitive to fluctuations of temperature due to adiabatic compression or expansion of the surrounding air, all of the other advantage of the apparatus illustrated in FIGS. 1 and 2 with any of the other modifiCations described with reference to FIG. 4 will still provide a radiometer which will perform satisfactorily for many purposes. Yet another modification which can be used with the embodiment illustrated in FIGS. 1 and 2, either by itself or in combination with other modifications, is the substitution of resistive thermometers for the thermocouples. For example, a first coil of thin wire made of material having a high thermal coefficient of resistance, such as platinum, may be provided in place of the cold junctions, and a second coil of the same wire may be provided in place of the hot junctions. The two

Claim 29

A radiometer comprising: a radiation-receiving cavity internally coated with a black matte, said cavity having an aperture and being made of metal having substantially high thermal conductance; a heat sink surrounding said cavity, said heat sink having an opening over the aperture of said cavity; a supporting member for holding said cavity stationary out of contact with said heat sink, said member having a predetermined resistance to heat flow from said cavity to said heat sink, said predetermined resistance being selected to provide a time constant of not more than a fraction of one minute; and means for measuring any difference in temperature between said cavity and said heat sink, said difference being a measure of radiation intensity being received by said receptor.

Claim 30

A radiometer comprising: a radiation-receiving cavity internally coated with a black matte, said cavity having an aperture and being made of metal having substantially high thermal conductance; a heat sink surrounding said cavity, said heat sink having an opening over the aperture of said cavity; a supporting member for holding said cavity stationary out of contact with said heat sink, said member having a predetermined resistance to heat flow from said cavity to said heat sink, said predetermined resistance being selected to provide a time constant of not more than a fraction of one minute; and means for measuring any difference in temperature between said cavity and said heat sink, said difference being a measure of radiation intensity being received by said receptor.

Claim 31

A radiometer as defined in claim 11 wherein said last named means comprises: a compensating mass having a thermal capacity substantially equal to the thermal capacity of said cavity, said mass being surrounded by said heat sink; a supporting member holding said mass stationary out of contact with said heat sink, said mass supporting member having a resistance to heat flow from said heat sink to said mass substantially equal to said predetermined resistance to heat flow of said cavity support member; and separate temperature sensing elements thermally connected to said cavity and said compensating means.

Claim 32

A radiometer as defined in claim 11 including: a heating coil in thermal contact with said cavity; means for applying power to said coil while said cavity is not receiving radiation; and means for measuring power applied to achieve a measured temperature difference, whereby said radiometer may be calibrated.

Claim 33

A radiometer as defined in claim 14 including a view-limiting tube attached to said housing and axially alIgned with said muffler, said view-limiting tube having a cap on the end thereof with an aperture of desired diameter, and annular baffles disposed along the internal wall of said tube.

Claim 34

In a radiometer, the combination comprising: an internally blackened receptor cavity comprising thermally connected parts; a heat sink; a supporting member spacing said receptor cavity away from said heat sink, said member having a given resistance to thermal conduction sufficient for a significant time lapse to attain steady heat flow from said cavity through said supporting member into said heat sink; a compensating cavity comprising thermally connected parts substantially identical to parts of said receptor cavity; and a supporting member spacing said receptor cavity away from said heat sink in a position shielded from radiation intended to be received by said receptor cavity, said member being substantially equal to said supporting member for said receptor cavity in both thermal capacity and thermal resistance.

Claim 35

In a radiometer, the combination comprising: an internally blackened receptor cavity comprising thermally connected parts; a heat sink; a supporting member spacing said receptor cavity away from said heat sink, said member having a given resistance to thermal conduction sufficient for a significant time lapse to attain steady heat flow from said cavity through said supporting member into said heat sink; a compensating cavity comprising thermally connected parts substantially identical to parts of said receptor cavity; and a supporting member spacing said receptor cavity away from said heat sink in a position shielded from radiation intended to be received by said receptor cavity, said member being substantially equal to said supporting member for said receptor cavity in both thermal capacity and thermal resistance.

Claim 36

The combination as defined in claim 16 including a heating coil having each turn thereof placed in close thermal contact with said receptor cavity.

Claim 37

The combination of claim 17 wherein said heat sink surrounds said receptor and compensating cavities except over said receptor cavity on the end thereof intended to receive radiation, said heat sink having an aperture of known area over said end.

Claim 38

The combination of claim 18 including means surrounding said heat sink except over said area for thermally insulating said heat sink from ambient thermal radiation.