IMPROVEMENTS IN PYRGEOMETERS

Abstract: A pyrgeometer for measuring infra-red radiation having a sensor which includes a thermopile comprising a plurality of thermocouple junctions (22) between at least two metallic compositions alternating along a thermocouple track (21) deposited on one surface of a planar substrate (13), hot junctions being located in infra-red absorbing regions of the substrate and cold junctions being located in infra-red reflecting regions of the substrate, the substrate being mounted across an essentially circular orifice of a body (10) acting as a heat sink and adjacent hot and cold junctions being located on the substrate substantially equidistant from the heat sink.

Description: IMPROVEMENTS IN PYRGEOMETERS The present invention is concerned with pyrgeometers suitable for measurement of infra-red radiation and improve¬ ments therein, more particularly with an instrument possessing a lower susceptablllty to internal thermal gradients than known instruments. In the known pyrgeometers adapted for the measurement of infra-red radiation the instruments employ a thermopile located beneath a dome that selectively transmits only longwave (over 4 μ_α) radiation. The thermopile hot junctions are exposed to the incident radiation whilst the cold junctions are shielded from the incident radiation. The thermopiles are normally in the form of wire wound thermocouples, wound on large metal formers, the cold junctions of which are in thermal contact with the metal former and located in the body of the instrument. This arrangement is susceptible to thermal gradients in the instrument which may in turn lead to spurious signals and results. In other forms of the instrument the thermopile takes the form of a track of two alternating metallic compositions deposited on a substrate mounted across an orifice in the body of the instrument, the hot junctions being generally located in the centre of the orifice and the cold junctions being located in proximity to the periphery of the substrate. The latter can be kept cold by coating with a radiation-reflecting substance and by use of the heat sink properties of the body. SUBSTITUTESHEET UK patents nos 1 342 710 and 1 603 363, and US patents 3 405 273 and 3 424 624, all describe such instruments in which one of the junction types is held at a reference temperature by location in proximity to a heat sink. In practice, these may be either the hot or the cold junctions. Such instruments suffer however from the disadvantage that they may provide an erroneous reading when the surround¬ ings are changing in temperature, the centre of the substrate being able to adapt more quickly than the body of the instrument. According to this invention a pyrgeometer for measuring infra-red radiation has a sensor which includes a thermopile comprising a plurality of thermocouple junctions between at least two different metallic compositions alternating along a thermocouple track deposited on one surface of a planar sub¬ strate, the hot junctions being located in infra-red absorbing regions of the substrate and the cold junctions being located in infra-red reflecting regions of the substrate, characterised in that the substrate is mounted across an essentially circular orifice of a body which can act as a heat sink and, as far as possible, adjacent hot and cold junctions are located on the substrate substantially equidistant from the heat sink. An advantage of the present invention is the provision of a sensor in the form of a flat planar assembly in which the thermocouple hot and cold junctions are located in the same plane, so minimizing the effect of the thermal gradients generated by heat sources other than the incident radiation, and thereby reducing the risk of spurious signals arising therefrom. According to a further aspect of the present invention there is provided a pyrgeometer in which the thermocouples comprise two different metals each interconnected by an intermediate metal, the intermediate metal portions of the track forming alternately hot and cold thermocouple junctions located in infra-red reflecting and absorbing regions SUBSTITUTE SHEET respectively, the infra-red absorbing regions being coated with an infra-red absorbing material, the infra-red reflecting regions being bare metal. The intermediate metal may be used with advantage because it provides a uniform consistency between junctions enabling the hot junctions to be easily covered by an infra-red absorbing matter, the cold junctions may be left uncoated because metals are good infra-red reflectors. According to a further aspect of the present invention the planar sensor assembly is a non-conducting substrate material, having on a first surface regions adapted to reflect infra-red radiation and regions adapted to absorb infra-red radiation and on a second surface a plurality of metal tracks formed by a plurality of metals running successively along the track, there being junctions between different types of metals forming thermocouples, successive thermocouple junctions located alternately in infra-red reflecting regions. An advantage of the present invention is the use of both surfaces of the substrate material, the thermocouples being disposed on one surface and the reflective and absorptive coatings being disposed on the obverse surface. According to a further aspect of the present invention there is provided a pyrgeometer with thermocouples comprising bismuth and antimony. The advantage of using bismuth and antimony as thermo¬ couple metals is their large thermoelectric voltage generated under the conditions of use of the instrument. According to a further aspect of the present invention there is provided a thermopile comprising a plurality of thermocouples, the hot junctions of the thermocouple located in an infra-red radiation absorbing region and the cold junctions are located in an infra-red radiation reflection layer, the reflection layer in the form of a layer of gold. An advantage of the present invention is the use of Infra-red absorbing layers and infra-red reflecting layers to SUBSTITUTESHEET enhance the performance of the thermocouples. Conveniently gold is a good reflector, having a fairly consistent performance in the radiation waveband of interest. According to a further aspect of the present invention there is provided a pyrgeometer for detecting infra-red radiation having a radiation sensor, the radiation sensor comprising a planar non-conducting substrate material, on a first surface of the substrate material and plurality of metal tracks having junctions of different types of metals, and forming thermocouples the hot junctions of the thermocouples located in infra-red reflecting regions, the second surface of the substrate being covered with infra-red absorbing material and infra-red reflecting material in the form of a layer of gold. An advantage of the present invention is the provision of a plurality of thermocouple junctions located on one side of a planar substrate thereby minimizing the thermal gradients in the sensor assembly and so decreasing its susceptablllty to spurious signals or radiative effects arising in the body of the instrument. In order that the present invention may be more easily understood it will now be described in greater detail and with reference to the accompanying drawings of which Figure 1 is a section of a completely assembled pyrgeometer, and Figures 2 and 3 are plan views of the sensor devices used in the pyrgeometer, the sensor of figure 3 being a modification of Figure 2. Figure 1 shows an Instrument body (10) mountable in any suitable location. The instrument has a domed silicon window (11) having substantially uniform transmission characteristics for permitting radiation to be incident upon the sensor. The sensor (12) is located in the body of the instrument. In this embodiment a hemispherical window is advantageous because SUBSTITUTESHEET incident radiation may be collected from an entire hemisphere and the distance from any point on the hemisphere to the central point of the sensor element is the same, thus the incident radiation will be uniformly transmitted across the hemisphere to the sensor and the possibility of distortion effects are thereby reduced. Figure 2 is a plan view of the sensor element (12). The sensor body (20) is formed from a substrate base of non¬ conducting material. On the surface of the substrate base is a metal track (21). The metal track comprises at least two different metals deposited sequentially around the track so as to form a plurality of thermocouples connected in series and comprising a thermopile. The thermocouple junctions (22) are located on the surface of the substrate and are the contact points of two dissimilar metals. In this emboidment it has been found to be advantageous to use antimony and bismuth as the thermocouple metals. The use of a plurality of thermo¬ couples is advantageous because it results in an algebraic summation of the thermally generated voltage thereby permitting easier and more accurate detection of a temperature gradient. The contact points (23) enable wires to be contacted to the end of the thermocouples, the wires then being connectable to remotely located voltage detection means. Further advantages arising from the use of a plurality of thermocouples located on different parts on the substrate are the minimizing of any spurious signals or temperature measurements caused by local temperature gradients. The substrate is separated into six segmented regions, three of which are coated with an infra-red radiation absorbing material, which in this embodiment is optical matt black paint, although any suitable infra-red absorber will suffice. The other regions are left uncoated and have only bare metal junctions which will reflect infra-red radiation. These separate regions are labelled H and C. The regions labelled H are those coated with the infra-red absorbing material and those labelled C are bare metal. The SUBSTITUTE SHEET effect of this is to reflect the incident infra-red radiation from the 'C regions and absorb it in the 'H' regions. As a consequence there arises a temperature differential between the two regions. It will be noticed that successive thermocouple junctions are located in the 'H' and 'C regions. The junctions in the 'H* region are the hot junctions and those in the 'C region form the cold junctions. The incoming radiation will cause the temperature of the hot junctions to rise, thereby establishing a temperature differential between the hot and cold temperatures of cold junctions of the thermocouples. This results in a voltage across the thermopile terminals which is dependent upon the level of incident radiation. From the differential voltage the level of incident radiation may be determined. A particular advantage of the present arrangement is that the thermocouples are all in close proximity and spurious effects due to thermal gradients in the body of the instrument due to spatial separation of the junctions are largely eliminated. The metal tracks may be conveniently deposited on the surface of the substrate by the use of masks and known vapour deposition processes. Figure 3 shows an alternative arrangement of thermo¬ couple tracks intended to improve the spectral consistency of the response over the region of particular interest (4-100 μ ). The layout of the tracks is the same as that of figure 2 but with the following alterations. A gold track is used to interconnect the antimony and bismuth thermocouple metals. The gold strips (30 and 31) link antimony and bismuth track elements. Thus each junction has gold as one of the metals. Gold is an excellent infra-red radiation reflector with a substantially constant coeffient of reflectivity in the 4-100 urn waveband and so particularly suitable as a reflecting material for this application. Thus the infrared reflecting regions C are those having bare metal gold tracks (31). The infra-red absorbing regions H, also have the interconnecting S UBST/ UTE gold layer (30) but in this instance it is painted with an optical matt black paint to enable it to absorb the infrared radiation. The differential heating is generated as explained previously because of the symmetrical nature of the junctions the combined thermo-electric properties of the gold/antimony and gold/bismuth junctions are equivalent to a single bismuth antimony junction. An alternative form of construction is one in which the substrate has two surfaces, the thermocouple track being deposited on the surface which will face away from the radiation to be measured and the hot and cold regions of the substrate being defined on the other, "front" surface. By this means, the track can be coated with a layer of lacquer or other protective substance. As before, the hot region of the substrate can be defined by a front coating of optical matt black paint, and the cold regions can be coated with gold. SUBSTITUTE SHEET

Claims: CLAIMS 1. A pyrgeometer for measuring infra-red radiation having a sensor which includes a thermopile comprising a plurality of thermocouple junctions between at least two different metallic compositions alternating along a thermocouple track deposited on one surface of a planar substrate, hot junctions being located in infra-red absorbing regions of the substrate and cold junctions being located in infra-red reflecting regions of the substrate, characterised in that the substrate (13) is mounted across an essentially circular orifice of a body (10) acting as a heat sink, and adjacent hot and cold junctions (22) are located on the substrate substantially equidistant from the heat sink. 2. A pyrgeometer as claimed in Claim 1, further characterised in that the deposited thermocouple track (21) follows a path generally circumferential about the centre of the orifice and the absorbing and reflecting regions of the substrate are arranged as alternating sectors extending from the centre of the orifice. 3. A pyrgeometer as claimed in either preceding claim further characterised in that the absorbing regions of the substrate are coated with an infra-red absorbing material and the cold junctions on the reflecting regions of the substrate are coated with gold. 4. A pyrgeometer as claimed in Claim 3 further characterised in that the gold coating forms part of a combined first metal - gold and gold - second metal cold junction. 5. A pyrgeometer as claimed in either Claim 1 or Claim 2 further characterised in that the substrate comprises a sheet of material and the thermocouple track is deposited on the surface of the substrate which will face away from the radiation to be measured, the hot and cold regions of the SUBSTITUTE SHEET substrate being formed by the deposition on its other surface of an infra-red absorber and a metallic reflector respectively. SUBSTITUTESHEET

Inventors: FOOT JOHN STUART, FOOT, JOHN, STUART
Applicants: SECR DEFENCE, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1986
Main classification: G01J5
IPC codes: G01J5