[0001] This invention relates generally to the monitoring and/or measurement of the orientation of multiple directional light sources. The invention enables measurement of radiation intensity over an array of ray angles and positions. One use is to simultaneously measure a pattern of intensity from each of multiple light sources. The invention is particularly, though not exclusively, useful in the calibration and control of the heliostats of a solar field. Such a solar field may typically be in a solar energy collection apparatus of the kind having a central solar energy receiver, typically on a tower, and an array of heliostats mounted for angular adjustment to optimally receive a beam of sunlight and direct it to the central receiver. Solar energy collection apparatus of the aforementioned kind is referred to herein as a central receiver solar energy collection system.

Background of the invention

[0002] A challenge with central receiver solar energy collection systems is the trade off, in relation to the heliostats, between manufacturing cost and manufactured accuracy. A large solar field may have many hundreds, even tens of thousands, of heliostats and so the overall economic performance of the system may be dependent upon achieving a low unit cost in the manufacture of each heliostat, including the actuator configuration for angularly adjusting the heliostat. On the other hand, inexpensive manufacture will generally come with high tolerances that will give rise to substantial variations in the optical characteristics across the heliostats of a large field. One way to address this issue is to obtain a geometrical calibration or

characterisation of each heliostat mirror, for example by measuring the location of the centre of the heliostat image. The location of this point can be used to calibrate or adjust each of the two angular positioning systems on each heliostat. Ideally the image shape could be measured on the receiving surface during operation. However, the receiver in a central receiver system is operating at high temperatures and is a hostile place for measuring equipment. The reflectivity can be non uniform and the surface can be non planar.

[0003] United States Patent 4,564,275 provides a calibration technique which eliminates resurveying and field work and provides a method of aligning a single heliostat or a number of heliostats at the same time. The technique relies on changing the aim point from the receiver to a reference position on a secondary target, with a radiometer used to determine the beam centroid error which is then used to re-align the heliostat. A problem with this technique is that it does not address the calibration of inexpensive and inaccurate heliostats.

[0004] PCT Patent Application Publication WO 2012/083383 by the present applicant discloses solar energy collection apparatus including a solar energy receiver defining a primary target to receive directed sunlight from a field of angularly adjustable heliostats. A controller operably coupled to an actuator arrangement for effecting angular adjustment of each heliostat is configured to sequentially cause, during operation of the apparatus, a temporary angular adjustment of the respective heliostats so as to direct the beam of sunlight received at each heliostat to a secondary target for a predetermined period of time. A representation of each diverted beam at the secondary target is recorded by a camera and deviation of a parameter of the image, for example the location of the centroid of the image, from a reference norm provides a basis for angularly adjusting the corresponding heliostat to improve its targeting accuracy on the primary target.

[0005] This latter calibration mechanism is suitable for small, inaccurately constructed heliostats. Frequent measurement of the image centroid on the secondary target during operation allows the system operator to maintain a model of the heliostat geometry.

[0006] United States Patent Application Publication 2012/0174909 describes a system for aligning heliostats using a ring of cameras around the central target. The cameras are designed to measure the imbalance of light reflecting off each heliostat from the circum solar region of the solar image. US Patent Application Publication 2013/0021471 describes a similar approach to US 2012/0174909.

[0007] Three issues have been identified with these approaches. Solar fields of tens or hundreds of megawatt capacity have been proposed, and these fields could consist of tens of thousands of small heliostats. In this case, the requirement to separately calibrate the heliostats regularly can be problematic because the calibration or secondary target is a shared resource. There will be a long time between calibration points for a heliostat while all of the other heliostats are being separately calibrated. There are practical limits to the number of calibration targets as they all need to be a similar size to the receiver making many targets unwieldy.

[0008] A second issue arises when a small heliostat is a long distance from the receiver: the image it forms on a white surface may be many times less intense than ambient light. Designing a system capable of accurately measuring the heliostat image under these conditions would present significant challenges.

[0009] The third issue is that, with reliance on calibration of heliostats via a secondary target, the system still needs a detailed model of the heliostat geometry to allow corrections made on the secondary target to be valid on the primary (i.e. receiver) target.

[0010] United States Patent Application Publication 2013/0139804 describes a system for characterising a surface of a single heliostat. Each heliostat is scanned incrementally across a multiple pixel camera, with an image snapshot taken at each scanned position. By associating pixels with sections of the heliostat surface, the geometry of the surface can be defined, and this information employed to optimise the energy flux of the solar receiver.

[001 1] PCT Patent Application Publication WO 2009/152573 describes a method and apparatus for calibrating heliostats which aims to address the third issue above but suffers from other limitations. In an online calibration procedure, calibration is performed by comparing, at a primary receiver, the flux distributions corresponding to a plurality of mirrors at first and second mirror angles. This technique requires movement of the heliostats during normal operation and therefore is susceptible to errors in the individual heliostat actuators. Further this technique does not permit spatially distinguishing the individual mirror contributions, only their overall contributed flux. In an offline calibration procedure WO 2009/152573 teaches only directing a single heliostat onto the primary receiver and adjusting the mirror angle for optimum power detection. This technique suffers from the heliostat downtime issue mentioned above.

[0012] It is a general object of the preferred embodiments of the invention to provide for the improved monitoring and/or measurement of the orientation of multiple direction light sources. In a particular application of interest, it is an object of the invention to provide for the calibration and adjustment of multiple spaced light sources in the form of heliostats in a solar field, in a manner that, at least in part, addresses the aforementioned problems with current calibration systems that entail the receipt of the solar light image from the heliostat onto a secondary target.

[0013] Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

[0014] In its application to a solar heliostat field, the present invention involves the concept of scanning or sweeping an array of sensors, each having multiple pixels, through a composite beam from multiple heliostats. Images are recorded and the resulting information may be used to measure the image from each heliostat simultaneously. Each heliostat can then be adjusted to illuminate the optimal part of the receiver target.

[0015] In one aspect, the invention provides an apparatus for monitoring and/or measuring multiple directional radiative sources, each radiative source directing radiation as a beam of limited solid angle, comprising:

a measuring device having the capability to angularly distinguish the directional radiative sources from one another; means to scan the measuring device across or through a zone on which at least 50% of the radiation beam from each radiative source impinges; and

means to record a set of multiple images respectively detected at the measuring device at successive positions of the measuring device during said scan,

wherein the radiation from the multiple directional radiative sources recorded at different positions of the measuring device as the measuring device is scanned across the zone and the positions are sufficiently distinguishable in said set of multiple images to permit simultaneous measuring and/or monitoring of the directional radiative sources during a single scan of the measuring device across or through said zone.

[0016] In a second aspect, the invention provides a method for monitoring and/or measuring multiple directional radiative sources, each radiative source directing radiation as a beam of limited solid angle, the method comprising:

(a) receiving radiation from the multiple directional radiative sources in a zone on which at least 50% of the radiation beam from each radiative source impinges;

(b) scanning a measuring device having the capability to angularly distinguish the directional radiative sources from one another across or through the zone; and

(c) recording a set of multiple images respectively detected at the measuring device at successive positions of the measuring device during said scan;

wherein the radiation from the multiple directional radiative sources is recorded at different positions of the measuring device as the measuring device is scanned across the zone; and

said positions are sufficiently distinguishable in said set of multiple images to permit simultaneous monitoring and/or measuring of the directional radiative sources during a single scan of the measuring device across or through said zone.

[0017] In a first embodiment of the first and second aspects, the measuring device preferably includes an array of sensors each have multiple radiation responsive pixels and, at each position of the array, radiation from the multiple directional radiative sources is preferably recorded at different respective subsets of the pixels and the subsets are sufficiently

distinguishable in said set of multiple images to permit simultaneous measuring and/or monitoring of the directional radiative sources during a single scan of the array of sensors across or through said zone.

[0018] In some embodiments the measuring device includes an array of tiltable mirrors configured to direct the radiation onto one or more sensors.

[0019] In one embodiment the measuring device includes an array of single pixel cameras having an associated computer controller. [0020] In some embodiments the subsets are mutually exclusive. In other embodiments the subsets share one or more common pixels.

[0021] In some embodiments the array of sensors is scanned by a first actuator. The scanning of the array of sensors is preferably achieved through mounting the array of sensors to an arm which is able to be incrementally moved through the composite beam by the first actuator. The arm preferably extends substantially vertically from the receiver and the direction of incremental movement of the arm is substantially horizontal. The position of the arm is preferably controlled by a controller. The controller is preferably also configured to control the velocity of the arm relative to the receiver. During a first period of time, the first actuator preferably scans the array of sensors across or through said zone and, during a second period of time, the first actuator moves the arm out of the path of said zone. In one embodiment, a second actuator is configured to selectively move the arm closer or further from the receiver.

[0022] In one embodiment, the array of sensors is a linear array of sensors. In another embodiment the array of sensors is a two dimensional array of sensors.

[0023] In a third aspect, the invention provides a solar energy collection apparatus comprising:

a solar energy receiver defining a target to receive directed solar radiation; a field of heliostats mounted for angular adjustment to optimally receive a beam of solar radiation and direct it to the target of the solar energy receiver, said beams together forming a composite beam incident on the target; and

an apparatus according to the first embodiment of the first aspect, wherein said multiple directional radiative sources include said heliostats and said zone includes an area defined by said composite beam.

[0024] In a fourth aspect, the invention provides a solar energy collection method, comprising:

receiving, at a target defined by a solar energy receiver, solar radiation directed from heliostats of a field of heliostats mounted for angular adjustment to optimally receive a beam of solar radiation and direct it to the target of the solar energy receiver, said beams together forming a composite beam incident on the target;

performing steps (b) and (c) of the method according to the first embodiment of the second aspect, wherein said multiple directional radiative sources include said heliostats and said zone includes an area defined by said composite beam.

[0025] In a fifth aspect, the invention provides use of an actuator in the monitoring and/or measuring of multiple directional radiative sources to scan a measuring device across or through a zone on which at least 50% of the radiation beam from each radiative source impinges.

[0026] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

[0027] As used herein, except where the context requires otherwise, the term "scanning" means the act of moving over or across an object with a detector (e.g. an array of sensors).

[0028] Use of terms "sunlight" "light" and the like in this specification are intended to refer to electromagnetic radiation covering one or more of the visible, ultra-violet and infra-red wavelength ranges.

[0029] As used herein, the terms "limited solid angle" mean a two-dimensional angle subtended in three-dimensional space as limited by a cone of acceptance of an optical receiver.

Brief description of the drawings

[0030] Preferred embodiments of the invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a simplified general diagram of an exemplary central receiver solar energy collection system;

Figure 2 is a perspective view of an embodiment of tower mounted central receiver for the system of Figure 1 incorporating an embodiment of the invention;

Figure 3 is a functional block diagram of the main components of a solar energy collection system according to an embodiment of the invention, including the controller;

Figure 4 is a rear perspective view of a typical heliostat;

Figure 5 is a schematic diagram illustrating the principle of the invention, applied to an embodiment in which a linear camera array scans or sweeps at right angles to its alignment; and

Figures 6 and 7 illustrate alternative embodiments with circular and cylindrical sweeps respectively.

Detailed description of the embodiments

[0031] The present invention has particular utility in the operation of a central receiver solar energy collection system utilising inexpensively manufactured heliostats. An exemplary such system 10 is depicted in Figure 1 . The system comprises a central solar energy receiver 12 mounted in cantilevered fashion from a tower 1 1 above and in front of a large array or field 18 of horizontally spatially separated heliostats 15. Heliostats 15 are mounted for angular adjustment to optimally receive a respective beam of sunlight 200 and to direct the beam, as respective directed beams 202, to the solar receiver 12. As best shown in Figure 2, receiver 12 has an aperture that defines a receiver target to receive the directed beams 202 of sunlight from the heliostats during operation of the system. Directed beams 202 together form a composite beam 203 incident on the receiver target.

[0032] An optimally receiving position in this context is the two dimensional angular position of the heliostat determined by a central controller, discussed further below, to be the appropriate position at the particular time on the particular date at which the respective heliostat makes a desired contribution to the energy flux incident on the receiver target. In general, the objective is to best approximate the desired flux levels and flux distribution at the receiver.

[0033] In general, each heliostat is a directional light source that directs light from its reflecting surface as a beam of limited solid angle, and at least 50%, typically close to 100%, of the reflected light beam impinges on the target and on a zone 38 in front of the target.

[0034] Receiver 12 is mounted atop tower 1 1 in an embodiment of the invention depicted in Figures 2 and 3. However, it will be appreciated that, in other embodiments, receiver 12 is supportively mounted to tower 1 1 by other types of frameworks including the cantilever framework illustrated in Figure 1 . Receiver target 13 is here shown as generally planar and rectangular. Other shapes may be substituted where appropriate, e.g. curved, conical or cylindrical.

[0035] Also projecting from above the receiver is a measuring device including a substantially horizontally extending mounting 33, which is integrally or releasably attached to a substantially vertically extending depending arm 31 . Mounted to arm 31 is a linear array 30 of substantially equally vertically spaced apart sensors 32, each having a sub-array of light response pixels. In a preferred embodiment, sensors 32 can be considered as digital camera sensors. Mount 33 includes an electronically actuatable mechanism (not shown) for scanning or sweeping camera array 30 across the composite beam 203 in zone 38 in front of target 13, in a straight line direction (indicated by arrow 35) at right angles to the alignment of the sensor array and substantially at right angles to the beams 202. In another embodiment, camera array 30 is not actively scanned but is allowed to scan passively under an external influence such as by wind or under gravity.

[0036] The actuatable mechanism is configured to incrementally move or progress arm 31 across receiver target 13 at a speed and direction determined by a control signal from a controller. During a predetermined period of time, different subsets of sensors 32 and their respective pixels detect the local optical power of the composite beam or individual beams from various heliostats. [0037] In the case of a flat receiver target 13, as illustrated in Figure 2, the actuatable mechanism is configured to sweep across target 13 in a linear fashion in both a forward and reverse direction. If and when necessary, arm 31 is able to be positioned out of alignment with target 13 so as to not be in the path of composite beam 203. In embodiments where the receiver is cylindrical, as illustrated in Figures 6 and 7, arm 31 is able to be incrementally progressed around the entire circumference of the receiver in a circular or cylindrical sweep.

[0038] In other embodiments (not illustrated), arm 31 extends substantially horizontally and is actuatably moveable in a vertical direction. In an embodiment having a cylindrical receiver surface, arm 31 is in the form of a substantially cylindrical collar which is moveable vertically about the receiver.

[0039] In some embodiments, an actuator system is provided for advanced movement of mount 33 and arm 31 . In one embodiment, mount 33 is telescopically or linearly actuatuable to move arm 31 closer or further from receiver target 13 in a radial or other plane. In another embodiment, arm 31 is hingedly or pivotally mounted to mount 33 and an actuator is configured to selectively rotate arm 31 vertically about the hinge/pivot point and out of the path of composite beam 203.

[0040] In general, sensors 32 need not be separated by the same distance but may be disposed in a linear array having different distances therebetween. In other embodiments, array 30 is two dimensional array having sensors 32 extending both vertically and horizontally.

[0041] In other embodiments, measuring devices other than pixel arrays are utilised. In some other embodiments, detectors at positions in array 30 are replaced with similarly sized and oriented measuring devices capable of measuring radiance as a function of two angular dimensions. In one alternative embodiment (not illustrated) a similarly sized and oriented two- dimensional array of electrically tiltable mirrors, lenses, mirror/lens combinations and or/a mirror or lens with different angular reflecting regions directing light to respective optical sensors or a single optical sensor having multiple radiation responsive pixels. The mirrors or lenses may convey light in series or in parallel to the optical sensor. The mirrors/lenses may be scanned to measure incoming light simultaneous to the scanning of the overall detector array. In another alternative embodiment (not illustrated) the detector includes an array of single pixel cameras, each capable of spatially and angularly distinguishing light through post-processing of the received light signal at a processor (for example, by way of compressive imaging techniques).

[0042] In a further alternative embodiment, the need for an array of sensors is dispensed by scanning a detector including only a single camera or sensor rapidly in two dimensions. The rate of two-dimensional scanning must be sufficiently fast so as to angularly distinguish the directional radiative sources from one another. [0043] It will be appreciated that other types of detector are able to be implemented provided that they allow angular selectivity and are capable of measuring irradiance as a function of two- dimensional angle.

[0044] As shown in Figures 3 and 4, each heliostat 15 has an individual actuator system 21 typically comprising a pair of linear actuators 60, 62 for respectively controlling the inclination and declination of the heliostat's reflecting surface. The angular position of each heliostat, both inclination and declination, is determined by a central controller 40, as illustrated in Figure 3, which may comprise a suitable computer system. This controller is operably coupled to the actuators 60, 62 of all of the heliostats, to magnetic sensors 80 by which the controller is kept informed of actual angular position co-ordinates of each heliostat, and to camera array 30. Controller 40 is also responsible for controlling the position and speed of arm 31 through actuatable mechanism. However, the scanning of arm 31 and sensors 32 is independent to the angular positioning of the heliostats.

[0045] A suitable heliostat 15 with respective actuators 60, 62 is illustrated in Figure 4.

Heliostat 15 is indicative of a typical device incorporated into system 10. However, it will be appreciated that other types and designs of heliostat or combinations of different heliostats can be incorporated into system 10. Heliostat 15 includes a large concave mirror fixed by adhesive to a backing frame 20 of rectangular profile. Frame 20 is mounted atop a stand or post 70, by means to be described, and comprises a central hub 23 and ribs 22 that extend radially from central hub 23 to peripheral edge beams 22a. The ribs 22 are fastened to corresponding flat radial arms 25 of hub 23. The mirror 14 lies on the concave side. The dimensions of the components are determined by a frame pattern which is generated by software. Fasteners such as thread forming screws, rivets, spot welding or bolted joints are used throughout the frame 20.

[0046] The mirror 14 is glued directly to the frame 20 using a polyurethane based adhesive applied to folded tabs on the inner edge flanges 28 of all the ribs 22, which collectively define a shallow concave paraboloid shape. The mirror is typically made of 3mm thick glass having a high reflectivity surface, such as a plastic composite, and a low iron content to reduce energy absorption. Suitable such glasses include those manufactured by Sencofein or "Miralite Solar Premium" manufactured by Saint Gobain.

[0047] The pair of linear screw actuators 60, 62 by which the heliostat orientation is controlled are positioned substantially parallel, so that they both extend generally perpendicular to the mirror 14. This prevents the actuators 60, 62 from colliding during operation whilst giving a greater range of optimal angle to the heliostat 12. The actuators 60, 62 include individual off- the-shelf DC motors 65. [0048] The actuators 60, 62 are arranged to provide control in two orthogonal directions so that the focusing point can be maintained for any angle of incident light. One axis is controlled east to west, i.e. side to side, and the other north to south, i.e. upward tilt. However, one axis is controlled relative to the other axis. Specifically, side-to-side rotation occurs about an intermediate mount in the form of a frame support bracket 66, which is itself rotated up or down: this arrangement minimises the amount of space taken up by each heliostat 12.

[0049] The frame 20 is connected to support bracket 66 at vertically spaced hinges for rotation about an upright axis joining the hinges. The first linear actuator 60 is mounted between the mirror frame 20 and an arm 64 that projects laterally rearwardly from support bracket 66, for controlling this side-to-side or east-west rotational movement. Bracket 66 is pivotally attached in turn, by a bracket pin 68, to the top of post 70. Pin 68 defines an inclination axis about which the tilt angle of bracket 66, and thereby of frame 20, is adjustable. The aforementioned upright axis is generally orthogonal to the inclination axis. Arm 64 is rigidly connected to the bracket 66 as close to the inclination axis as possible.

[0050] Second actuator 62 extends between post 70 and an attachment point at the lower end of bracket 66 for effecting adjustment of the tilt or inclination of the mirror. The angle of the arm 64 to the bracket 66 is selected to provide optimum actuator geometry, with different angles for each heliostat according to positions in the field. The bracket 66 may further include a number of different attachment points for the actuator 62 also selectable to provide the optimum angle according to the individual heliostat's position in the field.

[0051] As shown in Figure 3, magnetic sensors 80 mounted adjacent each heliostat are used to measure the respective orientation or positional angles of each heliostat, defined by inclination and lateral orientation or declination. In some embodiments, magnetic sensors 80 are only operatively associated with a subset of the heliostats. These sensors are inexpensive low precision (~8 bit) encoders, e.g. hall effect encoders. Furthermore these encoders are deployed to determine motor shaft position rather than directly measuring the angular location of the mirror assembly.

[0052] System 10 is activated into operation by controller 40 using the actuators 60, 62 to bring the heliostats to substantially their optimum orientation at which all optimally receive a respective beam of sunlight and direct it to the target 13 of the solar energy receiver 12. When this concentrated composite beam is not being utilised to provide heat energy for the power plant coupled to the receiver, the system must be deactivated out of operation by adjusting the heliostats away from their optimal position to random uncorrelated orientations that do not result in any collectively focused sunlight. [0053] According to an embodiment of the invention illustrated in Figure 5, each digital camera sensor 32 is a planar sub-array of light responsive pixels defined by individual or multiple photodiodes, in, for example, a charge coupled device (CCD) or complementary metal- oxide-semiconductor (CMOS) arrangement. The pixels are each responsive to provide a measure of light incident on the pixel. As depicted in Figure 2, the camera array 30 has twenty- five digital camera sensors 32, but depending on the application and desired sensitivity there may be more or fewer sensors. There will typically be at least two, and preferably at least ten sensors 32.

[0054] The pixel subarrays or digital camera sensors 32 are coupled to controller 40 with appropriate electronics for recording data representative of the intensity and location of light received at each sensor and at each pixel. As the camera array 30 is scanned or swept substantially horizontally across composite beam 203 in front of target 13, controller 40 is configured to record a set of multiple images respectively detected at the camera array at successive positions 100 of the array during the scan. At the same time, other subsets of sensors 32 are scanning across beams of other heliostats to simultaneously characterise those heliostats. During the horizontal scan of array 30, predetermined subsets of sensors 32 are exposed to light from respective heliostats for a predetermined period of time. The locations of the respective digital camera sensors or subarrays is depicted at 102 in Figure 5. In a given scan, there are preferably at least two successive positions 100 and more preferably at least ten.

[0055] The distance of the scan path from the receiver is not critical and might be either relatively close (as illustrated) or further away. This is a design decision that includes balancing many cameras over a larger area with lower beam intensity versus fewer cameras but higher beam intensity.

[0056] It will be appreciated that any given heliostat 15 in the field 18, will illuminate a respective different subset of the pixels, exemplified at 1 10 in Figure 5, as the camera array 30 is scanned across zone 38. With sufficiently small pixels in the digital camera sensors, and sufficient scanning positions, it is possible to simultaneously characterise all heliostats of the field. For example, if each digital camera sensor 32 is a 10 megapixel camera and there are 10,000 heliostats, this is still, on average, a reasonable number of pixels per heliostat (in fact, about 1 ,000).

[0057] Even where the pixels are not sufficiently small to simultaneously characterise all heliostats of the field, with sufficiently spaced heliostats in the field, the subsets of pixels - one or more at each location 102 of each of the digital camera subarrays 32 - are sufficiently distinguishable to permit simultaneous optical characterisation of the spaced heliostats . [0058] Another way of expressing this arrangement is to say that each heliostat will access a different area of each camera sensor. By knowing where each camera was when the image was taken, and calculating the intensity from each heliostat in each camera, the shape of the image from each heliostat can be calculated. The misalignment can then be calculated in turn and a corresponding angular position correction determined, as described below.

[0059] Where necessary, the required spacing between heliostats to allow simultaneous optical characterisation can be readily determined through correlating the spacing of the heliostats with the required minimum spacing between each of the subsets of pixels.

[0060] In some of the earlier described calibration and adjustment systems, each heliostat in the field must be singularly and in turn directed to the secondary target. With the arrangement of the present invention, a substantial number of sufficiently separated heliostats can be simultaneously characterised or monitored without any requirement for a secondary target and therefore for taking heliostats offline.

[0061] It will of course be appreciated that scanning is carried out quickly and infrequently so that the camera sensors are in composite beam 203 only for a proportionately short time during the scan or sweep and can be offset away from the actual receiver where the intensity is less, both of which make it easier to engineer a camera to survive and to reliably function. In some installations, it may be necessary to actively cool the sensor array to avoid damage as the array traverses the hot region. Additionally, or alternatively, residence time in composite beam 203 can be minimised by arranging for a high speed scan that employs a drive and control system of sufficiently high performance. A preferred duration of each scan is in the range 0.5 to 20 seconds, more preferably 1 to 15 seconds, for example about 2 seconds.

[0062] As to frequency of scanning, it will typically only be necessary to scan every sixty seconds or so, but it might be preferable to scan more often. It is conceivable that the array will sweep through a surface offset from the receiver by several times the dimension of the receiver. This offset would reduce the overall flux intensity incident onto the sensors and therefore prolong their useful life. A continually rotating system might be employed in systems where, say the receiver includes a continuous cylindrical surface, but this would preclude reducing the absorbed energy by intermittent movement.

[0063] As mentioned earlier, controller 40 is programmed to carry out a number of calibration and control tasks in order to optimise the convertible energy received at primary target aperture 13. The first of these tasks is the calibration of each heliostat and the second is to effect an angular adjustment of the heliostat in response to the calibration. During the calibration phase, by means of control signals from controller 40 to heliostat actuators 60, 62, one heliostat at a time has its angular position shifted. The system is calibrated by recording the relative magnitude of maximum response that can be generated by sweeping the camera array 30 across the individual heliostat beams 202 with a constant direct irradiance. The light responses from each sensor location 102 are then sampled, and these sample images are then assembled as a subset of pixels to produce a reference image for each heliostat - from which is deduced a reference position for the heliostat.

[0064] Repetition of this process across all heliostats in turn during commissioning of the apparatus constitutes an 'instantaneous' calibration of each heliostat.

[0065] The update phase of monitoring heliostat annular position and making on the run adjustment at regular intervals is effected by intermittently sweeping the sensor array 30 across the composite beam 203 during operation of the field, thereby receiving and recording updated data in relation to respective subsets of pixels simultaneously illuminated by the respective heliostats, comparing the updated data with the data in memory for the earlier determined reference positions of the heliostats, and if necessary outputting an angular position correction or adjustment signals responsive to differences detected in the comparing step.

[0066] In an optimum arrangement, the subsets of pixels are mutually exclusive, i.e there are no pixels common to any two subsets. It is preferable that there be at least a full pixel separation of adjacent pixels of respective subsets. This is because it would be impractical to ensure that pixel boundaries fall between heliostat beams. In an alternative arrangement, subsets of pixels might share one or more common pixels: these subsets can still be distinguishable provided there are sufficient pixels not common for the respective heliostats to be capable of being distinguished and characterised.

[0067] Because the camera pixels of interest are not influenced by ambient light, the earlier mentioned problem arising when heliostats are distant and the secondary target image much less intense than ambient light, is substantially resolved.

[0068] This system of calibrating during operation on the receiver target allows heliostats to be adjusted in a closed loop manner, removing all sources of error regardless of the properties of the actuation system, the only requirement being that the heliostats can be moved to intercept the swept area initially. This addresses the disadvantages of employing a secondary target, as discussed earlier as the "third issue" with prior configurations.

[0069] The method and apparatus of the invention may be employed to characterise the shape of an individual heliostat. If there are sufficient pixels per angle of incidence the described configuration can be used to measure the amount of light coming from various parts or elements of the mirror surface, on each camera. This allows the relative slope of elements across the mirror surface to be determined, effectively measuring the shape. [0070] Preferably, the angle per pixel is between 1 x10 ^"6 and 1 .0 degrees. The lower limit is likely to be limited by the pixel density of CCD elements in combination with the field of view.

[0071] A further application is to use these measurements to calculate adjustments in the 'canting' of large multi faceted heliostats, the relative angular adjustment of individual, sometimes flat, facets in a large heliostat.

[0072] In the above described embodiment, the sensor or camera array 30 is vertically linear and the scan or sweep is at right angles (horizontal) to its alignment. In alternative

arrangements, the array may traverse the composite beam by means of a rotational or part- circular sweep in front of a planar receiver, or a cylindrical sweep about a cylindrical or part- cylindrical receiver. In a further arrangement, the array traverses the composite beam in a vertical sweep in front of the receiver.

[0073] In the heliostat calibration application described above, it is desirable for the field of view of each camera to include the whole field and sufficient angular resolution to resolve individual heliostats. In the reflector surface characterisation case, it is desirable to provide higher angular resolution to be able to have many pixels over the mirror surface, with a field of view only covering a single heliostat. An implementation may have the sensors further from the aperture plane than for the heliostat calibration application and the whole assembly would need to be aimed at the heliostat of interest. Accordingly, in some embodiments, mounting 33 and/or arm 31 are able to be mechanically actuated to move arm 31 and sensor array 30 closer or further from the surface of receiver 12. This movement allows more or less sensor pixels to be included within the composite beams from the heliostats.

[0074] The invention more generally can be considered as an angularly selective imaging element - capable of measuring the intensity over a surface and over an angular range of incidence angles. It could be used for modern cinematography employing a mixture of computer generated imagery and conventional filming to accurately measure lighting patterns intended to intercept specular cgi surfaces. For example, if it was required to make a shiny CGI vehicle drive through a complex scene involving multiple light sources and shadow casting objects etc, instead drive an ordinary vehicle with a camera array on it to measure the patterns and directions of light so that they can be regenerated for CGI rendering. If implemented on a micro scale, the invention could provide a new type of camera which could form a focus algorithmically, and could refocus and change depth of field during the editing process.

[0075] Reference throughout this specification to "one embodiment", "some embodiments" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment", "in some embodiments" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0076] As used herein, the term "exemplary" is used in the sense of providing examples, as opposed to indicating quality. That is, an "exemplary embodiment" is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.

[0077] It should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, Figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed

embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0078] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0079] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0080] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected," along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0081] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. Functionality may be added or deleted from the schematic diagrams and operations may be interchanged. Steps may be added or deleted to methods described within the scope of the present invention.

Clauses

[0082] In a first aspect, the invention provides an apparatus for monitoring and/or measuring multiple directional light sources each directing light as a beam of limited solid angle, comprising:

an array of sensors each having multiple light responsive pixels;

means to scan the array of sensors across or through a zone on which at least 50% of the light beam from each light source impinges; and

means to record a set of multiple images respectively detected at the array at successive positions of the array during said scan,

wherein the light from the multiple directional light sources is recorded at different respective subsets of the pixels as the array is scanned across the zone and the subsets are sufficiently distinguishable in said set of multiple images to permit simultaneous measuring and/or monitoring of the directional light sources during a single scan of the array of sensors across or through said zone.

[0083] Advantageously, in one embodiment, the apparatus further includes a controller configured to receive and store in memory data in relation to said different subsets of the pixels for reference orientations of the respective light sources, to subsequently receive and record updated data in relation to respective subsets of pixels simultaneously illuminated by the respective light source, and to compare the updated data with the data in memory for the reference orientations of the light sources.

[0084] In an embodiment, the controller is further configured to output light source orientation correction signals responsive to differences detected in the comparing step.

[0085] In another embodiment, useful with multiple light sources that are elements of a larger, e.g. substantially continuous, light source, the apparatus includes a controller configured to receive and store in memory data in relation to said different subsets of the pixels, and to process the data to characterise the light sources, e.g. the amount of light emanating therefrom or the relative slope of the respective elements of a larger light source.

[0086] In a second aspect, the invention further provides a method for monitoring and/or measuring multiple directional light sources each directing light as a beam of limited solid angle, comprising:

receiving light from the multiple directional light sources in a zone on which at least 50% of the light beam from each light source impinges;

scanning an array of sensors each having multiple light responsive pixels across or through the zone; and

recording a set of multiple images respectively detected at the array at successive positions of the array during said scan;

wherein the light from the multiple directional light sources is recorded at different respective subsets of the pixels of the array as the array is scanned across the zone; and

said subsets are sufficiently distinguishable in said set of multiple images to permit simultaneous monitoring and/or measuring of the directional light sources during a single scan of the array of sensors across or through said zone.

[0087] Advantageously, the method further includes:

storing in memory, data in relation to said different subsets of the pixels for reference orientations of the respective light sources; and

subsequently receiving and recording updated data in relation to respective subsets of pixels simultaneously illuminated by the respective light sources, and comparing the updated data with the data in memory for the reference orientations of the light sources.

[0088] The method preferably further includes outputting light source orientation correction signals responsive to differences detected in the comparing step.

[0089] The multiple directional light sources may be discrete light sources, or elements of a larger, e.g. continuous, light source. The multiple directional light sources may be active light generators, or passive reflectors or transmitters of light. Heliostats are typical of the latter class.

[0090] In an application of the first aspect of the invention, the multiple directional light sources are heliostats in a solar field.

[0091] In a third aspect, the invention provides solar energy collection apparatus comprising:

a solar energy receiver defining a target to receive directed sunlight;

a field of heliostats mounted for angular adjustment to optimally receive a beam of sunlight and direct it to the target of the solar energy receiver, said beams together forming a composite beam incident on the target; an array of sensors each having multiple light responsive pixels;

means to scan the array of sensors across or through said composite beam; and means to record a set of multiple images respectively detected at the array at successive positions of the array during said scan, whereby light from plural, preferably multiple, heliostats of the field is recorded at different respective subsets of the pixels as the array is scanned across the composite beam and the subsets are sufficiently distinguishable in said set of multiple images to permit simultaneous measuring and/or monitoring of the plural heliostats during a single scan of the array of sensors across or through said composite beam.

[0092] Advantageously, the apparatus of the third aspect of the invention further includes a controller configured to receive and store in memory data in relation to said different subsets of the pixels for reference angular positions of the respective heliostats, to subsequently receive and record updated data in relation to respective subsets of pixels, simultaneously illuminated by the respective heliostats, to compare the updated data with the data in memory for the reference angular positions of the heliostats, and to output angular position correction signals for the respective heliostats responsive to differences detected in the comparing step.

[0093] In a fourth aspect, the invention further provides a method of solar energy collection, comprising:

receiving at a target defined by a solar energy receiver sunlight directed from heliostats of a field of heliostats mounted for angular adjustment to optimally receive a beam of sunlight and direct it to the target of the solar energy receiver, said beams together forming composite beam incident on the target;

scanning an array of sensors each having multiple light responsive pixels across or through the composite beam; and

recording a set of multiple images respectively detected at the array at successive positions of the array during said scan;

wherein the light from plural, preferably multiple, heliostats is recorded at different respective subsets of the pixels of the array as the array is scanned across the composite beam, and the subsets are sufficiently distinguishable to permit simultaneous optical

characterisation of said plural heliostats during a single scan of the array of sensors across or through said composite beam.

[0094] The array of sensors is preferably a linear array of sensors. The array may traverse the composite beam by means of lateral rotational or part-circular sweep in front of a planar receiver, or a cylindrical sweep about a cylindrical or part-cylindrical receiver. [0095] In some aspects of the invention, the subsets of pixels are preferably sufficiently distinguishable through no pixel of each subset being illuminated by more than one of the directional light sources.

[0096] Each of the subsets of pixels may be further distinguishable through having no overlap with adjacent subsets of pixels, i.e. no pixel common to any two or more of the subsets.

[0097] In an embodiment, the spacing between each subset of pixels is at least one pixel. This spacing may be at least two pixels, or it may be three pixels or more.

[0098] In both aspects of the invention, each sensor may conveniently be, for example, a discrete digital camera sensor array, e.g. a photodiode array in a CCD or CMOS arrangement. The pixels of each sensor may comprise a subarray of pixels, for example, an equi-spaced square array.

[0099] The light responsive pixels of each sensor are each preferably responsive to provide a measure of the light incident on the pixel. The array of light response pixels may comprise, for example, a photodiode array in a charge-coupled device (CCD) or CMOS arrangement.

[00100] The sensors of the array may be present in the composite beams for a proportionately short time during the scan or sweep and can be offset away from the actual receiver where the intensity is less, both of which make it easier to engineer a camera to survive.