FIELD OF THE INVENTION Typically, existing methods for testing mirrors reflect light from the mirror and then adjust the mirror, such as by grinding or mechanical adjustment of a mirror support, until the pattern of light so reflected meets some predetermined standard. However, doing so can be time consuming and adds expense to the manufacture of the mirror.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for characterizing the shape of a mirror so that the direction of light reflected therefrom can be predicted, such as so that an array of such mirrors can be aligned to achieve a desired pattern of reflection.

It is another object of the present invention to provide a method of aligning each of a plurality of mirrors within an array to achieve a desired pattern of reflection.

In a first broad aspect, therefore, the present invention provides a method of characterizing the shape of a mirror, comprising: characterizing each of a plurality of characterization locations on said mirror by observing reflection of a respective light beam from each of said

locations ; whereby further locations on said mirror can be characterized on the basis of said characterization locations.

Preferably said method includes characterizing said further locations on the basis of said characterization locations, such as by interpolating between characterization locations or extrapolation from individual characterization locations.

Thus, the manner in which light can be expected to be reflected from the mirror can be deduced from either merely the measured locations, or a combination of both the measured locations and other locations deduced from the measured locations. In the latter case, the characterizations of the further locations will generally approximate the actual characteristics of the mirror at those further locations.

The step of characterizing the further locations on the basis of the characterization locations might involve, for example, interpolating between two or more characterization locations. This might entail defining a polygonal region around each further location, with characterization locations at each vertex, and treating the mirror as flat within each such polygonal region. Alternatively, if the mirror has been fabricated to have a particular shape (such as spherical), this could entail interpolating on the basis of that intended shape. Still alternatively, the method may include defining for each characterization location a respective region and assigning to each region the characterization of its respective characterization location.

For example, if the characterization locations are arranged in a series of rows and columns on the mirror, so that each

is (at least approximately) at the centre of a rectangle or square (which may be curved, such as where the mirror is a spherical mirror), these rectangles or squares filling the area of the mirror, further locations falling within a particular rectangle or square may be assigned the characteristics of that rectangle or square\'s characterization location. As will be understood by those in the art, any other suitable combination of characterization locations and area filling regions around those characterization locations may be used in the same manner.

Preferably the method includes calculating how a beam of light would be reflected from said mirror and characterizing the reflected beam of light, on the basis of said characterization locations, on the basis of said further locations or on the basis of both said characterization locations and said further locations.

More preferably said method includes characterizing the shape of a plurality of mirrors, calculating how a beam of light would be reflected from said array of mirrors and characterizing the composite reflected beam of light reflected from said array, on the basis of said characterization locations of each of said mirrors, on the basis of said further locations of each of said mirrors or on the basis of both said characterization locations and said further locations of each of said mirrors.

Thus, the method may include predicting how a, for example, broad beam of light (such as sunlight) would be reflected from the mirror, or from an array of a plurality of mirrors.

The characterizing of the reflected beam or composite reflected beam may comprise predicting its shape at some desired distance, such as the distance at which-in use- a light receiver would be located relative to said mirror

or array of mirrors, whereby said composite reflected beam can be optimized.

Thus, once each of the mirrors has been characterized, an array can be constructed that has the optimal composite reflected beam without having to modify individual mirrors.

That is, each mirror can be put to the best use within the array.

Preferably the method includes adjusting the location, the attitude, or both the location and attitude, of one or more of said plurality of mirrors in said array of mirrors so that said composite reflected beam conforms more closely to a desired composite reflected beam.

Preferably the method involves reflecting a plurality of incident light beams from said mirror, each at a respective characterization location on said mirror, thereby producing respective reflected beams ; and detecting each of said reflected beams ; whereby said characterization locations can be characterized from the respective pairs of incident and reflected beams.

Preferably the method includes characterizing each characterization location by determining, from each respective pair of incident and reflected beams, the normal vector to said mirror at each of said respective characterization locations, or the gradient of said mirror at each of said respective characterization locations, or both the normal vector to said mirror and the gradient of said mirror at each of said respective characterization locations.

Thus, the method can include determining a vector, such as a normal vector, indicative of the gradient at each characterization location.

Preferably the method includes: 1) locating said mirror opposite a detecting surface ; 2) directing a plurality of incident light beams onto said mirror at said respective characterization location such that said beams are reflected to respective detection locations on said surface ; and 3) recording, for each of said incident beams, a set of data relating to the respective incident beam and a corresponding detection location ; and 4) characterizing from each of said data sets the respective characterization location.

Step 3) may comprise recording, for each of said incident beams, a set of data indicative of the origin of said respective incident beam, a corresponding characterization location, and a corresponding detection location.

Alternatively, step 3) may comprise recording, for each of said incident beams, a set of data indicative of the direction of each of said incident beams, a corresponding characterization location, and a corresponding detection location.

The plurality of incident beams may be directed onto the mirror simultaneously, consecutively, in consecutive groups of beams, each group comprising a plurality of beams, or otherwise. Preferably, however, the plurality of incident beams is directed onto said mirror either simultaneously or in consecutive groups of beams, each group comprising a plurality of beams.

The detecting surface may comprise a screen on which the reflected beams are visible so that their respective detection locations can be observed. Alternatively, the detecting surface could include detecting elements (such as

photodetectors) to detect the reflected beams at a plurality of respective detection locations.

Preferably each of said beams of light is a laser beam.

Preferably said method includes providing a plurality of light sources (each preferably a laser source), to provide the incident beams. More preferably said method includes performing steps 2) to 4), altering the relative locations of said plurality of light sources and said mirror, and repeating steps 2) to 4).

Thus, altering the relative locations of (in a preferred embodiment) the lasers could entail moving the lasers, moving the mirror, or moving both the lasers and the mirror.

Preferably the method includes recording the coordinates of the detection location at which each reflected beam is detected.

Thus, these coordinates, once recorded (such as on a computer), can be retrieved for later use.

In a second broad aspect, the present invention provides an apparatus for characterizing the shape of a mirror, comprising : a mirror support for supporting said mirror ; a detecting surface located opposite said mirror when said mirror is supported by said mirror support ; at least one light source for directing a plurality of incident beams of light onto said mirror at respective characterization locations on said mirror such that said incident beams are reflected as respective reflected beams to respective detection locations on said detecting surface ; data recording means for recording, for each of

said incident beams, a set of data indicative of the respective location of said light source, characterization location and detection location ; and data analysis means for characterizing from each of said data sets the respective characterization location.

Preferably said apparatus includes a plurality of collimated light sources, and more preferably each of said light sources is a laser source.

Preferably said apparatus includes means for altering the relative locations of said plurality of light sources and said mirror.

In a third broad aspect, the present invention provides a method of aligning each of a plurality of mirrors within an array, comprising : 1) determining a preferred pattern of light reflection from said array ; 2) obtaining a characterization of the shape of each of said mirrors ; 3) simulating said array and light reflection therefrom on the basis of said characterizations, and comparing said simulated light reflection with said preferred pattern of light reflection ; and 4) varying said simulated array and repeating step 3) until said simulated light reflection is within acceptable tolerances of said preferred pattern of light reflection.

The preferred pattern of light reflection from said array preferably includes preferred patterns of light reflection for each mirror in said array.

Thus, there may be a need to produce a certain"shape"of beam at the focus of a set of mirrors. This shape is dictated by the needs of the optical receiver placed at the

focus. Owing to production constraints, mirrors (such as those for solar concentrators in solar power generation systems) do not have perfect optical surfaces, so the theoretical alignment of a set of mirrors will not produce the anticipated result. Rather than attempting to improve production quality, the present invention determines the character of each mirror, and determines how each mirror should be oriented to produce a composite, reflected light beam that is substantially as prescribed.

Step 4) may comprise varying the simulated orientation of one or more mirrors, or varying the simulated location within said array of one or more mirrors, or both varying the simulated orientation of one or more mirrors and varying the simulated location within said array of one or more mirrors.

Preferably obtaining said characterization of each of said mirrors includes characterizing each of said mirrors.

Preferably the method includes obtaining a characterization of the shape each of said mirrors by measuring said characterization of each of said mirrors according to the method of characterizing the shape of a mirror described above.

Preferably the method includes simulating the location of each respective mirror within said array such that mirrors more closely approximating a theoretical shape are located closer to a centre of said array than mirrors less closely approximating said theoretical shape. For example, if the mirrors are designed to be spherical mirrors, those closest to spherical would be located at the edge of the array (where deviations from spherical would have the greatest deleterious effect owing to higher angles of incidence) and those furthest from spherical would be located at the centre of the array.

In one embodiment, step 1) includes determining preferred patterns of light reflection for each mirror in said array, and the method includes subsequently: 5) reflecting light from each of said mirrors and observing reflected light therefrom ; 6) comparing said reflected light with said preferred pattern of light reflection for each respective mirror ; and 7) varying the location, orientation or both location and orientation of one or more of said mirrors and repeating steps 5) and 6) until for each mirror said light reflection is within acceptable tolerances of said preferred pattern of light reflection from said respective mirror.

Preferably said array of said mirrors is for use in an energy conversion system, such as a solar power generation system.

In a fourth broad aspect, the present invention provides an apparatus for determining an alignment each of a plurality of mirrors within an array, comprising computational means (such as a computer provided with a computer program) for performing the method of aligning each of a plurality of mirrors within an array described above.

In a fifth broad aspect, the present invention provides a method of aligning one or more mirrors constituting an array, comprising: 1) determining a preferred pattern of light reflection from each of said mirrors ; 2) reflecting light from each of said mirrors and observing reflected light therefrom ; 3) comparing said reflected light with said preferred pattern of light reflection for each respective mirror ; and

4) varying the location, orientation or both location and orientation of one or more of said mirrors and repeating steps 2) and 3) until for each mirror said light reflection is within acceptable tolerances of said preferred pattern of light reflection for said respective mirror.

Thus, the array can be tested and the alignment of each mirror adjusted until satisfactory.

Preferably the method includes locating and orienting each of said mirrors within said array according to the results of simulating light reflection from said array.

The adjustment can be based on the results of a simulation of the array (as described above), and proceed until the results achieved in the simulation are substantially attained.

Preferably said method includes reflecting light from each of said mirrors by means of one or more light sources, and more preferably includes directing light from each of said light sources downwards onto said mirrors wherein said array is located with a substantially vertical optical axis and a focus generally above said array.

Thus, by arranging the light sources (preferably each a laser source) vertically, a straightforward way of achieving a light beam of known (i. e. vertical) direction is provided. This is particularly advantageous for adjusting arrays in the field.

Preferably said method includes reflecting said light onto a target located at a focus of said array.

In one embodiment, the array is the solar concentrator of a solar power generator.

In a sixth broad aspect, the present invention provides a method of aligning a mirror, comprising: 1) determining a preferred pattern of light reflection from said mirror ; 2) obtaining a characterization of the shape of said mirror ; 3) simulating said mirror and light reflection therefrom on the basis of said characterization, and comparing said simulated light reflection with said preferred pattern of light reflection ; and 4) adjusting a simulated orientation of said mirror and repeating step 3) until said simulated light reflection is within acceptable tolerances of said preferred pattern of light reflection.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the present invention may be more clearly ascertained, an embodiment will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic side view of an apparatus for determining the figure of a mirror according to one embodiment of the present invention ; Figure 2 is a schematic top view of the apparatus for determining the figure of a mirror of figure 1 ; and Figure 3 is a view of a solar power generator with an array of the type to be simulated by the simulation program of the apparatus of figure 1.

DETAILED DESCRIPTION OF THE INVENTION An apparatus for determining the figure of a mirror according to one embodiment of the present invention is shown generally at 10 in figures 1 and 2 with a mirror 12.

Figure 1 is a side view of the apparatus 10, which includes a bank 14 of laser sources, a detecting surface in the form of target screen 16, a digital camera 18 and a data

collection computer 20. Figure 2 is a top view, in which the individual laser sources 22a, 22b, etc. constituting the bank 14 of laser sources are shown. The mirror 12 is supported on a simple stand (not shown), while the target screen 16 and the laser sources 22a, 22b, etc. are mounted on a servo-motor driven, vertically translatable mount (also not shown), so that they can be translated vertically in concert. The laser sources 22a, 22b, etc. are mounted such that their respective laser beams 24a, 24b, 24c, etc. are horizontal.

In use, the bank 14 of laser sources is slowly translated downwards. The laser beams 24a, 24b, 24c, etc. are reflected by the test mirror 12 onto the target screen 16.

Periodically a screen grab is collected from the output of the camera 18 ; at the same time, the instantaneous locations of the laser sources 22a, 22b, etc. are also collected. Those locations are obtained from the servo- motor controller (not shown) controlling the servo-motor, and from the known geometry of the apparatus 10 overall.

At each measurement, the location on the test mirror 12 at which each beam 24a, 24b, etc. impinges the mirror 12 can be deduced from the locations of the laser sources 22a, 22b, etc., in view of the horizontal nature of the beams 24a, 24b, etc. From this information and the locations at which each reflected beam intersects the target screen 16 may be deduced the gradient of the mirror at each location at which a beam was incident when a measurement was made.

Consequently, the angle of reflection for any angle of incidence can subsequently be predicted for each of these locations.

It should be noted that as, in this embodiment, measurements are made progressively as the laser source/target screen assembly is translated vertically, the locations on the mirror 12 at which gradients are obtained

are arranged in a grid of rows and columns (though, where the mirror 12 is curved-as shown in figures 1 and 2, this grid will also be curved in space).

When this procedure has been completed, the behaviour of the mirror 12 in reflecting light from, say, the sun can be predicted. The apparatus includes a simulation program (not shown) running on computer 20 or on another computer networked to computer 20.

For a mirror 12 (such as a spherical mirror that might be employed in a solar power generation system), it may be desirable to predict how light falling on the mirror 12 will be focussed or reflected. Consequently, the simulation program receives the gradient values for the mirror 12, and calculates the intensity distribution of solar radiation (comprising an essentially broad but parallel incident beam) after reflection from the mirror, typically at a predetermined distance from the mirror corresponding to the location of, for example, a solar collector. The simulation program can perform this simulation on the basis of light rays impinging on the mirror at the locations on the mirror\'s surface at which these gradient values have been determined.

Optionally the simulation program performs the simulation with additional simulated rays impinging the mirror at the locations other than where gradient values have been determined. It does this by treating each measured location as being at the centre of a flat, essentially rectangular region. The regions are rectangular owing to the regular spacing of the locations at which the gradient values have been obtained. Even though the mirror may be curved, this approximation should generally be acceptable provided that the size of the regions (determined by the spacing of the gradient measurements) are relatively small.

The spacing of the gradient measurements can be selected to

ensure that this approximation is acceptable.

Subsequently, if the intensity distribution is less than adequate, the simulated orientation of the mirror can be adjusted and the intensity distribution recalculated by the simulation program. This can be repeated until an optimal or acceptable intensity distribution is obtained.

The simulation program can also be used to simulate an array of two or more such mirrors, such as an array of mirrors for a solar power generator. Such a solar power generator is illustrated generally at 30 in figure 3. The generator includes an array 32 of mirrors 34 and a collector 36 (comprising a square array of photovoltaic cells) approximately at the focus of the array 32. Once a grid of gradient values has been determined for each mirror and provided to the simulation program, the simulation program simulates the desired array by treating each mirror as being at a simulated location and orientation within the array. The intensity distribution of the array can then be simulated as described above for a single mirror.

One possible preferred simulation produces a substantially even intensity distribution over the collector so that, in an actual solar power generator, the energy is distributed and the collector does not develop hot-spots.

The simulated orientations of any of the mirrors can then be adjusted and/or the simulated locations of one or more mirrors can be modified, until the desired or an acceptable intensity distribution is obtained.

In running the simulation program, the mirrors simulated as located towards the periphery of the array of mirrors are preferably those which, when their gradient values were determined, to most closely conform to the design specifications for the mirrors. For example, if the

mirrors were intended to approximate spherical mirrors, those most closely spherical would be simulated as at the edge of the array of mirrors, where higher angles of incidence of sunlight will occur. Greater deviations from the intended spherical shape can be tolerated in individual mirrors where low angles of incidence (near the centre of the array) are expected.

If the array is to be installed in an actual installation (such as a solar concentrator of a solar power generator), each mirror in the array of mirrors can, according to the present invention, be aligned using a laser source or group of laser sources with respect to a target placed at or near the focal region of the array.

The array is assembled according to the results of the simulation. Then, at the installation site, the array is located with its optical axis pointing upwards, and with the laser source or group of laser sources suspended above the array such that their beams point vertically downwards.

The laser source or sources are shone onto each mirror in turn and the pattern of reflected light on the target observed. The orientation of each mirror is then adjusted until the pattern on the target agrees to an acceptable degree with that predicted in the simulation for that mirror. As the simulation sought to define locations and orientations for the mirrors to provide the optimal reflected intensity distribution or"focal shape"for energy conversion (in the case of a solar concentrator), this field alignment technique should ensure that that optimal arrangement is achieved.

Modifications within the spirit and scope of the invention may readily be effected by persons skilled in the art. It is to be understood, therefore, that the invention is not limited to the particular embodiments described by way of example hereinabove.

For the purpose of this specification the words "comprising","comprise"or"comprises"are understood to mean the inclusion of a feature but not necessarily exclusion of any other feature.

It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that that prior art forms a part of the common general knowledge in the art, in Australia or in any other country.