Patent Description:
Coded photography is a process that may be used to assist in the analysis of fabricated components. In a coded photography process, a number of images of an object are taken in succession, with one parameter being changed between images and the other parameters being held constant. One example of a parameter that may be changed between the images is the angle of illumination of the light used to illuminate the object.

Successive images (i.e. an image stack) can be obtained using various parameters and the greyscale value of the pixels of the images within the image stack can be used to give information regarding the geometry or other properties of the object being imaged.

Analysis of this image data can be used to identify defects, improving the manufacturing process of the components and helping to identify any components not suitable for use. However, the identification of certain defects in components can still prove difficult. It would therefore be desirable to provide an improved method for visual analysis of components.

Japanese patent application <CIT> discloses a method for evaluating the size and distribution of crystal grains in a polycrystalline superconductor thin film. The method comprises irradiating the surface of the polycrystalline superconducting thin film with polarised light, determining the angle at which the reflected light is brightest, and then altering the angle of illumination and monitoring the decrease in brightness to determine the spread of crystal grain alignments.

European patent application <CIT> discloses methods and an apparatus for examining an article or component and for optically determining the orientation of a grain structure in a component, particularly where the article or component is formed of a single crystal alloy.

European patent application <CIT> discloses methods and an apparatus for examining an article or component and for determining the value of a boundary between a primary grain and a secondary grain in a component, particularly where the article or component is formed of a single crystal alloy.

United States patent application <CIT> relates to a method for filtering undesired light reflections in a structured light measurement system during the inspection of shiny metal prismatic objects having uncoated prismatic surfaces, such as turbine blades, using polarized light.

There is provided a method of analysing a component formed from a metal alloy to identify a possible defect according to appended independent claim <NUM>. Particular embodiments of the invention are defined in the appended dependent claims.

Each pixel may be categorised as corresponding to the first crystal grain region by identifying a first region of the differences in polarisation in a first range or corresponding to the second crystal grain region by identifying a second region of the differences in polarisation in a second range, where the first range is different to the second range.

Each pixel may be categorised as corresponding to the second crystal grain region if the difference in polarisation exceeds a threshold value.

The method may further comprise determining the angle of orientation of the second crystal grain region relative to the angle of orientation of the first crystal grain region based on the difference in polarisation.

The method may further comprise determining the location of the boundary between the second crystal grain region and the first crystal grain region based on the difference in polarisation.

The method may further comprise determining the area of the second crystal grain region based on the difference in polarisation.

The component may be illuminated using the first polarisation state of light at the same perspective and orientation relative to the illumination using the second polarisation state of light.

The second image may be obtained at the same perspective and orientation relative to the component as the first image.

At least one of the first and second polarisation states may be a linear polarisation state.

Both of the first and second polarisation states may be a linear polarisation state and the first linear polarisation state is at a different polarisation angle with respect to the plane of incidence to the second linear polarisation state.

At least one of the first and second polarisation states may be a circular polarisation state.

Each of the first image and the second image may further comprise intensity data; and the method may further comprise the step of determining a difference in intensity for plural pixels of the first image between each pixel of the first image and a corresponding pixel of the second image; wherein the identification of pixels corresponding to the second crystal grain region is additionally based on the difference in intensity.

The method may further comprise obtaining a plurality of further images of the component wherein each of the further plurality of images is obtained using a different polarisation state to each of the other of the further plurality of images; and storing the polarisation data of each of the images of the component in a matrix of image data; wherein the identification of pixels corresponding to the second crystal grain region may be performed by analysis of the matrix of image data.

The component may be a component of a gas turbine engine.

The component may be a turbine blade of a gas turbine engine.

The gearbox may be a reduction gearbox (in that the output to the fan is a lower rotational rate than the input from the core shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "star" gearbox, as described in more detail elsewhere herein. The gearbox may have any desired reduction ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example greater than <NUM>, for example in the range of from <NUM> to <NUM>, or <NUM> to <NUM>, for example on the order of or at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The gear ratio may be, for example, between any two of the values in the previous sentence. Purely by way of example, the gearbox may be a "star" gearbox having a ratio in the range of from <NUM> or <NUM> to <NUM>. In some arrangements, the gear ratio may be outside these ranges.

Each fan blade may be defined as having a radial span extending from a root (or hub) at a radially inner gas-washed location, or <NUM>% span position, to a tip at a <NUM>% span position. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or on the order of) any of: <NUM>, <NUM>, <NUM><NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. The ratio of the radius of the fan blade at the hub to the radius of the fan blade at the tip may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>. These ratios may commonly be referred to as the hub-to-tip ratio. The radius at the hub and the radius at the tip may both be measured at the leading edge (or axially forwardmost) part of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e. the portion radially outside any platform. The radius of the fan may be measured between the engine centreline and the tip of a fan blade at its leading edge. The fan diameter (which may simply be twice the radius of the fan) may be greater than (or on the order of) any of: <NUM>, <NUM>, <NUM>, <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM> (around <NUM> inches), <NUM>, <NUM> (around <NUM> inches) or <NUM> (around <NUM> inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM> or <NUM> to <NUM>.

The rotational speed of the fan may vary in use. Generally, the rotational speed is lower for fans with a higher diameter. Purely by way of non-limitative example, the rotational speed of the fan at cruise conditions may be less than <NUM> rpm, for example less than <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> (for example <NUM> to <NUM> or <NUM> to <NUM>) may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm. Purely by way of further non-limitative example, the rotational speed of the fan at cruise conditions for an engine having a fan diameter in the range of from <NUM> to <NUM> may be in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm, for example in the range of from <NUM> rpm to <NUM> rpm.

In use of the gas turbine engine, the fan (with associated fan blades) rotates about a rotational axis. This rotation results in the tip of the fan blade moving with a velocity Utip. The work done by the fan blades on the flow results in an enthalpy rise dH of the flow. A fan tip loading may be defined as dH/Utip<NUM>, where dH is the enthalpy rise (for example the <NUM>-D average enthalpy rise) across the fan and Utip is the (translational) velocity of the fan tip, for example at the leading edge of the tip (which may be defined as fan tip radius at leading edge multiplied by angular speed). The fan tip loading at cruise conditions may be greater than (or on the order of) any of: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> (all units in this paragraph being Jkg-<NUM>K-<NUM>/(ms-<NUM>)<NUM>). The fan tip loading may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>, or <NUM> to <NUM>.

Gas turbine engines in accordance with the present disclosure may have any desired bypass ratio, where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core at cruise conditions. In some arrangements the bypass ratio may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The bypass ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of form <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The bypass duct may be substantially annular. The bypass duct may be radially outside the engine core. The radially outer surface of the bypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of a gas turbine engine as described and/or claimed herein may be defined as the ratio of the stagnation pressure upstream of the fan to the stagnation pressure at the exit of the highest pressure compressor (before entry into the combustor). By way of non-limitative example, the overall pressure ratio of a gas turbine engine as described and/or claimed herein at cruise may be greater than (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. The overall pressure ratio may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>.

Specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow through the engine. At cruise conditions, the specific thrust of an engine described and/or claimed herein may be less than (or on the order of) any of the following: <NUM> Nkg-<NUM>s, <NUM> Nkg-<NUM>s, <NUM> Nkg-<NUM>s, <NUM> Nkg-<NUM>s, <NUM> Nkg-<NUM>s, <NUM> Nkg-<NUM>s or <NUM> Nkg-<NUM>s. The specific thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> Nkg-<NUM>s to <NUM> Nkg-<NUM>s, or <NUM> Nkg-<NUM>s to <NUM> Nkg-<NUM>s. Such engines may be particularly efficient in comparison with conventional gas turbine engines.

A gas turbine engine as described and/or claimed herein may have any desired maximum thrust. Purely by way of non-limitative example, a gas turbine as described and/or claimed herein may be capable of producing a maximum thrust of at least (or on the order of) any of the following: 160kN, 170kN, 180kN, 190kN, 200kN, 250kN, 300kN, 350kN, 400kN, 450kN, 500kN, or 550kN. The maximum thrust may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). Purely by way of example, a gas turbine as described and/or claimed herein may be capable of producing a maximum thrust in the range of from 330kN to <NUM> kN, for example 350kN to 400kN. The thrust referred to above may be the maximum net thrust at standard atmospheric conditions at sea level plus <NUM> degrees C (ambient pressure <NUM>. 3kPa, temperature <NUM> degrees C), with the engine static.

In use, the temperature of the flow at the entry to the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the exit to the combustor, for example immediately upstream of the first turbine vane, which itself may be referred to as a nozzle guide vane. At cruise, the TET may be at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The TET at cruise may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds). The maximum TET in use of the engine may be, for example, at least (or on the order of) any of the following: <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM>. The maximum TET may be in an inclusive range bounded by any two of the values in the previous sentence (i.e. the values may form upper or lower bounds), for example in the range of from <NUM> to <NUM>. The maximum TET may occur, for example, at a high thrust condition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/or claimed herein may be manufactured from any suitable material or combination of materials. For example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a composite, for example a metal matrix composite and/or an organic matrix composite, such as carbon fibre. By way of further example at least a part of the fan blade and/or aerofoil may be manufactured at least in part from a metal, such as a titanium based metal or an aluminium based material (such as an aluminium-lithium alloy) or a steel based material. The fan blade may comprise at least two regions manufactured using different materials. For example, the fan blade may have a protective leading edge, which may be manufactured using a material that is better able to resist impact (for example from birds, ice or other material) than the rest of the blade. Such a leading edge may, for example, be manufactured using titanium or a titanium-based alloy. Thus, purely by way of example, the fan blade may have a carbon-fibre or aluminium based body (such as an aluminium lithium alloy) with a titanium leading edge.

A fan as described and/or claimed herein may comprise a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may comprise a fixture which may engage a corresponding slot in the hub (or disc). Purely by way of example, such a fixture may be in the form of a dovetail that may slot into and/or engage a corresponding slot in the hub/disc in order to fix the fan blade to the hub/disc. By way of further example, the fan blades maybe formed integrally with a central portion. Such an arrangement may be referred to as a bladed disc or a bladed ring. Any suitable method may be used to manufacture such a bladed disc or bladed ring. For example, at least a part of the fan blades may be machined from a block and/or at least part of the fan blades may be attached to the hub/disc by welding, such as linear friction welding.

The gas turbine engines described and/or claimed herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the exit area of the bypass duct to be varied in use. The general principles of the present disclosure may apply to engines with or without a VAN.

The fan of a gas turbine as described and/or claimed herein may have any desired number of fan blades, for example <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> or <NUM> fan blades.

As used herein, cruise conditions have the conventional meaning and would be readily understood by the skilled person. Thus, for a given gas turbine engine for an aircraft, the skilled person would immediately recognise cruise conditions to mean the operating point of the engine at mid-cruise of a given mission (which may be referred to in the industry as the "economic mission") of an aircraft to which the gas turbine engine is designed to be attached. In this regard, mid-cruise is the point in an aircraft flight cycle at which <NUM>% of the total fuel that is burned between top of climb and start of descent has been burned (which may be approximated by the midpoint - in terms of time and/or distance- between top of climb and start of descent. Cruise conditions thus define an operating point of, the gas turbine engine that provides a thrust that would ensure steady state operation (i.e. maintaining a constant altitude and constant Mach Number) at mid-cruise of an aircraft to which it is designed to be attached, taking into account the number of engines provided to that aircraft. For example, where an engine is designed to be attached to an aircraft that has two engines of the same type, at cruise conditions the engine provides half of the total thrust that would be required for steady state operation of that aircraft at mid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruise conditions are defined as the operating point of the engine that provides a specified thrust (required to provide - in combination with any other engines on the aircraft - steady state operation of the aircraft to which it is designed to be attached at a given mid-cruise Mach Number) at the mid-cruise atmospheric conditions (defined by the International Standard Atmosphere according to ISO <NUM> at the mid-cruise altitude). For any given gas turbine engine for an aircraft, the mid-cruise thrust, atmospheric conditions and Mach Number are known, and thus the operating point of the engine at cruise conditions is clearly defined.

Purely by way of example, the forward speed at the cruise condition may be any point in the range of from Mach <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example <NUM> to <NUM>, for example on the order of Mach <NUM>, on the order of Mach <NUM> or in the range of from <NUM> to <NUM>. Any single speed within these ranges may be part of the cruise condition. For some aircraft, the cruise conditions may be outside these ranges, for example below Mach <NUM> or above Mach <NUM>.

Purely by way of example, the cruise conditions may correspond to standard atmospheric conditions (according to the International Standard Atmosphere, ISA) at an altitude that is in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM> (around <NUM> ft), for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> (around <NUM> ft) to <NUM>, for example in the range of from <NUM> to <NUM>, for example in the range of from <NUM> to <NUM>, for example on the order of <NUM>. The cruise conditions may correspond to standard atmospheric conditions at any given altitude in these ranges.

Purely by way of example, the cruise conditions may correspond to an operating point of the engine that provides a known required thrust level (for example a value in the range of from 30kN to 35kN) at a forward Mach number of <NUM> and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of <NUM> (38000ft). Purely by way of further example, the cruise conditions may correspond to an operating point of the engine that provides a known required thrust level (for example a value in the range of from 50kN to 65kN) at a forward Mach number of <NUM> and standard atmospheric conditions (according to the International Standard Atmosphere) at an altitude of <NUM> (<NUM> ft).

In use, a gas turbine engine described and/or claimed herein may operate at the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (for example the mid-cruise conditions) of an aircraft to which at least one (for example <NUM> or <NUM>) gas turbine engine may be mounted in order to provide propulsive thrust.

However, any embodiment of a method in accordance with the present invention must fall within the scope defined by appended independent claim <NUM>.

The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines <NUM>, <NUM> before being exhausted through the core exhaust nozzle <NUM> to provide some propulsive thrust.

The planet carrier <NUM> constrains the planet gears <NUM> to process around the sun gear <NUM> in synchronicity whilst enabling each planet gear <NUM> to rotate about its own axis.

There are four planet gears <NUM> illustrated, although it will be apparent to the skilled reader that more or fewer planet gears <NUM> may be provided within the scope of the disclosure.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in <FIG> has a split flow nozzle <NUM>, <NUM> meaning that the flow through the bypass duct <NUM> has its own nozzle <NUM> that is separate to and radially outside the core exhaust nozzle <NUM>. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct <NUM> and the flow through the core <NUM> are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine <NUM> may not comprise a gearbox <NUM>.

<FIG> shows a schematic example of a component <NUM> that may present inside a gas turbine engine <NUM> as discussed above. The component may be a turbine blade of the gas turbine engine <NUM>.

The component <NUM> may be formed from a metal alloy, for example nickel, and the manufacturing process for the component <NUM> may be intended to result in a single crystal alloy. The component <NUM> may comprise a first crystal grain region <NUM>. The first crystal grain may be the intended crystal grain from which the component <NUM> is to be formed. The first crystal grain region <NUM> has a particular crystal grain axis <NUM>.

If the manufacturing process for the component <NUM> has been performed successfully, the first crystal grain region <NUM> may be the only crystal grain present in the component <NUM>. Alternatively, a second crystal grain region <NUM> may also be present in the component <NUM>. The second crystal grain region <NUM> has a crystal grain axis <NUM> that is different to the crystal grain axis <NUM> of the first crystal grain region <NUM>.

The presence of the second crystal grain region <NUM> may be considered a defect. The presence of the defect may be due to an error in the manufacturing process of the component <NUM>. Alternatively, the presence of defects may be unavoidable but it may be desirable to reduce the presence of defects in the component <NUM>.

It is possible to obtain information about the presence of such defects through optical analysis of the surface of the component. For example, when a component <NUM> formed of a nickel material is cast and goes through a blast and etch process, gamma prime precipitate blocks form the optical surface of the component. The presence of a second crystal grain <NUM> at the surface of the component <NUM> may cause a change in behaviour of light illuminating the component <NUM>. For example, the difference in the angle of the crystal grain axis <NUM> of the second crystal grain region <NUM> to the crystal grain axis <NUM> of the first crystal grain region <NUM> (known as the sheer angle) may cause a shift in the angular position of maximum reflectance from the surface of the component <NUM>.

The gamma prime precipitate blocks which form the surface of the component <NUM> may have a similar size distribution to the wavelength of light used to illuminate the surface. In this case, the surface of the component <NUM> may exhibit diffraction grating like properties. The presence of a second crystal grain region <NUM> at a different crystal grain axis <NUM> results in an in-plain rotation of the diffraction grating like surface, which causes a polarisation shift in light reflected from the surface. Therefore, the presence of a second crystal grain region <NUM> may cause a change in the polarisation angle of light illuminating the surface of the component <NUM> when compared to light illuminating the first crystal grain region <NUM>.

It is therefore possible to obtain information about the presence of a second crystal grain region <NUM> within a component <NUM> by imaging the component <NUM> using different polarisations states of light.

An example of the steps of such a method is shown in <FIG>. In a first step <NUM>, a first image of the component <NUM> is obtained. To obtain the first image, the component <NUM> is illuminated with light of a first polarisation state and a first image of the component <NUM> is recorded by a detector array comprising a plurality of pixels, where the first image includes polarisation data such as the polarisation angle recorded for each pixel.

In a second step <NUM>, a second image of the component <NUM> is obtained. When obtaining the second image, the component <NUM> is illuminated using light of a second polarisation state which is different to the first polarisation state. The second image also includes polarisation data such as the polarisation angle recorded for each pixel.

The method described above may be performed with various different polarisation states. For example, both the first polarisation state used to obtain the first image and the second polarisation state used to obtain the second image may be linear polarisation states. In this case, the polarisation angle of the first state may be different to the second state. Alternatively, one of the states may be a circular polarisation state. Different polarisation states may be used depending on the nature of the defect and the component being analysed.

In a third step <NUM>, a difference in polarisation is calculated. The first image of the component <NUM> and the second image of the component <NUM> are compared. A plurality of pixels in the first image may be compared to corresponding pixels of the second image. Corresponding pixels may be pixels from each image that represent the same point on the surface of the component <NUM>.

A difference in the measured polarisation between corresponding pixels may be calculated. For example, the change in the recorded polarisation angle between the corresponding pixels may be calculated.

In a fourth step <NUM>, pixels corresponding to the second crystal grain region <NUM> may be determined based on the calculated difference in polarisation. Using this method, regions of the second crystal grain region <NUM>, which may be a defect in the component <NUM> as discussed above, can be identified. Identification of various properties of such defect regions may allow selection of components with minimal or no defects. Identification of the properties may allow improvement of the manufacturing method of the component <NUM> by comparing the properties defects between different components manufactured using different methods.

The different regions in the component <NUM> may be identified in different ways. For example, particular regions of the component <NUM> may be characterised as being part of the first region <NUM> if the calculated polarisation difference falls into a first range. Particular regions of the component <NUM> may be characterised as being part of the second region <NUM> if the calculated polarisation difference falls into a second range. Alternatively, a region of the image may be categorised as being part of the second region <NUM> if the calculated in polarisation exceeds a threshold value.

The calculated difference in polarisation may be used to determine further information about the second crystal grain region <NUM>. For example, the calculated difference in polarisation may be used to calculate the angle of orientation of the second crystal grain axis <NUM> relative to the angle of orientation of the first crystal grain axis <NUM>. The boundary between the two regions may also be determined. The area of the second crystal grain region <NUM>, either as an absolute value or relative to the area of the first crystal grain region <NUM> may also be obtained.

Each of the first image and the second image may be obtained with at least one of the illumination source and the light receiver arranged at the same orientation relative to the component <NUM>. In this case, identifying corresponding pixels in the first and the second images may be simpler as corresponding pixels will be at the same location in each image.

Additional information may be used to assist with the identification of the second crystal grain region <NUM>. For example, when the polarisation information of the first image and the second image is obtained, intensity data may also be obtained for each pixel. A difference in intensity between different pixels within the first image or corresponding pixels of the first image and the second image may be calculated. The calculated difference in intensity may be used in addition to the calculated difference in polarisation when determining the properties of the second crystal grain region <NUM> as discussed above.

The method discussed above is not limited to the obtaining of only two images. Any number of images may be obtained, where each of the further plurality of images is obtained using a different polarisation state of light to each of the other plurality of images. For example, a plurality of images may be obtained, where linearly polarised light is used to illuminate the object <NUM> and the polarisation angle of the light is stepped through in each subsequent image. When a plurality of images has been obtained, the polarisation values of each of the pixels of the plurality of images may be stored as a matrix of image data. Identification of pixels corresponding to the second crystal grain region <NUM> may be performed by analysis of the matrix of image data. Data mining and deep learning techniques such as the analysis of the matrix of image data using trained neural networks may be used.

<FIG> shows a non-claimed schematic example of apparatus <NUM> which may be used to perform the method discussed above. A light source <NUM> is used to illuminate the component <NUM> in a plane of incidence. The light source <NUM> is capable of producing polarised light in various polarisation states, such as linear polarisation states at different angles and circular polarisation states as discussed above.

The light reaches the component <NUM>, is reflected in the plane of incidence and is received by a detector <NUM>. The detector <NUM> comprises a sensor with multiple pixels, where in each pixel is configured to detect the polarisation state of light incident on the pixel. Image data obtained by the detector <NUM> is passed to an analyser <NUM>. The analyser <NUM> compares the image data of multiple images obtained by the detector <NUM> to calculate the differences in polarisation as discussed above.

Claim 1:
A method of analysing a component (<NUM>) formed from a metal alloy to identify a possible defect, wherein the metal alloy comprises a first crystal grain region (<NUM>) and the possible defect comprises a second crystal grain region (<NUM>) aligned to a different axis to the first crystal grain region (<NUM>), the method comprising the steps of:
obtaining a first image of the component (<NUM>) using a multi-pixel sensor and illuminated using a first polarisation state of light, the first image comprising first polarisation data across a plurality of pixels within the multi-pixel sensor; and
obtaining a second image of the component (<NUM>) using a multi-pixel sensor and illuminated using a second polarisation state of light different to the first polarisation state, the second image comprising second polarisation data across a plurality of pixels within the multi-pixel sensor;
characterized in that the method further comprises the steps of:
determining a difference between the first and second polarisation data for the plurality of pixels of the first image and a corresponding plurality of pixels of the second image; and
identifying pixels corresponding to the second crystal grain region (<NUM>) based on the difference in first polarisation data from the plurality of pixels of the first image and second polarisation data of the corresponding plurality of pixels of the second image.