Field of Invention

The present invention relates to a method and apparatus for embedding means for determining film parameters of a film, a method and apparatus for determining parameters of a film and a film comprising means for determining film parameters.

Background

In the production of motion picture films, quality control is of great importance. After exposure in a camera or printer, motion picture films need to be developed on a processor. The processor contains a number of chemical baths and specific conditions which convert the light sensitive film having latent images formed by exposure to light into a stable "fixed" film which may be viewed (a positive) or printed (a negative).

To determine if a film processing operation has been performed to an acceptable standard, a control strip is commonly used. The control strip comprises a length of film in which a series of (typically 21 ) areas of the film are pre-exposed to differing known intensities of light, as shown in Figure 1. The control strips are provided unprocessed. The unprocessed control strips are then spliced in with the film reels being processed as they go through the film processor.

After processing, the control strips are separated from the production film and the density of the pre-exposed areas of the film are read using a densitometer. The densitometer is an instrument that measures the density of the film (absorption of light) by measuring areas of the control strip, which are typically circular and of a diameter between 1mm and 3mm. The density measurement may be in terms of grey scale ("visual") or in RGB (using filters to split the image into red, green and blue). The densitometer measures optical density at a series of circular areas and the results are expressed using a logarithmic scale of 0 to 4, where 0 is perfectly clear and 4 being the case where only 10 ^" of the light passes through. The densitometer measures this scale of 0 to 10,000 to a high degree of accuracy, typically with 1% error or less throughout the range.

CONFIRMATION COPY An indication of the quality of the processing may be obtained by measuring the density values for the pre-exposed areas of the film and comparing the measured values with expected values. However, with this procedure, there is no way of making sure that a control strip was processed on a given processor at a given time. Furthermore, there may be a considerable time interval between spiicing in a control strip and the control strip being processed, separated, read and assessed. This is generally a manual process and the delays can be anything from 10 minutes to hours. The value of production that can be generated by a motion picture processor at full speed may be extremely high, such that even 10 minutes delay can lead to loss of valuable asset time.

An object of the present invention is to address or mitigate at least one problem with the prior art.

Summary of invention

According to a first aspect of the present invention is an image carrier medium comprising at least a first section for use in determining film parameters and a further section for storing material for display, the first section defining a plurality of portions of differing predetermined density.

The image carrier medium may comprise a film strip. By providing a section of the film strip in which portions of differing predetermined density are provided, density measurement may be made directly from the customer film strips on the processor. Thus, the measurements may be automated, and feedback obtained in real time. The material required for making densitometric measurements may be embedded in the film strip. As such, it need not be necessary to rely on an external object, such as a control strip, for making the densitometric measurements. Furthermore, since the test pattern remains with the film strip, then these patterns may be useful for archiving.

The first and second portions may be contained within a unitary portion of the film strip, such as an unspliced portion. The material for display may comprise frames of a motion picture.

The film may comprise a second section for encoding information, such as quality metrics for the film and/or target values and/or reference values and/or calibration values for the film. The film may comprise a third section comprising conventional film header elements, such as a countdown and/or a title. The first and/or second and/or third portions may be comprised in a header of the film strip. The further section for storing material for display may comprise or be comprised in a fourth section. The fourth section may be comprised outwith the header of the film strip. The material stored in the fourth section may comprise frames of a motion picture.

The film strip may be configured to be fed through film strip handling apparatus in a running direction.

The portions of differing predetermined density may be elongated, preferably elongated in the running direction. The portions of differing predetermined density may comprise stripes. The stripes may extend in the running direction. A plurality of portions of differing predetermined density may be provided in a direction perpendicular to the running direction, for example, the film width direction.

By providing a plurality of stripes of varying predetermined densities that extend in the running direction, if the stripes are read by a camera and the camera shutter is open for a relatively long time, then the effect of any blurring in the running direction due to motion of the film during in-line reading of the film may be minimised as any blurring of the stripes due to long exposure times may not interfere with the other stripes.

The portions of differing predetermined density may be provided in monochrome or greyscale.

At least some of the portions of differing predetermined density may be spaced apart by varying predetermined amounts. The spacings between portions of differing predetermined density may define a range of resolutions. In this way, the sharpness of the edge between the stripe and the clear film may be calculated. For example, if a gap between stripes is less than the resolving power of the reading system, then the gap between stripes may not be detectable, or difficult to distinguish. By determining the stripe spacing at which adjacent stripes become distinguishable, then a determination of system resolution may be made.

The film may comprise a macromolecular matrix. A photosensitive material may be provided within the matrix. The film may comprise a material formed from fixing at least one photosensitive material. The portions of predetermined density may be formed from the photosensitive material or the material formed from fixing at least one photosensitive material.

The portions of differing predetermined density may be provided over a plurality of frames, preferably over three frames.

The plurality of portions of differing predetermined density may comprise at least two portions having different colours to each other. At least two portions of differing predetermined density and/or colour may be provided in the second section.

Portions of differing predetermined density and/or colour may be associated with or indicative of one or more bits, numbers or data items. Density ranges and/or colours of the portions of differing predetermined density may be associated with data such as bits or numbers. The data may encode parameters of the film such as quality metrics for the film and/or target values and/or reference values and/or calibration values for the film.

In this way, since the header remains part of the film strip, reference values and other data can be stored on the film in the second section, along with the density determination means of the first section. In this way, it is possible to assess parameters such as dye fade directly from the film using the header.

According to a second aspect of the present invention is an apparatus for analysing an image carrier medium according to a first aspect of the invention, the apparatus comprising an imaging device for capturing an image of the first and/or third portions of the image carrier medium, and processing means adapted to determine parameters of the image carrier medium using the plurality of portions of differing predetermined density. The imaging device may comprise a camera, preferably a digital camera. The digital camera may comprise a digital camera of at least 16Mpixel resolution. The apparatus may comprise an illumination source. The apparatus may comprise a macro lens located between a film strip receiving area and the imaging device. The apparatus may be comprised in, or adapted for use with, a film processor. The apparatus may be provided or be eatable after a film dryer of the processor, for example at a take-off elevator section of the processor. The apparatus may be configured to analyse the image of the first and/or second sections of the film strip, which may comprise analysing the portions of differing predetermined density. The apparatus may be configured to access calibration data and compare a value, such as a greyscale value, of portions of differing predetermined density in the image with the calibration data to determine a corresponding density value for one or more of the portions of differing predetermined density. The calibration data may be specific for the imaging device and/or the illumination source. The apparatus may be configured to determine the apparent width of one or more of the stripes from the image. The apparatus may be configured to determine a running speed of the film from the determined width(s).

The apparatus may comprise one or more reference areas comprising metallic based material, which may be in the form of stripes, preferably silver based stripes. The reference areas may be arranged so as to appear in images taken by the imaging device. The reference areas may be arranged so as to appear above and/or below the film in images taken by the imaging device. Since the density of these metallic based stripes does not change, they may be used to provide feedback to allow for routine variations in lamp or camera systems to be determined.

The apparatus may be configured to determine the density of the metallic based stripes and apply a correction factor to compensate for variations in the illumination source and/or imaging device based on the determined density of the metallic based stripes. Since the metallic based stripes do not change, comparison of measured density values for the metallic stripes can be compared to values stored with the calibration data to determine variations in the lighting system and/or imaging device since the calibration data was created and the density readings for the portions of differing predetermined density may be adjusted accordingly.

The apparatus may be adapted to detect gaps between portions of differing predetermined density, which may be used to determine the effective resolution. If a gap between portions of differing predetermined density is less than the resolving power of the system then the gap may not be visible or may be blurred. As the size of the gap increases, the system may be able to determine the presence of the gap. Therefore, the smallest detected gap sizes may be used to determine the resolving power of the system. The apparatus may be configured to determine density and/or colour of the elongated portions of the first and/or second sections of the film strip. The apparatus may be configured to access a look-up table that assigns data values to determined colours and/or density ranges. In this way, data may be encoded by providing multiple portions of differing predetermined density in various colours, the colour and density combinations of each portion being indicative of data.

A subset of the elongated portions of differing predetermined density may be configured to encode one or more classes or segments of data. A further subset of the portions of differing predetermined density may be configured to store a variation from the classes. In this way, the data that can be carried by the film strip is increased.

The film strip may be arranged such that the range of variation from the classes varies depending on the absolute density. The range of variation may be higher for a higher density portion than for a lower density portion.

The film strip may be configured to store data as a variation on the value stored by the previous portion. In this way, the amount of data that can be stored may be increased.

The apparatus may comprise a plurality of imaging devices. At least one of the imaging devices may be adapted to monitor a dial, display and/or control of a film processor. The apparatus may be configured to access a reference database of dials, displays and/or controls. The apparatus may be adapted to process an image obtained from the imaging device and compare at least a portion of the image and/or data extracted from the image with the reference database in order to determine a setting, parameter and/or condition of the film processor. According to a third aspect of the present invention is a method for analysing an image carrier medium according to the first aspect, the method comprising obtaining at least one image of the image carrier medium, and analysing the first and/or second sections of the image carrier medium in order to determine the density of at least one of the portions of differing predetermined density.

According to a fourth aspect of the invention is a display apparatus for displaying an image, the display apparatus comprising: display means for displaying one or more images;

an imaging device for capturing at least part of a displayed image; and

a processor adapted to analyse the captured image in order to determine one or more parameters of the image, compare the one or more determined parameters with target values and control the display means accordingly.

The displayed image may comprise or be comprised in a motion picture or moving image. At least some of the one or more images may comprise frames of a motion picture. The apparatus may comprise means for reading a film strip. The display means may be operable to display images from the film strip. The film strip may comprise at least one section representing at least one parameter of the film strip.

The display apparatus may be a projection system and the display means may comprise a projector for projecting an image.

The imaging device may comprise a digital camera.

The film strip may comprise means for determining at least one parameter of the film. The film strip may comprise at least one portion of predetermined density.

The film strip may be a film strip according to a first aspect of the invention. The display means may be adapted to display an image based on the first and/or second sections of the film strip of the first aspect. The image may contain at least one area generated from the portions of predetermined density on the film strip.

The processor may be configured to access calibration data.

The processor may be configured to determine at least one parameter, such as colour intensity and/or a greyscale value, of at least one area of the image, such as at least one area of the image representative of one or more portions of predetermined density of the film strip.

The processor may be configured to compare the determined parameter or parameters with the calibration data to determine a corresponding optical density value. The calibration data may be specific for the display apparatus. The processor may be operable to compare the determined optical density and/or one or more parameters of the image with one or more target parameters and control the display apparatus accordingly. In this way, the film strip may be provided with a header area that contains features that, when displayed in the image, may allow parameters of the image to be determined. The features from the image may be captured using the imaging device and analysed by comparison with calibration data to determine parameters of the image, which may then be compared to target parameter values and the display means adjusted accordingly. As such, a self calibrating display/projection system may be provided. Furthermore, by using a film strip of the first aspect, the header information used to determine the quality of the film processing may also be used to control the projector.

According to a fifth aspect of the invention is a method of operation of a display apparatus for displaying an image, the method comprising:

displaying at least one image;

capturing at least part of a displayed image; and

analysing the captured image in order to determine one or more parameters of the displayed image, comparing the one or more determined parameters with target values and controlling the display means accordingly.

According to a sixth aspect of the present invention is a method for controlling a film, processor, comprising obtaining at least one image of an image carrier medium according to the first aspect, and analysing the first and/or second sections of the image carrier medium in order to determine the density of at least one of the portions of differing predetermined density and controlling the film processor based on a difference between at least one determined density and at least one target density.

According to a seventh aspect of the present invention is an apparatus for writing to an image carrier medium according to the first aspect, the apparatus comprising a film writer and a processor, the apparatus being configured to apply at least a first section for use in determining film parameters, the first section comprising a plurality of portions of differing predetermined density and a second section for storing frames of film. The first section may comprise or be comprised in a film header. The apparatus may be further configured to apply material for display to a fourth section of the film.

According to a eighth aspect of the invention is a method for writing to an image carrier medium according to the first aspect, the method comprising applying at least a first section for use in determining film parameters to the image carrier medium, the first section comprising a plurality of portions of differing predetermined density.

The method may further comprise applying material for display to a fourth section of the film.

According to a ninth aspect of the invention is a computer program product for implementing the apparatus of the second, fourth or seventh aspects of the invention or the methods of the third, fifth, sixth or eighth aspects of the invention. According to a tenth aspect of the present invention is an apparatus when programmed with the computer program product of the ninth aspect.

According to an eleventh aspect of the present invention is a carrier medium comprising the computer program product of the ninth aspect.

It will be appreciated that features analogous to those described in relation to any of the above aspects may also be applicable to any of the other aspects.

Aspects and features defined in relation to a method may also provided as a corresponding apparatus and vice versa.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings of which:

Figure 1 shows a prior art control strip;

Figure 2 shows a schematic of a motion picture film strip according to an aspect of the invention;

Figure 3 shows a header portion of the motion picture film strip of Figure 2; Figure 4 shows a film processor according to an aspect of the present invention Figure 5 shows a first section of a film header;

Figure 6 shows a second section of a film header;

Figure 7 shows an alternative film processor according to an embodiment of the invention; and

Figure 8 shows a film projector according to an embodiment of the invention.

Detailed Description of the Drawings Figure 2 shows a motion picture film strip 5 comprising a leader or header portion 10. The leader or header 10 is an area of the film strip 5 that precedes the motion picture frames 15 and is conventionally used to provide a countdown and record information such as the title and usually contains a number of wasted or unused frames. The remaining part 15 of the film strip 5 is used to record the motion picture.

As shown in Figure 3, the header comprises a plurality of areas of predetermined density 20a, 20b, 20c. Eleven areas 20a, 20b, 20c of predetermined density are provided in a repeating pattern. In this example, the areas of predetermined density 20a, 20b, 20c are provided in the form of square blocks. The areas 20a, 20b, 20c collectively cover an operating range of densities. The densities of successive areas 20a, 20b, 20c vary in the film width direction. In this case, the film strip 5 is configured to pass through film handling equipment along an axis 25 shown in the horizontal direction in the page, i.e. perpendicular to the film width direction. The density of the blocks 20a, 20b, 20c may be determined by apparatus known in the art, such as a densitometer or a scanner. It is a requirement that measurement of density is carried out to a high degree of accuracy, typically to an accuracy of 1% or less.

By providing areas 20a, 20b, 20c of predetermined density in the header 10 of the film strip 5, then measurements of density may be made directly from the film strip 5 itself, rather than from a control strip (such as that shown in Figure 1 ). In this way, the determined density accurately reflects the density for the film strip 5 and minimises the possibility that incorrect density measurements are associated with a film strip 5 due to mixing up of control strips, which are separate from the film strip 5. Furthermore, the density data stays with the film strip 5 such that, if target/reference values are also encoded within the header 10, then factors, such as dye fade and shrinkage, that may affect the picture may be obtained from archived film alone and more accurate reproduction of the motion picture embodied in the film strip may be carried out.

In the example shown in Figure 3, the areas of predetermined density 20a, 20b, 20c are provided in the form of square blocks, with successive blocks in the film width direction varying in density. Whilst this arrangement is perfectly suitable for manual off-line measurements of density, it would be advantageous if these measurements could be conducted in-line.

Figure 4 shows an apparatus that handles film, in this example, a film processor 30. Whilst the film processor 30 shown is greatly simplified for reasons of clarity, the general principle of operation still applies. The film strip 5 is run through a tortuous route through successive treatment baths, such as developer baths 35, fixer baths 40 and wash baths 45. After the final wash bath 45, the film is passed through a drier 50 to dry the film and out via an elevator 55. An imaging device 60 in the form of a digital camera and a macro lens is provided after the drier 50 at the elevator section 55. Lighting means 65 is also provided. The camera 60 is connected to a processing device 70 such as a personal computer via a wireless or wired network connection.

The camera 60 is configured to capture an image of the header section 10 of the film strip 5, which may be moving at high speed, for example at 500 frames per minute or more.

In order to determine the density to a high degree of accuracy, the digital camera 60 is preferably of high resolution, such as 16Mpixel or over. A calibration process is performed wherein pixel values of the areas of predetermined density 20a, 20b, 20c are extracted from the image of the header information taken by the digital camera 60. The densities of the areas of predetermined density 20a, 20b, 20c are determined using a reference method, such as densitometer measurements. Correction factors may be applied in order to allow for variations due to thermal variation, lamp stability and aging. In use, the images of the areas having predetermined density 20a, 20b, 20c in the header 10 taken by the digital camera 60 may be analysed and the density extracted by reference to the calibration data. Since it is required to calculate the density to a high degree of accuracy, it is necessary to provide a large amount of light in order to evaluate the entire density range and the shutter opening time / exposure time of the camera is relatively long. In these circumstances, as the film header 10 passes the camera 60 at high speed, the image created tends to be blurred in the running direction of the film. In the film header 10 shown in Figure 3, this may result in differing areas of predetermined density being merged within the image taken by the digital camera 60, which may adversely affect the accuracy and operabiHty of the system.

A further example of a film header 10 is shown in Figure 5. In this example, the film header 10 comprises a plurality of stripes 75, each stripe 75 extending in an axis along the direction of the running direction 25 of the film strip 5. In this example, twenty one stripes 75 spaced apart from each other in the film width direction are provided.

Each stripe 75 is of a differing predetermined density so as to cover an operational density range. The stripes 75 appear as a grey-scale.

In this way, blurring of the stripes 75 in the running direction of the film strip 5 will not blur one area of predetermined density 20a, 20b, 20c onto another and the resultant determination of the density when the film strip 5 is run at speed and/or with a long shutter opening / exposure may be more accurate.

Longer stripes 75 will result in more density data being collected by the system, which may lead to more accurate determination of density. However, having stripes 75 that are too long will take up too much of the film strip 5. In this example, the stripes 75 preferably extend between one and seven frames and preferably extend for three frames.

The stripes 75 are spaced apart by predetermined amounts in the film width direction and a range of stripe spacings are provided. In this way, the header 10 can be used to determine both density and effective resolution.

If the gap 80 between stripes 75 is smaller than the effective resolution of the system 5, then the system 5 will not be able to detect the gap 80 and the adjacent stripes 75 will appear merged together. As the size of the gaps 80 increase above the effective resolution f the system 5, then the system 5 then becomes able to detect the gaps 80. Therefore, by determining the dimension of the smallest resolvable gap size, the detection of the gaps 80 between the stripes 75 may be used to determine the effecting system resolution. The projector 30 is provided with metallic based control stripes 85, in this case four silver based stripes, in the field of view of the imaging device 60 and extending in the running direction of the film strip 5. The control stripes are located to appear above and/or below the film strip 5 in the image collected by the imaging device 60. Since the density of the metallic based stripes 85 generally does not change, the density determined for these control stripes 85 can be compared with the density determined at the time the calibration data was collected, in this way, it is possible to determine and compensate for effects such as variations in the lighting source 65 or camera 60, for example through the use of variable correction factors.

In an optional embodiment, the stripes 75 of predetermined density are provided in a first section of the header 10 and a second section of the header 10 is used for storing reference values in such a way that they are readable at high speed. An example of the second section of the header is shown in Figure 6. In this case, the second section of the header is provided with a plurality of colour stripes 90 extending in the running direction of the film strip. The stripes 90 are of differing colours, for example red, green and blue, and are of differing densities. Combinations of colour and density may be associated with data or values, for example, by using a look-up table. In this way, reference and/or calibration data may be encoded within the film header 10 using the colour and density of the stripes. As an example, if red, green and blue stripes 95 are used and 8 densities, such as 0.1 , 0.4, 0.8, 1.2, 1.6, 2.0, 2.4 and 3.0 are provided in each colour. In this case, it is possible to distinguish 8 states per colour, which can encode 3 bits. If 30 stripes are provided, then each colour can store a 3x30 = a 90 bit number, which for 3 colours gives a total of 270 bits of data.

In an optional embodiment, the data may be encoded by providing a number of pre-defined classes and a portion of the data encoded may indicate a predefined class. Another portion of the data may define a difference from the predefined class. In this way, the amount of data that can be encoded may be increased.

For example, a control strip potentially has 21 steps in each colour, each step being in the range of 0 to 512, i.e. 21 x 9 = 189 bits per colour. For a given class of film, the control strip may be fixed, e.g. it starts at 0.08 and goes up to 4.0. The first 8 bits of the colour stripe code may be used to choose a template to use from a range of 256 classes and the remainder of the colour strip code can be used to encode differences from the template. If a maximum difference from the template is ± 0.32, then this gives 6 bits per step, i.e. 21 x 6 bits.

Furthermore, the maximum difference from a pre-defined class may be variable, for example, such that the maximum difference associated with lower densities may be lower than the maximum difference associated with higher densities.

For example, at a level of 0.08, the maximum difference from the class may be ± 4, i.e. 3 bits. At the high end, the difference may be ± 0.64, i.e. 7 bits.

In an optional embodiment, data is stored as a difference from a preceding data value.

For example, if the variation at a step (e.g. step 4) is +0.08, then the variation on the subsequent step (i.e. step 5) could not be ±0.08, but a likely maximum is between +0.02 and +0.14. Relative the previous step, the maximum variation is therefore ±0.08. Therefore, by storing values in terms of a difference from the previous value, the data for 21 steps can now be stored in around 84 bits per colour, i.e. within the storage capacity of a header second section comprising 30 stripes of each colour. An example of an encoded look-up table is provided below as table 1 :

Table 1

By combining the density/resolution stripes 75 in a first section of the header 10 and the coded reference data using the coloured stripes 90 in the second section of the header 10, all the information needed to evaluate the relative quality of the film strip 5 at any stage in its life is stored in the header 10.

The header information may be written using a suitably configured film writer to generate the stripe patterns 75, 90 in the header 10. The stripe patterns 75, 90 are formed using light sensitive material in the film strip 5. The film strip 5 is then processed to fix the light sensitive material into a light insensitive, stable form. The density of the stripe patterns 75, 90 may be determined after processing of the film strip 5 in order to determine the quality of the film processing and feed back the density information to the film processor 30 in order to control the film processing parameters.

The header reading apparatus may be stand alone or usable with or incorporated into any piece of film handling or processing equipment. As an example, the reading apparatus may be incorporated into film processor 30, as shown in Figure 4. B2011/001450

In an optional embodiment shown in Figure 7, the apparatus is configured to determine parameters associated with the film processor 30'. In this case, further image processing devices 95 are provided and arranged to obtain images of output devices such as displays 100, dials 05 and gauges of the film processor 30 and/or associated measurement devices. Furthermore, images of controls 110 and particular any settings thereof, may be collected. The image processing devices 95 are networked to a processing module 115 that is configured to receive the images from the image processing devices 95. The processing module 115 is configured to access a library 120 of output device types, which may be used in conjunction with the images from the further imaging devices 95 in order to determine parameters indicated or set using the output devices 100, 105 and/or controls 110. In this way, a unitary system is provided that is able to capture and assess in-line and in real time all the data necessary to monitor and control a motion picture processor 30 based on the determined parameters.

The header information encoded with the film strip can also be used by a projector system to control the projection system for optimal projection of the film.

As shown in Figure 8, the projector system 200 comprises a projector 205 as is known in the art, a controller 210 for controlling the projector 205 and an imaging device 215 in the form of a digital camera connected to the controller. The imaging device 215 is operable to capture images 220 generated from the header portion 10 of the film strip 5, including images from the first and/or second sections.

The controller 210 is operable to determine parameters of the captured image from the parts of the image 220 representative of the portions 75, 90 of predetermined density of the film strip 5. The determined parameters include the optical intensity. The controller 210 is operable to access calibration data that relates the optical intensity to density. The controller 210 is also operable to access target values and compare the determined values of intensity and density and adjust the setting of the projector 205 accordingly until the determined parameters match the target parameters.

Focus parameters may also be determined and the focus of the projector adjusted accordingly. The gaps between the stripes of the first and/or second sections of the film may

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I be used to determine the focus in analogous fashion to their use in determining resolution as detailed above. In particular, the image taken by the imaging means may be used to determine the minimum resolvable gap spacing between the stripes of predetermined density in the projected image generated from first and/or second sections of the film. The minimum resolvable gap size may be used as part of a feedback loop in order to optimise the focus of the projector. Optionally, the controller may be operable to retrieve a focus standard, and determine a satisfactory focus by cross-referring to the focus standard.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention.

For example, first and second header sections are advantageously both provided in the header, however it will be appreciated that either section may optionally also be provided individually.

Although the reading apparatus is preferably described as comprising a camera, it will be appreciated that other imaging devices, such as scanners or digital video cameras may be used. Although various example are described as having specified numbers of stripes, such as 2 , it will be appreciated that other numbers of stripes may be provided.

Furthermore, although the indicia used in the determination of density are advantageously stripes, it will be appreciated that other shapes and configurations may be used.

Furthermore, although various features of the invention are described in relation to apparatus features, it will be appreciated that corresponding method features are also intended, and vice-versa. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.