(19) d
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`Europaisches Patentamt
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`European Patent Office
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`Office européen des brevets
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`(11)
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`EP 1 536 399 At
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`(12)
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`EUROPEAN PATENT APPLICATION
`
`(43) Date of publication:
`01.06.2005 Bulletin 2005/22
`
`(51) Intcl.7; GO9G 3/20
`
`(21) Application number: 03078717.0
`
`(22) Dateoffiling: 26.11.2003
`
`(84) Designated Contracting States:
`AT BE BG CH CY CZ DE DK EE ES FIFR GB GR
`HU IE IT LI LU MC NL PT RO SE SISK TR
`
`(72) Inventor: Kimpe, Tom
`9000 Gent(BE)
`
`Designated Extension States:
`AL LT LV MK
`
`(74) Representative: Bird, Ariane et al
`Bird Goen & Co,
`Klein Dalenstraat 42A
`
`3020 Winksele (BE)
`(71) Applicant: BARCO NLV.
`8500 Kortrijk (BE)
`
`(64) Method and device for visual masking of defects in matrix displays by using characteristics
`of the human vision system
`
`rality of non-defective display elements in accord-
`(57)|Thepresent invention provides a methodfor re-
`ance with the representation of the human vision
`ducing the visual impact of defects present in a matrix
`system and the characterising of the at least one
`display comprising a plurality of display elements, the
`defect, to thereby minimise an expected response
`method comprising:
`of the human vision system to the defect, and
`driving at least some of the plurality of non-defective
`display elements with the derived drive signals.
`
`providing a representation of a human vision sys-
`tem,
`characterising at least one defect presentin the dis-
`play, the defect being surrounded by a plurality of
`non-defective display elements,
`deriving drive signals for at least some of the plu-
`
`The presentinvention also provides a correspond-
`ing system for reducing the visual
`impact of defects
`presentin a matrix display, and a matrix display with re-
`duced visual impact of defects presentin the display.
`
`
`
`Fig. 4a
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`Fig. 4b
`
`Fig. 4c
`
`Printed by Jouve, 75001 PARIS (FR)
`
`(Cont. next page)
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`EP1536399A1
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`EP 1 536 399 Al
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`host computer
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`matrix display pixel elements
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`mnatix display
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`hardware
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`firmware
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`hardware
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`EP 1 536 399 A1
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`Description
`
`Technical field of the invention
`
`[0001] The presentinvention relates to a system and method for visually masking of pixel or sub-pixel defects present
`in matrix addressed electronic display devices, especially fixed format displays such as plasma displays, field emission
`displays, liquid crystal displays, electroluminescent (EL) displays, light emitting diode (LED) and organic light emitting
`diode (OLED)displays, especially flat panel displays used in projection or direct viewing concepts.
`[0002]=The invention applies to both monochrome and colour displays and to emissive, transmissive, reflective and
`trans-reflective display technologiesfulfilling the feature that each pixel or sub-pixel is individually addressable.
`
`Background ofthe invention
`
`[0003] At present, most matrix based display technologies are in its technological infancy compared to long estab-
`lished electronic image forming technologies such as Cathode Ray Tubes (CRT). As a result, many domains of image
`quality deficiency still exist and cause problemsfar the acceptance of these technologies in certain applications.
`[0004] Matrix based or matrix addressed displays are composed of individual image forming elements, called pixels
`(Picture Elements), that can be driven (or addressed)individually by proper driving electronics. The driving signals can
`switch a pixel to a first state, the on-state (at which luminance is emitted, transmitted or reflected), to a second state,
`the off-state (at which no luminance is emitted, transmitted or reflected) - see for example EP-117335 - or for some
`displays, one or any intermediate state between on or off (modulation of the amount of luminance emitted, transmitted
`or reflected) - see for example EP-0462619 and EP-117335.
`[0005]
`Since matrix addresseddisplays are typically composed of many millions of pixels, very often pixels exist that
`are stuck in a certain state (on, off or anything in between). Where pixel elements comprise multiple sub pixels, indi-
`vidually controllable or not, then one or more of the sub-pixel elements may become stuck in a certain state. For
`example, a pixel structure may comprise three sub-pixel elements for red, green and blue colours respectively. If one
`of these sub-pixel elements becomes stuck in a certain state, then the pixel structure has a permanent colour shift.
`Mostly such problems are due to a malfunction in the driving electronics of the individual pixel (for instance a defect
`transistor). Other possible causes are problems with various production processes involved in the manufacturing of
`the displays, and/or by the physical construction of these displays, each of them being different depending on the type
`of technology of the electronic display under consideration. It is also possible that a pixel or sub-pixel element is not
`really stuck in a state, but shows a luminance or colour behaviour thatis significantly different from the pixels or sub-
`pixels in its neighbourhood. For instance, but not limited to: a defective pixel shows a luminance behaviour thatdiffers
`more than 20%(at one or morevideo levels) from the pixels in its neighbourhood, or a defective pixel shows a dynamic
`range (maximum luminance / minimum luminance) that differs more than 15% from the dynamic range of pixels in its
`neighbourhood, ora defective pixel shows a colour shift greater than a certain value comparing to an average or desired
`value for the display. Of course other rules are possible to determine whether a pixel or sub-pixel is defective or not
`(any condition that has a potential dangerfor image misinterpretation can be expressed in a rule to determine whether
`a pixel is a defective pixel). Bright or dark spots due to dust for example may also be considered as pixel defects. The
`exact reason for the defective pixel is not important for the present invention.
`[0006] Defective pixels or sub-pixels are typically very visible for the user of the display. They result in a significantly
`lower (subjective) image quality, can be very annoying or disturbing for the display-user and for demanding applications
`(such as medical imaging, in particular mammography) the defective pixels or sub-pixels can even makethe display
`unusable for the intended application, as it can also result in wrong interpretation of the image being displayed. For
`applications where image fidelity is required to be high, such as for example in medical applications, this situation is
`unacceptable.
`[0007] US-5,504,504 describes a method and display system for reducing the visual impact of defects present in an
`image display. The display includes an array of pixels, each non-defective pixel being selectively operable in response
`to input data by addressing facilities between an "on" state, whereat light is directed onto a viewing surface, and an
`“off" state, whereatlight is not directed onto the viewing surface. Each defective pixel is immediately surrounded by a
`first ring of compensation pixels adjacent to the central defective pixel. The compensation pixels are immediately sur-
`rounded by a second ring of reference pixels spaced from the central defective pixel. The addressing circuit-determined
`value of at least one compensation pixel in the first ring surrounding the defective pixel is changed from its desired or
`intended value to a corrective value, in order to reduce the visual impact of the defect. In one embodiment, the value
`of the compensation pixels is selected such that the average visually defected value for all of the compensation pixels
`and the defective pixel is equal to the intended value of the defective pixel. In another embodiment, the values of the
`compensation pixels are adjusted by adding an offset to the desired value of each compensation pixel. The offsetis
`chosen such that the sum of the offset values is equal to the intended value of the defective pixel.
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`It is a disadvantage of the solution proposed in the above documentthat a trial and error method is required
`[0008]
`for every other display in order to obtain a reasonable correction result.
`
`Summaryof the invention
`
`It is an object of the present invention to provide a methed and device for making pixel defects less visible
`[a009]
`and thus avoid wrong image interpretation, the method being usable for different types of matrix displays withouta trial
`and error method being required to obtain acceptable correction results.
`[0010] The above objective is accomplished by a method and device according to the presentinvention.
`[0011]
`Ina first aspect, the present invention provides a method for reducing the visual impact of defects presentin
`a matrix display comprising a plurality of display elements, the method comprising:
`
`providing a representation of a human vision system,
`characterising at least one defect presentin the display, the defect being surrounded by a plurality of non-defective
`display elements,
`deriving drive signals for at least some of the plurality of non-defective display elements in accordance with the
`representation of the human vision system and the characterising of the at least one defect, to thereby minimise
`an expected response of the human vision system to the defect, and
`driving at least some of the plurality of non-defective display elements with the derived drive signals.
`
`[0012] Minimising the response of the human vision system to the defect may comprise changing the light output
`value of at least one non-defective display element surrounding the defect in the display.
`[0013] Characterising at least one defect present in the display may comprise storing characterisation data charac-
`terising the location and non-linear light output response of individual display elements, the characterisation data rep-
`resenting light outputs of an individual display element as a function of its drive signals.
`[0014]
`A method according to the present invention may further comprise generating the characterisation data from
`images captured from individual display elements. Generating the characterisation data may comprise building a dis-
`play element profile map representing characterisation data for each display elementof the display.
`[0015]
`Providing a representation of the human vision system may comprise calculating an expected response of a
`human eye to a stimulus applied to a display element. For calculating the expected response of a human eye toa
`stimulus applied to a display element, use may be made of anyof a point spread function, a pupil function, a line spread
`function, an optical transfer function, a modulation transfer function or a phase transfer function of the eye. These
`functions may be described analytically, for example based on using anyofTailor, Seidel or Zernike polynomials, or
`numerically.
`[0016]
`In amethod according to the present invention, when minimising the response of the human vision system
`to the defect, boundary conditions may be taken into account.
`[0017] Minimising the response of the human vision system may be carried out in real-time or off-line.
`[0018]
`A defect may be causedby a defective display element or by an external cause, such as dust adhering on or
`between display elements for example.
`[0019]
`In asecond aspect, the present invention provides a system for reducing the visual impact of defects present
`in a matrix display comprising a plurality of display elements and intended to be looked at by a human vision system,
`first characterisation data for a human vision system being provided, the system comprising:
`
`a defect characterising device for generating second characterisation data for at least one defect present in the
`display, the defect being surrounded bya plurality of non-defective display elements,
`a correction device for deriving drive signals for at least some of the plurality of non-defective display elements in
`accordancewith the first characterisation data and the second characterising data, to thereby minimise an expected
`response of the human vision system to the defect, and
`means for driving at least some of the plurality of non-defective display elements with the derived drive signals.
`
`[0020] The correction device may comprise means to changethe light output value of at least one non-defective
`display element surrounding the defect in the display.
`[0021] The defect characterising device may comprise an image capturing device for generating an image of the
`display elements of the display. The defect characterising device may also comprise a display element location iden-
`tifying device for identifying the actual location of individual display elements of the display.
`[0022]
`Ina system according to the present invention, for providing the first characterisation data, a vision charac-
`terising device having calculating means for calculating the response of a human eyeto a stimulus applied to a display
`element may be provided.
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`Ina third aspect, the present invention provides a matrix display device for displaying an image intended to
`[0023]
`be looked at by a human vision system, the matrix display device comprising:
`
`a plurality of display elements,
`a first memory for storing first characterisation data for a human vision system,
`a second memory for storing second characterisation data for at least one defect presentin the display device,
`a modulation device for modulating, in accordance with the first characterisation data and the second characteri-
`sation data, drive signals for non-defective display elements surrounding the defect so as to reduce the visual
`impact of the defect presentin the matrix display device.
`
`The first and the second memory mayphysically be a same memory device.
`[0024]
`Ina fourth aspect, the present invention provides a control unit for use with a system for reducing the visual
`[0025]
`impact of defects present in a matrix display comprising a plurality of display elements and intended to be looked at
`by a human vision system, the control unit comprising:
`
`a first memory for storing first characterisation data for a human vision system,
`a second memory for storing second characterisation data for at least one defect presentin the display, and
`modulating means for modulating, in accordance with the first characterisation data and the second characterisa-
`tion data, drive signals for non-defective display elements surrounding the defect so as to reducethe visual impact
`of the defect.
`
`By visually masking is meant minimising the visibility and negative effects of the defect for the user of the
`
`[0026] The present invention thus solves the problem of defective pixels and/or sub-pixels in matrix displays by
`making them almost invisible for the human eye under normal usage circumstances. This is done by changing the
`drive signal of non-defective pixels and/or sub-pixels in the neighbourhood of the defective pixel or sub-pixel.
`[0027]
`Inthe following description the pixels or sub-pixels that are used to mask the defective pixel are called "mask-
`ing elements" and the defective pixel or sub-pixel itself is called "the defect".
`[0028]
`By a defective pixel or sub-pixel is meant a pixel that always shows the same luminance, i.e. a pixel or sub-
`pixel stuck in a specific state (for instance, but notlimited to, always black, or alwaysfull white) and/or colour behaviour
`independentof the drive stimulus applied to it, or a pixel or sub-pixel that shows a luminanceor colour behaviour that
`showsa severedistortion compared to non-defective pixels or sub-pixels of the display. For example a pixel that reacts
`to an applied drive signal, but that has a luminance behaviour that is very different from the luminance behaviour of
`neighbouring pixels, for instance significantly more darkor bright than surrounding pixels, can be considered a defective
`pixel.
`[0029]
`display.
`[0030] The present invention discloses a mathematical modelthat is able to calculate the optimal driving signal for
`the masking elements in order to minimise the visibility of the defect(s). The same algorithm can be used for every
`display configuration because it uses some parameters that describe the display characteristics. Amathematical model
`based on the characteristics of the human eye is used to calculate the optimal drive signals of the masking elements.
`The model describes algorithms to calculate the actual response of the human eye to the superposition of the stimulus
`applied (in casu to the defect and to the masking pixels). In this way the optimal drive signals of the masking elements
`can be described as a mathematical minimisation problem of a function with one or morevariables. It is possible to
`add one or more boundary conditions to this minimisation problem. Examples when extra boundary conditions are
`neededare in case of defects of one or more masking elements, limitations to the possible drive signal of the masking
`elements, dependenciesin the drive signals of masking elements...
`[0031] The present invention cannot repair the defective pixels but makes the defects (nearly) invisible and thus
`avoids wrong image interpretation.
`[0032] The above and other characteristics, features and advantages of the present invention will become apparent
`from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way
`of example, the principles of the invention. This description is given for the sake of exampleonly, without limiting the
`scope of the invention. The reference figures quoted below refer to the attached drawings.
`
`Brief description of the drawings
`
`[0033]
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`1 aillustrates a matrix display having greyscale pixels with equal luminance, and Fig.Fig. 1billustrates a matrix
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`display having greyscale pixels with unequal luminance.
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`EP 1 536 399 Al
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`Fig. 2a illustrates an LCD display having an RGB-stripe pixel arrangement: one pixel comprises three coloured
`sub-pixels in stripe ordering, and the display has a defective green sub-pixel thatis alwaysfully on, and a defective
`red sub-pixel that is alwaysoff. Fig. 2b illustrates a greyscale LCD based matrix display having unequal luminance
`in sub-pixels.
`Fig. 3a illustrates an analytical point spread function (PSF) in case the optics is considered to bediffraction-limited
`only; Fig. 3b and Fig. 3cillustrate numerical PSFs that are measured on test subjects.
`Fig. 4a showsthe eye response to a single pixel defect in the image plane if no masking is applied. Fig. 46 shows
`the eye response to the same pixel defect but after masking with 24 masking pixels has been applied. Fig. 4c
`showsthe centre locations of the PSFsin the image plane of the masking pixels and the pixel defect.
`Fig. 5a illustrates nine pixels each having three sub-pixels and two domains. Fig. 5b shows one of such pixels in
`detail.
`
`Fig. 6 illustrates the transformation from a driving level to a luminancelevel.
`Fig. 7a shows a real green sub-pixel defect present in a display, and Fig. 7b shows the same green sub-pixel
`defect and artificial red and blue sub-pixel defects introduced to retain a colour co-ordinate of the pixel which is
`as close to the correct colour co-ordinate as possible.
`Fig. 8 illustrates possible locations for a real-time correction system according to any embodimentof the present
`invention.
`
`[0034]
`
`In the different figures, the same reference signs refer to the same or analogous elements.
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`[0035] The present invention will be described with respect to particular embodiments and with referenceto certain
`drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic
`and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale
`for illustrative purposes. Where the term "comprising"is used in the present description and claims, it does not exclude
`other elements or steps.
`[0036]
`Inthe present description, the terms "horizontal" and "vertical" are used to provide a co-ordinate system and
`for ease of explanation only. They refer to a co-ordinate system with two orthogonal directions which are conveniently
`referred to as vertical and horizontal directions. They do not need to, but may, refer to an actual physical direction of
`the device.
`In particular, horizontal and vertical are equivalent and interchangeable by means of a simple rotation
`through and odd multiple of 90°.
`[0037]
`A matrix addressed display comprisesindividual display elements. The display elements, either themselves
`or in groupings, are individually addressable to thereby display or project an arbitrary image. In the present description,
`the term "display elements"is to be understood to comprise any form of element which modulatesa light output, e.g.
`elements which emit light or through which light is passed or from which light is reflected. The term "display" includes
`a projector. A display element may therefore be an individually addressable element of an emissive, transmissive,
`reflective or trans-reflective display, especially a fixed format display. The term "fixed format’ relates to the fact that an
`area of any image to be displayed or projected is associated with a certain portion of the display or projector, e.g. ina
`one-to-one relationship. Display elements maybe pixels, e.g. in a greyscale LCD, as well as sub-pixels, a plurality of
`sub-pixels forming one pixel. For example three sub-pixels with a different colour, such as a red sub-pixel, a green
`sub-pixel and a blue sub-pixel, may together from one pixel in a colour display such as an LCD. Wheneverthe word
`pixel is used, it is to be understood that the same mayhold for sub-pixels, unless the contrary is explicitly mentioned.
`[0038] The invention will be described with referenceto flat panel displays but is not limited thereto. It is understood
`that a flat panel display does not haveto be exactly flat but includes shaped or bent panels. A flat panel display differs
`from a display such as a cathode raytube in that it comprises a matrix or array of "cells" or "pixels" each producing or
`controlling light over a small area. Arrays of this kind are called fixed format arrays. There is a relationship between
`the pixel of an image to be displayed and a cell of the display. Usually this is a one-to-one relationship. Each cell may
`be addressed and driven separately. It is not consideredalimitation on the present invention whether the flat panel
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`displays are active or passive matrix devices. The array of cells is usually in rows and columns but the presentinvention
`is not limited thereto but may include any arrangement, e.g. polar or hexagonal. The invention will mainly be described
`with respectto liquid crystal displays but the present invention is more widely applicable to flat panel displaysof different
`types, such as plasma displays, field emission displays, EL-displays, OLED displays etc.
`In particular the present
`invention relates not only to displays having an array of light emitting elements but also displays having arrays of light
`emitting devices, whereby each device is made up of a number of individual elements. The displays may be emissive,
`transmissive, reflective, or trans-reflective displays.
`[0039]
`Further the method of addressing and driving the pixel elements of an array is not considereda limitation on
`the invention. Typically, each pixel element is addressed by meansof wiring but other methods are known and are
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`useful with the invention, e.g. plasma discharge addressing (as disclosed in US-6,089,739) or CRT addressing.
`[0040]
`A matrix addressed display 12 comprises individual pixels 14. These pixels 14 can takeall kinds of shapes,
`e.g. they can take the forms of characters. The examples of matrix displays 12 given in Fig.
`1 a to Fig. 2b have rec-
`tangular or square pixels 14 arrangedin horizontal rows and vertical columns. Fig. 1 a illustrates an image of a perfect
`display 12 having equal luminance response in all pixels 14 when equally driven. Every pixel 14 driven with the same
`signal renders the same luminance. In contrast, Fig. 1 6 illustrates an image of a display 12 where the pixels 14 of the
`display 12 are also driven by equal signals, but wherethe pixels 14 render a different luminance, as can be seen by
`the different grey values. Pixel 16 in the display 12 of Fig. 1 b is a defective pixel. Fig. 1 b shows a monochromepixel
`structure with one defective pixel 16 that is always in an intermediatepixel state.
`[0041]
`Fig. 2a showsa typical RGB-stripe pixel arrangement of a colour LCD display 12: one pixel 14 consists of
`three coloured sub-pixels 20, 21, 22 in stripe ordering. These three sub-pixels 20, 21, 22 are driven individually to
`generate colour images. In Fig. 2a there are two defective sub-pixels present: a defective red sub-pixel 24 that is always
`off and a defective green sub-pixel 25 that is always fully on.
`[0042]
`Fig. 2b shows an asymmetric pixel structure that is often used for high-resolution monochrome displays. In
`Fig. 2b, one monochrome pixel 14 consists of three monochrome sub-pixels. Depending on the panel type and driving
`electronics the three sub-pixels of one pixel are driven as a unit or individually. Fig. 2b shows 3 pixel defects: a complete
`defective pixel 16 in "always on" state and two defective sub-pixels 27, 28 in "always off" state that happen to be located
`in asame pixel 14.
`[0043] The spatial distribution of the luminance differences of the pixels 14 can be arbitrary.It is also found that with
`many technologies, this distribution changes as function of the applied drive to the pixels indicating different response
`relationships forthe pixels 14. For a low drive signal leading to low luminance, the spatial distrioution pattern can differ
`from the pattern at higher driving signal.
`[0044]
`The optical system of the eye, in particular of the human eye, comprises three main components: the cornea,
`the iris and the lens. The cornea is the transparent outer surface of the eye. The pupil limits the amountof light that
`reachesthe retina and it changes the numerical aperture of the optical system of the eye. By applying tension to the
`lens, the eye is able to focus on both nearby and far away objects. The optical system of the eye is very complex but
`the process of image formation can be simplified by using a "black-box" approach. The behaviour of the black box can
`be described by the complex pupil function:
`
`P(x.y) - exp[-i(27/2) - WOx,y)].
`
`In this formula i stands for V-1 and 1 is the wavelengthofthe light. The pupil function consists of two parts: the amplitude
`component P{x,y) which defines the shape, size and transmission of the black box; and the wave aberration W(x,y)
`which defines how the phaseof the light has changed after passing through the black box.
`[0045]
`Oncethe nature of the light (that passed through the black box, in this case the eye) is known, the image
`formation process can be described by the point spread function (PSF). The PSF describes the image of a point source
`formed by the black box. Most lenses, including the human lens, are not perfect optical systems. As a result when
`visual stimuli are passed through the cornea and lens the stimuli undergo a certain degree of degradation or distortion.
`This degradation or distortion can be represented by projecting an exceedingly small dot of light, a point, through a
`lens. The imageofthis point will not be the same asthe original becausethe lens will introduce a small amountof blur.
`[0046] The PSF of the eye can be calculated using the Fraunhofer approximation:
`
`PSF(x' y')=K-|FT{ P(x.y)-expl-i(2n/a)WO%y)IH°
`
`where FT standsfor the two-dimensional Fourier transform, usually denoted as F(x',y')= FT{f(x,y)}, and K is a constant.
`The || represents the modulus-operator. In case of the human eye, the PSF describes the image of a point source on
`the retina. To describe a complete object one can think of an object as a combination or a matrix of (a potentially
`exceedingly large number or infinite number of) point sources. Each of these point sources is then projected on the
`retina as described by the same PSF (this approximation is strictly only valid if the object is small and composed of a
`single wavelength). Mathematically this can be described by means of a convolution:
`
`I(x.y'}= PSF © O(x',y')
`
`where |(x',y’) is the resulting image on the retina, PSF the point spread function and O(x',y') the object representation
`at the image-plane. Typically this convolution will be computed in the Fourier domain by multiplying the Fourier trans-
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`forms of both the PSF and the object and then applying the inverse Fourier transform to the result.
`[0047]
`‘It is common practice in vision applications to describe the wave aberration W(x,y) mathematically by means
`of a set of polynomials. Often Seidel polynomials are used, but also Taylor polynomials and Zernike polynomials are
`common choices. Especially Zernike polynomials haveinteresting properties that make wave aberration analysis much
`easier. Often unknown waveaberrations are approximated by Zernike polynomials; the coefficients of the polynomials
`are typically determined by performing a least-squarefit.
`[0048]
`For the presentinvention, it is not considered a limitation on the invention how the complex pupil function or
`the PSF is described. This can be done analytically (for instance but not limited to a mathematical function in Cartesian
`or polar co-ordinates, by means of standard polynomials, or by means of any other suitable analytical method) or
`numerically by describing the function value at certain points.
`It is also possible to use (instead of the PSF) other
`(equivalent) representations of the optical system such as but notlimited to the ‘Pupil Function (or aberration)’, the
`‘Line Spread Function (LSF)’, the ‘Optical Transfer Function (OTF})', the 'Modulation Transfer function (MT F)' and'Phase
`Transfer Function (PTF)'. Clear mathematical relations exist between all these representation-methods so that it is
`possible to transform one form into another form. Fig. 3a shows an analytical PSF in case the optics is considered to
`be diffraction-limited only.
`It is to be noted that the PSF is clearly not a single point, i.e. the image of a point source is
`not a point, the central zone of the diffraction-limited PSF is called an airy disc. Fig. 3b and Fig. 3c show (numerical)
`PSFs that were measured on test subjects. Here again it can be seen that the PSF is not a point.
`[0049] As the PSF of each optical system may be different, correction according to the present invention can be
`made user specific by using eye characteristics, and thus PSFs, which are specific for that user.
`[0050] Based on the PSF ofthe optical system, according to an aspect of the present invention, the response or
`expected response of the eye to a defective pixel can be mathematically described. Therefore the defective pixel is
`treated as a point source with an "error luminance" value dependent on the defectitself and the image data that should
`be displayedat the defect location at that time. For instanceif the defective pixel is driven to have luminance value 23
`but due to the defect it outputs luminance value 3, then this defect is treated as a point source with error luminance
`value -20.
`It is to be noted that this error luminance value can have both a positive and a negative value. Supposing
`that some time later this same defective pixel is driven to show luminance value 1 but due to the defectit still shows
`luminance value 3, then this same defective pixel will be treated as a point source with error luminance value +2.
`[0051] As described above, this point source with a specific error luminance value will result in a response of the
`eye as described by the PSF. Because this response is typically not a single point, it is possible to use pixels and/or
`sub pixels in the neighbourhood of the defective pixel to provide some image improvement. These neighbouring pixels
`are called masking pixels and can bedriven in such a wayas to minimise the responseof the eye to the defective
`pixel. According to the presentinvention, this is achieved by changing the drive signal of the masking pixels such that
`the superposition of the image of the masking pixels and the image of the defective pixel results in a lower or minimal
`response of the human eye. Mathematically this can be expressed asfollows:
`
`C1, C),...5C,
`[
`1>
`“2ortto |
`
`|= min
`
`ee
`J |cosfunction
`ele2,...en ay
`
`C, PSF(x'—x1', y'-yl) +
`C,.PSF(x'—x2', y'—y2')4+..+
`C,,PSF