in the exemplary embodiments described below , components that are alike in function and structure are designated as bar as possible by alike reference numerals . therefore , to understand the features of the individual components of a specific embodiment , the descriptions of other embodiments and of the summary of the invention should be referred to . fig1 schematically illustrates , in a simplified representation , an oct system 1 , which is used for examination of an human eye 2 . in particular , the oct system 1 may be used during an eye surgery , in particular during a cataract surgery . in the embodiment , which is schematically illustrated in fig1 , the oct system is a fourier domain oct system ( fd - oct system ), which is also referred to as spectral domain oct system . further embodiments comprise a swept source oct system ( ss - oct system ) or a time domain oct system ( td - oct system ). the fd - oct system 1 comprises a light source 3 , which generates measuring light 5 of a certain spectrum . the light source 3 comprises a super luminescent diode , which is designed such that oct measuring light 5 is generated having a spectrum with a mean wavelength and a spectral width . the mean wavelength is about 1000 nm and has a spectral width of 20 to 30 nm . alternatively , instead of using a super luminescent diode , the light source 3 may comprise a white light source and spectral filters , which are arranged in the path of oct measuring light for approximately providing the above described spectrum . through the optical fiber 4 , the oct measuring light 5 which is generated by the light source 3 is guided to the divider / coupler 7 . the fiber optic divider / coupler 7 is configured such that oct measuring light 5 is divided into two light portions 9 and 15 . light portion 9 is guided to a reflecting reference surface 11 by an optical fiber 4 , at which the light portion 9 is reflected to constitute the light portion 9 ′. the reference surface 11 is disposable in directions which are indicated by double arrow 12 . thereby , a path length , which is traveled by light portions 9 and 9 ′ is variable . the other light portion 15 of the oct measuring light 5 is guided by an optical fiber 4 to a scanner 8 having an illumination optical system . the scanner 8 including the illumination optical system is designed such that a focused beam of oct measuring light 16 is formed having a defined cross - sectional extent in the examination area ( i . e . an object region ) of about 10 to 50 μm . the scanner 8 having the illumination optical system may also be designed such that a beam of measuring light 16 is formed , which consists of parallel light beams , in particular , for investigating a posterior portion of the eye , such as the retina . the scanner 8 including the illumination optical system is further designed such that the bundle of oct measuring light 16 is laterally guided over the examination area of the human eye 2 . for this purpose , the scanner 8 may comprise one or more mirrors , which are pivotable about different axes . the bundle of oct measuring light 16 interacts with structures of the human eye 2 , such as the cornea 13 , the iris 14 , the capsular bag 17 , the intraocular lens 19 , in particular the mark 20 of the intraocular lens 19 , and with the suction tube 21 . the suction inlet 21 ′ of the suction tube 21 is arranged close to the capsular bag 17 . the interaction of the bundle of oct measuring light 16 comprises different physical processes such as scattering , reflection and absorption . a portion of the incident bundle of oct measuring light 16 is reflected into a substantially reverse direction ( i . e . reverse to the direction of the incident light ), captured by the scanner 8 and directed again into the optical fiber 4 as light 16 ′. the light 16 ′ carries structure information of the examination area of the eye into which the bundle of oct measuring light 16 has been penetrated . light 16 ′ is guided to the fiber optic divider / coupler 7 , where it is superposed on the light portion 9 ′, which has been reflected at the reference surface 11 . thereby , superposed light 25 is formed . the superposed light 25 is guided via the optical fiber 4 to the spectrometer 27 . the spectrometer 27 comprises a dispersion device 29 for spectrally dispersing the superposed light into spatially separated light portions 30 . each of the light portions 30 comprises light waves having wavelengths of a certain wavelength range . the wavelength ranges of different light portions may be different . the spatially separated light portions 30 are detected by a position sensitive detector 31 , which comprises a plurality of pixels for separately detecting intensities of different spatially separated light portions 30 and for generating electrical signals . the electrical signals are led via signal line 39 to a control and processing system 33 , which is designed such that the electrical signals are processed and date representing a structure of the examination area of the eye are determined . namely , the intensities of the detected spectral light portions 30 represent a spectrum of the superposed light 25 . from the spectrum of superposed light 25 , structure information along a depth direction 23 is determinable after having applied background subtraction , spectral resampling and determining of a fourier transform . the control and processing system 33 may be designed such that via a signal line 35 , a change in the characteristics of the light source 3 in view of its spectrum is controlled . the control and processing unit may further be designed such that via a signal line 37 , a dispersion strength of a dispersion device 29 is varied . from data which represents the structure of the examination area of the eye 2 , an image of the examination area of the eye 2 may be obtained , which may be displayed on a monitor ( not illustrated ). this representation may comprise for example a volumetric view or a cross - sectional view of the object . embodiments provide articles , which may be applied during an examination or a surgery in which an oct system 1 is used . on the one hand , the articles have a suitable reflectivity for being imaged by the oct system 1 , on the other hand , the bundle of oct measuring light 16 is attenuated by the extinction of the article only such that anatomical structures of the eye 2 which are located downstream of the article are detectable and hence imagable by the oct system 1 . the suction tube 21 , which is illustrated in fig1 , comprises nanoparticles which are disposed on it surface . thereby , the signal of the suction tube 21 is increased which leads to an improved detection by the oct system 1 compared to a suction tube 21 without any disposed nanoparticles . the suction tube 21 is comprised by a suction device 22 , which is provided for suctioning off an emulsified natural lens from the capsular bag 17 during a cataract surgery . the suction device 22 may be designed in the form of a phaco handpiece for phacoemulsification . thereby , a suctioning off of lens fragments may be performed in an integrated way with the phaco handpiece . the surgeon approaches the suction device 22 and in particular the suction inlet 21 ′ of the suction tube 21 to the location where the surgery is performed . thereby , the suction inlet 21 ′ is approached as close as possible to an incision , which is made at the capsular bag 17 for suctioning off an emulsified natural lens . the increase in contrast of the imaged suction tube 21 which is caused by the dispersed nanoparticles provides a facilitated positioning of the suction inlet 21 ′ of the suction tube 21 for the surgeon . fig2 schematically illustrates in more detail a portion of the suction tube 21 during an examination of the eye 2 by using the oct system 1 . the suction tube comprises a cylindrical substrate body having a diameter a of 2 mm and a wall thickness of about ⅕ mm . the diameter a may also have values of between 1 mm and 3 mm , and the wall thickness may also have values of between 1 / 20 mm to ½ mm . the cylindrical substrate body is made of glass or plastics . on the outer surface of the substrate body , there are located nanoparticles 26 which have a substantially constant surface concentration , i . e . the number of nanoparticles per unit area is substantially constant on the outer surface of the cylindrical substrate body . the nanoparticles have been deposited by dipping the substrate body into a solution of nanoparticles having a concentration of about 7 × 10 8 nanoparticles per ml . in other embodiments of the suction tube , the nanoparticles 26 are not only deposited on the surface of the substrate body , but rather are also located in the bulk of the cylindrical substrate body . alternatively or additionally , the nanoparticles 26 may be deposited on the cylindrical inner surface of the suction tube 21 . during examination of the eye 2 , the suction tube 21 is arranged in a beam path of the bundle of measuring light 16 of the oct system 1 , which is illustrated in fig1 . an intensity of the bundle of oct measuring light 16 , which is incident on the suction tube 21 , is denoted as i e . an intensity of the bundle of oct measuring light , which transmits the suction tube 21 is denoted as i t . because of the presence of the nanoparticles on the surface of the cylindrical substrate body of the suction tube 21 , the intensity i t of the transmitted oct measuring light is smaller than the intensity i e of the incident oct measuring light . the extinction , which is defined in equation ( 2 ) above may assume values of up to 10 . notwithstanding this significant attentuation of the intensity of the bundle of oct measuring light 16 after having been transmitted through the suction tube 21 , an anatomical structure of the eye 2 , which is located downstream of the suction tube 21 , ( in this case the capsular bag 17 ) is imagable by the oct system 1 illustrated in fig1 because of the high sensitivity of the oct system 1 . generally , the sensitivity of an oct system is defined as the minimal reflectivity of the probe arm of the interferometer at which the signal to noise ratio is 1 . instead of determining the minimal reflectivity of the probe arm , the maximum attenuation or extinction of the bundle of oct measuring light at which an ideal mirror is detectable by an intensity i t of oct measuring light . this results in the sensitivity which is given in decibel sensitivities of modern oct systems have been investigated in the publication “ performance of fourier domain vs . time domain optical coherence tomography ”, r . leitgeb et al , optics express . vol . 11 , no . 8 , pages 889 to 894 . it is possible to obtain a sensitivity of up to 108 db . using equations ( 2 ) and ( 5 ) and taking into account that the path goes through the article two times , and structures which are located downstream of the article have to be detectable , the maximum extinction is given by : thereby , in case the extinction of the suction tube 21 along a transmission direction , which is given by the direction of the incident handle of oct measuring light 16 , is smaller than the maximum extinction , which is defined in equation ( 6 ), in particular smaller than 5 , a boundary layer or interface of the capsular bag 17 is detectable by the oct system 1 . oct measures light 16 ′, which emanates from the examination area of the eye 2 , which is located downstream of the suction tube 21 , has covered an additional optical path d ·( n − n m ). d denotes a thickness of the transmitted substrate material of the suction tube 21 having an index of refraction n on the way to and fro , and n m represents an index of refraction of the medium , which in this case the aqueous humor within the eye . without a correction of structural data taking into consideration this additional optical path , the capsular bag 17 would be illustrated according to the dashed line 17 ′. the control and processing system 33 , however , is designed such that based on knowledge of the geometry of the suction tube 21 , the refractive index of the suction tube 21 and the orientation and position of the suction tube 21 , a correction is performed . thereby , the capsular bag 17 which is erroneously depicted downstream of the suction tube 21 as contour 17 ′, is displayed as corrected contour 17 . this correction may be performed in a real space of the volume data set , for example by displacing pixel values according to the additional optical path of the oct measuring light 16 , 16 ′ for portions of the object , which are located downstream of the suction tube 21 . embodiments of an article , may have an index of refraction , which is adapted to a medium of the area of examination . in this case a correction as described above is not required . for example , the article may be made of plastics , having an index of refraction of about 1 . 3 to 1 . 4 in the wavelength range of the used oct measuring light . a deviation of the index of refraction of the article from a mean index of retraction of the medium of the area of examination may be chosen to be smaller than the ratio of the resolution of the oct system to the extent of the article of the portion through which the oct light is transmitted . fig3 shows a further article 41 , which may be used in a method by using the oct system 1 . the article is configured as tweezers made of glass or plastics . nanoparticles 26 are dispersed in the volume . different embodiments of the article which are described in this application may comprise different nanoparticles or a mixture of different nanoparticles . the nanoparticles may comprise for example alo 3 , wherein the nanoparticles have an extent of between 100 nm and 100 μm . the nanoparticles may also comprise au , wherein the nanoparticles have an extent of between 2 nm to 250 nm . the nanoparticles may also comprise ag , wherein the nanoparticles have an extent of between 20 nm to 80 nm or of between 20 nm and 300 nm . additionally or alternatively , the nanoparticles may comprise other metals and / or silicon . the nanoparticles may consist completely of metal or may comprise a core made of silicon or silicon oxide around which a shell of metal is formed . for given articles having a given geometry and a given substrate material , the extinction along any transmission direction of the articles may be calculated depending on a material , a shape , a size and a concentration of nanoparticles . in the calculation , the nanoparticles may be approximated by a sphere . since the scattering process of light at a homogenous sphere cannot be expressed analytically , numerical procedures such as mie theory have to be applied for a given configuration of the article . the so - called mie coefficients a n and b n are calculated for a given configuration of the article and given boundary conditions , for example by applying the program “ miecale ” ( bernhard michel ) and / or “ mie scattering calculator ” ( scott prahl ). based on these mie coefficients and based on the radius of the sphere of the nanoparticles , the efficiency of the extinction q ext may be calculated for a given article . the calculation of the mie coefficients is also based on the complex index of refraction n = n r − i · n i of the nanoparticles . by way of example , at a wavelength of 840 nm of the oct measuring light , gold nanoparticles have a real index of refraction n r = 0 . 18 and an imaginary index of refraction n e = 5 . 36 . the linear coefficient of extinction τ is related to the efficiency of extinction q ext by the following equation : herein , r denotes the radius of the nanoparticle , q ext denotes the efficiency of extinction and n d denotes the density of the nanoparticles in the article ; in particular in the portion of the article , which is transmitted by light . after the efficiency of extinction q ext has been calculated based on the infinite series according to the mie theory , the efficiency of extinction τ may be obtained ; and by using equation ( 3 ) the extinction of the article may be obtained in the at least one light transmissive portion , in which nanoparticles are dispersed . the extinction therefore linearly depends on the path d , through which light has been transmitted through the articles . for nanoparticles , which are made of gold , which have a concentration of n d = 7 . 0 · 10 8 / ml and a radius of r = 100 nm and further in case of a given sensitivity of 108 decibel ( db ) of the oct system 1 , and a maximum extent d of the articles in the light transmissive portion in which the nanoparticles are dispersed of d max = 32 cm results . according to equation ( 7 ) above in combination with equation ( 3 ) above , at a density , which is 10 times higher , i . e . n d = 7 . 0 · 10 9 / ml , a value of d max = 3 . 2 cm results . based on this description , the person skilled in the art is able to determine from a given sensitivity of the oct system , a given geometry of the article , some or all of the relevant properties of the nanoparticles , such as material , extent , concentration , etc . such that equation ( 6 ) is fulfilled . thereby , structures of the area of examination , which are located downstream of the article and in the beam path of the bundle of oct measuring light 16 , are detectable by the oct system 1 . furthermore , based on the size , structure and / or concentration of the particles , characteristics of backscattering of the nanoparticles may be determined by using suitable calculations . thereby , by adapting the parameters of the nanoparticles , a higher reflectivity of the nanoparticles may be achieved for an improved detection by the oct system . the tweezers 41 comprise two legs 41 a and 41 b which can be moved towards and away from each other , such that teeth which are located in the jaws 42 a and 42 b may grab and hold tissue by applying a pressing force . the legs 41 a and 41 b comprise glass or plastics as a substrate and nanoparticles 26 which are dispersed in the volume having a concentration of about n d = 7 . 0 · 19 8 / ml . in a method for using the tweezers 41 and by applying the oct system 1 , which is illustrated in fig1 , the tweezers 41 are imaged and the spatial orientation and position relative to the surrounding tissue of the eye 2 is determined . thereafter , a correction of structure information which represent anatomical structures which are located downstream of the tweezers 41 is applied , in analogy to the correction method which has been described referring to fig2 . the tweezers , which are schematically illustrated in fig3 may be used for epiretinal membrane peeling . tissue of the retina , is peeled using the tweezers , wherein an accurate positioning of the tweezers in relation to the epiretinal membrane is possible by imaging with the oct system . in particular , a distance between the tweezers and the epiretinal membrane may be determined by using the oct system . fig4 illustrates a further embodiment of an article 43 , which is used in a method which makes use of the oct system 1 . the article 43 is configured as a surgical scalpel , which comprises a metal plate 44 in a portion 43 a . the substrate body of the scalpel 43 is made of a transparent material like plastics or glass and comprises a section 43 a having a smaller cross - sectional extend d 1 and a section 43 b having a greater cross - sectional extent d 2 . in the substrate body of the scalpel 43 , nanoparticles are distributed , having a higher concentration in the section 43 a than in the section 43 b . the inhomogenous concentration of the nanoparticles 26 in the sections 43 a and 43 b is chosen such that an extinction along a transmission direction defined by the direction of the bundle of oct measuring light 16 in the section 43 a is substantially equal to an extinction in the portion 43 b . thereby , it is ensured , that an intensity i t , a of oct measuring light 16 , which is transmitted through the section 43 a is substantially equal to an intensity i t , b of oct measuring light 16 , which is transmitted through the section 43 b , i . e . i t , b = i t , a . thereby , structures of the eye 2 , which are located downstream of the scalpel 43 may be imaged substantially with the same signal to noise ration , independent of a position along a direction of longitudinal extent , which is in this case approximately perpendicular to the transmission direction of the scalpel 43 . fig5 a schematically shows a further embodiment 45 of the article , which may be used in combination with the oct system 1 , which is illustrated in fig1 . the article 45 is a surgical thread , which may be inserted into the schlemm &# 39 ; s canal 47 of a human eye during an eye surgery . the surgical thread may in particular be used in combination with the method of viscocanaloplasty , wherein a liquid is injected for extending the schlemm &# 39 ; s canal . the schlemm &# 39 ; s canal , which is limited in the cross - sectional view of fig1 by lines 47 a and 47 b , and which is denoted in fig1 by reference sign 47 , contributes to the fluid regulation of the anterior chamber of the eye between the iris and the cornea . during an eye surgery , the tread 45 may be inserted into the schlemm &# 39 ; s canal 47 , 47 a , 47 b . thereby , either the geometry of the schlemm &# 39 ; s canal , which is otherwise only difficult to be imaged , may be determined or the schlemm &# 39 ; s canal may be deformed in a suitable way , for example by stretching . for increasing the contrast of the image generated by the oct system 1 , the surgical thread 45 , which is made of a material such as prolene comprises nanoparticles 26 which are deposited on the surface of the surgical thread by dipping the thread of prolene into a solution of nanoparticles having a concentration of 7 · 10 8 nanoparticles per ml . the thread of prolene 45 has a cross - sectional diameter of about 30 μm . fig5 b illustrates results of two oct measurements , wherein in the upper portion , there is shown a thread of prolene without nanoparticles and in the lower portion , there is shown thread of prolene with nanoparticles as described above , wherein the thread of prolene is imaged by an oct system 1 as shown in fig1 . the upper image 48 of the thread of prolene without nanoparticles shows a lower signal which makes it difficult to obtain an accurate image of the thread for determining its position . in contrast thereto , the lower image of the thread of prolene , in which nanoparticles are dispersed , has a signal which is higher by a factor of four than in the upper image 48 . thereby a more accurate measurement or a more definite manipulation of the schlemm &# 39 ; s canal during an eye surgery is possible by using an oct system . other articles according to embodiments comprise portions of surfaces , which are provided with one or more layers which increase a reflectivity or which are roughened through a process , for increasing the imageability of the surfaces by an oct system . fig6 a and 6 b schematically illustrate in a top view embodiments of an intraocular lens . the intraocular lens 19 a , which is shown in fig6 a comprises an optical element 50 a , which is substantially transparent in the visible wavelength range . the optical element 50 a has a positive refractive power and an optical axis 50 a , which is perpendicular to the drawing layer . the optical element 50 a may be a spherical and / or toric lens . furthermore , the intraocular lens 19 a comprises two holding elements 52 a and 53 a , which extend from inside to outside in a radial direction and which serve to hold the intraocular lens 19 a in the capsular bag 17 of the human eye 2 . at the holding element 52 a , there is provided a mark 54 a and at the holding element 53 a , there is provided a mark 55 a . the mark 54 a or 55 a may for example comprise a reflecting foil and / or information on optical properties of the intraocular lens 19 a , treatment data , patient data and the like . the mark 54 a or 55 a may for example comprise characters or a barcode and information about an alignment of the main axis of the optical elements 50 a in case it is a toric optical element . the optical element 50 a has a radius r 0 which defines a circular area , in which the optical power of the optical element 50 a has a positive refractive power . the marks 54 a and 55 a are located outside of the circular area defined by the radius r 0 , in a portion between the radii r 1 and r 2 . the radii r 1 and r 2 are chosen under consideration of the anatomy of the eye 2 which is to be examined . in particular , it has to be ensured , that the intraocular lens 19 , which is inserted into the capsular bag 17 is held within the capsular bag 17 by the holding elements 52 a and 53 a and bather that the marks 54 a and 55 a are located along an optical axis of the eye behind ( i . e . downstream ) of the iris , without limiting the field of view of the eye . for example , r 1 may be chosen to be greater or equal to 3 mm and r 2 may be chosen to be smaller or equal to 6 mm . the intraocular lens 19 a may be used for an improved cataract surgery by using the oct system 1 , which is illustrated in fig1 . thereby , the high sensitivity of the oct system 1 may be used for imaging marks 54 a , 55 a of the implanted intraocular lens 19 a , which is located in the capsular bag 17 behind the iris 14 , whereby it is possible to determine the exact position of the intraocular lens 19 a within the capsular bag 17 . an inaccurate position or inaccurate orientation of the implanted intraocular lens 19 a may be determined , for example a tilt of the optical axis 51 a of the intraocular lens 19 a in relation to an optical axis of the eye or an inaccurate azimuthal orientation of the intraocular lens 19 a relative to the optical axis 51 a in case the optical element 50 a is a toric optical element , the orientation of which is given by a main axis . an accurate positioning of an intraocular lens has been difficult by using common imaging systems and common intraocular lenses . fig6 b illustrates a further embodiment 19 b of an intraocular lens . such as the intraocular lens 19 a , which is illustrated in fig6 a , the intraocular lens 19 b , which is illustrated in fig6 b comprises an optical element 50 b having an optical axis 51 b and two holding elements , which are located outside of the optical element 50 b in a radial direction for holding the intraocular lens 19 b in the capsular bag of a human eye 2 . contrary to the intraocular lens 19 a , which is illustrated in fig6 a , there are provided no marks on the holding elements 52 b and 53 b . instead , a mark 54 b is provided on an outwardly directed portion 56 , wherein the outwardly directed portion 56 extends in a direction from inside to outside of the optical element 50 b in a radial direction . other embodiments of intraocular lenses comprise further marks on the holding elements or further supporting elements , such as outwardly directed portions . the marks max comprise nanoparticles , which are arranged within a volume or a surface of a corresponding supporting element . in addition to the articles as described above , other commonly used items or items which , are necessary in a surgery may be provided with nanoparticles , such as visco elastic materials , rinsing liquids , catheters or the like . liquids , which contain nanoparticles , may be used to increase the contrast after having been injected into a tissue which is examined during being imaged by an oct system . liquids containing nanoparticles , such as visco - elastic materials may for example be injected into the anterior chamber of the eye or posterior chamber of the eye or into the schlemm &# 39 ; s canal for increasing contrast . fig7 a , 7 b and 7 c illustrate the use of a liquid , which contains nanoparticles for increasing the contrast of the image of the anterior chamber of a trout &# 39 ; s eye by using an oct system . fig7 a shows an image obtained by an oct system , which is illustrated in fig1 , of a trout &# 39 ; s eye , wherein in the image which is inserted in the lower left corner , the eye of the trout is shown in top view . the image in fig7 a of the natural eye of a trout shows the outer limit of the cornea as a line 13 , whereas the anterior chamber of the eye which is located below seems to have no limiting structures below the cornea . fig7 b shows an image of the same eye of the front shortly after an injection of a liquid , which contains nanoparticles below the cornea . portions within the anterior chamber of the eye , which are enriched with the liquid which contains nanoparticles , show an increased oct signal . fig7 c shows an increased oct signal after a short time of waiting in a band 58 below the cornea 13 , which is generated by reflection of the oct measuring light at the nanoparticles . the fact , that the liquid , which contains nanoparticles has only spread out within a limited band , indicates that the anterior chamber of the eye is limited by structures below the cornea 13 , which has not been recognizable in images of the natural eye of the trout . thereby , using liquids containing nanoparticles which are injected into a tissue to be examined , an improved examination of this tissue is made possible . an injected liquid which contains nanoparticles may be used together with articles according to fig2 to 5 a and fig6 a and 6 b during an examination by an oct system . thereby , examinations and surgeries may further be improved . while the invention has been described with respect to certain exemplary embodiments thereof , it is evident that many alternatives , modifications and variations will be apparent to those skilled in the art . accordingly , the exemplary embodiments of the invention set forth herein are intended to be illustrative and not limiting in any way . various changes may be made without departing from the spirit and scope of the present invention , as defined in the following claims .
an article for use in an oct method , the article comprising a solid substrate and nanoparticles dispersed in or on the substrate in at least one light transmissive portion of the article such that the nanoparticles result in an increased extinction of the light transmissive portion along a transmission direction of the light transmissive portion compared to the substrate being free of nanoparticles . the extinction of the light transmissive portion along the transmission direction is less than 6 , wherein the extinction is defined as a negative decadic logarithm of a ratio of an intensity of light which is transmitted through the light transmissive portion to an intensity of light which is incident on the light transmissive portion , wherein the light is in at least one of a visible and a near infrared wavelength range .