the following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention or its application or uses . fig1 shows in a highly schematized diagram the essential components of an inventive analysis system designated on the whole by 1 . the light source is a laser 2 ( preferably a semiconductor laser ) which emits visible light or light from the near infrared . the primary light generated by the light source 2 is coupled into an inbound light guide 4 through a coupling device 3 . the inbound light guide 4 is formed for a part of its length by the central fiber of an optical fiber bundle 5 whose distal section is designed as a scattered - light percutaneous sensor 7 piercing through the skin surface and into the skin . the distal end of the percutaneous sensor which is inserted into the skin 8 is designated sensor head 9 . details of the generation and detection of the raman - scattered light are shown in fig2 in an enlarged detail diagram which is also highly schematized . further details may be taken from fig7 and 8 . the sensor head 9 penetrates through the skin surface and the underlying layers of skin , namely the epidermis 11 and the dermis 12 , into the subcutis 13 so that the test volume 15 contains interstitial fluid at the distal end of the sensor head in the direction of insertion . the primary light passes through a light irradiation surface 18 , which is formed by the distal end face of the inbound light guide 4 , into the test volume 15 . to eliminate raman scattering and fluorescence occurring in the inbound light guide 4 , it is provided with a bandpass filter , preferably on its most distal end , to ensure that the primary light penetrating into the test volume 15 has an extremely narrow bandwidth . due to scattering , the secondary light occurring in the test volume 15 contains raman components as well as rayleigh components and fluorescence components . a large portion of the scattered secondary light 19 penetrates through a light receiving surface 20 into a detection light guide 21 ( see fig7 ). in the preferred embodiment depicted here , the detection light guide 21 consists of a plurality of optical fibers 22 which are arranged at least on both sides of the inbound light guide 4 . this embodiment shows that the term “ detection light guide ” must not be interpreted in a restrictive sense as pertaining to a single fiber - optic element , in particular an optical fiber . instead the term “ detection light guide ” refers to the totality of fiber - optic elements that serve in an inventive system to transport the scattered light from a light receiving surface 20 ( which may consist of a plurality of partial surfaces ) to the spectrometer . consequently , the detection light guide may consist of a plurality of fiber - optic elements running in parallel as well as a plurality of fiber - optic elements arranged in succession , made of a suitable transparent material in which the detection light is transported . the same is also basically true for the inbound light guide , although this is , in the area of the sensor head , preferably formed by a single central optical fiber . in the optical fibers 22 of the detection light guide 21 reflection takes place at a reflective surface 23 ( see fig7 ) which is inclined obliquely to the axis of the primary light 16 . it runs on the side of the detection light guide 21 which faces away from the primary light beam 16 a . the detected secondary light is conducted in the detection light guide 21 to a wavelength - selective detection device 24 having at its input a notch filter ( negated bandpass filter ) which eliminates as much as possible of the wavelength of the primary light and thus the elastically scattered rayleigh components of the secondary light . in the detection device 24 the detected light is divided spectrally and the resulting spectrum , which contains the information for determining the glucose concentration , is recorded digitally . the digitized spectrum is transmitted to an electronic analyzer device 26 , e . g ., a computer and is analyzed there . this is preferably done by means of a multivariate analysis method such as used in spectroscopy ( e . g ., principle component regression , partial least squares ). such methods are described , for example , in h . martens et al ., “ multivariate calibration ,” john wiley & amp ; sons , new york , n . y . 1989 a . höskuldsson , “ prediction methods in science and technology ,” thor publishing , denmark 1996 . generation of primary light , spectral analysis of the secondary light and further analysis of the spectrum may be performed in separate parts . in a highly integrated portable analysis system , they are preferably integrated into a common instrument housing . fig3 through 11 illustrate various preferred embodiments of the sensor head 9 . the distal end of sensor head 9 is always enclosed by a semipermeable membrane 30 such that macromolecules having a molecular weight above the exclusion limit of the membrane cannot penetrate into the test volume 15 which is enclosed by the membrane 30 . it has been found within the context of the present invention that through this measure , in particular taking into account the preferred values of the exclusion limit mentioned above , a significant reduction in the interfering fluorescence is achieved . this is true in particular in the preferred range of the primary light wavelength , i . e ., in the visible and very near infrared portion of the spectrum . according to the test results by the present inventors , suppression of the fluorescence background is optimal in the spectral range between 550 and 750 nm , in particular between 570 and 650 nm . with each of the embodiments depicted in fig3 through 11 , measures were taken to increase the intensity of the raman - scattered light detected by the detection light guide 21 , in comparison with interfering light components , by means of a reflective surface . in the embodiments illustrated in fig3 and 4 , the detection light guide 21 comprises six optical fibers 22 which are arranged in a ring pattern around a central optical fiber forming the inbound light guide 4 . the totality of the optical fibers 22 thus forms a detection fiber - optical ring 33 which surrounds the central inbound light guide 4 . all fibers are surrounded by cladding 32 ( as is customary in fiber - optic technology ) which has a lower refractive index than the fiber material . therefore , the light transport in the fibers is achieved by total reflection in known manner and the fibers are optically separated from one another . in the embodiment depicted in fig3 , the light receiving surface 20 of the detection light guide 21 is formed by the end faces of the optical fibers 22 running normal to the fiber axis (“ flat face ” arrangement ). a reflective surface 23 is formed by the inside surface of a reflector element 34 which form a lateral limitation of the test volume 15 and is not part of the detection light guide 21 . it is inclined at an angle α to the axis a of the primary light beam 16 a emerging from the light irradiation surface 18 such that the scattered secondary light 19 is reflected in the direction of the light receiving surface 20 of the detection light guide 21 , i . e ., it penetrates to an increased extent into the detection light guide 21 . the reflector element 34 is in the preferred embodiment depicted in fig3 and 5 designed as a reflecting sleeve 35 which surrounds the primary light beam 16 a and has slightly conically inclined side walls . the reflector element 34 preferably consists completely of a thin film of a highly reflective metal such as gold . in principle , however , it may also be made of a material that is not itself reflective ( such as a plastic ) but has a reflective ( metallic ) coating — preferably on the inside facing the primary light beam 16 a . holes 36 are provided in the wall of the reflecting sleeve 35 to improve the liquid exchange with the volume enclosed thereby — in addition to the distal sleeve opening 37 through which the primary light beam 16 a penetrates . again in this embodiment , suitable filter coatings are preferably provided , namely a bandpass filter 17 at the light irradiation surface 18 and a notch filter 27 on the light receiving surface 20 ( for the sake of simplicity , shown only with one optical fiber 22 in fig3 ). the embodiment depicted in fig5 and 6 differs from that of fig3 and 4 only in that the light guide ring 33 is not formed by a plurality of individual fibers 22 but instead by a fiber - optic tube 38 . thereby the light receiving surface is increased ( at a given outside diameter ) and the cross - section available for conducting the detection light is also increased . however , production is substantially more difficult than with the embodiment depicted in fig3 and 4 . the preferred dimensions of the reflecting sleeve 35 . the angle of inclination α of the reflective surface 23 to the axis a of the primary light beam 16 a should be very acute , with values of less than 10 degrees being preferred . the axial length of the reflecting sleeve is preferably between 1 mm and 20 mm , with values of 3 mm or 5 mm being preferred as the lower limit and 10 mm as the upper limit . the low inclination of the reflecting sleeve results in the test volume 15 extending forward beyond the distal sleeve opening 37 . therefore , the test volume 15 is large , so the detected raman intensity is increased . the size of the test volume 15 is influenced in a positive sense by the fact that a thin - walled reflecting sleeve 35 is used , its proximal diameter ( facing the detection light guide ) approximately matching that of the connected section of the detection light guide . this maximizes the test volume which is possible at a given diameter of the sensor head 9 . fig7 through 11 have in common the fact that ( as is also the case in fig2 ) the reflective surface 23 is formed by an outer boundary surface 40 of the detection light guide 21 which surface faces away from the primary light beam 16 a emerging from the light irradiation surface 18 . fig7 shows the geometry according to fig2 more clearly . the light receiving surface 20 of the detection light guide 21 is formed by lateral surfaces of the optical fibers 22 which run axially parallel and face the primary light beam 16 a . the inbound light guide 4 is shorter than the detection light guide 21 so that the test volume 15 is rearwardly enclosed by the light irradiation surface 18 of the inbound light guide 4 and toward the sides it is enclosed by the light receiving surface 20 of the detection light guide 21 . here again , the aforementioned bandpass filters and notch filters 17 and 27 are provided . filtering of the detection light is also improved by integrating an additional notch filter 39 into the detection light guide as close as possible to the light receiving surface 20 . in the variant depicted in fig9 and 10 , the most distal end of the detection light guide 21 is formed by a transparent ring segment body 41 whose conical outside surface functions as a reflective surface 23 . it has a central recess 42 which is aligned with the inbound light guide 4 . the bordering wall of the recess 42 forms the light receiving surface 20 of the detection light guide 21 and encloses the test volume 15 . the term “ ring segment body ” in this context is understood to refer to a transparent body having a conical outside boundary surface 40 and a central recess of the type shown here . its base surface 44 which faces the adjacent part of the detection light guide 22 , is aligned with the detection light guide ring 33 . the ring segment body must not be a complete truncated cone , however . instead it is advantageous if , as shown here , it is interrupted to form a plurality of segments between which there are gaps forming liquid exchange openings 43 through which the liquid exchange between the test volume 15 and the space outside the ring segment body 41 is improved . the desired reflection properties of the reflective surface 23 can be achieved , for example , by coating it with a cladding so that total reflection occurs on the boundary surface of the detection light guide 21 . however , a metallic reflective coating on the boundary surface 40 forming the reflective surface 23 is preferred . according to a particularly preferred embodiment , the boundary surface 40 is coated with a filter coating 45 which reflects the raman - scattered light but is permeable for the wavelength of the primary light . this improves the s / n ratio . the embodiments depicted in fig9 / 10 and 11 differ ( similarly to fig3 / 4 and 5 / 6 ) through the use of different types of light guide rings 33 , namely a plurality of optical fibers 22 or a tubular fiber - optic element 38 . according to the investigations conducted in the context of the present invention , the angle of inclination β of the reflective surface 23 to the axis of the primary light beam 16 a should preferably be between 10 ° and 40 ° in the embodiment depicted in fig7 through 11 , where the reflective surface 23 is formed by an outer boundary surface of the detection light guide 21 . the results of investigations of this type are depicted graphically in fig1 through 14 . fig1 shows the results of simulation experiments for different measurement geometries , showing the dependence of the absolute raman power p ra ( right ordinate ) on the angle β as a series of crosses (+) that are not connected , and the relationship of the raman power p ra to the sum of the detected power of the incident light p pr ( rayleigh scattered and reflected primary light ; left ordinate ) as crosses (+) connected by a dotted line . the results shown here are based on the measurement geometry according to fig7 , where the reflective surface is coated with a filter coating so that the raman - scattered light is reflected on the reflective surface 23 but primary light is allowed to pass through . it can be seen here that very good results are achieved with regard to the absolute intensity of the raman light as well as the ratio between p ra and p pr at relatively small angles , in particular between approx . 15 ° and approx . 35 °. as any person skilled in the art will recognize from the previous description and from the figures and claims , modifications and changes can be made to the preferred embodiment of the invention without departing from the scope of the invention as defined in the following claims .
the present invention is generally directed towards a system for reagent - free determination of the concentration of an analyte in vivo . the system comprises a light transmitter for generating monochromatic primary light , a scattered - light percutaneous sensor which includes an inbound light guide and a detection light guide , a wavelength - selective detection device that is connected to the detection light guide for detection of raman - scattered components of the secondary light and an evaluation device for determining the concentration of the analyte from the raman - scattered components of the secondary light .