Abstract:
An optical measuring device ( 10 ) for detecting the coloring of test fields of a test strip ( 12 ), which is to be wetted with a liquid for detecting substances in said liquid, whereupon the reflectivity of the test fields changes depending on the concentrations of the substances to be detected, comprising: a measuring plane ( 14 ) wherein said test strip ( 12 ) is to be placed; an illumination device ( 16 ) for illuminating the measuring plane ( 14 ); a planar image sensor ( 36 ); an optical system for imaging said measuring plane ( 14 ) onto the image sensor ( 36 ); and an electronic evaluation unit ( 52 ) for evaluation of the signals detected by said image sensor ( 36 ), said illumination device ( 16 ) comprises light sources ( 18, 20, 22 ) or other means of different colors which serve to alternatively illuminate said measuring plane ( 14 ) in different colors, wherein said electronic evaluation unit ( 52 ) detects the coloring of the test fields from the images obtained under different color illuminations.

Description:
BACKGROUND OF THE INVENTION 
   The present invention concerns an optical measuring device for detecting the coloring of test fields of a test strip, which is wetted with a liquid for detecting substances in said liquid, whereupon the reflectivity of the test fields changes depending on the concentration of the substances to be detected, comprising
         a measuring plane wherein said test strip is to be placed,   an illumination device for illuminating the measuring plane,   a planar image sensor,   an optical system for imaging said measuring plane onto the image sensor, and   an electronic evaluation unit for evaluation of the signals detected by said image sensor.       

   Optical measuring devices of this kind are e.g. known as part of test strip analysis apparatus, which are used for examining urine or blood test strips in doctor&#39;s practices, hospitals or medical laboratories. The test strips to be analyzed often comprise a plurality of test fields, each of which serves to detect another substance. In such measuring devices, the test strip is illuminated as a whole and imaged onto the image sensor. From the image obtained, the electronic evaluation unit may then determine the coloring of all test fields across the test strip. Thus, it is not necessary to scan the individual test fields and the measurement of the test strip may be conducted in a shorter time, wherein the structure of the measuring device is simpler. 
   For detecting the coloring of a test field, its reflectivity for light of three different wavelengths has to be determined. In a conventional measuring device, the test strip is illuminated with white light and imaged onto an image sensor comprising sensor elements, each of which is sensitive to one of three wavelengths (colors). The light-sensitive elements are arranged in a planar matrix whose lines each comprise sensor elements sensitive to the light of the same color. Thus, there are three types of such color lines comprised in the image sensor, the order of adjacent color lines being such that one color line of the first type is followed by one of the second type and one of the second type is followed by one of the third type and one of the third type is followed by one of the first type. 
   Thus, three images are effectively detected by the image sensor sequentially, each time one of the entirety of color lines of one type, which represent the intensity of three spectral components of the same colored picture. From the intensity of the spectral components in every image point, basically a colored image may be calculated. 
   However, a problem is posed by the fact that these three images do not really represent the spectral components of the same image points since the color lines are offset from each other. If now these three images are taken for color evaluation, a systematic error occurs due to the finite width of the color lines. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention is to solve the problem of providing an optical measuring device of the above-mentioned kind, which avoids this systematic error. This problem is solved by an optical measuring device of the above kind in that the illumination device comprises light sources of different colors or other means that serve to alternatively illuminate the measuring plane with different colors and in that the electronic evaluation means detects the coloring of the test fields from the images obtained under different illumination. 
   Thus, the measuring device of the present invention does not require an image sensor having color lines but an image sensor whose light-sensitive elements detect the intensity of the light received independent from its wavelength. In the images obtained under different color illumination, the same image point (i.e. the signal received under different color illumination at the same sensor element) is generated by a light that has been reflected on the same point on the test strip. From the relative reflectivity of these points under different color illumination, the respective color may be calculated without any systematic errors occurring. 
   In a preferred embodiment, the image sensor comprises light-sensitive CMOS components arranged in a planar matrix. 
   Preferably, the illumination unit uses colored LEDs as light sources. In a preferred embodiment, the light sources comprise blue, green and orange LEDs, in particular LEDs having wavelengths of 450 nm, 530 nm and 620 nm. 
   In order to uniformly illuminate a test strip in a measuring plane, it is advantageous to arrange the light sources in a series on a transmitter board. It is especially advantageous if the arrangement density of light sources on the transmitter board increases within the series from the center in an outward direction. Due to such inhomogeneous distribution of light sources on the transmitter board, the test strip may be almost homogeneously illuminated in the measuring plane. 
   In a further preferred embodiment, on both sides of the series of light sources in parallel to the longitudinal direction of the series, screens are employed which focus the light emitted by the light sources on a strip-shaped portion in the measuring plane in which the test strip is to be placed. The screens preferably extend substantially over the length of the series of light sources and comprise a plurality of surface segments extending over the entire length of the transmitter board, wherein said surface segments&#39; inclination angle relative to the transmitter board increases in correspondence with an increasing distance therefrom. With these screens, a test strip in the measuring plane may be illuminated with high intensity but nevertheless almost homogeneously. 
   The screens preferably consist of parts that have been milled or manufactured by injection molding, said parts having a reflecting layer or being laminated with a reflecting film. Thus, the screens are stable and can be manufactured at low cost. 
   As has been mentioned above, the color of a test field is detected by the relative reflectivity of the test field for light of three different wavelengths. If, for example, a test field is illuminated three times in a row with lights of different colors that, however, have different intensities, the relative reflectivity corresponds to the relation of intensities that are detected by the image sensor in the image of the test field. However, it is not indispensably necessary to permanently illuminate the test strip with light of the same intensity. What is important is that the intensity of illumination is known, thus, the relative reflectivity can be calculated from the measured intensities at the image sensor. Further, it may occur that the intensity of illumination changes in operation. If, e.g. colored LEDs are used for the illumination means, these are subject to wear which diminishes their performance. 
   For detecting a change of illumination intensity during operation it is advantageous to provide a first reference surface that is arranged such that it is illuminated by the illumination unit together with the test strip and imaged by the optical system onto the image sensor. By means of the image of this reference surface, the electronic evaluation unit may determine any illumination changes, such as a change in intensity of illumination with one of the colors as a whole or only within a portion of the measuring plane. In case such change is determined, it is necessary to newly calibrate the measuring device. 
   For calibration of the measuring device, the local intensity distribution on the measuring plane for all three colors has to be known. Preferably, the optical measuring device therefore has a second reference surface adjustable between a first position in which it takes the position of a test strip during measuring and a second position in which it cannot be imaged by the optical system onto the image sensor. The optical measuring device may thus calibrate itself by an adjustment of the second surface to the first position, an illumination subsequently with the different colors and by using the images provided by the evaluation unit as calibration standard. Such calibration procedure may for example routinely be carried out by the measuring device upon switching on the measuring device or after a predetermined amount of measurements has been made. 
   In an advantageous embodiment, the second reference surface is formed by a surface of a strip-shaped plate whose first end is equipped with a first arm at least almost perpendicular with respect to said second reference surface and pivotable about an axis parallel with respect to the reference surface. By adjusting the arm, the second reference surface can be adjusted between its first and second positions. Preferably, a second arm is further mounted to the second end of the strip-shaped plate, said arm being substantially parallel with the first arm and pivotable about the same axis as the first arm. The first arm is discharged by the second arm. 
   Preferably, a bar is pivotally supported with its first end by the first arm for adjusting the second reference surface between its first and its second position. In a preferred embodiment, this bar is mounted with its second end to a first lever pivotable about a first lever axis and biased into a first lever position by means of a biasing element, in which position the reference surface takes its second position and the lever is adjustable against the biasing force of the biasing element to a second lever position in which the reference surface takes its first position. Preferably, the first lever is actuated by means of an eccentric drive. 
   The device according to the invention may be used as part of a test strip analysis apparatus in which the test strips usually are advanced towards the measuring means by means of belts or the like. However, when measured, the test strip has to be in a static condition. This is most simply obtained when the conveying surface of the transport means of the test strip analysis apparatus lies in the measuring plane and the test strip is shortly kept in its measuring position on the conveying surface. In an advantageous embodiment, the optical measuring device therefore comprises a means for holding and aligning a test strip in its measuring position. 
   Preferably, these means are defined by two at least almost parallel pins movable along their longitudinal axis between a first position in which they protrude into said measuring plane and a second position in which they are entirely outside said measuring plane. In their first position, these pins protrude into the measuring plane so that a test strip conveyed in the measuring plane is caught at the pins and aligns relative to these pins in a measuring position. Preferably, the pins are biased into their second position and adjustable against the biasing force to their first position by a lever element. Preferably, the lever element is adjusted by the same eccentric drive as the first lever. 
   In a particularly preferred embodiment, the eccentric drive is formed by a rotating disc driven by a motor, on the surface of which a pin is arranged perpendicularly thereto such that upon rotation of the disc in its first rotational direction, said pin engagingly moves said first lever into its second lever position and that upon rotation of the disc in its second rotational direction, said pin engagingly moves said lever element such that said lever element moves said pins into their first position. 
   Even though, the optical measuring device according to the invention makes use of an illumination means that is able to alternatively illuminate the measuring plane with different colors, all of the features concerning the arrangement of the light sources, the screens, the first and second reference surfaces, the adjustment mechanism of the second reference surface and the means for holding and aligning a test strip in its measuring position may also be advantageously used with conventional measuring means, in which white light for illuminating and filters, i.e. image sensors having color lines are used. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Further advantages and features of the solution according to the invention will become apparent from the following description which outlines the invention by means of an embodiment with reference to the accompanying drawings, wherein 
       FIG. 1  is a perspective view of the optical measuring means according to the invention; 
       FIG. 2  is a cross-section through a portion of the optical measuring device along A-A′ in  FIG. 1 , 
       FIG. 3  is a perspective view of the section of  FIG. 2  with the second reference surface in its first position, 
       FIG. 4  corresponds with  FIG. 3 , wherein the second reference surface is in its second position and a test strip in its measuring position, and 
       FIG. 5  is a perspective view of the illumination unit. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a perspective view of the optical measuring device  10  according to the invention, in which a test strip  12  extends in the measuring plane  14  (cf.  FIG. 2 ) in its measuring position. 
     FIG. 2  shows a cross-section through the measuring device of  FIG. 1  along A-A′. An illumination means  16  is illustrated by means of which a strip-shaped portion of the measuring plane, in which the test strip  12  is in its measuring position, may be illuminated with different colors.  FIG. 5  shows a perspective view of the illumination means  16 . The illumination means  16  comprise blue  18 , green  20  and orange  22  LEDs having wavelengths of 450 nm, 530 nm and 620 nm in a serial order on a transmitter board  24 . This serial arrangement of the LEDs is particularly suitable for illuminating the strip-shaped portion on the measuring plane  14 . Within the series, blue  18 , green  20  and orange LEDs  22  are respectively placed closely adjacent to one another in one group so that the measuring plane is uniformly illuminated in all three colors with the same intensity distribution. The groups of LEDs are arranged in a series such that their arrangement density of light sources on the transmitter board increases within the series from the center in an outward direction. This inhomogeneous arrangement of LEDs in a series results in an almost homogeneous illumination intensity on the measuring plane in the field of the test strip. 
   In parallel to the longitudinal direction of the LED series, screens  26  are disposed which focus the light emitted by the LEDs onto a strip-shaped portion, in which the test strip extends in its measuring position. The screens  26  extend over the entire length of the LED series. They comprise four surface segments  28 ,  30 ,  32  and  34 , extending over the entire length of the screen  26 . The inclination angle of the surface segments increases relative to the transmission board  24  in correspondence with an increasing distance therefrom. The screens  26  are made of parts having been manufactured by injection molding and whose surfaces  28 ,  30 ,  32  and  34  are provided with a reflecting layer. 
   The light emitted by the illumination unit  16  is diffusely reflected at the test strip  12  and the test strip is imaged via an optical system onto a planar image sensor  36 , which is disposed in a housing  38  that is impermeable against light. The optical system comprises a mirror  40 , a lens  42  and a screen  44 . In  FIG. 2  it is not the test strip  12  but the second reference surface  58  that is imaged onto the image sensor  36 . Since the second reference surface  58 , as will be described in more detail below, takes the position of a test strip during measuring, the position of the image  46  of the test strip is the same as the one of the image of the reference surface. In  FIG. 2 , the generation of the image  46  at the image sensor  36  is illustrated by means of two exemplary rays of light  48 ,  50 . The optical path is folded at the mirror  40  thus maintaining the measuring device in its entirety compact. 
   The image sensor  36  comprises a plurality of light sensitive CMOS parts generating a signal depending on the intensity of the light illuminating them. The light sensitive elements are arranged in a planar matrix and each of them serves to generate an image point (pixel) of the image  46 . 
   In the following, it shall be described how to detect the coloring of the test fields of the test strip  12  by means of the measuring device according to the invention. For this purpose, one of a plurality of light sensitive sensor elements is selected. Light is imaged onto the sensor element by the optical system, the light being diffusely reflected from a certain point, i.e. a very small portion, in one of the test fields. Thus, the sensor element detects the intensity of the light reflected at that point. If now this point is illuminated three times sequentially with blue, green and orange light having the same intensity, the relations of intensities measured at the sensor element represent the relative reflectivity for light of these three colors. Thus, however, the color of the test field is clearly determined. The signals generated at the image sensor  36  are transmitted to an evaluation unit  52  that detects the color of the original image, i.e. of the point or small portion on the test field, from these three signals generated upon a three-time illumination at each of the image points. Due to the plurality of light sensitive sensor elements which are comprised in the image sensor  36 , the determination of color may be performed sequentially for a corresponding number of pixels in the test plane. 
   For determining the relative reflectivity it is certainly not necessary to illuminate each pixel in each color with light having the same intensity. What is important is that the evaluation unit  52  gains information about the relation of intensities of the different colors at one pixel in the measuring plane so that it may be taken into account when calculating the relative reflectivity. By resorting to this information, the measuring device may be calibrated according to the illumination conditions. However, the illumination of the measuring plane may vary in the course of time for a great number of reasons, e.g. due to the wear of the LEDs, defective LEDs or even only due to contaminations in the illumination means. As a consequence, the measuring device has to be calibrated anew. 
   For immediately determining during operation a change of intensity of illumination of the measuring plane, the measuring device  10  according to the invention is provided with a first reference surface  54 , which is arranged such that it is illuminated by the illumination unit  16  together with the test strip and imaged by the optical system onto the image sensor  36 . This first reference surface  54  is illustrated in  FIG. 2  and its image on the image sensor  36  has been designated  56 . As soon as there is a change of illumination, the image  56  changes on the image sensor  36 , which in turn is detected by the evaluation unit  52 . Then, the evaluation unit  52  causes the self-calibration of the measuring device. 
   However, for calibrating the measuring means, the image  56  of the first reference surface  54  cannot be resorted to since the illumination intensity distribution on the first reference surface  54  is not identical with the one in the measuring plane. Therefore, a second reference surface  58  is provided which is adjustable from a second position, in which it is not illuminated by the illumination means  16  and not imaged by the optical system onto the image sensor  36 , to a first position, in which it takes the position of a test strip during measurement. The image of this second reference surface  58  serves to calibrate the measuring means. 
   As shown in  FIG. 1  and  FIG. 4 , the measuring device according to the invention may be used for a test strip analysis apparatus, in which the test strips are advanced towards the measuring device placed across transport belts  60 . It is particularly advantageous to provide a construction in which the transport belts  60  form a conveying surface which coincides with the  35  measuring plane. Thus, the test strip  12  may be measured lying on the transport belt  60 . However, this requires holding and aligning the test strip in its measuring position. 
   With regard to the illustrated measuring device two pins  62  are provided that are movable along their longitudinal axis between a first position in which they protrude into said measuring plane (cf.  FIG. 1  and  FIG. 4 ) and a second position in which they are entirely outside said measuring plane (cf.  FIG. 2  and  FIG. 3 ). The pins  62  are biased into their second position by a spring and adjustable to their first position by a lever element  66  against the biasing force. 
   In the following, the adjustment mechanism of the pins  62  and the second reference surface  58  is described. The second reference surface  58  is defined by a surface of a strip-shaped plate  68  whose first end is equipped with a first arm  70  perpendicular relative to the reference surface  58 , this arm being pivotable with respect to an axis  72  (cf.  FIG. 2 ) parallel to reference surface  58 . A second arm is mounted to the second end of the strip-shaped plate, this arm being parallel with respect to the first arm and also pivotable about the axis  72 . This arm is not shown in the drawings since it extends in a region of the measuring device, that is cut away in  FIGS. 2 to 4 . The strip-shaped plate  68  and the first and second arms form a U-shaped part. 
   For adjustment of the second reference surface between its first and its second position, a bar  74  is pivotally mounted with its first end via a joint  76  to a free end of the first arm  70 . This bar is also pivotally mounted with its second end via a joint  76  to a first lever  78 . This first lever  78  is pivotable about an axis  80  between a first and a second position. This axis  80  is realized by a bolt, which, however, is not shown in  FIGS. 3 and 4 . In  FIG. 4 , the first lever  78  is in its first lever position. The lever is biased into this position by means of a spring  82 . Upon the lever  78  taking this first lever position, the second reference surface  68  is adjusted via the bar  74  and the first arm  70  into its second position. 
   The first lever  78  is adjustable from its first into its second position by means of an eccentric drive against the biasing force of the spring  82 . The eccentric drive is formed by a rotating disc  84  that is driven by a motor, not shown, and on which a pin  68  is mounted perpendicularly thereto. Upon rotation of the disc in its first rotational direction (cf.  FIG. 1 to 4 , in a clockwise direction) the pin  86  engages the first lever  78  and moves it into the second lever position, as shown in  FIGS. 2 and 3 , against the biasing force of the spring  82 . Thus, the second reference surface  68  is moved into its first position by means of the bar  74  and the first arm  70 . 
   The lever element  66  is pivotable about an axis  88 . This axis  88  is realized by a bolt which is not shown in  FIGS. 3 and 4 . The spring  82  is fixed to this bolt. In a position of the disc  84 , in which the pin  86  holds the second lever  78  in its second lever position against the biasing force of the spring  82 , as is the case in  FIGS. 2 and 3 , the end  90  of the lever element  66  is free so that the springs  64  of the pins  62  relax and take their second position in which they lie completely outside the measuring plane. However, upon rotation of the disc  84  from this position in its second rotational direction (cf.  FIG. 1 to 4 , in a clockwise direction), the first lever  78  is moved by the spring  82  into its first position, whereby the reference surface  68  moves into its second position. On the other hand, the pin  86  engages the end  90  of the lever element  66  after rotation about a certain angle and pivots the lever element about the axis  88  such that the pins  62  are moved into their second position against the biasing force of the spring  64 . This is the position taken during measurement of a test strip, namely in which the second reference surface  68  is turned out of the optical path and the pins  62  protrude into the measuring plane, so that a test strip  12  may be aligned and held by them. 
   Therefore, by using a single eccentric drive, the adjustment mechanism described facilitates adjustment between a calibration position, in which the second reference surface  68  takes its first position and in which the pins  62  take their second position, and a measuring position, in which the second reference surface  68  takes its second position and the pins  62  take their first position.