GB2115175A - Fibre optics head featuring core spacing to block specular reflection - Google Patents

Abstract

A fibre optics head (10) comprises a light-emitting fibre (12) and light-collecting fibres (14) having transparent cores (15), and light- trapping means (28, 31) between them. The thickness of the light- trapping means (28, 31) is predeterminedly constructed at the ends of the fibres (12, 14) so that at least 50% of the specular reflection coming from the light ingress surface (27) of the indicator layer (34) of a test element (E) is blocked. <IMAGE>

Description

SPECIFICATION Fibre optics head featuring core spacing to block specular reflection The present invention relates to a fibre optics head adapted to detect optical density from a test element.

Fibre optics have been used in reflectometers that detect the amount of optical density available from a test element. Examples of such reflectometers are described in US Patent No 3 562 539. In general, test elements generate optical density in proportion to the amount of analyte present. To detect the level of this optical density, light is directed into the indicator zone of the test element, usually a layer, and the light is preferably reflected back from a reflective surface behind the indicator layer. The greater the optical density that is present, the greater the amount of light that is absorbed within the layer and the less that is detected therefrom. Thus, the signal detected by the instrument is that amount of light reflected from the interior of the indicator layer, and which is not absorbed by the indicator dye produced in proportion to analyte present.

In contrast, a noise factor is introduced by light reflected from the surface of the indicator layer facing the fibre optics. Light reflected from this surface never encounters the interior of the indicator layer, and accordingly remains constant regardless of the amount of analyte present and the amount of dye produced. It is this noise factor that needs to be removed.

This is: done in the device of the above-noted US Patent No. 3 562 539, for example, by taking a blank reading in additon to the test reading. There are no means in such a device for blocking such reflected light. The blank reading is then subtracted from the test reading. Such a corrective process is time consuming and requires a separate blank-reading head.

Thus, there has been a need prior to the present invention for a fibre optics reflectometer that automatically eliminates unwanted reflection created at non-test areas, such as at the surface of the indicator layer that faces the fibre optics reflectometer.

In accordance with the present invention there is provided a fibre optics head adapted for detecting optical density of a test element indicator layer that is spaced therefrom, which head comprises a) a first light-transmitting fibre having an optically transparent core and a light-emitting end, b) a second lighttransmitting fibre having an optically transparent core and a light-collecting end adjacent said light-emitting end of said first fibre, and c) material located between said adjacent ends of said fibres for preventing light incident thereon from entering the core of the fibre having said light-collecting end, wherein said material has a thickness effective to block at least 50% of the specular reflection generated at the light ingress surface of such a spaced indicator layer by said first fibre from entering the light-collecting end of said second fibre.

In accordance with the present invention there is also provided a fibre optics head adapted for detecting optical density of a test element via an indicator layer and comprising a) a first light-transmitting fibre having a lightemitting end, b) at least one other lighttransmitting fibre having a light-collecting end adjacent to said light-emitting end of said first fibre, and c) a light trapping material located between said fibres at said adjacent ends, wherein said first fibre has a core having a maximum acceptance angle, and the thickness of said light-trapping material is at least equal to twice the distance from said end of said first fibre to the operative position of a test layer light ingress surface times the tangent of said maximum acceptance angle.

It is an advantage of the present invention that readings are automatically corrected for specular reflection without requiring blank readings to be taken.

The present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an isotropic view of a fibre optics head constructed in accordance with the present invention; Figure 2 is a section taken generally along the line ll-ll of Fig. 1; Figure 3 is a perspective, partially schematic, view illustrating three of such heads in use with a test element; Figure 4 is a partially schematic, vertical section taken generally along the line IV-IV of Fig 2, illustrating the manner in which specular reflection is blocked; Figure 5 is a fragmentary section similar to that of Fig. 4, but illustrating a comparative example outside the scope of the present invention; Figure 6 is a fragmentary section taken generally along the line VI-VI of Fig. 5; and Figure 7 is a graph of BUN readings taken both by a reflectometer having a fibre optics head in accordance with the present invention, and by a conventional reflectometer.

The present invention is useful with a variety of test elements, particularly those that produce reflection at the surface of the indicator layer that is non-representative of the interior optical density of the layer. Ideally all such reflection is to be avoided, as it is all "noise". However, the primary component of such reflection is specular reflection. Furthermore, as will become apparent, only the specular reflection from such facing surface is readily locatable, and therefore, controllable.

As used herein, "specular reflection" and derivatives thereof refer to reflected radiation the angle of which, measured from the nor mal, equals the angle of incidence, both angles lying in a plane common with the normal.

The preferred embodiments are hereinafter described in connection with preferred test elements containing a transparent support for the indicator layer. It is this support that is placed in contact with the fibre optics head, creating the spacing between the fibre optics head and the indicator layer. In addition, the present invention is useful wherein such spacing is created only by a protective cover of any kind, either on the fibre optics head or the indicator layer. The present invention is further useful by providing an air gap, preferably of constant dimensions, between the indicator layer and the exterior surface of the fibre optics head.

Thus, useful test elements include those of US Patent No. 3992 1 58, wherein a support is coated with one or more indicator layers. A particularly useful example E of such a test element, as shown in Fig. 4, comprises a support 24, an indicator layer 34, and a reflective spreading layer 38. Layer 34 and support 24 form an interface 27, and layers 34 and 38 form an interface 29. Element E is inverted after contact with the test liquid, to permit scanning by head 10 to detect the optical density of layer 34.

Depending upon the materials selected for the support 24 and a binder of the indicator layer 34, the optical characteristics of the interface 27 creates specular reflection. For example, in some instances a significant difference in index of refraction occurs at the interface between the layers. Such a difference causes at least a portion of incoming radiation to be specularly reflected. For example, if subbed polystyrene is used as a support and gelatin is used as the binder for the indicator layer, the indices of refraction are 1.60 and 1.52, respectively. In such a case, some specular reflection can be expected to be generated at interface 27, so that only a fraction of the light emitted by a fibre 1 2 enters the light ingress surface (interface 27) of layer 34.

A fibre optics head 10 of the present invention comprises (Figs. 1 and 2) a light-emitting fibre 1 2 and a plurality of light-collecting fibres 14 that gather the reflected light. Each of these fibres (Fig. 4) comprises, as is conventional, an optically transparent core 1 5 surrounded by a layer of cladding 28. The fibres terminate in a face 16, and a spacer 26 is optionally included. The dimensions of the cores 1 5 are not critical so long as the cores 1 5 of fibres 14 are wide enough to collect the desired reflection from interface 29, indicated by arrow 36 in Fig. 4.As is readily apparent (Fig. 4) the necessary width for such collection depends upon the distance of interface 29 from face 1 6. This distance tends to vary, depending upon the analyte being assayed.

Plastics materials such as polymethyl methacrylate are useful as the core 15, and polystyrene and polymethyl methacrylate are useful materials for the cladding 28.

The cladding thickness as shown in Fig. 4 has been exaggerated for clarity, and is generally only a small fraction of the total fibre thickness. The total thickness, measured as a diameter, of each fibre, including the cladding 28, is preferably between 0.5 and 2 mm.

The cladding 28, though "transmissive" to light, is effective to prevent light which enters it, from passing into the core 1 5. Practically, such material transmits light only until it encounters the exterior wall of the light fibre, where it is absorbed. For convenience, this property is hereinafter referred to as a lighttrapping property, and the associated material, light-trapping material.

The number of light-collecting fibres is not critical, six being particularly preferred in that six fibres of equal diameter is an optimum number for a given cylindrical volume, when used with one light-emitting fibre. The seven fibres are cemented together by a filler 31 in the form of a suitable opaque, and therefore, light-trapping, material, such as an epoxy resin such as is available from Ciba-Geigy under the trademark "Araldite", filled with 3% carbon black.

The ends of the fibres 12, 14 are ground and polished to present a generally flat terminal face 16, as shown in Figs. 1 and 4, adapted to contact the support 24 of a test element E to be measured, either directly, or indirectly through a spacer 26. The end of fibre 1 2 at face 1 6 (Fig. 4) is the lightemitting end, and the ends of fibres 14 at face 1 6 are the light-collecting ends, as is conventional.

An alternative useful configuration of head 10 is one in which terminal face 1 6 is very slightly convex, (not shown) having a radius of curvature of no less than 50 mm. When head 10 contacts the test element E, it forces the layers of that element to curve accordingly.

To measure a single wavelength, fibre 1 2 is preferbly coupled to a LED light source 18 (Fig. 1) that emits at a selected wavelength such as 565, 590, or 670 nm, and the lightcollecting fibres are cemented as a bundle to a detector 20 such as a silicon PIN photodetector. The light-emitting fibre 1 2 is also useful if coupled to alternative light sources, such as a filament lamp, quarz halogen lamp, or xenon flash tube, filtered to the wavelength of interest either at the source or at the photodetector.

The signal from the detector 20 is pro cessed by a conventional microprocessor (not shown) that generates either a density readig, or a concentration of analyte if element E is an element such as is described in the afore said US Patent No. 3992158.

By using two or more fibre optics heads (Fig. 3) it is possible to measure the optical density of adjacent areas of a single test element E. In such an embodiment, the lightemitting fibres 1 2 are preferably connected to a commn light source 18'; As noted, face 1 6 of head 10 (Fig. 4) is placed, preferably under slight pressure, either in direct positive contact with the support 24 of the test element E, or it is placed under such pressure in indirect contact through transparent spacer 26 mounted on the end of the head 10. The spacer 26 has a thicknews x so that face 1 6 is spaced from interface 27 by a predetermined distance x'.Distance x' is generally maintained even if face 1 6 is slightly convex, since element E flexes in response to the applied pressure and interface 27 remains generally parallel to face 1 6.

In accordance with the present invention, the amount of light-trapping material used to separate the cores 1 5 of the fibres at face 1 6 is selected so as to a) block from reception by fibres 14, at least 50% of the specular reflection designated as arrows 30 and 32, and b) transmit to fibres 14 the reflection indicated by 36 from which the optical density produced in layer 34 is detected. The total amount of the specular reflection that would be detected from interface 27 if the cladding 28 of the fibres were infinitesimally thin, as shown in Fig. 5, and no opaque filler were present, is determinable as follows.For a fixed distance x' that is determined by the test element E (and spacer 26, if any), and a fixed maximum acceptance angle a dictated by the material of the cores 15, specular reflection indicated by arrow 30 will extend out a maximum distance S from the edge of fibre 1 2.

For example, if x' equals 1 75 m and angle a equals 30 , S is about 200 jum. (The maximum acceptance angle a remains the same whether the core 1 5 is receiving light or emitting it.) Distance S in turn represents the thickness of an annular zone of specular reflection around fibre 1 2. Each fibres 1 4 subtends a portion 40 of this zone (Fig. 6). It is the specular reflection detectable within portion 40 that is the noise from interface 27 that is to be avoided. Of course, specular reflection also arises from interface 27 along directions such as indicated by arrow 42 (Fig. 5) created by incoming light reflected within fibre 1 2 at a very shallow angle a'.However, such specular reflection 42 extends out a distance much less than S, and can be ignored compared to the reflection produced by angle a.

For fibres having cores 1 5 with a radius that is much larger than distance x', as in the present case, S will be less than the radius of light-collecting fibre 1 5.

It has been discovered that at least 50% of the specular reflection from interface 27, and most preferably, substantially 100%, can be blocked by the proper selection of the thick ness T of the light-trapping material between the cores (Fig. 4). Considering first the most preferred embodiment in which all of the specular reflection is blocked, the view of Fig.

4 represents the points at which fibres 1 2 and 14 are closest together. Therefore, if the light trapping material blocks all reflection 32 and 30 at these points around the circumference of fibres 12, it blocks all of it elsewhere since the thicker portions of the filler 31 present elsewhere along the circumference of fibre 1 2 act as an additional thickness of light-trapping material. For arrow 30 to just clear cladding 28 of fibre 14, the dimensions for thicknesses x' and T of the light-trapping material be tween fibres 1 2 and 14 form the relationship T/2 1 /x' = tan a, where a is the maximum acceptance angle of the cores 1 5.

The total thickness of the light-trapping ma terial between fibres equals the sum of the cladding thicknesses of those fibres plus the filler, if any. If the filler thickness is absent or negligible at points where fibres 1 2 and 1 4 are closest together, and the cladding thick nesses are a constant value Y for all the fibres, then it is readily apparent that the total thickness T equals twice the cladding thick ness Y and that hence Y/x' equals tan a.

Therefore, to block reception by fibres 14 of reflection from interface 27, cladding thick ness Y is selected in this case to at least equal x' times tan a.

For example, a preferred test element E features a support 24 having a thickness x' of 175 ym. For a head 10 having no spacer 26 (x = 0) fibres 1 2 and 1 4 in contact, generally equal cladding thicknesses and a maximum acceptance angle a of he core 1 5 equal to 30 , Y is selected to be at least about 100 item.

Alternatively, if cladding thickness Y is se lected to be some lesser value, but a value that is at least 1/6 that of S, only a minor portion of the specular reflection (less than 50%) will be detected. It can be shown graphically that for Y equal to S/6, the area of portion 40 that is blocked by the cladding of fibres 1 2 and 14 is about one half that of original area 40, assuming the cladding thick ness Y to be equal for all fibres. This relation ship is assumed to be approximately valid for distances S that are less than the radius of the cores 15.Because S/2x' = tan a, and S = 6Y, then 3Y/x' = tan a or Y = x' tan a l 3 For the previously described example wherein a = 30 and x' = 175 ,um, if Y is at least one third that of the previous example, or 33.3 ,um, then at least 50% of the specular reflection 30 is blocked.

The fibre optics head of the present invention is useful as a reading station or relecto meter in an analyzer for determining analytes of liquids. The additional parts of the analyzer do not constitute the present invention, and accordingly no further description is necessary.

Example A head 10 according to the present invention was prepared as described above, particu larly for Figs. 2 and 4, in which T/2 for the section view along line IV-IV of Fig. 2 was between 50 ,um and 100 ,um, a = 30 , and x' = 175 jum. An LED 18 was selected (Fig. 1) to emit light at 658 nm. As a control, a conventional reflectometer using a lens system similar to that shown in Fig. 9 of U.S.

Patent No. 4,224,032 was prepared. A bandpass filter was used to permit detection of radiation by the control reflectometer at only about 660 nm. To test the effectiveness of the head 10 of the Example, and of the control, identical test elements were prepared as described in U.S. Reissue Patnt No.

30,267, using a support having a thickness of 1 75 ,um. These elements were spotted with a drop of a BUN calibrator liquid, seven different levels of known BUN concentrations being chosen to plot the performance of the reflectometer. For the Example, the test element was held with the support in contact with the head 10 as shown in Fig. 4, but without a spacer 26 being present. An automatic clocking system was used to "freeze" the reflection data at the completion of 5 minutes time measured by a timer. Reflection density was calculated as DR = - log (Rtest/ Rwhite reference) where the white reference was a white surface viewed by the reflectometer before each test. All readings were made at about 37"C. Eight replicates were made at each concentration level. To avoid ammonia carryover, the eight replicates were run for a given level all at one time.

Fig. 7 indicates the results. Curve 100 is the results of the control, and curve 200 is the results of the head 10. As indicated, at any given level of BUN, the head 10 was able to detect greater density than was the conventional reflectometer, and the measured difference increased as the density increased. This is the expected result, because the greater density levels represent less light being reflected. A reflectometer such as is provided by the head 10 increases the signal-to-noise ratio by screening out the specular reflection 30 (Fig. 4). This increase in ratio becomes more and more significant is less and less light is being reflected.

Claims (10)

1. A fibre optics head adapted for detecting optical density of a test element indicator layer that is spaced therefrom, which head comprises a) a first light-transmitting fibre having an optically transparent core and a light-emitting end, b) a second light-transmitting fibre having an optically transparent core and a light-collecting end adjacent said lightemitting end of said first fibre, and c) material located between said adjacent ends of said fibres for preventing light incident thereon from entering the core of said second fibre wherein said material has a thickness effective to block at least 50% of the specular reflection generated at the light ingress surface of such a spaced indicator layer by said first fibre from entering the light-collecting end of said second fibre.

2. A head according to Claim 1, wherein said fibres are each provided with a layer of cladding encircling said core, and wherein said thickness of said material comprises at least the sum of the thicknesses of said cladding.

3. A head according to Claim 1 or 2 wherein i) the core of said first fibre has a maximum acceptance angle, ii) said head is adapted to be positioned in contact with such test element with a terminal face of said head extending generally parallel to, and at a predetermined distance from, said surface of such indicator layer, and iii) said thickness of the cladding is the same for each of said fibres and is at least equal to said predetermined distance times the tangent of the maximum acceptance angle of the core of said first fibre.

4. A head according to Claim 3 further comprising a transparent spacer disposed on said terminal face and wherein said predetermined distance equals the thickness of said spacer plus the thickness of a support of such test element.

5. A head according to any one of the preceding claims, further comprising at least another light-transmitting fibre having a lightcollecting end and core.

6. A head according to any one of the preceding claims, wherein said thickness of said material is effective to block substantially all specular reflection from said surface of said indicator layer from entering said light-collecting end or ends.

7. A head according to Claim 1 substantially as hereinbefore described with reference to, and as shown in, Figs. 1-4 of the accompanying drawings.

8. A fibre optics head adapted for detecting optical density of a test element via an indicator layer and comprising a) a first lighttransmitting fibre having a light-emitting end, b) at least one other light-transmitting fibre having a light-collecting end adjacent to said light-emitting end of said first fibre, and c) a light trapping material located between said fibres at said adjacent ends, wherein said first fibre has a core having a maximum acceptance angle, and the thickness of said lighttrapping material is at least equal to twice the distance from said end of said first fibre to the operative position of a test layer light ingress surface times the tangent of said maximum acceptance angle.

9. A head according to Claim 8, further comprising at least another light-transmitting fibre having a light-collecting end and having a core separated from the core of said first fibre by said thickness of said light-trapping material.

10. A head according to Claim 8 substantially as hereinbefore described with reference to, and as shown in, Figs. 1-4 of the accompanying drawings.