GB2348949A

GB2348949A - Optical analytical instrument

Info

Publication number: GB2348949A
Application number: GB0004682A
Authority: GB
Inventors: Klaus Deuter
Original assignee: LRE Technologies Partner GmbH
Current assignee: LRE Technologies Partner GmbH
Priority date: 1999-04-16
Filing date: 2000-02-28
Publication date: 2000-10-18
Anticipated expiration: 2020-02-28
Also published as: GB2348949B; GB0004682D0; DE19917306A1

Abstract

An optical analytical instrument comprises a light-proof housing (34) with a measuring chamber (36) for holding a measurement sample, a measuring light receiver (40), arranged in the housing (34) for measuring the light emanating from a sample, a reference light source with an optotransmitter (20) for calibrating the measuring light receiver (40), and a control and evaluation device (58, 62) connected to the measuring light receiver (40) and the reference light source, the reference light source has a diffuser (14) which receives the light of the optotransmitter (20) and is connected to the measuring chamber (36) via an optical conductor (26).

Description

2348949 optical analytical instrument The invention relates to an optical

analytical instrument comprising a light-proof housing with a measuring chamber for holding a measurement sample, a measuring light receiver, arranged in the housing for measuring the light emanating from a sample, a reference light source with an optotransmitter for calibrating the measuring light receiver, and a control and evaluation device connected to the-measuring light receiver and the reference light source.

An analytical instrument of the aforesaid type is disclosed, for example, in DE 44 22 580. Such an analytical instrument is used, for example, for the analytical investigation of medical samples. It is the aim in this case, for example, to determine the concentration of a substance which is provided with an optically detectable marking, as described in DE 44 22 580. The measurable light quantity is very small in this case.

Generally, a photomultiplier is used as the measuring light receiver with such instruments. Although such a receiver has a high photosensitivity, the latter is not stable over time, given temperature changes, and also as a function of the electronic operating conditions. The reference light source serves to eliminate this deficiency in order to calibrate the measuring light receiver. In order to calibrate the measuring light receiver, the latter is illuminated with light of a known and/or constant intensity, and the output signal of the measuring light receiver is measured in this case. From this there is determined a calibrating quantity to which the output signal of the measuring light receiver can be referred when the sample is being measured.

In the known analytical instrument, the reference 2 light source has a light-emitting diode. In the case of operation with currents which can be effectively manipulated and with which the light- emitting diode operates as stably as possible, the latter generates light with an intensity which is too high by several orders of magnitude for a photomultiplier. The photomultip lier would be overdriven if all the light of the light-emitting diode is fed to it. Consequently, only a portion of the light generated by the light-emitting diode may reach the photomultiplier. Furthermore, the optical emission of the light-emitting diode itself is not stable over the service life thereof, and is, moreover, a function of temperature.

In the known solution, the light generated by the light-emitting diode is therefore split by means of a beam splitter, a first portion of the light being led to the measuring chamber, and the other portion of the light reaching a calibration receiver which itself is sufficiently stable. This receiver measures the light emanating from the light-emitting diode and compares it with a stored value. If the measured light deviates from the stored value, the measured value is corrected computationally.

The splitting of the light intensity of the light-emitting diode onto the photomultiplier and the calibration receiver is undertaken via prescribed apertures in the case of the known solution. In this solution, a change in the spatial characteristic of the optical components has a very strong effect. The consequence is a strong scattering of the effective intensity of the reference light source at the photomultiplier. Consequently, the optical components must be adjusted mechanically, if appropriate. This is complicated.

It is the object of the invention to design the reference light source in the case of an optical 3 analytical instrument of the type mentioned at the beginning such that it can be produced cost-effectively, and that it is possible to achieve a reproducible splitting of the radiation generated by the light-emitting diode in a f ashion as independent as possible from the optical emission and reception characteristics of the -components in a simple way and without mechanical adjustment.

This object is achieved according to the invention by virtue of the fact that the reference light source has a diffuser which -receives the light of the optotransmitter and is- connected to the measuring chamber via an optical conductor.

In the solution -according to the invention, the diffuser has an integrating effect. Its brightness depends on the light flux of the irradiated light, but not on the exact direction of irradiation. It is possible in a simple way to determine via the aperture of the optical conductor, for example an optical fibre, the flux of the light which is to be led to the photomultiplier. A further parameter can also be the location of the coupling of the optical fibre on the diffuser relative to the irradiation site-of the optotransmitter. In the case of the solution according to the invention, not only is complicated adjustment eliminated, but it is also possible to use standard components which do not have to be put through a strict quality inspection.

When, as in the case of the known solution, the optotransmitter is a light-emitting diode, it is expedient to assign the diffuser an optoreceiver element which is connected to a control circuit which controls the brightness of the light- emitting diode as a function of the scattered light received by the optoreceiver element. Such an optoreceiver element can, for example, be a photodiode which has a very stable sensitivity given suitable electric circuitry (preferably in current-source 4 mode) The diffuser is preferably a block made from an opaque material which has cut-outs for holding the respective light-exit or light-entry end of the optotransmitter, the optoreceiver element and the optical conductor. It is possible to ensure by suitable selection of the position of the cut- outs in the block that the scattered light passes in the quantities respectively required on the one hand to the optoreceiver element, that is to say the photodiode, for example, and on the other hand to the optical conductor. The block of the diffuser can consist, for example, of a suitable opaque plastic which scatters the light effectively.

The end of the optical conductor which faces the measuring chamber can be introduced into the measuring chamber via a further diffuser. The second diffuser is expediently arranged at a location of the measuring chamber in alignment with the sample location, so that the light of the reference light source incident in the measuring chamber simulates the sample light as accurately as possible.

It is preferable to arrange in the measuring chamber a transparent sample holder which has a cut-out on its wall facing the second diffuser. This measure also serves the purpose of ensuring that the same conditions prevail as far as possible when measuring the reference light and the sample light, so that the two measured values can be effectively compared with one another.

The solution according to the invention renders it possible for the high intensity of the light of a light-emitting diode in the reference light source to be split stably, simply and cost- effectively such that a large component impinges on the optoreceiver element, and a very much smaller component on the photomultiplier.

This permits the photodiode to receive the quantity of light required for stable and linear operation, and thereby compensation of the instability of the light-emitting diode via the optoreceiver element and the control circuit. The splitting of the intensity of radiation of the light-emitting diode is largely independent of the optical emission and/or reception characteristics of the -optical components, for which standard elements can be used. Mechanical adjustment of the elements or components is not required.

In the solution according to the invention, it is possible, furthermore, for the reference light source to be spatially separated from the actual measuring chamber, with the result that the overall space requirement can be minimized. The individual components can be arranged wherever is optimal in terms of instrument design. In addition, the design of the reference light source is independent of the particular form of the measuring chamber.

Further features and advantages of the invention emerge from the following description, which explains the invention with the aid of an exemplary embodiment in conjunction with the attached figures, in which:

Figure 1 shows a- diagrammatic cross section through a reference light source, Figure 2 shows a diagrammatic section through a measuring chamber housing, and Figure 3 shows a diagrammatic combination of the individual components - of the analytical instrument according to the invention.

The reference light source represented in Figure 1 comprises an opaque housing 10 which is connected in a light-proof fashion to a backing plate 12, preferably a printed circuit board. Arranged inside the housing 10 is a diffuser 14 which consists of an opaque material, for example, a suitable plastic, which scatters light as uniformly as possible. On the side facing the board 12, the diffuser 14 has two cut-outs 16 and 18 for 6 respectively holding a light-emitting diode 20 and a photodiode 22 which are mounted on the board 12. On its top side remote from the board 12, the diffuser 14 has a further small bore 24 into which there is plugged one end of an optical fibre 26 which is surrounded by a light-proof sheath 28. The optical fibre 26 in this case penetrates an opening 30 in the housing 10. The optical fibre is fixed in the opening 30 by means of a lightproof sealing compound 32. At the same time, the sheath 28 is thereby sealed with respect to the housing 10, thus producing an optically closed formation.

The light-emitting diode 20 emits light from its upper surface into the opaque diffuser 14. This produces in the volume of the diffuser a light distribution which is diffuse to a first approximation. Owing to the integrating effect of the diffuser, the intensity of said light distribution depends only slightly on the angle of emission and direction of emission of the particular light-emitting diode.

The light emerges at all the boundary surfaces of the diffuser 14, and is partially reflected and partially absorbed at the inner sides of the housing 10 and at the surface of the board 12. The light likewise emerges at the cut-out 18 and thereby illuminates the light-sensitive surface applied to the top of the photodiode 22. This produces in the photodiode a photocurrent which is a direct measure of thelight intensity generated by the light-emitting diode 20 in the diffuser 14.

Since the integrating effect, described for the light-emitting diode 20, of the diffuser 14 applies likewise for the irradiation of the light into the photodiode 22, an extensive independence from the spatial sensitivity distribution of the photodiode 22 obtains here, as well. Consequently, the entire transfer function between the current through the light-emitting diode 20, 7 the light intensity in the diffuser 14 and the photocurrent in the photodiode 22 is largely independent of the scattering of the emission or reception characteristics of the individual components, which 5 depends on the particular items.

Light also emerges from the diffuser 14 at the base surface of the cutout 24. This light is partially launched into the end face of the optical fibre 26. only a very low light intensity is achieved in the optical fibre 26, because -ofthe large losses produced thereby in launching into a very small effective aperture of the optical fibre 26.

For a given intensity of the diffuse light in the diffuser 14, this light intensity is essentially dependent on the geometrical dimensions of the overall arrangement, on the optical properties of the diffuser material, and on tive-effective aperture of the optical fibre 26. Regarded per se, each of these influences is admittedly subject to manufacturing and material tolerances, but is constant with time. Consequently, for a constructed referencelight source the entire transfer function between the current through the light-emitting diode 20, the launched light intensity in the optical fibre 26 and the photocurrent in the photodiode 22 is constant.

Figure 2 shows an instrument housing, denoted in general by 34, with a measuring chamber 36. An extension 38 of the instrument housing 34 contains a photomultiplier 40. Inserted into the measuring chamber 36 through an opening 42 in the instrument housing 34 is a tubular cell holder 44 which consists of a transparent plastic. The opening 42 and the cell holder 44 are covered in a light-proof fashion by a cover 46.

On the wall of the measuring chamber 36 opposite the photomultiplier 40, the instrument housing 34 has an opening 48 through which the other end of the optical 8 fibre 26 is inserted into the measuring chamber 36. The optical fibre 26 is plugged in this case in a second diffuser 50 which, like the first diffuser 14, is likewise produced from opaque material. The opening 48 5 is also sealed by a light-proof sealing compound 52.

The quantity of light emerging from the exit surface of the optical fibre 26 is launched into the diffuser 50. This also gives rise to losses which are, however, once again constant in time. The diffuser 50 emits approximately diffuse light into the measuring chamber 36, so that a portion of the light in turn impinges on a photocathode 54 of the photomultiplier and generates an electric measuring signal therein.

The light emitted by the diffuser 50 in the case of a reference measurement passes in part directly through the cell holder, in which no cell may be located at the instant of the reference measurement, and in part indirectly by reflection on the inner walls of the measuring chamber 36 onto the photocathode 54 of the photomultiplier 40. In order for the propagation path of the light which is emitted from the sample towards the photocathode 54 during a real-time measurement to be simulated as accurately as possible in the case of the reference measurement, an opening 56 is provided on the side of the cell holder 44 facing the diffuser 50. Consequently, the preponderant portion of the light emitted by the diffuser 50 passes through the opening 56 and through the wall of the cell holder 44 facing the photocathode 54. The propagation of the sample light is simulated very accurately in this way. It is therefore possible during reference measurement also to take account of and compensate changes in theoptical transparency of the cell holder 44, which constitute a direct error for the real-time measurement.

Figure 3 shows a combination of all the elements of the optical analytical instrument in the form of a 9 luminometer. An electronic controller 58, illustrated as a block, has the task of stabilizing the light intensity in the diffuser 14. This is achieved by virtue of the fact that the controller 58 compares the photocurrent, which is generated by the photodiode 22 as a function of the received light, with a reference variable 60 which is provided by an electronic master controller 62. The master controller 62--is generally equipped with a microprocessor and can undertake the overall control and evaluation of the luminometer. The comparison with the reference variable 60 supplies a differential signal which is used in a simple control circuit as manipulated variable for the current set by the controller 58, which flows through the light-emitting diode 20. The light-emitting diode current is set in this case such that the difference vanishes, and thus the light intensity in the diffuser is stabilized such that it corresponds to the reference variable 60.

The reference variable 60 can therefore also be used for the purpose of varying the light intensity of the reference light source and adapting it to the circumstances of the photomultiplier 40. The reference variable 60 is __ expediently generated by an analogue-to- digital converter present in the microprocessor of the master controller 62. This renders it possible to control the light intensity via a programme.

The master controller 62 controls the sequence of a measurement. For this purpose, a measurement is firstly carried out with the reference light source switched on when calibrating the instrument without a cell inserted, and the output signal of the photomultiplier 40 is measured. The master controller 62 now varies the reference variable 60 such that the photomultiplier output signal falls into a defined range of values. It is not important in this case to hit a prescribed target value precisely. Thereafter, the reference variable 60 and the associated photomultiplier output signal are stored as calibration data in a permanent memory 64.

After calibration has been performed, the measuring instrument can be used to measure actual samples. For this purpose, a measurement with the reference light source switched on is carried out at a suitable moment, for example after the instrument has been switched on, without a cell inserted, and the output signal of the photomultiplier 40 is measured. For this purpose, the reference variable 60 stored during the calibration is set. The master controller 62 now compares the photomultiplier output signal thus measured with the calibration photomultiplier signal, likewise stored during the calibration. If there is a deviation, either the sensitivity of the photomultiplier 40 or the optical transmission path has changed (for example owing to contamination of the cell holder). The master controller 62 can now determine from the difference between the measured output signal and the output signal, stored as calibration value, of the photomultiplier 40 a correction variable which corrects the final result of the actual measurements subsequently carried out with regard to the changes in sensitivity or optics which have occurred. If the deviations exceed a fixed threshold value, it is also possible to output an error message which lets it be known that owing to an impermissible state the analytical instrument is not ready for measurement and requires to be checked.

The above described calibration and measurement procedure can also be extended to the effect that it is carried out at several different intensities. For this purpose, the master controller 62 can prescribe by means of a change in the reference variable 60 a plurality of intensities for which the calibration operation described is ca rried out in each case. Thus, it is possible later, 11 for example during a self-test routine, for the instrument itself to establish whether, for example, the linearity within the measuring range, or the limits of the measuring range still correspond to prescribed 5 criteria.

The described calibration and measurement procedure can, furthermore, also be used for the purpose of establishing by means of the additional attenuation of the reference light caused by an inserted cell whether a cell had been inserted or not.

In many analytical instruments, use is made of stop arrangements between the photocathode and measuring chamber which prevent the incidence of light on the photocathode of the photomultiplier when the sealing cover is open. Such a stop arrangement can be tested very easily for correct functioning by means of the reference light source described.

12

Claims

1. An optical analytical instrument comprising a light-proof housing with a measuring chamber for holding a measurement sample, a measuring light receiver, arranged in the housing for measuring the light emanating from a sample, a reference light source with an optotransmitter for calibrating the measuring light receiver, and a control and evaluation device connected to the measuring light receiver and the reference light source, wherein the reference light source has a diffuser which receives the light of the optotransmitter and is connected to the measuring chamber via an optical conductor.

2. An analytical instrument according to Claim 1, wherein the optical conductor is an optical fibre.

3. An analytical instrument according to Claim 1 or Claim 2, wherein the optotransmitter is a light-emitting diode, and the diffuser is assigned an optoreceiver element which is connected to a control circuit which controls the brightness of the light-emitting diode as a function of the scattered light received by the optoreceiver element.

4. An analytical instrument according to Claim 3, wherein the optoreceiver element is a photodiode.

5. An analytical instrument according to Claim 3 or 4 30 wherein the diffuser is a block made from an opaque material which has cut-outs for holding the respective light-exit or light-entry end of the optotransmitter, the optoreceiver element and the optical conductor.

6. An analytical instrument according to Claim 5, wherein the block is made from a plastics material.

7. An analytical instrument according to any one of Claims 1 to 6, wherein the end of the optical conductor on the side of the measuring chamber is introduced into 5 the measuring chamber via a second diffuser.

8. An analytical instrument according to Claim 7, wherein the second diffuser is arranged at a location of the measuring chamber in aligrment with the sample 10 location.

9. An analytical instrument according to any one of Claims 1 to 8, wherein arranged in the measuring chamber, is a transparent sample holder which has a cut-out on its wall facing the second diffuser.

10. An analytical instrument substantially as hereinbefore described with reference to and as shown in the accompanying drawings.