Measurement system and method

Abstract

An optical measurement system for a liquid sample test kit 10, comprises; one or more sample reading chambers to receive test cuvettes 100, a temperature control system, and optical instrumentation co

Description

Field of the Invention The present invention relates to an assay kit for water sample measurements, in particular, a self-contained, portable field kit for water testing or other sample testing. More specifically, the present invention relates to practical aspects of providing and operating a low-power consumption field kit, including temperature management and self-calibration. Background Kinetic bioassays are typically used in laboratory settings in which there is ready access to reagents, to devices for controlled reaction conditions (water baths or dry bath blocks), measurement systems, data processing and back-up systems, as well as provisions for reagent and sample storage under defined conditions such as temperature and humidity. A typical analysis process of a biological sample involves obtaining a sample, for instance by way of a swab, performing an isolation step via filtration or similar processes, processing the sample to ensure there is sufficient material for an analytic test (such as incubation of microorganisms, or carrying out amplification such as polymerase chain reaction to provide sufficient analyte material), and carrying out an analytic test. The processing steps may often take several hours, e.g. 18 hours or more incubation time, and the time from obtaining a sample to receiving a result may take a few days. It has been accepted in practice that samples may be sent to a laboratory by courier or routine postal services, and this may be taken as an indication of the time scales that may be involved. The present invention seeks to provide more practical alternatives to using laboratory-based bench top analytical devices. Summary of the Invention In accordance with a first aspect of the invention, there is disclosed an optical measurement system as defined in claim 1. The optical measurement system is of a type suitable for use as a liquid sample test kit and comprises one or more sample reading chambers, a temperature control system, and optical sensor instrumentation including an optical emitter and an optical sensor positioned to allow an optical measurement to be taken via an optical path through the sample reading chamber. The system is configured to apply warm-up conditions, to bring components of the one or more sample reading chambers to a target temperature for incubation, and to apply incubation conditions to the one or more sample reading chambers under the control of the temperature control system. It will be appreciated that both warm-up and incubation conditions may be controlled by the temperature control system, and that incubation conditions are applied to the same chambers that were under warm-up conditions. The warm-up conditions may be applied prior to insertion of a sample or sample container, to bring components of the sample reading chamber to a target temperature. Components of the sample reading chamber may be wall structures, optical instrumentation placed within or in proximity of the sample reading chamber, or other instrumentation such as temperature sensors. The warm-up conditions and incubation conditions may be the same conditions. For instance, the warm-up conditions may be intended for a phase before sample insertion, and the incubation conditions may be intended for a phase after sample insertion. However, in embodiments, the conditions may differ. E.g., warm-up conditions may be using a relatively higher temperature, and / or continuous heating, to more rapidly heat up the sample reading chamber to reach a target incubation temperature. Likewise, the warm-up condition may warm the sample reading chamber to a target temperature that is other than the incubation temperature or incubation temperature profile. For instance, incubation conditions may be using a relatively higher, or even a lower temperature, and / or discrete heating periods. To provide another example, incubation conditions may be intended to maintain a target incubation temperature whereas warm-up conditions maybe intended to alter a temperature of a chamber and / or components constituting the chamber and / or components sufficiently close to the chamber such that they may affect temperature control of the chamber, to create defined incubation conditions. The system is further configured to control operation of the optical instrumentation to maintain at least one of the optical emitter and the optical sensor deactivated while warm-up conditions are applied, and to activate both the optical emitter and the optical sensor for optical measurements to obtain one or more reference measurements, and to obtain one or more incubated measurements after incubation conditions have been applied for a predetermined period of incubation time. As such, the system is configured to obtain at least one reference measurement, representative of warmup conditions prior to incubation, and at least one measurement of the incubated sample, herein called “incubated measurement”, after incubation, i.e. after temperature conditions have been applied for the purpose of sample incubation. The operation of the optical instrumentation may be the same for a reference measurement and for an incubated measurement, the terms “reference” and “incubated”, in relation to a measurement, referring to a sample reading during a reference condition or after incubation conditions, respectively. In this manner, the system may expose either or both of the optical emitter and the optical sensor to warm-up conditions, and / or to incubation conditions, while either or both of the optical emitter and sensor is inactive. For optical instrumentation with temperature dependent behaviour and / or components exhibiting drift, it was found that drift-dependent performance variations can be reduced by deactivating the optical components for a period of time. Likewise, it was found that ambient temperature-dependent performance variations can be controlled by providing a controlled temperature profile in the form of warm-up conditions. A suggestion made in this disclosure is to bring optical instrumentation to a reference temperature before a sample is warmed and / or incubated. A further suggestion made in this disclosure is to maintain the optical instrumentation deactivated, during warmup and / or during incubation. In this manner, heat generation from an active optical component can be reduced and practically eliminated. It will be appreciated that the effect is stronger for optical components whose performance is stronger correlated with temperature. For instance, an emission performance, or brightness, of an LED may be temperature dependent, and may be influenced by heat from the temperature control system and by heat originating from operation of the LED itself. In contrast, the detection range of an optical sensor may be less strongly affected by temperature. In such a situation, the emitting LED may be deactivated during warmup and / or incubation prior to measurements. As such, a typical measurement protocol comprises keeping inactive either or both of the optical emitter and the optical sensor during warmup, to activate both optical emitter and optical sensor for at least one reference measurement, to deactivate either or both of the optical emitter and the optical sensor during incubation, and to active both optical emitter and optical sensor for at least one incubated measurement. The system is configured to create an output based on at least one incubated measurement and at least one reference measurement. The output may be a calibrated measurement or adjusted measurement, based on the incubated measurement and corrected or adjusted on the basis of the reference measurement. It will be understood that the sample reading chamber may be designed to receive a samplecontaining vial, such as a cuvette or other sample container, suitable for optical measurements, which allows a sample to remain in the sample reading chamber for incubation and for the duration of a measurement. To this end, the sample reading chamber may comprise or provide a socket into which a sample container may be inserted. As such, it will be appreciated that the sample can remain in a sample container for the duration of a measurement, whereas the sample reading chamber can be warmed prior to insertion of the sample container. Typical optical sample containers, such as cuvettes, usually comprise an external quadrilateral footprint and at least two opposing walls that are transparent for a particular radiation wavelength or wavelength range, e.g. UV transparent. For such a type of sample container, the sample reading chamber may comprise a quadrilateral socket for receiving a cuvette of quadrilateral footprint. Likewise, the chamber may comprise a round socket for a sample container of round or cylindrical footprint, and or more complex geometries such as rounded octagons to be suited for both quadrilateral and round sample containers. Herein, the expression “optical sensor instrumentation” includes instrumentation for visible radiation and optical radiation outside the visible spectrum, such as UV and near IR radiation, as may be used in conventional bench top systems for evaluation of a sample in a cuvette. Herein, the expression “optical sensor instrumentation” includes diodes that may be operated as light emitting diodes or light sensitive diodes. The invention is believed to enable use of a wider range of optical components, including LEDs with strong temperature-dependent behaviour and / or LEDs (or other emitters) that might create inherent heat during their operation that would otherwise negatively affect the accuracy of measurements. When a sample container is placed in the sample reading chamber, optical measurements are taken through a sample container. An optical measurement path is therefore defined between the optical source and the optical sensor. This allows a sample placed in the optical path to be interrogated via an optical measurement. The optical measurement may be in the form of an emission measurement, to determine the emission of radiation from the sample in response to optical excitation caused by the optical emitter. The optical measurement may be an attenuation or transmission measurement, to determine the optical attenuation of radiation by the sample of radiation emitted by the emitter. The optical measurement may be a nephelometric measurement indicative of the turbidity of the sample. In some embodiments, the system comprises a radiator arrangement operable under the control of the temperature control system. In such embodiments, the temperature control system is understood to comprise heating elements, such as radiator plates, arranged to transfer heat in order to control the temperature of a sample placed in the sample measurement chamber. Typical temperatures to which the sample is to be heated and at which the sample temperature is to be maintained may be in the region of between 35° C and 50° C, although other temperature ranges may be used depending on the control system. The temperature control system allows a sample measurement to be carried out while the sample temperature is kept at a predefined temperature, or within a predefined temperature range. The sample reading chamber can therefore be understood to be a temperature-controlled sample reading chamber. The arrangement allows an optical measurement to be carried out on a sample while it is located in a temperature-controlled environment, e.g., on or between radiator body portions constituted by plates. The arrangement avoids a need to transfer a warmed sample from a heater location to another location to carry out a measurement. In addition, embodiments of the present invention may allow the performance of kinetic assays while a sample temperature is modulated in a controlled manner, e.g. gradually increased from 25°C to 37°C, and / or maintained at a pre-determined temperature. It will be appreciated that a sample may be warmed from a lower temperature, e.g. from an ambient or room temperature as a starting temperature at, e.g. between 10°C to 25°C, and sometimes from frozen. As such, the time for the sample to be warmed to a measurement temperature may vary. It will be appreciated that the system is therefore configured to apply a warm-up condition. The warmup condition may be used to reach a target temperature, and may be used to warm a chamber prior to insertion of a sample container, or after insertion of a sample container. The target temperature may be an incubation temperature, i.e. a temperature at which the incubation phase of the sample is to begin. Alternatively, the target temperature may be other than the incubation temperature, e.g., a few degrees below or above the incubation temperature. The optical instrumentation may be deactivated, or switched off, for some or all of the warm-up phase. In this manner, influences of heat generated by optical instrumentation on the temperature control can be reduced, and practically avoided. Thereby, the temperature control system can be assumed to also determine the operating temperature of components of the optical instrumentation that are within range of the temperature control system. In some embodiments, the radiator arrangement comprises radiator surfaces positioned on two sides of each sample reading chamber, the radiator surfaces comprising openings between the optical emitter and the optical sensor. In such a configuration, radiator plates are positioned, with reference to the optical source, in front of the sample chamber and behind the sample chamber. It will be appreciated that a radiator plate may comprise, in that case, a passage for optical radiation. The passage may be a hole, in the radiator plate, such that the radiator plate effectively surrounds the optical path, and / or may surround components of the optical instrumentation on one or both sides of the sample reading chamber. In some embodiments, the temperature control system comprises two radiator plates positioned across the optical path. In some embodiments, the radiator surfaces are positioned across the optical path. In some embodiments, the temperature control system comprises two radiator plates positioned laterally of the optical path. In such a configuration, the radiator plates are positioned in the form of paired elements, one each laterally of the optical path of the sample reading chamber, and therefore practically laterally of a cuvette or sample located therein. The radiator plates may be provided by ribs of a radiator body in the form of a profiled block, for example a profiled metal block formed from a material with high thermal conductivity such as aluminium, copper, or alloys thereof. It is preferred that the radiator has relatively high thermal conductivity at 25°C, of no less than 100 W / mK, 125 W / mK, 150 W / mK, 175 W / mK, or 200W / mK. Aluminium alloys may have conductivity values in the region of 160 W / mK to 230 W / mK, although other materials such as copper or composite materials may be used. Aluminium is thought to be a material of suitably high conductivity properties while also being available at moderate cost. However, the invention may be embodied using other suitable materials. By using relatively low masses and high thermal conductivity of the radiator, an arrangement of one or more heater elements can be used to bring the radiator body to a desired temperature, throughout the body, in relatively short time. To provide illustrative values, the radiators may bring the sample reading chamber, and thereby a sample in a cuvette seated therein, to a temperature of about 37°C within a few minutes. Typical incubation temperatures may be 25°C, representative of room temperature, or 37°C, corresponding to a human core body temperature. Other temperatures such as 45°C or higher may be used. The radiator plates may be provided by a planar metal sheet, or by a sheet bent to provide radiator plate portions. This allows components to be used that can be obtained with relatively simple manufacturing methods. In one embodiment, the radiator plates may be provided by two metal plates extending along an array of sample chambers. This avoids the need for complex forming operations that might otherwise be required, for instance, compared to a block. The radiator plates may be provided by bending wings of a metal sheet. Multiple radiator elements may be provided by a unitary body such as a metal sheet. Radiator plates provided in this manner may be spaced apart, in profile, such that a measurement chamber is defined between radiator plates. To provide an illustrative value, a measurement chamber may be spaced to accommodate a cuvette of 12.5 mm width. The spacing may be close to 12.5 mm, e.g. no more than 12.5 mm, 13 mm, 13.5 mm, 14 mm or 14.5 mm. Alternatively, a sample reading chamber may be provided in a block, particularly by a recess in a block, e.g. a machined recess, socket, or slot in a block formed of conductive material, such as from aluminium, the block being heated under control of the temperature control system to thereby warm up and / or incubate a sample container placed in the reading chamber. It was an appreciation by the inventors that a higher conductivity material such as aluminium or copper can be used in a more energy efficient way despite the relatively faster dissipation of heat. On the one hand, using the material as radiator for smaller volumes of fluids reduces the overall amount of heat energy required to reach a desired temperature. Furthermore, the heat can be controlled and maintained with relatively quick response times, meaning the sample used in a field kit according to the invention can be warmed up to a required temperature faster. Yet another benefit appreciated during the development of the present invention is that the relatively fast energy dissipation also results in a faster cooling rate after use, improving safety aspects for practical handling and transport. Embodiments of the invention may be operated without requirement for an active cooling instrumentation, although cooling means such as a fan may be integrated with the kit. In some embodiments, the optical emitter and / or the optical sensor are positioned on or within the radiator arrangement. The sample reading chamber may be provided by a recess in a radiator body. The radiator arrangement may be shaped as a non-planar body to provide one or more receptacles for a cuvette, thereby providing a sample reading chamber. Conveniently, as the radiator body is provided in the form of pairs of ribs or plates, to be located on opposite sides of a sample. For instance, the optical emitter and / or optical sensor may be LEDs located, or at least partially inserted, in apertures, or between radiator plates. As such, the temperature of an optical component will be understood to be influenced by, e.g. rise, during warm-up conditions and incubation conditions. However, if the optical component is not active, it is assumed that the temperature of the optical component is influenced by the temperature control system and practically not influenced by heat otherwise generated by the optical component as it might otherwise be while active. In some embodiments, the system is configured to obtain at least one reference measurement before warm-up conditions are applied. In some embodiments, the system is configured to obtain at least one reference measurement while warm-up conditions are applied and before the incubation temperature is reached. In some embodiments, the system is configured to obtain at least one reference measurement after a target incubation temperature has been reached. In some embodiments, the system is configured to obtain at least one reference measurement within 1 second, 2 seconds or 3 seconds after activation of the optical instrumentation. In this manner, performance variations of the optical emitter / sensor can be measured. To provide an illustrative example, if an optical emitter is turned off during part of a warm-up phase, and a measurement is taken immediately after switching on the optical emitter, a measurement may be characteristic of the performance at 0 seconds operating time, which may be at point in time during a warm-up phase or once a target incubation temperature has been reached. The inventor found that this allows standardised conditions to be provided for measurements that may simplify calibration, or may even avoid a need for more complex calibration. Light-emitting diodes (LEDs) may exhibit performance variation depending on ambient temperature and depending on time of active operation (drift). By providing a controlled temperature profile, it was found that ambient temperaturedependent behaviour can be standardised. By controlling the time of activation relative to the incubation time, the active measurement time, and therefore drift behaviour, can be reduced. The reference measurements allow a subsequent measurement after incubation to be calibrated against an earlier measurement of the same sample. In some embodiments, the system is configured to activate the optical emitter and the optical sensor for a reference measurement and to maintain continuously active the optical emitter and the optical sensor until the incubated measurement. In some embodiments, the system is configured to activate the optical emitter and the optical sensor for a reference measurement and to deactivate at least one of the optical emitter and the optical sensor before the incubated measurement. Furthermore, the system may be configured to acquire, for use as calibration data, optical measurements obtained at different temperatures, and / or at different durations of activity of the optical instrumentation. In that case, the system may also be configured to acquire for use as calibration data a first optical measurement or first series of measurements, to apply incubation conditions under the control of the temperature control system for a predetermined period of time, and to acquire a second optical measurement or second series of measurements after the predetermined period of incubation time, wherein either the first or the second optical measurement, or series thereof, respectively, may be used as calibration data. In some embodiments, the system is configured to apply warm-up conditions under the control of the temperature control system prior to the optical measurement. Warm-up conditions may be used for preheating a sample chamber or components thereof prior to the introduction of a sample, or to preheat a sample prior to incubation, e.g. to a temperature close to incubation temperature but not suitable for inducing incubation reactions. The warm-up conditions may be applied for a predetermined period of time. Alternatively, the warm-up conditions may be applied until a target start temperature is reached. E.g., the target start temperature may be an incubation temperature or start temperature of an incubation range. E.g., if the ambient temperature is 20°C, and a desired incubation temperature is 37°C, then the temperature control system may carry out a warm-up phase, to reach the desired incubation temperature of 37°C, or 1-3 degrees below incubation temperature, before a sample is introduced. The system may provide a notification, e.g. a visual or audible notification, that the warm-up phase is concluded. Alternatively, the notification may indicate that a desired incubation temperature has been reached and is being maintained. A notification may provide a prompt for a user to introduce a sample once the sample reading chamber has reached incubation temperature. An appreciation underlying the embodiment was that the optical emitter and / or optical sensor may be located closely to the heating or radiator elements. As such, the optical elements may be exposed to the warm-up temperature and / or to the incubation temperature. In this manner, measurements may provide a reference value of the optical emitter and / or sensor operating for a predetermined period of time at a warm-up temperature and / or at an incubation temperature. In some embodiments, the system is configured to reactivate both of the optical emitter and the optical sensor after a predetermined period of inactive time. The system may deactivate and reactivate the optical emitter and / or sensor multiple times if appropriate, e.g. for a kinetic assay obtaining multiple measurements during an incubation period. In some embodiments, the temperature control system is controllable to maintain a pre-determined temperature for the duration of a measurement. In some embodiments, the temperature control system is controllable to maintain at least a predetermined minimum temperature for the duration of a measurement. As the radiator body material may be selected for its high thermal conductivity, it will be appreciated that it dissipates heat relatively quickly. Typical kinetic measurements may be carried out for a duration of several minutes, e.g. for 10 minutes, 15 minutes or sometimes longer, in the region of several hours. In order to maintain a predetermined temperature, the temperature control system may be controlled to apply heat via the radiator plates. The temperature profile versus time may be chosen according to the heat dissipation behaviour of the radiator body. The temperature control may be operated to activate the heater elements repeatedly during an incubation period and during a measurement period. The temperature control may be operated to overshoot above a target temperature that is higher than a predetermined temperature, to ensure the sample temperature does not drop below a minimum predetermined temperature. The temperature, and the rate of decrease in temperature, may be monitored by a temperature sensor to activate the heater element before the measurement chamber temperature drops below the target temperature. It will be appreciated that the temperature profile, created by operation of the heating elements and / or the radiator plates, may not necessarily be static, even if a target temperature is fixed. For instance, the temperature profile may follow a sawtooth profile around or above the predetermined temperature. In some embodiments, the portable test kit comprises a temperature sensor configured to measure a sample temperature from a sample reading chamber. The temperature sensor may be a contactless sensor such as an infrared sensor. The sensor may be a thermocouple element. A temperature sensor may be located at each one of the sample reading chambers. However, in some embodiments only one or a few sample reading chambers may be provided with a temperature sensor. The temperature sensor provides an independent measurement of the temperature in the measurement chamber, avoiding reliance on a temperature estimate provided by controls of the heater element. It will be appreciated that one or more temperature sensors may be operatively connected to a feedback control of the temperature control system. In some embodiments, the temperature sensor is arranged to obtain a measurement at an oblique angle relative to the vertical extension of the sample reading chamber. This provides a longer measurement path length relative to a path crossing a cuvette perpendicularly to the vertical axis. In some embodiments, the system comprises a processor and memory controllable by the processor, configured to store calibration data for each sample reading chamber. The calibration data may comprise one or more of an optical emitter calibration and / or and optical sensor calibration. In some embodiments, the system is configured to store calibration data for each sample measurement. In some embodiments, the system comprises a configuration allowing it to drive individual ones of the optical emitters and / or the optical sensors with different parameters. Such a configuration may be used to reduce performance variation between optical emitters and / or optical sensors, respectively. In some embodiments, the system is configured to store driver calibration values for the power source driving one or more optical emitters and / or one or more optical sensors. For instance, memory of the system may be controllable by a processor to allow driver calibration values to be updated as may be required from time to time following a calibration or re-calibration. The driver calibration values may be voltage parameters, for instance for a programmable voltage regulator, to supply a driving current required to achieve a desired optical emitter performance. As will be appreciated, a higher power (e.g. current) provided to an optical emitter may result in a stronger emission and a lower power (e.g. current) provided to an optical emitter may result in a weaker emission. In this manner, the power of each optical emitter may be adjusted and modulated to provide a more homogeneous emission from the optical emitters. In some embodiments, the system comprises a configuration allowing it to drive individual ones of the optical emitters and / or optical sensors for a predetermined measurement period followed by a period of inactivity. The predetermined measurement period may be chosen as a length of time during which the optical emitters and / or optical sensor show little, or practically no drift. In this regard, due to manufacturing tolerances and supplier variation, some components may exhibit no, or less pronounced, drift whereas other components may exhibit noticeable drift when compared against a reference. By providing a mechanism that increases resilience against drift behaviour, a wider range of components may be used in the design of a device. A portable test kit for multiple samples may process samples in turn such that the delay introduced by an inactive period may not by noticeable in practical use during the measurement of several samples. In some embodiments, at least one of the one or more reference measurements and at least one of the one or more incubated measurements are obtained for the same sample reading chamber. As such, the sample reading chamber may be measured at least a first time, for a reference measurement, for instance before or after insertion of a sample to be tested, and at least a second time, for an incubated measurement. In some embodiments, at least one of the optical emitter and the optical sensor is used as a common optical component for multiple sample reading chambers, for instance by being coupled into a light guiding system. In some embodiments, the light guiding system is provided by one or more light pipes, and or by one or more optical fibre arrangements. In this manner, a common optical emitter may be used for the interrogation of multiple sample reading chambers, and / or a common optical sensor may be used for obtaining measurements from multiple sample reading chambers. It will be appreciated that a system configuration with light guiding system using a common emitter or common sensor may require a controller to individually drive or read the respective other of the emitter and sensor, to facilitate interrogation of one chamber without interference from optical interrogation of other chambers, to allow the system to distinguish measurements from different sample reading chambers. In some embodiments, at least one of the one or more reference measurements and at least one of the one or more incubated measurements are obtained from a reference arm, wherein, optionally, the reference arm is provided by another sample reading chamber. In system configurations that permit using the same optical emitter and / or the same optical sensor for different sample reading chambers, the system may be provided with a reference arm. A reference arm may be a dedicated measurement path not used for measurement of an introduced sample. Alternatively, or in addition, a reference arm may be provided by another sample reading chamber. The calibration may be carried out using one or more measurements from a different sample reading chamber than the chamber used for a sample measurement. A reference arm may, in that case, permit calibration to account for variations in optical properties of a sample container. Such an arrangement permits the use of a wider range of sample containers, including glass tubes that may not have clearly defined optical properties. In systems configured to use a reference arm measurement, a single reference measurement may be used to provide reference data for multiple sample reading chambers. Such a setup permits introduction of one or more samples before the reference measurement, whereas the samples are located in the sample reading chambers used for the incubated measurement, and a reference (e.g., empty) reading chamber is used for the reference measurement. It will be appreciated that the different sample reading chambers and / or reference arm are temperature controlled such that their temperature-dependent behaviour is for practical purposes identical. For instance, in a practical embodiment, the two or more different sample reading chambers, or sample reading chamber and reference arm, are provided in the same block of material. In this manner, the system setup provides reassurance that the measurements from the one or more sample reading chambers are carried out under the same temperature conditions as the measurements from the reference arm. A group of measurements may comprise four measurements, i.e. a reference measurement and incubated measurement each from the sample reading chamber and from the reference arm. In some embodiments, the test kit further comprises one or more optical reference elements for calibration measurements, the reference elements removably positionable in the one or more sample reading chambers. In accordance with a second aspect of the invention, there is disclosed an optical measurement method for a system for measuring liquid samples, wherein the system comprises one or more sample reading chambers, a temperature control system, and optical sensor instrumentation including an optical emitter and an optical sensor positioned to allow an optical measurement to be taken via an optical path through the sample reading chamber, the method comprising: controlling the temperature control system to apply warm-up conditions, to bring components of the one or more sample reading chambers to a target temperature for incubation, and to apply incubation conditions to the one or more sample reading chambers, controlling operation of the optical instrumentation to maintain at least one of the optical emitter and the optical sensor deactivated while warm-up conditions are applied, and controlling operation of the optical instrumentation to activate both the optical emitter and the optical sensor for optical measurements to obtain one or more reference measurements, and to obtain one or more incubated measurements after incubation conditions have been applied for a predetermined period of incubation time; and creating an output based on at least one incubated measurement and at least one reference measurement. One or more embodiments of the second aspect may comprise steps to carry out or use features disclosed in relation to any one or more embodiments of the first aspect. For instance, embodiments of the method may comprise deactivating and reactivating the optical instrumentation before both the reference measurement and the incubated measurement. Description of the Figures Exemplary embodiments of the invention will now be described with reference to the Figures, in which: Figure 1 is an isometric view of an exemplary portable test kit in a first condition; Figure 2 is an isometric view of the Figure 1 test kit in a second condition; Figure 3 is longitudinal vertical section of the exemplary portable test kit; Figure 4 is a lateral vertical section of the exemplary portable test kit; Figure 5 is a schematic illustration of a radiator arrangement of one embodiment; Figure 6 is a schematic illustration of cuvettes arranged with a radiator arrangement; Figure 7 is a schematic illustration of a control arrangement; Figures 8 to 10 illustrate another radiator arrangement; and Figure 11 is a flow chart illustrating exemplary steps of a method. Description Referring to the Figures, Figure 1 shows a portable test kit 10 in a closed condition for transport, and Figure 2 shows the portable test kit 10 in an open condition for use. The portable test kit 10 is an example of an optical measurement system and comprises a carry case of generally cuboid form with generally rectangular footprint, comprising a body member 12 and a lid member 14 comprising a circumferential seal 16, the lid member 14 being connected to the body member 12 via a hinge 18. The circumferential seal 16 provides a fluid tight engagement when the lid member 14 is shut. The circumferential seal 16 may also improve the heat insulation of the portable test kit 10. On its outer surface, i.e. on top of the test kit 10 when the lid member 14 is shut, the test kit 10 comprises a power button 20, a control interface 22 in the form of an array of buttons, and an output field 24. The output field 24 may be provided in the form of a display screen. Both control interface 22 and output field 24 may be provided in the form of a touch screen. In that case, a single touch screen may comprise both control interface 22 and output field 24. Power and / or data connections may extend via the hinge 18 to link the body member 12 and the lid member 14. The body member 12 comprises a generally flat work surface 32 in which there is a centrally positioned sample compartment 34 for a plurality of test cuvettes 100 to be provided. The sample compartment 34 is dimensioned shorter than standard cuvettes, such that top ends of the cuvettes 100 protrude by a small amount beyond the flat work surface 32 so as to be more easily accessible for a user or transfer mechanism. The present exemplary embodiment is dimensioned for ten cuvettes 100 seated side-by-side, although other arrangements may be used to hold fewer or more cuvettes. The inside of the lid member 14 comprises an array of pads 26 located corresponding to the cuvette holder positions, to engage and abut against the tops of the cuvettes 100 when the lid member 14 is closed. The pads 26 may comprise resiliently deformable material to better hold the cuvettes in position during transport, and to maintain the cuvettes 100 immobilised during measurements and / or shaking. The engagement pads 26 may provide a fluid tight engagement to reduce the likelihood of spills. The test kit 10 may comprise a lid closure sensor to determine whether or not the lid is closed, such as a contact switch or other suitable arrangement, located for instance in the lid member 14, the body member 12, the hinge 18, or other suitable location. Data from the lid closure sensor may be used in the control of heating or analysis modules of the test kit 10, for instance to warn a user that the lid member 14 is not closed, or to prevent the activation of a shaking mechanism while the lid member 14 is not closed. Figures 3 and 4 show sections of the portable test kit 10. The same reference numerals are used throughout the figures for like elements without repeating the description thereof. Located within the sample compartment 34 is a radiator body 54 comprising on its underside a contacting surface 56 which is in thermally conducting contact with a plurality of heater elements 52a, 52b that are located underneath the radiator body 54 and operable to heat the radiator body 54. Herein, the radiator body 54 is illustrated in the form of a monolithic block comprising a plurality of (here: twenty) ribs 58, each rib 58 constituting a radiator body portion providing a radiator plate. The twenty ribs 58 are arranged in ten pairs, each pair of ribs 58 providing a radiator arrangement for a cuvette 100 to be located on the pair of ribs 58. The body member 12 contains a rechargeable battery 50 providing a power source for the components located in the test kit 10. The battery 50 is a portable power source and may be a 12V battery or of other suitably dimensioned power source. The heater elements 52a, 52b may be resistive heaters that generate heat when powered by the battery 50, and in turn by way of thermal connection warm up the radiator body 54 and thereby the ribs 58 to provide a temperature-controlled environment. The battery 50 may be chargeable via a mains power supply. The battery 50 may be removable for charging and / or may be chargeable while retained in the body member 12. A PCB 38 comprises a controller and memory for operation of the test kit 10. The test kit 10 comprises a motor 48 that is mechanically coupled to the radiator body 54 and operable to vibrate the radiator body 54. The motor 48 constitutes a shaker integrally provided with the test kit 10. The motor 48 may not be provided in every embodiment, which may be appropriate for use scenarios that do not require shaking and / or when manual shaking suffices. In embodiments comprising an integral shaker, the shaker such as the motor 48 may be operated by a controller, such as the PCB 38 or separate controller, to coordinate any vibrations with a sample-mixing and measurement sequence. This allows a sample to be optically measured after a pre-determined settling period after shaking, or immediately after shaking, or if desired during shaking. The test kit 10 comprises an optical source 40 and an optical sensor 42 at opposite sides of the sample measurement chamber. The direct line between the optical source 40 and the optical sensor 42 defines an optical path 41. The optical source 40 and the optical sensor 42 are set up such that the optical path 41 extends between two ribs 58 of the radiator body 54, such that the ribs 58 are positioned laterally on both sides and along the optical path 41. As will be appreciated, the setup allows a sample to be positioned between two ribs 58, within the optical path 41, to be measured. However, the radiator arrangement may also be provided in the form of two opposite plates crossing the optical path 41, with openings, such as holes, for the optical path 41, or as recesses in a block. A temperature sensor 44 is located directly next to the optical setup. In the present example, the temperature sensor 44 is located above the optical sensor 42 to reduce, and practically avoid, interference with the optical sensor 42. The temperature sensor 44 is, in this example, oriented to measure at an angle between the vertical axis and the horizontal axis of the sample measurement chamber, and therefore relative to a cuvette 100 seated in the sample measurement chamber, such that a sensing field 45 of the temperature sensor 44 comprises an increased path length in the sample compared to a perpendicular orientation. For instance, the angle may be at least 45, 50, 55, 60, 65, 70 or 75 degrees from the vertical axis. Likewise, the optical sensor 42 may be vertically offset (higher or lower) than the optical source 40, whereby the optical path 41 is angled relative to the vertical line, providing a longer optical path through the sample 1. However, this need not always be the case, and, as shown in Figure 4, the optical path 41 between the optical source 40 and the optical sensor 42 may be perpendicular to the cuvette seat, and thus perpendicular to the cuvette walls, so as to conform to optical standard distances when used with standard-dimension cuvettes. The test kit 10 comprises an optical source 40 and an optical sensor 42 for each cuvette location, i.e. for each pair of ribs defining a measurement chamber, allowing a plurality of cuvettes to be measured simultaneously, each measurement chamber being provided with individual optical sensor instrumentation. By way of the ribs 58, arranged in pairs for each cuvette chamber, each cuvette chamber can be temperature controlled. Likewise, individual temperature sensors 44 may, too, be provided for each cuvette location. However, it has been observed in trials that the temperature control is relatively reliable and a single thermometer may provide a sufficiently reliable measurement for an array of cuvettes. While being able to measure samples simultaneously, the system may be configured to measure samples sequentially, to avoid interference from adjacent contemporaneous measurements. The test kit 10 may be designed for micro-cuvettes or smaller sample volume cuvettes such as ultramicrocuvettes that are designed for very small volumes of fluid to be held in a cuvette reservoir, compared to a standard quadrilateral cuvette footprint. Although there are several cuvette designs, typical cuvettes have a common footprint so as to be compatible with standardised equipment. A standard cuvette has a square footprint of 12.5mm x 12.5mm, and a height of 45mm, the sample reservoir having a smaller footprint of 10mm x 10mm due to the cuvette wall thicknesses. Micro cuvettes have a narrow elongate sample reservoir extending, in the optical path dimension, from one cuvette wall to the opposite cuvette wall, to provide a standard optical measurement path length of 10mm permitted by the maximum cuvette inner wall-to-wall distance. Compared to standard cuvettes, micro cuvettes have a narrower side wall distance of 4.5mm or 2mm, respectively, providing a correspondingly smaller sample reservoir which requires a smaller sample volume while still providing a standard optical path length of 10 mm. Micro-cuvettes of this type comprise a characteristic tapered portion at the transition from a less narrow cuvette section to a micro-volume section. The centre height of the micro-volume chamber may also be standardised at 8mm from the cuvette floor or 15mm from the cuvette floor, for sub micro cuvettes, semi micro cuvettes, or ultra micro cuvettes. Likewise, while the exemplary embodiment refers to cuvettes, which are in common use, the invention is not intended to be limited to cuvettes. As such, the measurement chamber may be designed for other sample containers that have any other suitable geometry, e.g. for cylindrical vials, test tubes and the like. In the exemplary embodiment of Figure 3, the ribs 58 are shaped corresponding to the tapered portion of a micro cuvette. Two ribs 58 forming a pair are spaced apart by a distance of 3mm to 6mm that allows insertion of the narrower cuvette reservoir, such as an ultra micro cuvette reservoir, between the ribs 58 of a pair, and comprise a taper widening corresponding to the micro cuvette taper. As will be appreciated from the side view section of Figure 3, each pair of ribs 58 sits within the footprint of a standard cuvette. As such, a pair of ribs 58 is located within a footprint of 12.5mm x 12.5mm, extending into the void between the micro-cuvette outer circumference and the sample chamber walls. By using a smaller sample volume, less energy demand is placed on the temperature control system, which in turn allows a constant sample temperature to be maintained more reliably. It will be appreciated that the radiator plates in the form of ribs may be shaped differently than shown in Figure 3, according to a sample container geometry with which the design is intended to be used. Figures 5 and 6 each show a radiator body arrangement 60 in the form of two plates 62a, 62b to be positioned along the array of measurement chambers. The two plates 62a, 62b may be of identical form, or may have different form. The plates 62a and 62b each comprise an array of (here: six) apertures 64a, 64b, 64c, 64d, 64e, 64f, the apertures of the plates 62a and 62b being aligned to provide an optical path 66a, 66b, 66c, 66d, 66e, 66f. The apertures 64a-64f are illustrated as circular apertures. In embodiments, the apertures may have a different shape, and / or may be provided as recesses extending from a free edge into the body of the plates 62a, 62b. The apertures 64a-64f have a diameter of at least .0.5 mm, 1.0 mm, 1.5 mm, 2 mm, 2.5 mm, or 3 mm, to provide a sufficiently sized aperture for the optical beam. The two plates 62a, 62b may be spaced apart at least 12.5 mm, 13 mm, 13.5 mm, yet no more than 15 mm, 14.5 mm, 14 mm, 13.5 mm, or 13 mm, allowing cuvettes 100a to be positioned between and closely to the plates 62a, 62b. If the plates 62a, 62b are provided as separate components, it will be appreciated that each plate may be heated by a separate heater in thermal connection with a plate. Heater elements may be located laterally of the plates 62a, 62b, underneath, or at a suitable position. Alternatively, the plates 62a, 62b may be provided as elements of a unitary component, or may be connected by a connecting body portion to provide a unitary radiator body. The connecting body portion may provide heat transfer from underneath a sample. While the apertures 64a-f of one side will be understood to be aligned with the opposite apertures 66a-f, the apertures may have different sizes. To provide an example, depending on the type of optical emitter and optical sensor, one of the apertures may have a diameter of 0.6 mm (+ / - 0.1 mm) and the other aperture may have a diameter of 2 mm. As such, along an optical path, one aperture may be smaller than another aperture. The apertures may have a narrowing cross-section, for instance a conical cross-section, with a smaller opening on the sample-facing (inner) side and a larger opening on the outer side, to accommodate optical components such as LEDs or ends of a light guide. In practice, an optical emitter or optical sensor may be placed closely to the apertures and / or within a recess of a radiator body. As such, it will be appreciated that the optical emitter and / or optical sensor may be exposed to temperature emitted by the radiator body during its operation. Figures 8 to 10 illustrate a radiator body arrangement 90 in the form of a non-planar body formed from a sheet 92 comprising a central body 95 and laterally extending wings 96a, 96b arranged in pairs. Each wing 96a comprises an aperture 94a and each wing 96b comprises an aperture 94b. By folding the wings 96a, 96b perpendicular to the plane of the central body 95, the wings 96a, 96b provide opposing wall portions. The apertures 94a, 94b of a pair are aligned to provide an optical path. In line with the description for the apertures 64a-f and 66a-f, the apertures 94a, 94b may have a contour other than round, e.g. may be slots or cutouts, and they may have different sizes for the optical emitter and the optical sensor, and / or may have a different cross-section on the sample-facing side. A cuvette 100c may be inserted in the space between two wings 96a, 96b. While the wings 96a, 96b are shown spaced apart from their respective adjacent wings, this need not be the case in all embodiments, instead wings may be separated by slots. In another variant, the opposing wall portions are not separated, and the radiator body arrangement is provided in the form of a U-profile with apertures for the optical path. The radiator body arrangement may in that case comprise several recesses and apertures to provide a regular lattice structure or irregular lattice structure to reduce the overall mass of the radiator body arrangement. In the above radiator body arrangements, the opposing wall portions may be practically parallel to each other, however it will be understood that they may be folded at a non-parallel angle, for instance if this is desired as an insertion taper to facilitate insertion of a sample vial. The sheet thickness of the sheets 62, 92 may be at least a millimetre. The sheet thickness may be no more than 4, 3, 2, or 1.5 millimetres. The sheet thickness is chosen such that the radiator body is sufficiently sturdy to avoid deformation during normal use, while being sufficiently thin to reduce its overall mass and heat retention properties. Parts of the sheets 62, 92 may be provided with reinforcing rib structures or gusset structures. Such reinforcing structures allow a relatively thinner sheet thickness, and thereby lower mass, to be used than a comparable sheet without reinforcing structures. The radiator body arrangements shaped from sheet material enable economic manufacturing. The design tolerates that a sample is not necessarily heated from all sides. Rather, a sample is heated from two opposing sides, and may also be heated from underneath. In initial trials, the design allowed a sufficient heating performance for small sample volumes. As a variant, the opposing wall portions may be of arcuate profile, for instance the wings 96a, 96b may be provided in curved form, following a forming operation carried out on a flat sheet, so as to better conform to the contour of a round vial or test tube, e.g. to partially surround a cylindrical sample container. The radiator bodies described in Figures 5 to 6 and 8 to 10 may be used in place of the radiator body 54 illustrated in Figures 3 and 4. It will be understood that the radiator bodies may be made from the material described in relation to the radiator body 54 and that they may be heated using heaters in the above-described manner. As an alternative to folded sheet bodies, the radiator arrangement may use blocks, e.g., monolithic blocks or a plurality of radiator block bodies, formed with sample reading chambers and apertures for the optical path. For instance, the radiator body may be a monolithic block into which sample reading chamber and optical path apertures have been machined or drilled. With reference to Figures 3 and 4, the test kit 10 comprises a controller for controlling the temperature of the sample reading chamber and, when provide, the sample, as measured by the temperature sensor 44. The controller may be located on a printed circuit board (PCB) 38. The temperature control may be considered a closed-loop feedback control. The controller operates the heater elements 52a, 52b to heat the radiator body 54 to maintain its temperature above a target temperature. The temperature controller may take into account the relatively quick heat dissipation from the radiator body 54. As such, the controller may comprise a configuration allowing it to operate the heater elements 52a, 52b to repeatedly warm up the radiator body 54 to overshoot the target temperature, in the manner of a sawtooth profile or similar profile (when considering a graph showing temperature versus time). The repeated warming up may be carried out multiple times during an incubation interval, e.g. for the duration of an incubation phase and / or for the duration of an optical measurement. The overshoot range may be a small amount above the target temperature, so that the temperature may be considered to be constant for practical purposes. For instance, the temperature may fluctuate less than 10°C, 9°C, 8°C, 7°C, 6°C, 5°C, 4°C, or 3°C. Using an appropriately designed radiator body, a target temperature can be reached and maintained throughout the radiator body 54 relatively quickly. The target temperature may be reached in a warm-up phase. For instance, a warm-up phase for increasing the radiator body temperature from ambient temperature of about 20°C to an incubation temperature of 37°C or 45°C may take in the region of a few minutes. The high thermal conductivity of the radiator body 54 helps to ensure an even heat distribution also at the ribs 58. The controller is operative to maintain a target temperature as a warmup temperature and / or as an incubation temperature for a predetermined period of time, e.g. for 10, 15 or 20 minutes at a practically constant level, or within a pre-defined range. The period of time during which the incubation temperature is kept at a practically constant level is understood to be an incubation period. During the incubation period, the optical measurement system may carry out optical measurements by controlling the optical source 40 to emit radiation in a controlled manner, e.g. at a predetermined wavelength, intensity, and sampling frequency (number of times a measurement is made). Likewise, the optical measurement system may control the optical sensor 42 to read an optical sensor response. The optical measurements may be made continuously or in discrete intervals in order to obtain an optical response curve over time. However, the optical measurements are not necessarily carried out throughout the incubation period, and may, depending on the type of assay, be carried out before, during and / or at the end of an incubation period. In addition, optical measurement may be carried out throughout a warm-up period, e.g. before, during and / or after the warm-up period. It is an advantage of providing optical instrumentation integrated with a temperature control element that measurements can be carried out without the need to transfer a sample to an optical measurement station. Alternatively, or in addition, the system may be configured to accept, as an input, calibration data for the optical emitter, the optical sensor, or both the optical emitter and the optical sensor. The calibration data may be applied to an output of optical measurements to provide a calibrated output. Such a calibration arrangement allows the system to reduce the effect of performance variation between different optical components. To provide an illustrative example, some types of photodiode or LED elements may exhibit varying performance for several reasons. Depending on use conditions, variations in performance of 30% and more have been observed in prototypes. In addition, photodiodes and LEDs may exhibit a temperature-dependent performance variation and may exhibit drift, i.e. a variation in performance over their active time. Drift behaviour can be specific to individual LEDs or batches, and may be practically negligible, or may amount to a few percent difference, e.g. about 6% difference over time. To mitigate against such manufacturing tolerances, the LEDs may be calibrated using a reference. The reference may be a reference sample. To this end, the system may comprise a configuration allowing each optical emitter and / or each optical sensor to be driven independently, e.g. with a different power level or current level. In this manner, the emission strength of each optical emitter may be modulated, e.g. increased by driving it with higher current, or decreased by driving it with lower current. The current supply to each optical emitter may be set depending on the unadjusted performance of the optical emitter, whereby each optical emitter and / or optical sensor may be operated at a similar or practically the same level. Preferably, the power source and / or voltage regulation is selected such that it provides output regulation (e.g. current) in a linear range. This simplifies the calibration by avoiding non-linear effects. The calibration may be carried out such that a threshold difference is not exceeded between the lowest emission power and the highest emission power. The threshold difference may be 30%, meaning that the lowest emission power is no less than 30% of the highest emission power. Alternative threshold difference values may be 25%, 20%, 15%, 10%, or 5%. Figure 7 illustrates schematically an exemplary arrangement for driving an LED to provide a calibrated output. In Figure 7, a voltage source 80 is provided to provide current to an LED 40a constituting an optical emitter, which may be used to emit light through a test sample 88 to be sensed by a photo diode 40b constituting an optical sensor. The arrangement includes a voltage regulator 82 and a first feedback loop 84 to allow the voltage regulator 82 to modulate the current suppled to the LED 40a to thereby modulate its emission power. The arrangement includes a second feedback loop 86 to allow the voltage regulator 82 to modulate the current relative to the performance of the photodiode 40b. For an array of paired optical emitters and sensors, the calibration may be repeated for each one in the array to minimise variation between the optical emitters and / or sensors to remain within a predetermined threshold. As will be appreciated, other arrangements may be used to modulate the optical emission and sensors. In some prototypes, the use of a programmable voltage regulator has been found to allow performance variation between different LEDs and photodiodes to be reduced to less than 20%, and even less than 10%, compared to unmodulated performance. To mitigate against drift behaviour, the optical emitters may be controlled to be emitting for a predetermined measurement period after an inactive period, to practically “reset” drift behaviour. The predetermined measurement period may be a length of time with no, or only negligible, drift behaviour. The inactive period is a period of time allowing the optical emitter to effectively settle, to return to a pre-drift performance it is expected to have when turned on from an off-state. To provide an illustrative example, an optical emitter may be active for a predetermined measurement period of no more than 0.5, 1, 1.5, 2, 2.5, 3, 4 or 5 seconds, and may be inactive for a period of no less than 0.5, 1, 1.5, 2, 2.5, 3, 4, or 5 seconds. Preferably, the predetermined measurement period is sufficiently long for a full reading cycle, e.g. this may be about one second. It is also preferred that the inactive period is at least about twice the length of an active measurement period. However, it will be appreciated that the precise durations may depend on sample type, reagents and test types in use, whether the measurement is static or carried out to monitor reaction kinetics, and other factors. An appreciation underlying embodiments of the invention was that performance variation due to ambient temperature may be corrected by warming the optical emitter and / or optical sensor to incubation temperature, while turned off. In that case, a measurement with a just activated LED may be dependent on the LED temperature, but will be less influenced by drift. In this manner, the temperature conditions affecting temperature-dependent behaviour can be standardised. Likewise, maintaining optical components inactive was found to reduce influences on the temperature profile that may otherwise be observed as a result from heat generation by active optical components. Figure 11 shows a method 110 of obtaining a sample measurement, calibrated for temperaturedependent and / or drift-dependent performance variations. The method 110 may be used as alternative to, or in combination, with other calibration methods such as LED driving power calibration and / or calibration using reference specimens. In step 112, the temperature control arrangement is used to bring the temperature to a predetermined reference temperature, such as 37°C. Step 112 may be considered a warm-up phase. Depending on the temperature control mechanism, the temperature may be kept within a range such as +1- 3°C, +1- 2°C, or +1- 1°C of the reference temperature. In step 114, at least one of, or both of, the optical emitter and / or optical sensor is maintained inactive. For instance, the optical emitter and / or optical sensor may be switched off after a previous measurement. In this manner, the likelihood of heat emission and / or drift arising from a continued active period is reduced. It will be appreciated that step 114 may be carried out before, during, or concurrently with step 112. In step 116, a sample is placed in the sample compartment. It will be understood that the sample may be held in a sample container such as a test vial or cuvette or other suitable sample measurement container. The sample is the sample to be measured, but is at this stage an unincubated sample. For a typical water matrix sample, the liquid is typically expected to be transparent, and may be expected to be colourless and transparent. In step 118, both of the optical emitter and optical sensor are activated, or reactivated, respectively, i.e. switched on. The optical emitter and / or optical sensor may be switched on after a warm-up period. In a variation of the method 110, step 118 is carried out before or simultaneously with step 116, e.g., the optical emitter and sensor are activated before, e.g. briefly before, the sample is placed in the sample compartment. The activation of the optical emitter and / or optical sensor may be carried out in response to a manual activation by a user. Alternatively, or in addition, the activation of the optical emitter may be triggered by a switch, e.g., by a switch system activated by placement of a sample container in the sample reading chamber. In step 120, the sample is incubated, e.g., by using the temperature control arrangement to maintain an incubation temperature for the duration of an incubation period. The incubation temperature may be the same as the warm-up target temperature set in step 112. The incubation temperature may be lower or higher than the warm-up target temperature set in step 112. The incubation period may be at least 5, 10, or 15 minutes. In some embodiments, the incubation period may no more than 15 minutes, 20 minutes, 25 minutes, 30 minutes, 35 minutes, 40 minutes, or 45 minutes. Depending on the presence of a target analytic material and type of test reagent being used, the sample may change its appearance, e.g. change colour and / or turbidity. In step 122, one or more reference measurements are obtained at different stages before or during the incubation of the sample. For instance, a reference measurement may be taken just after the sample is placed into the sample reading chamber, practically prior to incubation. To provide another example, a reference measurement may be taken after about 50% of an expected incubation period, e.g. after 10 minutes if the incubation period is 20 minutes. The reference measurement is an optical measurement using the optical emitter and the optical sensor for a given sample compartment. It will be appreciated that the optical emitter and optical sensor are at this stage operating at incubation temperature conditions. However, their active time, and therefore heat emission and / or drift behaviour, may be correlated with the start of the incubation period. The reference measurements may be considered indicative of the performance of the optical emitter and / or optical sensor at the incubation temperature, after a known period of activity. It will be appreciated that step 122 may be carried out multiple times during the incubation step 120. In practice, it was found that a single measurement, e.g. just at the beginning of the incubation period, provides a suitable reference measurement. However, the method may comprise taking more than one reference measurements. In variations of the method 110, further reference measurements are obtained from a reference arm. The reference arm may be a dedicated reference arm or another sample reading chamber. The reference measurements may be obtained as a parallel reference measurement and / or a parallel incubated measurement. In an optional step 124, at least one of, or both of, the optical emitter and / or optical sensor is deactivated, again, prior to another measurement. For instance, the optical emitter and / or optical sensor may be switched off after a previous measurement. In this manner, the likelihood of heat emission and / or drift arising from a continued active period is reduced. After step 124, the method may return to step 118 to reactive both emitter and sensor for another measurement. In step 126, once the incubation period is completed, an incubated measurement is obtained from the incubated sample. It will be appreciated that the incubated sample may have changed its appearance, e.g. colour or turbidity. In step 128, a calculation is applied to calibrate the post-incubation measurement from step 126 with one or more reference measurements from step 122. The calculation may be a simple baseline subtraction or other suitable calculation. The calculation may apply a weighting factor, e.g. depending on incubation time, extent of colour change or otherwise. As will be appreciated, the exact manner of calculation is not necessarily essential to all embodiments. However, the method 110 avoids reliance on a separate reference sample by using a measurement from the sample-to-be measured as reference for the post-incubation measurement of the same sample, measured from the same sample chamber. The method 110 was found to allow measurements to be standardised taking into account performance variation of different LEDs in the same system 10. The method 110 may be used for samples, as well as for blind comparison (e.g., a reference bioassay carried out without active test material) and reference comparison (e.g., a reference bioassay carried out with a known test material). It was found that the method 110 allows the same blind comparison and reference comparison to be used for multiple contemporaneously measured samples, despite such samples being measured with LEDs that may have different drift characteristics and / or different temperaturedependent behaviour. The test kit 10 may comprise an internal memory to store sample data and measurement data, including reference data and calibration data. The test kit 10 may comprise a communications link, which may be wired (such as a USB port) or wireless to communicate measurement data to a local computer workstation, laptop, and / or a remote processing centre. The remote processing centre may be a cloud-based processing system. The data processing system may be provided on a handheld portable device, such as a mobile telephone or tablet computer. The optical measurement system described above may be used for samples undergoing a kinetic bioassay in which a biological reaction such as an enzyme reaction influences optical measurements differently according to the progress of the reaction over time. The bioassay may be of the type used for the detection of analytes characteristic for certain microorganisms. Exemplary analytic tests are disclosed in the United Kingdom Patent Publication GB2571613 by the present applicant. For instance, the test may be a test detecting the presence of lipopolysaccharide (LPS) characteristic of bacterial matrix. The detection method may be a bacterial endotoxin test such as a Limulus Amebocyte Lysate (LAL) or Tachypleus Amebocyte Lysate (TAL) test. Other detection methods detecting the presence of, for example, lipoteichoic acid or peptidoglycan may be used, if desired, or beta glucan-based techniques may be used to detect the presence of fungal organisms. It will be appreciated, therefore, that the testing method may be used in the detection of either Gram-negative or Gram-positive bacteria or in the detection of fungi. In the context of such analytic tests, it will be understood that “incubation” relates to the application of conditions promoting a kinetic reaction such as a change of colour in the presence of a reagent. By arranging the test parameters such that small volumes of sample fluid are used together with a heat-dissipating radiator body, surprisingly this leads to a more economical energy usage than would conventionally be expected. The arrangements disclosed herein allow a relatively large number of samples to be incubated outside a laboratory. Furthermore, the arrangement allows multiple (e.g., ten) samples to be measured contemporaneously, or sequentially in one measurement cycle assessing samples in different measurement chambers. The test kit may be designed as a kit of parts comprising all reagents, kinetic solutions and / or stop solutions required to carry out the tests of interest. The test kit can be designed as a liquid and splash proof kit to reduce the likelihood of contamination. The test kit may be provided with calibration elements such as slides or cuvettes with designed properties, e.g. reference colours, to allow calibration of the individual sample measurement chambers, optical sources and / or optical sensors. Calibration values may be stored in a memory of the system to provide a correction taking into account the performance and any drift behaviour of the optical sources and the optical sensors. Correction values may be specific for the optical sensor instrumentation of a chamber. In a simple form, the measurement kit may be provided with a single chamber, or two or three chambers, for instance in the form of a “pocket” device. A further advantage of embodiments disclosed herein is that calibration of the optical instrumentation may be carried out while the temperature control arrangement is brought to and / or while it maintains an incubation temperature, as measured by a separate temperature sensor, to take into account temperature influences on drift or other performance parameters. In some embodiments, the system may store calibration values indicative of the optical performance of the optical instrumentation at different temperatures. It will be appreciated that the description hereinbefore is exemplary in accordance with embodiments the invention and that a wide range of modifications and alterations may be made thereto without departing from the scope of the invention as defined by the appended claims.

Claims

1. An optical measurement system for a liquid sample test kit, the system comprising one or more sample reading chambers, a temperature control system, and optical sensor instrumentation including an optical emitter and an optical sensor positioned to allow an optical measurement to be taken via an optical path through the sample reading chamber,wherein the system is configured to apply warm-up conditions, to bring components of the one or more sample reading chambers to a target temperature for incubation, and to apply incubation conditions to the one or more sample reading chambers under the control of the temperature control system,wherein the system is further configured to control operation of the optical instrumentation to maintain at least one of the optical emitter and the optical sensor deactivated while warm-up conditions are applied, and to activate both the optical emitter and the optical sensor for optical measurements to obtain one or more reference measurements, and to obtain one or more incubated measurements after incubation conditions have been applied for a predetermined period of incubation time;wherein the system is configured to create an output based on at least one incubated measurement and at least one reference measurement.

2. The system according to claim 1, wherein the system comprises a radiator arrangement operable under the control of the temperature control system.

3. The system according to claim 2, wherein the radiator arrangement comprises radiator surfaces positioned on two sides of each sample reading chamber, the radiator surfaces comprising openings between the optical emitter and the optical sensor, wherein, optionally, the radiator surfaces are positioned across the optical path.

4. The system according to claim 2 or 3, wherein the optical emitter and / or the optical sensor are positioned on or within the radiator arrangement.

5. The system according to any one of the preceding claims, configured to obtain at least one reference measurement before warm-up conditions are applied.

6. The system according to any one of the preceding claims, configured to obtain at least one reference measurement while warm-up conditions are applied and before the incubation temperature is reached.

7. The system according to any one of the preceding claims, configured to obtain at least one reference measurement after a target incubation temperature has been reached.

8. The system according to any one of the preceding claims, configured to obtain at least one reference measurement within 10 seconds, 20 seconds or 30 seconds after activation of the optical instrumentation.

9. The system according to any one of the preceding claims, configured to activate the optical emitter and the optical sensor for a reference measurement and to maintain continuously active the optical emitter and the optical sensor until the incubated measurement.

10. The system according to any one of claims 1 to 8, configured to activate the optical emitter and the optical sensor for a reference measurement and to deactivate at least one of the optical emitter and the optical sensor for at least some time during which incubation conditions are applied, before reactivating the optical emitter and optical sensor for the incubated measurement.

11. The system according to claim 10, configured to reactivate both of the optical emitter and the optical sensor after a predetermined period of inactive time.

12. The system according to any one of the preceding claims, wherein the temperature control system is controllable to maintain a pre-determined temperature for the duration of a measurement.

13. The system according to any one of the preceding claims, wherein the temperature control system is controllable to maintain at least a pre-determined minimum temperature for the duration of a measurement.

14. The system according to any one of the preceding claims, comprising a temperature sensor configured to measure a sample temperature from a sample reading chamber.

15. The system according to claim 14, wherein the sample reading chamber has a vertical extension and wherein the temperature sensor is arranged to obtain a measurement at an oblique angle relative to the vertical extension of the sample reading chamber.

16. The system according to any one of the preceding claims, comprising a processor and memory controllable by the processor, configured to store calibration data for each sample reading chamber.

17. The system according to claim 16, configured to store calibration data for each sample measurement.

18. The system according to any one of the preceding claims, comprising a configuration allowing it to drive individual ones of the optical emitters and / or the optical sensors with different parameters.

19. The system according to claim 18, configured to store driver calibration values for the power source driving one or more optical emitters and / or one or more optical sensors.

20. The system according to any one of the preceding claims, comprising a configuration allowing it to drive individual ones of the optical emitters and / or the optical sensors for a predetermined measurement period followed by a period of inactivity.

21. The system according to any one of the preceding claims, wherein at least one of the one or more reference measurements and at least one of the one or more incubated measurements are obtained for the same sample reading chamber.

22. The system according to any one of the preceding claims, wherein at least one of the optical emitter and the optical sensor is used as a common optical component for multiple sample reading chambers, for instance by being coupled into a light guiding system, wherein, optionally, the light guiding system is provided by one of a light pipe or an optical fibre arrangement.

23. The system according to any one of the preceding claims, wherein at least one of the one or more reference measurements and at least one of the one or more incubated measurements are obtained from a reference arm, wherein, optionally, the reference arm is provided by another sample reading chamber.

24. The system according to any one of the preceding claims, comprising one or more optical reference elements for calibration measurements, the reference elements removably positionable in the one or more sample reading chambers.

25. An optical measurement method for a system for measuring liquid samples, wherein the system comprises one or more sample reading chambers, a temperature control system, and optical sensor instrumentation including an optical emitter and an optical sensor positioned to allow an optical measurement to be taken via an optical path through the sample reading chamber, the method comprisingcontrolling the temperature control system to apply warm-up conditions, to bring components of the one or more sample reading chambers to a target temperature for incubation, and to apply incubation conditions to the one or more sample reading chambers,controlling operation of the optical instrumentation to maintain at least one of the optical emitter and the optical sensor deactivated while warm-up conditions are applied, andcontrolling operation of the optical instrumentation to activate both the optical emitter and the optical sensor for optical measurements to obtain one or more reference measurements, and to obtain one or more incubated measurements after incubation conditions have been applied for a predetermined period of incubation time; andcreating an output based on at least one incubated measurement and at least one reference measurement.

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