Abstract:
The invention relates to an apparatus and method for optically analyzing samples contained in sample sites of a sample holder by means of fluorescence. The apparatus comprises a first light source comprising a plurality of individual light sources having narrow wavelength bands, means for further limiting wavelength bands of the light emitted by the individual light sources, means for guiding the reduced-wavelength light to the sample sites of the sample holder, and a detector for detecting light from the sample sites. According to the invention said means for further reducing the wavelength bands emitted by the individual light sources comprise a wavelength-tunable single monochromator. The invention allows manufacturing of a microplate reader having the capability for fluorescence measurements at a continuous wavelength range, while maintaining the cost of the device at a reasonable level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to optical sample detectors used in various fields of life and analytical sciences, in particular medical and biomedical assays. In particular, the invention relates to fluorescence-based sample analysers. 
     2. Prior Art 
     Fluorescence analysers or the present kind are commonly adapted to detect light successively from each one of a plurality of sample sites contained in the microplate. Therefore, they are frequently called as microplate “readers”. One example or such a system is the EnVision-reader and its variants available from PerkinElmer. 
     The present commercial microplate readers typically include a broad-band excitation light source. Before guiding the light emitted by the light source to a sample, the bandwidth of the light is regulated by filters, monochromators, or both. In prior art devices, high-quality dichroic filters and double monochromators are used. 
     US 2003/0081207 and U.S. Pat. No. 6,232,608 disclose devices including a plurality of optical filters and a monochromator arranged in series for selecting the desired wavelength. A disadvantage of such devices is that they require a large number of filters in order to be operable on a wide wavelength range. Therefore, if a new fluorescent marker, for example, is taken into use, also the filter bank of the device must be updated accordingly in order to get the best possible measurement results. However, the right filter having a sufficiently high quality may not always be at hand. 
     Devices having double monochromators installed in both the excitation and emission sides are disclosed in WO 00/63680 and US 2002/0109841. Double monochromators increase the wavelength-selectivity to a reasonable level but at the same time greatly increase the costs of the device. 
     WO 2005/057187 discloses a device having several different wavelength LEDs mounted to a rotatable wheel used as a moveable support, a rotatable wavelength filter wheel or moveable slide, which includes a plurality of optical filters having predefined transmit wavelengths. Therefore, the device is provided with a conventional filter bank, from which a filter having predefined transmission properties is chosen. Thus the abovementioned problem relating to using and availability of filters remain. 
     BioTek Instruments Inc. has published a Synergy™ 4 Multi-Detection Microplate Reader. The system is provided with a Xenon flash as its light source and it contains two double monochromators. 
     US 2007/0098594 discloses a fluorescence measurement apparatus where the excitation light is produced using discrete wavelengths obtainable by lasers. Thus, there is no need for wavelength filtering. A drawback of this arrangement is that the set of wavelengths which can be used for excitation is very limited. 
     Light emitting diodes have also been used in apparatuses designed for other purposes than fluorescence measurements. U.S. Pat. No. 5,475,221 discloses an apparatus having an array of LEDs and an Acousto-Optic Tunable Filter (AOTF) controlled by multiplexing means to obtain a broadband spectrometer. DE 19709377 discloses a luminescence measurement system having an ultraviolet LED as a light source. Light may be filtered before guiding it to a sample. The system allows for improved measurement of luminescence decay times. Neither of these apparatuses is suitable for fluorescence measurements. 
     SUMMARY OF THE INVENTION 
     It is an aim of the invention to achieve an improved microplate reader having the capability for fluorescence measurements at a continuous wavelength range, while maintaining the cost of the device at a reasonable level. In particular, it is an aim of the invention to provide a plate reader which has an improved optical performance in contrast to known devices in the same price category. 
     The invention is based on the idea of providing, in the excitation side of the apparatus, a first light source comprising a plurality of individual relatively narrow-band light sources, such as light emitting diodes (LEDs), having different emission bands, and a wavelength-tunable monochromator for further limiting the wavelength of the light originating from one of the narrow-band light sources conveyed to the sample. Further, the apparatus comprises means for conveying the light passing through the tunable monochromator to a sample plate or the like. 
     According to one embodiment, the narrow-band light sources are narrow-band LEDs (light emitting diodes). The LEDs may be further arranged in a multiple-wavelength module, from which one light from one LED at a time can be lead to the monochromator. The selection of the source LED may be carried out using a optical source selector separate from the LED module or by incorporating the LEDs into a combined narrow-band light source/selector module. 
     The narrow-band light sources typically have an emission band less than 100 nm, in particular 30-70 nm (at 10% intensity level). 
     According to one embodiment, the monochromator is a single monochromator, in contrast to double or higher order tandem monochromators typically used in high-performance plate readers. 
     According to one embodiment, the light directed to the sample can be individually selected to originate from one of the narrow-band light sources at a time. 
     In addition the first light source, the device may include wide-band light sources for enabling absorption measurements further to fluorescence measurements. For selecting the desired mode of operation, there may be provided an optical relay suitable for optically coupling one of more optical input lines to an optical output line or to several output lines at a time. 
     The narrow-band light source used for excitation, i.e., in the exemplary embodiment the LED used, is chosen based on the properties of the marking agent (marker) used in the sample. That is, the emission wavelength band of the source is such that it covers the absorption wavelength of the marker but does not overlap with the emission wavelength of the marker. In practice, an optimization algorithm taking into account both these criteria may be used such that the best possible signal-to-noise ratio is achieved. 
     On emission side of the device, there are means for collecting the light emitted by the sample due to the excitation, and means for detecting the intensity of the collected light. 
     According to one embodiment, a light relay is provided also on the emission side in order to allow for selecting a light detector most suitable for the intensity and/or wavelength of the emitted light and/or the goals of the measurement. Thus, a plurality of detectors may be used. 
     The emission side typically has a second monochromator for efficiently picking from the emission signal the marker emission wavelength of interest. 
     The monochromators on the excitation and on the emission sides may be diffraction grating-based adjustable monochromators. For example, Czerny-Turner monochromators are suitable for the purposes of the invention. On the excitation side it is the tuning range of the monochromator should extend over the usable bandwidth range of the narrow-band light sources. 
     Because grating-based monochromators, as is commonly known, pass through also higher diffraction order wavelengths, a filter or filters may be provided to prevent these undesired wavelengths from propagating to the sample. According to one embodiment, the apparatus automatically selects a right band-pass filter, depending on the wavelength of the narrow-band excitation light source used, for preventing any expected higher order interfering radiation from the excitation and/or the emission monochromator to pass the sample and/or the detector. However, conventional band-pass filters are in this embodiment not used for any other purpose. 
     According to one embodiment, light is produced at such a narrow wavelength band that even higher order diffraction blocking is not needed at the excitation and/or the emission side of the apparatus. 
     The invention offers significant benefits, as we have found that a very clean excitation signal can be formed at a significantly reduced cost level. The cost benefit results mainly from the fact that there is no need for second-order blocking of undesired wavelengths, and a single monochromator suffices. The quality of the excitation signal reflects to the emission (detection) side of the device as a more marker-specific emission signal. Ultimately, the throughput of the apparatus can be improved as the measurement time of a single sample well can be kept short. 
     The invention allows one to use an excitation light source which does not emit light practically at all at the emission wavelength of the marking agent. This is a significant benefit because in conventional apparatuses the excitation light source produces most of the undesired stray light hitting the sample and ultimately the detector. Thus, the combination of narrow-band light sources, such as LEDs and a single monochromator has proven to be surprisingly good as far as the amount of stray light and the costs of the device are concerned. 
     The present invention offers the benefit that the excitation wavelength can be continuously selected, in addition to the abovementioned benefits of low cost and good performance. Thus, the disadvantage of having to limit the possible excitation light at discrete wavelengths only, as when dichroic filters are used, is overcome. On the other hand, relatively inexpensive single monochromators can be used. Single monochromators provide monochromatization of the optical signal of only about 4 orders of magnitude, which is insufficient when used with broadband light sources. This is why prior art devices include additional band-limiting filters or double/tandem monochromators having two or more monochromators arranged in series. 
     In a further embodiment of the invention, the apparatus comprises
         a first light source, which is the multiple-narrow band light source described above, and at least one second light source,   a monochromator having an input to which first light source is optically connected or connectable and an output for monochromatized light,   light guiding means for guiding light originating from the first and from the at least one second light sources to the sample sites,   a detector for detecting light from the sample sites, and   a light relay comprising   a first input optically connected to the output of the monochromator,   at least one second input optically connected or connectable to a second light source, and   a first output for guiding light from selected input of the light relay to the sample sites.       

     According to one embodiment, the light relay further comprises a second output, the first and second outputs being adapted for guiding light from selected input of the light relay to the sample sites optionally from above or from below of the sample holder. 
     According to one embodiment, the light relay includes a plurality of light inputs arranged in circumferential manner to an input member and a plurality of light outputs arranged in circumferential manner on an output member. Further, the relay allows the inputs to be optically connected to the outputs one at a time, depending on the desired measurement mode, by the relative rotation of the input and output members. 
     According to one embodiment, the input member or the output member is a rotatable wheel comprising a plurality of connection slots for optic fibers. 
     According to one embodiment, the input and output members are arranged coaxially such that at least one of the inputs and one of outputs, respectively, can be optically connected to each other at a time. According to a further embodiment, a plurality of such input/output pairs can be formed at a time for providing two or more optical pathways. 
     The light source and light relay arrangement described above has the advantage that several different kinds of light sources can be conveniently incorporated into a single device without considerably increasing the amount of other optics in the device. Thus, the light relay acts as “an optical control centre” of the apparatus, guiding light to the measurement optics in a centralized manner. The measurement flexibility increases, as the optical relay may serve so as to guide light from the light sources to several measurement subunits, such as fluorescence measurement optics, absorption measurement optics, and/or to either the upper or lower side of the microplate. 
     The number of second light sources is typically 1-10, in particular 2-4. 
     By stating that two subunits of the apparatus are “optically connected or connectable”, we mean that there is provided a direct optical link between the subunits, for example, by optical fibers or through direct visual connection, or that the device comprises means for easily connecting and disconnecting said link by suitable optical means, such as pivotable or movable mirrors, optical fiber connections, prisms or the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The embodiments of the invention will be described more closely with reference to the attached drawings, in which 
         FIG. 1  represents an exemplary measurement set-up as a system-level scheme, including necessary equipment for both fluorescence and absorption measurements, 
         FIG. 2  illustrates another embodiment of the system, 
         FIG. 3  illustrates still another embodiment of the system, 
         FIG. 4  depicts a multiple-narrow-band light source according to one embodiment of the invention,  FIG. 5  illustrates in a perspective view in detail the light relay according to one embodiment of the invention, and 
         FIG. 6  illustrates exemplary spectra of commercially available LED light sources. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1 , the multiple narrow-band light source module used in fluorescence measurements is denoted with the reference numeral  102 . The module is connected by an optical fiber or fiber bundle to a first light source selector module  108 . To the first selector module  108  is also connected a wide-band light source  104 . In this example, the selector module comprises also a third input  106  for a third light source type. The light is directed to a movable mirror  112 , which is used for choosing the mode of operation. In absorbance measurements, wide-band light is conveyed to a sample site of a microtiter plate  126 . In fluorescence measurement, narrow-band light is conveyed to a monochromator  114 , from which the light is further guided to the sample site of the microtiter plate  126 . 
     For selecting whether the absorption/excitation light is directed to the sample site from above or from below, a light relay  118  is provided in the optical path between the mirror  112  and the microtiter plate  126 . 
     Accordingly, the apparatus is provided with means for collecting the transmission/emission light from the sample sites. Such means may comprise a concave light-focusing mirror  124 , as in the present example on top of the sample sites, or a lens element  132 , as in the present example below the sample sites. In fluorescence measurements, the emission light is directed to a second monochromator for preventing wavelengths outside the region of interest to be blocked. In absorption measurements, the transmitted light is not conveyed to the monochromator. A second light switch  134  may be provided in order to optically connect the transmission/monochromatized emission light to a detector  142 . Suitable optical means  140 , such as mirrors, lenses and collimators may be provided before the detector. 
     The sample holder is schematically denoted with the reference numeral  130  in the Figure. The sample holder is adapted to move two-dimensionally (on XY-track) between the excitation/emission optics such that any of the plurality of sample sites contained in a microtiter plate placed on the holder can be subjected to measurement. 
     The narrow-band light sources within the first light source may be light-emitting diodes (LEDs), the emission bands of which are less than 100 nm, typically 30-70 nm (at 10% intensity level). According to one embodiment, the emission bands of the LEDs at least slightly overlap with each others emission bands. That is, the wavelength bands of at least two of the plurality of LEDs overlap, the light intensity at the overlap wavelength being at least 10% of the peak wavelength of the dimmer of the LEDs. If two or more LEDs are arranged this way, a continuous emission light spectrum on a range having a width of at least 100 nm, typically at least 200 nm, even more than 500 nm, can be produced by choosing a right LED for light production. Together with using a single tunable monochromator, this embodiment practically allows for selection of any narrow wavelength within that range to be used for excitation of the sample. The excitation light has been found to be of high quality. That is, practically no light is directed to the sample at its emission wavelength. The excitation and emission wavelengths of typical markers used in biomedical analyses differ from each other by 10-100 nm. 
     Both ultraviolet and visible light sources can be used within the present invention either separately or in combination. According to one embodiment, the LEDs substantially cover the wavelength range 260-1000 nm, in particular 365-940 nm, typically at least 450-600 nm. Thus, the most common excitation wavelengths of marking agents can be covered. According to one embodiment, there are provided LEDs having peak wavelength at one, several or all of the following: 365 nm, 375 nm, 450 nm, 460 nm, 500 nm, 525 nm, 590 nm, 630 nm, 640 nm Typically LEDs of the “Power LED”-type are used. 
       FIG. 6  illustrates the spectrum achievable using a light source comprising a series of PHILIPS LUXEON III Star power LEDs (for further details of the characteristics of the LEDs, see Technical Datasheet DS46/Luxeon/Philips. Another usable LED series for the present device are the Nichia Power LED series, which also cover UV wavelengths (e.g. the i-LED series). 
       FIG. 2  depicts another embodiment of the present system. The apparatus comprises, as the first light source  206 , a narrow-band light source as described above. Light emitted by the first light source  206  is guidable to a monochromator  214  via optical fiber  280  for further reducing the wavelength band of the light. From the monochromator  214 , monochromatized light is guided via optical fiber  289  to a first input of a light relay  218 . 
     The input side of the light relay comprises an input member  218 A, which contains a plurality of inputs  218 A′ and the output side of the light relay comprises an output member  218 B, which contains a plurality of outputs  218 B′. Further, the input member  218 A and the output member  218 B are movable with respect to each other such that at least one of the inputs  218 A′ in aligned with at least one of the outputs  218 B′ at a time for providing and optical connection between them. Also several such connections may be formed at a time. 
     The apparatus comprises means for guiding light from the light relay  218  successively to each of the sample sites of the sample plate  226  and for detecting light individually from said sample sites is turn. For this purpose, the sample plate  226  may be movable in two dimensions for allowing measurement of all sample sites of the sample plate  226 . For this purpose, there may be provided a XY-track  230 . 
     From the output side of the light relay  218 , light is guided, depending on the measurement mode, either to upper or to lower side of the microtiter plate  226  for upper or lower sample excitation, respectively. In  FIG. 2 , the optical fiber  283  is used for upper excitation and the optical fiber  287  for lower excitation. Optical blocks  256 ,  252 , respectively, are provided for directing the light excitation light towards the sample well to be measured. They also allow emission light to be passed to a detector  242 . For this purpose, the blocks  256 ,  252  may contain a hinged or semitransparent mirror or the like optical arrangement. Lenses  258 ,  254  are typically provided between the blocks  256 ,  252  for giving the light beam hitting the sample a desired shape. 
     On emission side of the device, there are means for collecting the light emitted by the sample due to the excitation, and means for detecting the intensity of the collected light. The emission light collected from the sample by the upper of lower optical block  256 ,  252 , depending on the measurement mode, is guided by to the detector  242 . As the detector typically has only one input route, there may be provided a hinged or semitransparent mirror  244 . In this example, emission light collected from below the plate  226 , is guided via an optical fiber and reflected from the mirror  244 , which is in first position, to detection optics Emission light collected from above the plate  226  is guided directly to the detection optics. The detection optics may comprise a lens  245  and a filter bank  248  containing a plurality of emissions filters  246 . 
     As illustrated in  FIG. 2 , the apparatus may comprise also one or more second light sources  202 ,  204 ,  262  and  264 . These may include one or more wide band or monochromatic light sources, or both. The wide band light sources are arranged such that their emission light is guided to the optical relay partially along the sample optical path than light from the first light source, that is, in a light source entity comprising only unmonochromatic light sources. For this purpose, there may be provided a light source selector  208  having a turnable mirror or mirrors. Light from the wide band light sources is typically used for absorption/transmission measurements, whereby it is guided to the samples unmonochromatized. For achieving this, there is provided a first hinged mirror  212 A, which can be placed in a position that reflects light either to the monochromator (first light source used) or directly to the light relay (second light source used). Further, there may be provided a second hinged mirror  212 B, whose purpose is to guide light to the light relay or directly to the upper of lower optical block  256 ,  252 , thus by-passing both the monochromator and the light relay. 
     According to one embodiment, the apparatus comprises at least one second light source that is optically connected directly to a second input of the light relay. Such a light source may be a laser source, which can, in addition to the first light source, be used for fluorescence or ALPHASCREEN measurements. 
     According to one embodiment, the apparatus comprises at least one wide band second light source  202 ,  204 , and at least one monochromatic second light source  262 ,  264 . According to a further embodiment, the apparatus comprises both a continuous and a flashing second wide band source, such as a CERMAX unit and a Xenon flash. According to a further embodiment, the apparatus comprises both a continuous and a pulsed monochromatic second light source, such as a laser. 
     There may be provided, before the monochromator  214 , optics, such as a lens  211  and a filter bank  209 , which comprises one or more individual filters  210 . The purpose of these filters is to allow selection of more exact wavelength band of the wide band second light sources, that may be present. 
     The present embodiments allow several kinds of measurements can be carried out, including fluorescence and time-resolved fluorescence, absorption, transmission, ALPHASCREEN using one apparatus only. 
     As the light initially has relatively narrow band (100 nm at maximum measured at 10% level), we have found that a single monochromator is sufficient for producing high-quality light whose wavelength can be selected not to overlap significantly with the emission wavelength of the sample. A single monochromator has the benefit of keeping the costs of the device at lower level. Of course, double or higher order monochromators can be used too. Such monochromators may be desirable, if also light from a wide-bandwidth lamp, such as a flash lamp (e.g. Xenon) or continuous lamp (e.g. CERMAX) or the like is to be monochromatized by the monochromator by a suitable light-guiding arrangement. 
     Output of the monochromator is connected to the first input the light relay. For this purpose, there may be provided an optically transparent quartz bar or the like rigid light guide. According to one embodiment, the monochromator and the optical relay are formed as a single unit, wherein there is a direct optical visibility between the output of the monochromator and the first input of the relay. 
     The light relay comprises a first input optically connected to the output of the monochromator, and at least one second input optically connected (by direct optical fiber  284  or  285 ) or connectable (by means of the hinged mirror  212 A) to a second light source. As shown in  FIG. 2 , one or more of the outputs of the light relay are in optical connection to the sample sites for providing measurement light therein. According to one embodiment the light relay comprises two outputs being adapted for guiding light from selected input of the light relay to the sample sites optionally from above or from below of the sample plate  226 . 
     According to one embodiment, light directed to the sample can be individually selected to originate from one of the light sources at a time. According to one embodiment, the inputs  218 A′ of the light relay are arranged in circumferential manner to an input member  218 A and the outputs  218 B′ of the light relay are arranged in circumferential manner on an output member  218 B facing the input member  218 A and being rotatable with respect to the input member  218 A for achieving selection of light to be guided to the sample sites. According to a further embodiment, the input and output members  218 A,  218 B are arranged coaxially such that at least one of the inputs and one of the outputs, respectively, can be optically connected to each other at a time. The input member  218 A or the output member  218 B, typically the output member  218 B, may comprise a gearing, toothing or the like, which is connected to an electrically actuated motor, such as a stepper motor, for rotating the member into a desired position. 
     According to one embodiment, the optical relay allows the inputs  218 A′ of the light relay to be optically connected freely with any of the outputs  218 B′ of the relay one at a time, depending on the desired measurement mode. In some special measurement modes, also several outputs may be utilized at a time. 
     Light is guided to the inputs  218 A′ of the optical relay  218  and from the outputs  218 B′ of the optical relay  218  by means of optical fibres, whose ends can be aligned with each other in order to provide optical connection between a selected input and a selected output. According to one embodiment, the diameter of the input fibres is chosen to be smaller than the diameter of output fibres. Thus, all light emitted by the ends of the input fibres is collected by the output fibres. In particular, when a rotating light relay construction is used, this feature has been found to be of importance for securing lossless light transmission through the light relay  218 , because of the achievable tolerances of the rotation mechanics and overall thinness of the fibres 
     According to one embodiment, the monochromator  214  and the optical relay  218  are in firm mechanical contact with each other. That is, they are mounted in fixed position relative to each other. This allows a special kind of optical connection between the units, namely by a rigid optical fiber, such as a quartz rod. This is beneficial, as it makes the optical connection between the units very robust. Manufacturing these units as a single mechanical entity allows also control electronics of the monochromator and the light relay to be manufactured in a single control unit located in their vicinity. 
       FIG. 3  illustrates still another embodiment of the system. The reference numerals 3xx mainly correspond to those of  FIG. 1  (1xx). However, in this system, the light relay and one light detector on the emission side of the device is replaced with two individual light detectors  344  and  342 , from which the first is connected to a monochromator and the other one is not. This allows for more versatile measurements, as detectors having different properties can be used. 
     As illustrated in  FIG. 3 , the apparatus may comprise an optics module  357  placed typically on top of the sample plate and comprising an input for excitation light (from fibre  383 ) and an output for emission light (to fibre  389 ). According to one embodiment, the optics module  357  comprises an excitation lens for focusing the excitation light to an individual sample space of the sample plate and an emission lens, separate from the excitation lens, for collecting emission light from the sample space. That is, the excitation and emission light are conveyed through separate optical channels in the optics module  357 . However, there may be provided also one or more mirrors in the optics module that is/are common to both channels. In addition to focusing lenses, the channels may separately also contain mirrors and/or optical fibres. 
     As illustrated in  FIG. 3 , according to one embodiment, the output fibre of the optics module  357  is connected to an output monochromator  336  and further to the detector  344 . The detector  344  downstream the monochromator  366  may also be employed for detecting emission light excited through a second optics module  332  placed below the sample plate and collected using the first optics module  357 . On the other hand, the other detector  342  not typically coupled to a monochromator may be used to detect emission light excited through fibres  382  or  388  or in absorption measurements using, for example, the fibre  388  for transmitting broadband light to the sample through the second optics module  388  and collecting the transmitted light by a third optics module  356 . It is clear from  FIG. 3 , that the optical relay  318  is the key element, in addition to the light source and light source selecting arrangement  302 - 314 , in selecting the desired measurement mode out of the several possibilities herein described. 
     With reference to  FIG. 4 , the first light source comprises means for selecting which one of the individual light sources is optically coupled to the sample sites. According to one embodiment, the individual light sources  41 ,  42 ,  43  (optionally also more) are optically connected by means of optical fibers to a narrow-band light source selector unit  45  that provides an optical pathway between one of the optical fibres and the output  452  of the first light source. There may be one or more further input channels  44  for additional individual light sources (in addition to the three explicitly illustrated in  FIG. 4 ). The output  452  may also comprise a fiber optic connection  46  to further optics of the device (that is, typically another light source selector  47 , corresponding to the light source selectors  108 ,  208 ,  308  introduced with reference to  FIGS. 1 ,  2  and  3 , respectively). The individual light sources  41 ,  42 ,  43  may be mounted on a separate light source module having an optical fiber connector for each of the individual light sources  41 ,  42 ,  43 . 
     The narrow band light source selector may comprise a first rotating or translating element and a second static element, whereby the selection of the light source is determined by the relative positions of the first and second elements. For example, the ends of the optical fibres can be mounted circumferentially on an input wheel, which is rotatable such that one of the fibers at a time is aligned with an output fibre. Alternatively, the output fibre may be movable. Such an arrangement is illustrated in  FIG. 4 , wherein the inputs  451  and output  452  are mounted on elements, whish may be rotatable or translatable with respect to each other such that the mutual optical connection between the light sources  41 ,  42 ,  43  and the output  452  changes. 
     The plurality of light sources can also be directly mounted on a movable (typically translatable or rotatable) frame, whereby each one of the light sources can be individually optically coupled with the sample sites by movement the movable frame, for example, by aligning the light source with an output fibre, one end of which is arranged in the vicinity of the frame. 
     It has become apparent from the above that the apparatus may include a cascade of light source selectors: one in the first light source for choosing the individual narrow band light source; one downstream the first light source for selecting whether the first light source or some other light source is used; and one (the light relay) for finally choosing the desired measurement mode. It has shown that this kind of arrangement provides cost-effective manufacture, and versatile and flexible use of the device, allowing not only fluorescence measurement, but also other types of common measurements. 
       FIG. 5  shows an exemplary embodiment of the optical relay which can be used for changing the mode of operation of the device (i.e., a relay corresponding to parts  118 ,  218  and  318  of  FIGS. 1 ,  2  and  3 , respectively). The relay comprises body  53 , which comprises a plurality of inputs. Show in the figure is input  52 , to which a quartz rod  51  from the excitation monochromator is connected. On the output side there is a toothed output wheel  54 , which is rotatable. The rotation is achieved by rotating a toothed gear wheel  56 , which co-operates with the output wheel  54 . The gear wheel  56  is rotatably connected to a stepper motor  57 . A plurality of outputs  55  are arranged on the output wheel  54 . The outputs  55  are suitable for engagement with optical fibers or optical fiber connectors. 
     According to one embodiment, the detector of the apparatus is a photomultiplier tube (PMT). Other kinds of detectors may be employed too. 
     Also several detectors may be arranged in parallel and means for optically switching between the detectors may be provided. This allows for the most suitable detector for the wavelength/intensity to be used. 
     Also several detectors may be arranged in parallel and means for optically switching between the detectors may be provided. This allows for the most suitable detector for the wavelength/intensity to be used. 
     The embodiments described above and presented in the attached drawings are provided for illustrative purposes and do not limit the scope of the invention defined in the appended claims. The embodiments may be combined in order to achieve even more versatile systems. In particular, the light source arrangements, in particular, teachings concerning the structure and operation of the multiple-narrow-band light source, as well as the structure and operation of the light relay for selecting the operation mode of the device can be freely applied to each of the exemplified embodiments and also those variations not herein discussed in detail.