Abstract:
The present invention relates generally to the field of biochemical laboratory. More particularly the invention relates to the improved and more efficient instrumental features of equipment used as e.g. fluorometers, photometers and luminometers. The object of the invention is achieved by providing an optical measurement instrument where there is an interface ( 218, 223, 233   a,    233   b   , 238 ) for a changeable optical module ( 240 ), the interface being adapted for at least one excitation beam and at least two emission beams. This allows performing various types of measurements by changing an optical module. The change of module and related parameters can be performed automatically controlled by software. It is also possible to easily upgrade the instrument for new types of measurements by just providing the instrument with a new optical module and the related software.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of biochemical laboratory instrumentation for different applications of measuring properties of samples on e.g. microtitration plates and corresponding sample supports. More particularly the invention relates to the improved and more efficient instrumental features of equipment used as e.g. fluorometers, photometers and luminometers. 
     The routine work and also the research work in analytical biochemical laboratories and in clinical laboratories are often based on different tags or labels coupled on macromolecules under inspection. The typical labels used are different radioactive isotopes, enzymes, different fluorescent molecules and e.g. fluorescent chelates of rare earth metals. 
     The detection of enzyme labels can be performed by utilizing its natural biochemical function, i.e. to alter the physical properties of molecules. In enzyme immunoassays colourless substances are catalysed by enzyme to colourful substances or non-fluorescent substances to fluorescent substances. 
     The colourful substances are measured with an absorption, i.e. photometric measurement. In the photometric measurement the intensity of filtered and stabilized beam is first measured without any sample and then the sample inside one plate is measured. The absorbance i.e. the absorption values are then calculated. 
     The fluorescent measurement is generally used for measuring quantities of fluorescent label substance in a sample. The most photoluminescence labels are based on molecular photoluminescence process. In this process optical radiation is absorbed by the ground state of a molecule. Due to the absorption of energy the quantum molecule rises into higher excited state. After the fast vibrational relaxation the molecule returns back to its ground state and the excess energy is released as an optical quantum. Due to losses in this process the average absorbed energies are higher than the average emitted energies. 
     A further measurement method is chemiluminescence measurement where emission of a substance is measured from a sample without excitation by illumination. Thus any photoluminometer can also be used as a chemiluminometer. 
     The typical instruments in analytical chemical research laboratories are the different spectroscopic instruments. Many of them are utilizing optical region of electromagnetic spectrum. The two common type of instruments are the spectrophotometers and the spectrofluorometers. These instruments comprise usually one or two wavelength dispersion devices, like monochromators. The dispersion devices make them capable to perform photometric and fluorometric measurements throughout the optical spectrum. 
     FIG. 1 illustrates an advanced prior art optical analyser, especially the optical components and the different optical paths. The instrument has two illumination sources, a continuous wave lamp (cw-lamp)  112   a  and a pulse lamp  112   b . The cw-lamp can be used for continuous wave photoluminescence excitation and for absorption measurements. 
     Infrared part of radiation from the cw-lamp  112   a  is absorbed by a filter  104 , and after transiting a stray-light aperture plate  105 , the optical radiation is collimated with a lens  115   a  through an interference filter  114   a  located in a filter wheel  114 . 
     The light beam is focused with a lens  113   a , similar to the lens  114   a , into a light guide  118 , which isolates the measuring head thermally and mechanically. It also shields the measuring unit for the stray light from the cw-lamp. The optical radiation from an output aperture plate  106  of a light guide  118  is collimated with a lens  107 , similar to the lens  115   a . The radiation beam is reflected by a beam-splitter mirror  141  inside a mirror block  140 , and passed through a sample well  181  and through an entrance window  122  of a photometric detector unit  132 . 
     The mirror block  140  is located on the upper side of the sample. Its function is to reflect the horizontal light beam from the selected lamp downwards to the sample and to reflect a portion of this beam by a mirror  143  into a reference photodiode  119 , and also to allow the emission from the sample to travel upwards to the detector  132 . 
     The emission unit comprises optical components, which are lenses  133 ,  135 , a filter  134   a  in filter slide  134 , a combined shutter and aperture slide  136  and a detector  132 , such as a photo-multiplier. The detector  132  is used in the fast photon counting mode where the pulses from photo-multiplier anode are first amplified and then fed through a fast comparator  191  and gate  192  counter  193 . The comparator rejects the pulses, which are lower than the pre adjusted reference level. The fast counting electronics is equipped with a gate in the front of the counter. This gate is used in overall timings of the measurements. 
     The pulse-lamp unit is used in time-resolved photoluminescence measurement for long-living photoluminescence emission. It comprises a second lamp  112   b , lenses  115   b ,  113   b , and optical filters  114   b  in a filter slide for wavelength isolation. When this second lamp is used the mirror  141  must be rotated by 90 degrees in order to reflect the radiation to the sample. This can be achieved by using different optical modules for the two lamps. 
     There are certain limitations related to the prior art technology. It is often required to make several measurements from same samples, e.g. measuring of two or more photoluminescence emissions, as well as absorption and chemiluminescence measurements may be required. With the prior art instruments it is necessary to make the different measurements successively, and it may be necessary to make changes in the optics of the instrument between the different measurements. Therefore performing such measurements from a large number of samples tends to take a very long measurement time with the prior art instruments, and the reliability of the measurement results is not optimal. 
     There are also instruments, which have two measurement heads; a top measurement head and a bottom measurement head. Such instruments are disclosed e.g. in documents U.S. Pat. No. 6,187,267 and U.S. Pat No. 5,933,232. With this kind of instrument it is possible to make measurements also from below the sample, so this kind of instrument is more versatile for performing different measurements. However, the prior art instruments are not capable of performing different measurements simultaneously, nor capable of performing dual emission measurements. Performing different measurements successively from a large number of samples tends to take a long time. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an optical instrument for laboratory measurements, wherein the described disadvantages of the prior art are avoided or reduced. The object of the invention is therefore to achieve a measurement instrument with improved efficiency for performing measurements from samples. 
     The object of the invention is achieved by providing an optical measurement instrument where there is an interface for a changeable optical module, the interface being adapted for at least one excitation beam and at least two emission beams. The object is further achieved by a changeable optical module for a measurement instrument, the module comprising a preferably dichroic mirror for dividing an emission beam into two emission beams, and a preferably dichroic mirror for separating the optical paths of emission and excitation beams. The invention allows performing various types of measurements by changing an optical module. The change of module and related parameters can be performed automatically controlled by software. It is also possible to easily upgrade the instrument for new types of measurements by just providing the instrument with a new optical module and the related software. 
     An optical measurement instrument according to the invention for measuring samples, comprising an illumination source for forming an excitation beam, a first detector for detecting a first emission beam, an interface for a changeable optical module directing the excitation beam received from the illumination source into the sample and directing an emission beam received from the sample to the first detector, is characterized in that the interface further comprises means for receiving a second emission beam from a same optical module. 
     The invention also applies to a changeable optical module for an optical measurement instrument, the module comprising means for receiving an excitation signal from an illumination source and means for directing the excitation to a sample, means for receiving an emission beam from the sample and means for outputting the emission beam received from the sample to a detector, which is characterized in that the module further comprises means for separating the emission beam into a first emission beam and a second emission beam, and means for outputting the first emission beam for a first detector, and means for outputting the second emission beam for a second detector. 
     The invention also applies to a process for measurement of samples with an optical measurement instrument comprising means for providing excitation of a sample and means for measuring two emissions from the sample, the process comprising the phases of 
     selecting a measurement mode, 
     selecting a possible excitation filter, 
     selecting a first emission filter for a first detector, 
     selecting at least one optical module for guiding the excitation beam into the sample and for guiding the first emission into the first detector, 
     performing the optical measurement, which is characterized in that a process for measuring two emissions from the sample comprises the phases of 
     selecting a second emission filter for a second detector, 
     selecting one and same optical module for guiding the excitation beam into the sample, for dividing the emission beam into first emission beam and a second emission beam, for guiding a first emission beam into the first detector and for guiding a second emission beam into the second detector. 
     A method according to the invention for optical measurement of samples comprising the steps of: 
     forming an excitation beam, 
     directing the excitation beam to a sample with an optical module, 
     acquisition of an emission beam from the sample, is characterized in that the method further comprises the steps of: 
     dividing the emission beam into a first emission beam and a second emission beam within said optical module, 
     guiding the first emission beam to a first detector, 
     guiding the second emission beam to the second detector, 
     converting the emission beams into emission signals in said detectors, and 
     processing the signals for providing measurement results. 
     Some preferred embodiments are described in the dependent claims. 
     An important advantage of the invention relates to achieving high measurement efficiency. Measurements of two emissions can be made simultaneously, and the time needed for the measurement is thus halved. Further efficiency is achieved due to the minimal attenuation of the optical paths. 
     There are also other important advantages related to the idea of placing into a same changeable optical module the mirror for dividing the emission into two emission beams and the mirror for separating the optical paths of emission and excitation beams. This way one measurement head can be used for both one-emission measurement and for two-emission measurement in an optimal way. If a second emission is not measured with the same measurement head as the first emission, the optical module in use can be easily changed into a module, which does not include the mirror for the second emission beam. This way it is possible to have one emission measurement without unnecessary attenuation caused by the mirror. 
     A further advantage relates to the ability to offer optional functions in measuring equipment. Equipment with a measurement head for one emission measurement can be easily upgraded into equipment, which has a measurement head for one emission or two emission measurements. For the upgrade it is only necessary to provide the equipment with an optical module, which includes a mirror for the second emission, en providing the equipment with the second detector, if not readily available in the equipment. The basic version of the equipment preferably includes the required optics for guiding the second emission beam from the optical module to the second detector. 
     A further advantage relates to the possibility to have a filter combined with the mirror; different types of measurements can be optimized by selecting mirror that substantially transmits the wavelength of the first emission beam and substantially reflects the wavelength of the second emission beam. This way the attenuation of the emissions can be minimized, and there is less need for further filtering of the emission beams. 
     One further advantage of the present invention is related to the fact that two emissions can be measured without changing the connections of the optical fibres. This way the measurement modes can be changed by software without any need for manual work such as connecting and disconnecting optical cables. 
     The invention also allows the use of direct optical coupling in emission detection in the top measurement head of the equipment; attenuation caused by optical fibres is thus avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the invention will become apparent from the following detailed description and by referring to the drawings where: 
     FIG. 1 is a schematic block diagram of a prior art optical unit of a measurement instrument, 
     FIG. 2 is a schematic illustration of optical paths and main components of an exemplary optical unit for a measurement instrument according to the invention, 
     FIG. 3 is a schematic block diagram of an exemplary measurement instrument according to the invention, 
     FIG. 4 is a schematic block diagram of an optical unit showing a first embodiment for a double emission measurement according to the invention, 
     FIG. 5 is a schematic block diagram of an optical unit showing a second embodiment for a double emission measurement according to the invention, 
     FIG. 6 is a schematic block diagram of an optical unit showing a third embodiment for a double emission measurement according to the invention, 
     FIG. 7 is a schematic block diagram of an optical unit showing a fourth embodiment for a double emission measurement according to the invention, 
     FIG. 8 is a schematic block diagram of an optical unit showing a fifth embodiment for a double emission measurement according to the invention, 
     FIG. 9 is a schematic block diagram of an optical unit showing a sixth embodiment for a double emission measurement according to the invention, 
     FIG. 10 illustrates a perspective view of an exemplary top optical module according to the invention, 
     FIG. 11 illustrates a perspective view of an exemplary bottom optical module according to the invention, 
     FIG. 12 illustrates an exemplary four-position wheel with four optical modules according to the invention, 
     FIG. 13 illustrates a first exemplary top head optical module for implementing the invention, 
     FIG. 14 illustrates a second exemplary top head optical module for implementing the invention, 
     FIG. 15 illustrates a third exemplary top head optical module for implementing the invention, 
     FIG. 16 illustrates a fourth exemplary top head optical module for implementing the invention, 
     FIG. 17 illustrates a fifth exemplary top head optical module for implementing the invention, 
     FIG. 18 illustrates a first exemplary bottom head optical module according to the invention for implementing the invention, 
     FIG. 19 illustrates a second exemplary bottom head optical module for implementing the invention, 
     FIG. 20 illustrates a third exemplary bottom head optical module for implementing the invention, 
     FIG. 21 illustrates a fourth exemplary bottom head optical module for implementing the invention, 
     FIG. 22 illustrates a fifth exemplary bottom head optical module for implementing the invention, 
     FIG. 23 illustrates an exemplary process for performing a measurement with an optical measurement instrument according to the invention. 
     FIG. 24 illustrates an exemplary method for performing a measurement according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 was already explained in the description of the prior art. In the following, the principle of the invention is first described referring to FIG.  2 . Then, an example of a more detailed implementation is described referring to FIG. 3, which is a block diagram of exemplary analyser equipment according to the invention. Next there are some exemplary embodiments described for using an analyser according to the invention for double emission measurements, referring to FIGS. 4-12. After this, relating to FIGS. 13-22 there is a description of exemplary optical cubes that can be used e.g. for the measurements referred to in FIGS. 4-12. Finally, examples of a process and a method for performing a measurement according to invention are described referring to flow diagrams in FIGS. 23 and 24. 
     FIG. 2 illustrates main components and optical paths of an exemplary optical analyser instrument according to the invention. The instrument comprises an illumination source  211  for the excitation of a sample. The radiation from the lamp  211  is collimated with lens  215  and directed through an interference filter  214 . Different filters can be selected for different wavelengths. The excitation beam is then focused to an end of a fibre optic guide  218 , which guides it to an aperture of an optical module. The fibre optic guide is preferably a bundle of fibres, such as 200 pieces of fibres with a diameter of 100 μm. One important purpose of the fibre optic guide is to mix the light of the illumination source in order to avoid an uneven distribution of excitation beam within the sample volume to be measured. The excitation beam is reflected by a dichroic mirror  241  inside the optical module  240 , and directed into the sample  281  with a lens system  223 . A part of the illumination light is reflected by a beam splitter mirror  243  into a reference detector  219  in order to give a reference information on the actual illumination intensity. While the reference mirror is located in the changeable mirror block, the excitation filter differences can be compensated by modifying the properties of the reference mirror. This way high feedback accuracy is achieved. A beam splitter mirror can be produced e.g. by forming reflecting coating for the mirror to be e.g. stripes or dots, which cover only a part of the mirror surface. 
     The emission beam from the sample  281  is directed with the lens system  223  into the optical module  240 , where it passes the (preferably) dichroic mirror  241 . The dichroic mirror is preferably designed for each label so, that it reflects excitation wavelength but transmits emission wavelengths. The emission beam is then divided inside the optical cube into two beams by a second dichroic mirror  242 . The dichroic mirror preferably functions as a filter so that a beam with a wavelength of the first emission is transmitted through the first detector  231   a,  and a beam with a wavelength of the second emission is reflected to the second detector  231   b.  The second dichroic mirror is therefore also preferably designed for each label/pair of labels so that it transmits first emission wavelengths but reflects second emission wavelengths. 
     The first emission beam is collimated with a lens  233   a  and directed through an interference filter  234   a  in order to prevent light with a wavelength outside the first emission from passing to the first detector. The first emission beam is then focused with lens  235   a  to the first detector  231   a . The second emission beam is reflected with a mirror  238  to a lens  233   b  where the beam is collimated and directed through a second interference filter  234   b  in order to prevent light with a wavelength outside the second emission from passing to the second detector. The second emission beam is then focused with lens  235   a  to the first detector  231   a . The signals received from the detectors are then amplified and processed to achieve a value for the intensities of the first and second emissions. The excitation and emission parts of the instrument can be used, except for photoluminescence measurements, also to e.g. photometric and chemiluminescence measurements. 
     As already mentioned, an essential feature of the invention is that the excitation beam as well as two emission beams are adapted to be interfaced with a single changeable optical module. This allows performing various types of measurements by changing just one optical module, and the change of module and related parameters can be performed automatically controlled by software. This advantage becomes more apparent in the following more complete example of an optical instrument according to the invention. 
     FIG. 3 illustrates in more detail an exemplary optical instrument according to the invention. The instrument has a top measurement head  320 , which includes components for providing an excitation beam and for detecting emissions from above the sample. The instrument has also an optional bottom measurement head  360 , which includes components for providing an excitation beam and for detecting emissions from below the sample. The instrument further comprises a sample platform  380 , which has means for moving and a sample tray  389  in order to position successive samples  381  into the measurement volume. There may also be means provided for adjusting the vertical position of the sample platform relative to the top and bottom measurement heads. 
     The instrument has one or two illumination sources. The main illumination source  312   a  includes a pulse lamp, and the optical energy of each pulse is preferably equal. The excitation beam generated by the pulse lamp is collimated with a lens  315  and directed through an interference filter  314 . The filter is placed on a filter slide, so that the excitation filter to be used in a measurement can be selected from several filters. The excitation beam is then focused to an end of a fibre optic guide  318 , which mixes the excitation beam and guides it to an aperture of an optical module  340  according to the invention. The optical module  340  and the lens system  323  directs the excitation beam into the sample  391 . The optical module is not described here in more detail because it is explained in relation to other Figures. 
     The equipment may also include a second pulse lamp  312   b ,  311   b , which may be a low power lamp, e.g. for simultaneous photometric measurements. The instrument has an optical fibre guide  312   a  for guiding the light from the second lamp. The light can be distributed for the photometric measurement into three filters  314   h ,  314   j  and  314   k  with fibre branches  377   h ,  377   j  and  377   k.    
     The light beams are collimated with lenses  375   h ,  375   j  and  375   k  before directing the beams through the filters. The filters can be located on the same or different filter slide as the filter  314   e  for the first illumination source. If the same filter slide is used for filters of both lamps, the simultaneous measurement modes must be taken into account when the location of the filters is planned. After filtering, the beams are collimated into ends of three optical fibre cables  378 , which are led to the bottom measurement head for the photometric measurement. The light beams from the optical cables  378  are focused to three samples  384  with a lens system  379  including lenses for each three beams. After transmitting through the samples the beams are measured with three detectors  322   d ,  322   e  and  322   f , which are e.g. a photo diodes. The three ends of the fibre optic cables, three lenses, three simultaneously measured samples and three detectors are in this case located in a row perpendicular to the plane of the drawing and thus only one of them can be seen in the drawing. 
     It is preferable to have a separate optics for the photometrics measurement so that a luminescence measurement and a photometrics measurement can be performed simultaneously from different samples. If simultaneous photoluminescence and photometric measurements are required, the analyzer is preferably equipped with two pulse lamps. However, it is also possible to use an instrument with one lamp for photometrics measurements. For example, an optical switch  317  may have an output for an optical fibre  378   a , which leads light from the lamp  312   a  to the photometries measurement optics  379 . It is then possible to control the optical switch either to guide the light for providing excitation for an emission measurement or to guide the light the a photometric measurement. 
     An optical fibre  318 T is used for guiding the excitation beam from the optical switch  317  to the optical module  340  of the top measurement head. An optical fibre  318 B is used for guiding the excitation beam from the optical switch  317  to the optical module  350  of the bottom measurement head. The instrument may also have a further lamp so that different lamps can be selected for providing the excitation beam of the top head and the bottom head. In this case, a more versatile optical switch system is required. 
     The emission beam from the sample  381  is directed with the lens system  323  into the optical module  340  where the emission beam is divided into to two beams. A dichroic mirror in the optical module preferably functions as a filter so that a beam with a wavelength of the first emission is transmitted through the to the first detector  331   a , and a beam with a wavelength of the second emission is reflected to the second detector  331   b . The detector can be e.g. a photo-multiplier tube, which may be used in analogue mode or in photon count mode, or in both modes simultaneously. When the equipment includes two photoluminescence detectors they may be of different types and the detection modes may be different during a measurement. 
     The first emission beam is collimated with a lens  333   a  and directed through an interference filter  334   j  in order to prevent light with a wavelength outside the first emission from passing to the first detector. The first emission beam is then focused with lens  335   a  to the first detector  331   a . The second emission beam is reflected with a mirror  338  to a lens  333   b  where the beam is collimated and directed through a second interference filter  334   k  in order to prevent light with a wavelength outside the second emission from passing to the second detector. The second emission beam is then focused with lens  335   a  to the first detector  331   a . The filters  334   j  and  334   k  are located on same filter slide or they may locate on different filter slides. The filter slide(s) is movable so that the filters used in the measurement can be selected from a number of filters with different pass-band wavelengths. 
     In an instrument also comprising a bottom measurement head there are optical switches  337   a  and  337   b  for selecting the detected emission beam from the top or bottom measurement head. An optical fibre  338   a  is used for guiding the first emission beam from the optical module  350  of the bottom measurement head  360  to the optical switch  337   a . Another optical fibre  338   b  is used for guiding the second emission beam from the optical module  350  of the bottom measurement head  360  to the optical switch  337   b.    
     The signals received from the detectors are then amplified and processed to achieve a value for the intensities of the first and second emissions. Measurement signals and reference signals are amplified and read after each excitation pulse and signal corrections are calculated. Basic references are determined with standard solvents after the analyzer has been assembled. If there are more than one excitation pulses used for one well, the corresponding emission signals are digitally integrated. 
     The instrument has also an optional detector  332   c ,  331   c  for chemiluminescence measurements. The detector receives the chemiluminescence radiation from the sample via a thick bundle of optical fibres  318   c . It is preferable to have a separate optics for the chemiluminescence measurement so that a photoluminescence measurement and a chemiluminescence measurement can be performed simultaneously from different samples. In FIG. 3 the chemiluminescence measurement is made from a sample located behind sample  381 . A photo-multiplier tube can also be used as a detector for the chemiluminescence. The detector can be used in analogue mode or digital mode, or if the properties of the tube allow, both modes may be used simultaneously. 
     The instrument comprises a carousel wheel  328  for the attachment of optical modules  340   a ,  340   b , . . . . The wheel can be rotated around its fixing point  329 , and the optical module used in a measurement can thus be selected by controlling the position of the wheel. According to the present invention, the equipment has an optical interface of at least two emission beams and at least one excitation beam for a single optical module. 
     If the instrument is equipped with a bottom measurement head, there may be a similar optical module  350  used in the bottom measurement head as in the top measurement head. The excitation and emission beams are lead between the two measurement heads with optical fibres  338   a ,  338   b  and  318 B. There is also a lens system  363  for focusing the beams to the sample and ends of the optical fibres. Since the optical module of the bottom measurement head needs not be so frequently changed, it may be manually changeable. Alternatively a processor-controlled carousel can also be used in the bottom measurement head. 
     The optical modules are shown essentially enlarged in FIG. 3 in order to better illustrate the optical paths in the instruments. The actual size of the optical modules may be as small as 20 mm×20 mm×20 mm. 
     The optical modules may be equipped with machine readable codes, such as bar codes, so that the processor of the equipment can check with a code reader, which types of optical modules are installed in each location. This way it can be certified that a correct type of optical module is used for each measurement. The bar code reader or related electronics are not shown in FIG.  3 . 
     The instrument is also equipped with electronics for amplifying and processing the signals from the detectors, as well as electronics for driving the lamp(s). There is also control electronics provided for controlling the measurements, such as selecting filter(s), selecting the optical module(s), controlling optical switch(es), controlling the position of the sample tray  389  for selecting the sample to be measured, and controlling the positions of the measurement heads  320  and  360  relative to the sample platform  380 . The electronics is not shown in FIG. 3, as the required electronics can be designed by a skilled person in the art. 
     In the preferred embodiment the user can adjust various parameters of a measurement. The excitation pulse energy is adjusted by the discharge voltage and by the capacitors of the flash lamp power supply. Total excitation energy of one measurement is controlled by measuring every pulse and comparing the sum to a reference level of the integrator. The parameters of measurements are preferably user adjustable. 
     Next some embodiments of possible measurement modes are described referring to FIGS. 4-9. These exemplary embodiments show how the interface according to the invention for an optical module with two emission outputs gives a possibility for a large variety of different measurement modes. These measurement modes are available with an automatic selection and control of filters, optical switches and just one changeable optical module in each measurement head. The described measurement modes are related but not restricted to photoluminescence measurements. 
     FIG. 4 illustrates a first embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment both excitation and detection is made from the above the sample using the top measurement head of the instrument. One of the possible alternative excitation sources  411  gives an excitation pulse, which is guided through an optical system  413  to an optical fibre  418 . The optical system may include filters, lenses and mechanical components as was shown in FIG.  3 . The excitation beam is mixed in the optical fibre and lead to the optical module  450 . The excitation beam is reflected from the mirror  441  and collimated in the optical system  423  into the sample  481  on the sample plate  480  to be measured. The excitation beam provides excitation for two simultaneous measurements. 
     The excited sample  481  gives two emissions that are measured with detectors A and B. The emission beams are first collimated in the optical system  423 , and the beams lead to the optical module  440 . The emission beams first transmit the dichroic mirror  441 , where after the second dichroic mirror  442  separates the two emission beams. The separation may be based on the wavelength of the emissions, polarization etc. The first emission beam is substantially transmitted through the second dichroic mirror  442  and further collimated and filtered in the optical system  433   a  to be measured in the detector  431   a . The second emission beam is substantially reflected by the second dichroic mirror  442 , and further reflected by the mirror  438 . The beam is collimated and filtered in the optical system  433   b  to be measured in the detector  431   b.    
     One advantage of this first embodiment is that the emissions are guided to both detectors directly i.e. without optical fibre cables. This way an optimal sensitivity of the measurement is achieved. 
     In the first embodiment illustrated in FIG. 4 the whole measurement is made with the top measurement head, and so it is not necessary to have a bottom measurement head in the instrument in order to perform the double emission measurement. The use of an optical module according to the invention gives therefore a possibility to make versatile measurements efficiently even with a basic instrument, which is not equipped with a bottom measurement head. In the further described embodiments for using the instrument according to the invention, also the bottom measurement head is used. 
     FIG. 5 illustrates a second embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment both excitation and detection is made from the below the sample using the bottom measurement head of the instrument. One of the possible alternative excitation sources  511  gives an excitation pulse, which is lead to the optical module of the bottom measurement head with an optical fibre (not shown in the FIG.  5 ), wherein the excitation beam is mixed. The excitation beam is reflected from the mirror  551  and collimated in the optical system  563  into the sample  581  on the sample plate  580  to be measured. The excitation beam provides excitation for two simultaneous measurements, or alternatively two successive excitations with different wavelengths are made with successive excitation pulses (successive excitation is preferably used only in bottom measurements). 
     The excited sample  581  gives two emissions that are measured with detectors A and B. The emission beams are first collimated in the optical system  563 , and the beams are lead to the optical module  550 . The emission beams first transmit the dichroic mirror  551 , where after the second dichroic mirror  552  separates the two emission beams. The separation may be based on the wavelength of the emissions, polarization etc. The first emission beam is substantially transmitted through the second dichroic mirror  552  and further lead to the detector  531   a  through an optical fibre (not shown in FIG.  5 ). The second emission beam is substantially reflected by the second dichroic mirror  552 , and lead to the second detector  531   b  through an optical fibre (not shown in FIG.  5 ). The emission beams are then measured in the detectors  531   a  and  531   b.    
     In the second embodiment illustrated in FIG. 5 the whole measurement is made with the bottom measurement head. This embodiment is useful for making measurements where the substance to be measured lies essentially on the bottom of the sample tube. With this embodiment it is possible to measure simultaneously two emissions from the bottom surface of such substance and thus the measurement can be performed with optimal efficiency. This embodiment also makes it possible to use the top measurement head for a chemiluminescence measurement. This way both the photoluminescence measurement and the chemiluminescence measurement can be performed the samples without changing the locations of the optical modules or cables between the measurements. In the embodiments that are described in the following, both the top measurement head and the bottom measurement head are used for the photoluminescence measurement. 
     FIG. 6 illustrates a third embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment the excitation is made from the above the sample using the top measurement head, and the detection is made from below the sample using the bottom measurement head of the instrument. One of the possible alternative excitation sources  611  gives an excitation pulse, which is guided through an optical system  613  to an optical fibre  618 . The optical system may include filters, lenses and mechanical components as was shown in FIG.  3 . The excitation beam is mixed in the optical fibre and lead to the optical module  650 . The excitation beam is reflected from the mirror  641  and collimated in the optical system  623  into the sample  681  on the sample plate  680  to be measured. The excitation beam provides excitations for two simultaneous measurements. 
     The excited sample  681  gives two emissions that are measured with detectors A and B. The emission beams are first collimated in the optical system  663 , and the beams are lead to the optical module  650  of the bottom measurement head. The emission beams first transmit the dichroic mirror  651 , where after the second dichroic mirror  652  separates the two emission beams. The separation may be based on the wavelength of the emissions, polarization etc. The first emission beam is substantially transmitted through the second dichroic mirror  652  and further lead to the detector  631   a  through an optical fibre (not shown in FIG.  6 ). The second emission beam is substantially reflected by the second dichroic mirror  652 , and lead to the second detector  631   b  through an optical fibre (not shown in FIG.  6 ). The emission beams are then measured in the detectors  631   a  and  631   b.    
     The third embodiment illustrated in FIG. 6 gives some advantages compared to the second embodiment of FIG.  5 . When the excitation pulse is exposed from the top measurement head the length of the optical fibre within the optical route of the excitation pulse can be made optimally short. This way the attenuation of the optical fibre can be minimized, and consequently a maximum illumination intensity is achieved. 
     Another advantage of the embodiment of FIG. 6 is that it is possible to use an optical module where there is no first mirror  651  in the module. This way the attenuation of the emission beam caused by the excitation mirror  651  can be totally avoided. 
     FIG. 7 illustrates a fourth embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment the excitation is made from below the sample using the bottom measurement head, and the detection is made from above the sample using the top measurement head of the instrument. One of the possible alternative excitation sources  711  gives an excitation pulse, which is lead to the optical module of the bottom measurement head with an optical fibre (not shown in the FIG.  7 ), wherein the excitation beam is mixed. The excitation beam is reflected from the mirror  751  and collimated in the optical system  763  into the sample  781  on the sample plate  780  to be measured. The excitation beam provides excitation for two simultaneous measurements. 
     The excited sample  781  gives two emissions that are measured with detectors A and B. The emission beams are first collimated in the optical system  723 , and the beams lead to the optical module  740 . The emission beams first transmit the dichroic mirror  741 , where after the second dichroic mirror  742  separates the two emission beams. The separation may be based on the wavelength of the emissions, polarization etc. The first emission beam is substantially transmitted through the second dichroic mirror  742  and further collimated and filtered in the optical system  733   a  to be measured in the detector  731   a . The second emission beam is substantially reflected by the second dichroic mirror  742 , and further reflected by the mirror  738 . The beam is collimated and filtered in the optical system  733   b  to be measured in the detector  731   b.    
     The fourth embodiment illustrated in FIG. 7 gives some advantages compared to the first embodiment of FIG.  4 . When the excitation beam is exposed from the bottom measurement head it is possible to use in the top measurement head an optical module where there is no first mirror  741  in the module. This way the attenuation of the emission beam caused by the excitation mirror  741  can be totally avoided. 
     FIG. 8 illustrates a fifth embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment the excitation is made from the above the sample using the top measurement head. The detection if the first emission is made from above the sample using the top measurement head, and the detection of the second emission is made from below the sample using the bottom measurement head of the instrument. 
     One of the possible alternative excitation sources  811  gives an excitation pulse, which is guided through an optical system  813  to an optical fibre  818 . The optical system may include filters, lenses and mechanical components as was shown in FIG.  3 . The excitation beam is mixed in the optical fibre and lead to the optical module  850 . The excitation beam is reflected from the mirror  841  and collimated in the optical system  823  into the sample  881  on the sample plate  880  to be measured. The excitation beam provides excitations for two simultaneous measurements. 
     The excited sample  881  gives two emissions that are measured with detectors A and B. The first emission beam is first collimated in the optical system  823  and lead to the optical module  840  of the top measurement head. The first emission beam is substantially transmitted by the first dichroic mirror  841  and the second dichroic mirror  842 . The first emission beam is then collimated and filtered in the optical system  833   a  to be measured in the detector  831   a.    
     The second emission beam is first collimated in the optical system  863 , and the beam is lead to the optical module  850  of the bottom measurement head. The emission beam first transmits the dichroic mirror  651 , where after it is substantially reflected in the second dichroic mirror  852 . The second emission beam is lead to the second detector  831   b  through an optical fibre (not shown in FIG.  6 ). The emission beams are then measured in the detectors  831   a  and  831   b.    
     One advantage of the embodiment of FIG. 8 is that it is possible to measure simultaneously emissions from both above and below the sample simultaneously. 
     There is also another advantage related to the embodiment illustrated in FIG.  8 . When the two emissions are measured with different measurement heads it allows the use of only one mirror within the path of the emission beam. In the top measurement head it is possible to use an optical module, which has no second mirror  842 . In the bottom measurement head it is possible to use an optical module, which has no first mirror  851 . It is also possible to use a non-dichroic mirror  852  in the bottom measurement head. This way a very small attenuation is achieved in the measurement of the both emissions. Especially the measurement of the first emission can be measured with high sensitivity, because of the direct optical path between the sample and the detector  831   a.    
     FIG. 9 illustrates a sixth embodiment of performing a photoluminescence measurement with a measuring instrument according to the present invention. In this embodiment the excitation is made from below the sample using the bottom measurement head. The detection of the first emission is made from below the sample using the bottom measurement head, and the detection of the second emission is made from above the sample using the top measurement head of the instrument. 
     One of the possible alternative excitation sources  911  gives an excitation pulse, which is lead to the optical module of the bottom measurement head with an optical fibre (not shown in the FIG.  9 ), wherein the excitation beam is mixed. The excitation beam is reflected from the mirror  951  and collimated in the optical system  963  into the sample  981  on the sample plate  980  to be measured. The excitation beam provides excitation for two simultaneous measurements. 
     The excitated sample  981  gives two emissions that are measured with detectors A and B. The first emission beam is first collimated in the optical system  963 , and the beam is lead to the optical module  950  of the bottom measurement head. The first emission beam is substantially transmitted by the first dichroic mirror  951  and the second dichroic mirror  952 . The first emission beam is further lead to the detector  831   a  through an optical fibre (not shown in FIG.  9 ). The first emission beam is finally measured in the detector  931   a.    
     The second emission beam is first collimated in the optical system  923 , and lead to the optical module  940 . The second emission beam first transmits the dichroic mirror  941 , where after the second emission beam is substantially reflected by the second dichroic mirror  942 , and further reflected by the mirror  938 . The second emission beam is collimated and filtered in the optical system  933   b  and measured in the detector  931   b.    
     Also the embodiment of FIG. 9 has the advantage that it is possible to measure simultaneously emissions from both above and below the sample simultaneously. 
     There is also another advantage related to the embodiment illustrated in FIG.  9 . When the two emissions are measured with different measurement heads it allows the use of only one mirror within the path of the emission beam. In the top measurement head it is possible to use an optical module, which has no first mirror  941 . In the bottom measurement head it is possible to use an optical module, which has no second mirror  952 . It is also possible to use a non-dichroic mirror  941  in the top measurement head. This way a very small attenuation is achieved in the measurement of both emissions. 
     FIG. 10 illustrates a perspective view of an exemplary optical module  1040  according to the invention. It is designed for a top measurement head, but it is also possible to design a bottom measurement head, which is adapted for such a module. The Figure shows an aperture  1046  for the excitation beam from the lamp, an aperture  1044  for the emission beam to the first detector and an aperture for the emission beam to the second detector. There is also a bar code  1049  shown on the optical cube for a possible automatic recognition of the module type. 
     FIG. 11 illustrates a perspective view of another exemplary optical module  1150  according to the invention. It is designed for a bottom measurement head, but it is also possible to design top and bottom measurement heads, which are adapted for similar modules. The Figure shows an aperture  1156  for the excitation beam from the lamp, and an aperture  1158  for providing an optical interface to the sample. There is also a bar code  1159  shown on the optical cube for a possible automatic recognition of the module type. 
     FIG. 12 illustrates a top view of an exemplary arrangement where four optical modules are attached to a carousel  1228  in a top measurement head. The optical modules are cited  1240   a ,  1240   b ,  1240   c  and  1240   d  with their apertures for the emission to the first detector cited as  1244   a ,  1244   b ,  1244   c  and  1244   d . The instrument preferably has means for turning the carousel around its axis  1229  so the one of the four optical modules can be selected for use by the program of the instrument. If the optical modules are equipped with a code, such as bar code, the control unit of the instrument may check, which modules are available in each position of the carousel. It is preferable that there is an attachment arrangement for the optical modules, which allows the optical modules be easily removed and attached when necessary. Although the carousel of FIG. 12 is designed for a top measurement head, a bottom measurement head may of course also be equipped with such a carousel for an automatically controlled change of the optical module. Although there are four optical modules shown in FIG. 12, there may naturally be a different number of optical modules. Considering the preferable small size of the optical modules, it is possible provide a carousel with e.g. 8 or 16 optical modules. 
     It is also possible to use another kind of mechanical arrangement for the optical modules instead of a carousel. For example, there may be a slide for the optical modules, wherein the optical modules are placed parallel in a line, and a module to be used can be changed by shifting the slide into a corresponding position. If a slide is used, there may be slides of different lengths with a different number on locations for optical modules. 
     In the following some embodiments of possible optical modules are described refering to FIGS. 13-22. These exemplary embodiments show optical modules, which can be used in an optical instrument according to the invention including an interface for receiving two emissions from an optical module. These optical modules can also be used for implementing the measurement modes described in FIGS. 4-9, and generally for implementing the process and method according to the invention. 
     FIG. 13 illustrates a cross section view from the side of an exemplary optical module for a top measurement head. This optical module  1340  comprises three dicroic mirrors. The module receives an excitation beam from the aperture  1346 , and mirror  1343  reflects a part of the excitation beam into a reference sensor through the aperture  1347 . The main part of the excitation beam is reflected from the mirror  1341  and thus directed to a sample through the aperture  1348 . 
     The emission from the sample is received into the module through the aperture  1348 . The emissions transmit the dichroic mirror  1341  and reach the further dichroic mirror  1342 . The mirror  1342  splits the emission beam into a first beam that is led to the first detector through the aperture  1344 , and a second beam that is led to the second detector through the aperture  1345 . 
     The optical module illustrated in FIG. 13 is very suitable for the double emission measurement, which was described in FIG.  4 . However, this optical module can be used also in many other types of measurements, such as those described in FIGS. 6-9 or single emission measurements, if an optimized performance is not required. 
     FIG. 14 illustrates a cross section view from the side of another exemplary optical module for a top measurement head. This optical module  1440  comprises one dichroic mirror  1443  and one non-dichroic mirror  1441 . The module receives an excitation beam from the aperture  1446 , and mirror  1443  reflects a part of the excitation beam into a reference sensor through the aperture  1447 . The main part of the excitation beam is reflected from the mirror  1441  and thus directed to a sample through the aperture  1448 . 
     This optical module is designed for measurements where emission measurement is made using the bottom measurement head. The measurement illustrated in FIG. 6 is an example of such a measurement. Therefore this optical module for the top measurement head does not have any optical paths for emission beams. One advantage of this optical module is that attenuation of the excitation beam is minimal. 
     FIG. 15 illustrates a cross section view from the side of a third exemplary optical module for a top measurement head. This optical module  1540  comprises one dichroic mirror. This optical module is designed for measurements where bottom measurement head is used for excitation. An example of this kind of measurement is illustrated in FIG.  7 . Therefore this optical module does not have any optical paths for an excitation beam. 
     The emission from the sample is received into the module through the aperture  1548 . The mirror  1542  splits the emission beam into a first beam that is led to the first detector through the aperture  1544 , and a second beam that is led to the second detector through the aperture  1545 . 
     Although this optical module illustrated in FIG. 15 is very suitable for the double emission measurement, which was described in FIG. 7, this optical module can also be used in many other types of measurements, such as single emission measurements, if an optimized performance is not required. 
     FIG. 16 illustrates a cross section view from the side of a fourth exemplary optical module for a top measurement head. This optical module  1640  comprises two dichroic mirrors. The module receives an excitation beam from the aperture  1646 , and mirror  1643  reflects a part of the excitation beam into a reference sensor through the aperture  1647 . The main part of the excitation beam is reflected from the mirror  1641  and thus directed to a sample through the aperture  1648 . 
     The emission from the sample is received into the module through the aperture  1648 . The emission transmits the dichroic mirror  1641 , and it is led to the first detector through the aperture  1644 . 
     The optical module illustrated in FIG. 16 is very suitable for a double emission measurement, where the first emission is measured with the top measurement head and the second emission is measured with the bottom measurement head. This kind of measurement was described in FIG.  8 . However, this optical module can be used also in many other types of measurements, such as single emission measurements. 
     FIG. 17 illustrates a cross section view from the side of a fifth exemplary optical module for a top measurement head. This optical module  1740  comprises one non-dichroic mirror. This optical module is designed for measurements where excitation is made using the bottom measurement head. An example of this kind of measurement is illustrated in FIG.  9 . Therefore this optical module does not have any optical paths for an excitation beam. 
     The emission from the sample is received into the module through the aperture  1748 . The mirror  1742  reflects the emission beam, which is further led to the second detector through the aperture  1745 . 
     The optical module illustrated in FIG. 17 is very suitable for a double emission measurement, where the second emission is measured with the top measurement head and the first emission is measured with the bottom measurement head. This kind of measurement was described in FIG.  9 . However, this optical module can be used also in many other types of measurements, if an optimised performance is not required. 
     Although the optical modules illustrated in FIGS. 13-17 are designed for the top measurement head, it is also possible to design the bottom measurement head be adapted to the use of these modules. 
     FIG. 18 illustrates a cross section view from the side of an exemplary optical module for a bottom measurement head. This optical module  1850  comprises three mirrors. The module receives an excitation beam from the aperture  1856 , and mirror  1853  reflects a part of the excitation beam into a reference sensor through the aperture  1857 . The main part of the excitation beam is reflected from the mirror  1851  and thus directed to a sample through the aperture  1858 . 
     The emissions from the sample are received into the module through the aperture  1858 . The emissions transmit the dichroic mirror  1851  and reach the further dichroic mirror  1852 . The mirror  1852  splits the emission beam into a first beam that is led to the first detector through the aperture  1854 , and a second beam that is led to the second detector through the aperture  1855 . 
     The optical module illustrated in FIG. 18 is very suitable for the double emission measurement, which was described in FIG.  5 . However, this optical module can be used also in many other types of measurements, such as those described in FIGS. 6-9 or single emission measurements, if an optimized performance is not required. 
     FIG. 19 illustrates a cross section view from the side of another exemplary optical module for a bottom measurement head. This optical module  1950  comprises one dichroic mirror. This optical module is designed for measurements where excitation is made using the top measurement head. An example of this kind of measurement is illustrated in FIG.  6 . Therefore this optical module does not have any optical paths for an excitation beam. 
     The emissions from the sample are received into the module through the aperture  1958 . The mirror  1952  splits the emission beam into a first beam that is led to the first detector through the aperture  1954 , and a second beam that is led to the second detector through the aperture  1955 . 
     Although this optical module illustrated in FIG. 19 is very suitable for the double emission measurement, which was described in FIG. 6, this optical module can also be used in many other types of measurements, such as single emission measurements, if an optimized performance is not required. 
     FIG. 20 illustrates a cross section view from the side of a third exemplary optical module for a bottom measurement head. This optical module  2050  comprises one beam splitter mirror  2053  and one further mirror  2051 . The module receives an excitation beam from the aperture  2056 , and mirror  2053  reflects a part of the excitation beam into a reference sensor through the aperture  2057 . The main part of the excitation beam is reflected from the mirror  2051  and thus directed to a sample through the aperture  2058 . 
     This optical module is designed for measurements where emission measurement is made using the top measurement head. The measurement illustrated in FIG. 7 is an example of such a measurement. Therefore this optical module designed for the bottom measurement head does not have any optical paths for emission beams. One advantage of this optical module is that attenuation of the excitation beam is small. 
     FIG. 21 illustrates a cross section view from the side of a fourth exemplary optical module for a bottom measurement head. This optical module  2150  comprises one non-dichroic mirror. This optical module is designed for measurements where excitation is made using the top measurement head. An example of this kind of measurement is illustrated in FIG.  8 . Therefore this optical module does not have any optical paths for an excitation beam. 
     The emission from the sample is received into the module through the aperture  2158 . The mirror  2152  reflects the emission beam, which is further led to the second detector through the aperture  2155 . 
     The optical module illustrated in FIG. 21 is very suitable for a double emission measurement, where the second emission is measured with the bottom measurement head and the first emission is measured with the top measurement head. This kind of measurement was described in FIG.  8 . However, this optical module can be used also in other types of measurements, if an optimised performance is not required. 
     FIG. 22 illustrates a cross section view from the side of a fifth exemplary optical module for a bottom measurement head. This optical module  2250  comprises two dicroic mirrors. The module receives an excitation beam from the aperture  2256 , and mirror  2253  reflects a part of the excitation beam into a reference sensor through the aperture  2257 . The main part of the excitation beam is reflected from the mirror  2251  and thus directed to a sample through the aperture  2258 . 
     The emission from the sample is received into the module through the aperture  2258 . The emission transmits the dichroic mirror  2251 , and it is led to the first detector through the aperture  2254 . 
     The optical module illustrated in FIG. 22 is very suitable for a double emission measurement, where the first emission is measured with the bottom measurement head and the second emission is measured with the top measurement head. This kind of measurement was described in FIG.  9 . However, this optical module can be used also in many other types of measurements, such as single emission measurements. 
     Although the optical modules illustrated in FIGS. 18-22 are designed for the bottom measurement head, it is also possible to design both the top measurement head and the bottom measurement head be adapted to the use of these modules. 
     FIG. 23 illustrates a flow diagram of an exemplary process according to the invention for using an optical instrument for a photoluminescence measurement. In phase  11  the type of measurement is selected. The excitation source and interference filter is then selected according to the measurement type in phase  12 . Either the top measurement head or bottom measurement head is selected for providing the excitation beam into the sample, phase  13 . This is made e.g. with an optical switch. 
     In phase  14 , the emission filter is selected for the detector A. Either the top measurement head or the bottom measurement head is then selected in step  15  for receiving the emission A and for guiding the emission beam A into the detector A. The optical path is connected to the selected measurement head e.g. by controlling an optical switch. If two emissions are measured the emission filter is also selected for the detector B, steps  16  and  17 , and either the top measurement head or the bottom measurement head is selected in step  18  for receiving the emission B and guiding the emission beam into the detector B. The optical path can be connected to the selected measurement head also by controlling an optical switch. 
     If excitation or emission of the measurement is made from above the sample, ie. the top measurement head is used, then the optical module of the top measurement head is selected and placed into the measurement location, phases  19  and  20 . If excitation or emission of the measurement is made from below the sample, ie. the bottom measurement head is used, then the optical module of the bottom measurement head is selected and placed into the measurement location, phases  21  and  22 . According to the invention, if two emissions are measured simultaneously, an optical module providing an emission beam for the second detector B is selected to either top or bottom measurement head. Especially, if the same measurement head is used for receiving two emissions, an optical module with an output for both detectors is selected in the measurement head. 
     After the optical paths have been selected, the first sample to be measured is selected, phase  23 . The selected sample is then measured,  24 , and the signals received from the detector(s) are processed to produce measurement result(s) for the measured sample, phase  25 . Samples are successively measured by repeating phases  23 - 26  until all samples have been measured. 
     One should note that several variations of the measurement process according to the invention can be applied. For example, the order of the process phases can be different from the one described above. Also, if an instrument without a bottom measurement head is used, the selection between top/bottom measurement head or selection of the optical module for the bottom measurement head are not required. And if only one excitation source is available, a selection between excitation sources is not required. 
     FIG. 24 illustrates a flow diagram of an exemplary method according to the invention for optical measurement of a sample. In phase  41  an excitation beam is formed in an illumination source, and the excitation beam is filtered with an interference filter in phase  42  to include wavelength(s) for the excitation of two substances in the sample. The filtered excitation beam is guided to an optical module according to the invention, wherein the beam is reflected, phase  43 . The excitation beam is then focused into the sample within a volume that is to be measured,  44 . The excitation beam may be an excitation pulse, succession of pulses or a continuous wave beam, depending on the type of measurement. 
     After the (fluorescent) label substances in the sample have been excited, they release emissions which are received into the optical module according to the invention, phase  45 . The emissions may be in the form of bursts or continuous emissions depending on the excitation. In the optical module the emission beam may first transmit an excitation mirror, and the emission beams are then divided with a dichroic mirror into two emission beams e.g. according to their wavelength in phase  46 . The splitting may be performed, according to the invention, in same optical module. 
     The first emission beam received from the first substance of the sample is first filtered in phase  47  by transmitting the first emission beam and blocking other light, e.g. light with different wavelength. The first emission beam is then guided to a first detector in phase  48 . Simultaneously with receiving the first emission, the second emission beam is received from the second substance of the sample, guided through the optical module and filtered in phase  49  by transmitting the second emission beam and blocking other light, e.g. light with different wavelength. The filtered second emission is then guided to a second detector in phase  50 . The emissions are then converted into electrical signals in the detectors, phase  51 , and the signals are processed in order to provide measurement results showing the quantity of the first and second substances within the sample, phase  52 . 
     One should note that the inventive method is not restricted to the measurement of two emissions of two substances, but there may be further means for splitting the emission into several emission beams and further detectors for measuring the emission beams. 
     Above, examples of a general measurement process and method were described. Next some typical measurements are described in more detail. In this description the use of an optical instrument according to FIG. 3 is referred to. 
     FI and TRF Measurements 
     In a prompt photoluminescence, i.e. FI measurement, one excitation pulse is given for each sample to be measured. In a FI measurement an excitation filter and an emission filter are selected as was described above. A suitable optical module is also selected; the optical module may be a general-purpose module, or it may be a module that is especially designed for a determined label substance. 
     After a sample has been chosen for the measurement an excitation pulse is transmitted, and reference R 1  is read wherein R i  is the amount of light that has been used in the excitation of the label. The illumination reference is received from a reference detector  319 . Emission signals S 1   A  and S 1   B  are then read from the detectors. A correction factor for the signals is calculated on the basis of the illumination reference value. The long-term stability of the equipment is fixed to this amount of light when using a determined excitation filter and mirror block. 
     If several excitation pulses are used for one sample, the sequence is repeated and the results are summed or averaged. This leads to improved signal-to-noise ratio of the measurement. 
     A time resolved photoluminescence measurement, i.e. TRF measurement, is equal to the FI measurement except that several excitation pulses are formed for each sample and corresponding emissions are measured. The measurement signals and reference signals are read after each excitation pulse and signal corrections are calculated. Basic references are determined with standard solvents after the analyzer has been assembled. After receiving all emission signals from a sample, the results are preferably digitally integrated. Finally, a linear correction can be made for the digital signal using a reference. 
     Chemiluminescence Measurement 
     In a chemiluminescence measurement no excitation pulse is given. A separate detector can be used for the chemiluminescence measurement, if it is desirable to make chemiluminescence measurements simultaneously with a photoluminescence measurement. In this case the simultaneous chemiluminescence and photoluminescence measurements are made from different samples. However, if a simultaneous measurement is not required, same detector as used for photoluminescence measurements can be used for the chemiluminescence measurement. 
     An emission filter is not needed in a chemiluminescence measurement, so the filter slide can be moved outside the measurement beam. An optical module is selected according to the label; a TR module can be used, but a better measurement quality can be achieved with a block designed for the chemiluminescence measurement. The analogue gates or a digital window for the measurement period is set. After a sample is chosen a first period for measuring illumination is triggered. The length of the measurement period is e.g. 1 ms. Detected signals are read, further measurement periods are triggered, and the corresponding signals are read. The measurement periods are repeated for e.g. 1000 times, which gives 1 second for the total measurement time. Finally the measured signals are summed to achieve the result of the total measurement. 
     The process of photometric measurement was already described in relation to FIG.  3 . 
     In this patent specification the structure of the components in an optical measurement instrument is not described in more detail as they can be implemented using the description above and the general knowledge of a person skilled in the art. 
     An optical instrument includes control means for performing the optical measurement process. The control of the measuring process in an optical measurement instrument generally takes place in an arrangement of processing capacity in the form of microprocessor(s) and memory in the form of memory circuits. Such arrangements are known as such from the technology of analyzers and relating equipment. To convert a known optical instrument into an equipment according to the invention it is necessary, in addition to the hardware modifications, to store into the memory means a set of machine-readable instructions that instruct the microprocessor(s) to perform the operations described above. Composing and storing into memory of such instructions involves known technology which, when combined with the teachings of this patent application, is within the capabilities of a person skilled in the art. 
     Above, an embodiment of the solution according to the invention has been described. The principle according to the invention can naturally be modified within the frame of the scope defined by the claims, for example, by modification of the details of the implementation and ranges of use. 
     It is especially to be noted that the invention is not in any way restricted to the applications of the photoluminescence measurement. An experienced user is able the use the present invention also in other measurement technologies in common use in biochemical laboratories. For example, e.g. reflectance, turbidimetric and nephelometric measurement can be measured using a fluorescent measurement technology with the exception that the emission filter must be a gray filter. 
     In the field of photometric sample measurements the present invention is not in any way limited to applications where sample excitation is used, but the invention can also be used in measurements that are based, for example, on chemiluminescence. 
     Although the invention has been described with reference to the different microtitration plates it is equally applicable to any form of sample matrix like gels and filter. 
     Although the invention is described with the arrangement where the illumination sources and detectors are located on the top measurement head, there is no reason why their location on the bottom measurement head should not work.