Abstract:
An apparatus, a microscope having an apparatus, and a method for calibration of a photosensor chip ( 19 ) are disclosed. The apparatus has a photosensor chip ( 19 ) which has a multiplicity of light-sensitive elements. A reference light source ( 30 ) is provided and directs the light at at least one part of the photosensor chip ( 19 ). In addition, an open-loop or closed-loop control unit ( 19   a ) is provided and determines and corrects variances between the individual light-sensitive elements.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
   The right of foreign priority is claimed under 35 U.S.C. §119(a) based on Federal Republic of Germany Application No. 10 2006 000 976.2, filed Jan. 7, 2006, the entire contents of which, including the specification, drawings, claims and abstract, are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to an apparatus for calibration of a photosensor chip. The photosensor chip is equipped with a multiplicity of light-sensitive elements. 
   The invention also relates to a microscope having an apparatus for calibration of a photosensor chip. In this case, the microscope has at least one light source which emits an illumination light beam which illuminates a sample point-by-point, in the form of a point grid or line-by-line through the optics and via a scanning device, and having a dispersive element which spatially spectrally splits a detection light beam which originates from the sample, and directs it at a light-sensitive area on a photosensor chip. 
   The invention also relates to a method for calibration of a photosensor chip 
   German Patent Application DE 102 53 609 discloses a scanning microscope which has a detector, which is arranged in a detection beam path, for reception of detection light originating from a sample. An optical shutter means is arranged between the sample and the detector, and can be used to block the detection beam path. A control means is provided for open-loop control of the shutter means. The detection beam path can be blocked automatically outside the scanning process and when the detection light power is excessively high. 
   US Patent Application 2003/0223531 discloses the signal gain of a charge multiplication detector or of a CMD (Charge Multiplying Device) being checked with high precision. The CMD has a multiplicity of sections, each of which in turn represents one CMD, so that charge multiplication can be carried out in this way. 
   German Patent Specification DE 100 38 526 discloses a method and an arrangement for fluorescence microscopy, in particular for laser scanning microscopy, fluorescence correlation spectroscopy and scanning near-field microscopy, in order to investigate primarily biological samples, preparations and associated components. The changeover from the detection of a small number of broad spectral color fluorescence bands to the simultaneous recording of complete spectra opens up new capabilities for identification, separation and association of the generally analytical or functional sample characteristics with respect to three-dimensional structure elements or dynamic processes. Simultaneous investigations of samples with multiple fluorophores are thus possible with overlapping fluorescence spectra, even in three-dimensional structures with thick samples. The data recording rate is not reduced by the arrangement. 
   The German translation DE 690 32 582 T2 of a European Patent discloses a silicon avalanche photodiode matrix, which comprises a body composed of n-type silicon semiconductor material, which has phosphorous distributed uniformly in it by means of neutron conversion doping. The resistivity has a maximum discrepancy of 0.5% over at least one active area. The body has a first and second opposite surface, which enclose a radiation input surface, and which are separated from one another by a side surface. Furthermore, a p-n junction is provided and extends between the surfaces, with the side surface being inclined in order to maximize the breakdown voltage. A multiplicity of signal contacts which are arranged on the first body surface form the individual photodiodes. Furthermore, a common electrode is provided, and is arranged on the second body surface. A means for reading signals from the signal contacts is likewise provided. 
   The German translation DE 692 31 482 T2 of a European Patent discloses a CCD image sensor with small pixels. So far, the increasing demand in this field from the consumer market for CCD cameras with ever higher resolution requires that the developers increase the number of pixels of the CCD image sensors that are used in these products. At the same time, the competitive pressure requires not only for the price of these sensors to be maintained or to be reduced, but also for the chip size to be reduced, in consequence resulting in a reduction in the size of the active pixel area. However, this leads to two major problems: the reduction in the sensitivity of the image sensor, and the reduction in the signal-to-noise ratio. The sensitivity of the image sensors is proportional to the pixel area, to the aperture efficiency, to the quantum efficiency, to the integration type and to the charge/voltage conversion factor. The maximum sensor sensitivity of present-day image sensors is normally governed by the noise level of the charge detection amplifiers which convert the charge that has been collected to an output voltage. Without this restriction, it will be possible to detect the individual photons, and in consequence to achieve a photon counting mode, the maximum capability of an image sensor, which is governed by the fundamental laws of physics. 
   British Patent GB 2 348 070 discloses a photocathode CCD sensor having a separate high-field multiplication register. The imaging device is suitable for counting photons, or for applications in low lighting conditions. The apparatus comprises a CCD sensor, which is arranged in such a manner that it receives the electrons which are emitted from a photocathode. The CCD component has a separate high-field multiplication register, to which signals are transferred from an output register. The aim of this is to achieve a better signal-to-noise ratio, and better resolution. 
   German Patent Application DE 10 2004 003 993 discloses an apparatus for verification of the photons in a light beam which originates from a physically limited source, in particular for use in a fluorescence microscope. For this purpose, a detection device is provided in order to increase the maximum counting rate which can be processed by the detection device. The detection device comprises at least two detectors. A component is provided in the beam path of the light beam, by means of which the light beam is split in such a manner that the photons are distributed over the detectors, for verification purposes. 
   In the case of EMCCD chips, electrons that are generated by light in the photo-active area are amplified again in an amplifier register before they are read. The gain of this amplification register depends on various parameters, such as the voltage, temperature, time etc., in a complex manner. The data obtained about the illumination intensity of the EMCCD chips is correspondingly difficult to assess. Problems occur even if the EMCCD chip has a single amplification register (as is the normal case), and a plurality of measurements carried out successively are intended to be compared with one another. Particularly serious problems occur in particular when the EMCCD chip has a plurality of amplification registers, which may behave differently for various reasons. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object of providing an apparatus for calibration of a photosensor chip, by means of which the gain response of an amplification register can be kept constant over time by adjustment and/or closed-loop control of suitable parameters, or the gain response of a plurality of amplification registers can be matched to one another. A further aim is to correct relevant defects caused by the gain response of the amplification register by subsequent processing of the recorded data, as well. 
   The object is achieved by an apparatus which has the features of claim  1 . 
   A further object of the invention is to provide a microscope which has an apparatus for calibration of a photosensor chip, in order that the gain response of an amplification register is kept constant over time by adjustment or closed-loop control of suitable parameters. It is likewise intended to be possible to correct for the effects, which are dependent on the amplification registers, caused by the gain response, by subsequent processing of the recorded data. 
   The object is achieved by a microscope which has the features of claim  23 . 
   In addition, another object of the invention is to provide a method by means of which the gain response of an amplification register can be kept constant over time by adjustment of suitable parameters, or the gain response of a plurality of amplification registers can be matched to one another. It is likewise intended to be possible to correct for the relative effects that are caused by the gain response of the amplification register by subsequent processing of the recorded data. 
   The object is achieved by a method which has the features of Patent claim  44 . For calibration purposes, it is advantageous to provide a reference light source which directs light at at least one part of the photosensor chip. Furthermore, an open-loop or closed-loop control unit is provided, and determines and corrects variances between the individual light-sensitive elements. 
   The reference light source illuminates all of the light-sensitive elements on the photosensor chip. It is likewise feasible for the reference light source to illuminate a limited number of the light-sensitive elements on the photosensor chip. The limited number of light-sensitive elements on the photosensor chip are those light-sensitive elements which make no contribution to the actual measurement. 
   The reference light source defines a reference beam path, and a detection beam is likewise directed at the photosensor chip, with the detection beam path and the reference beam path being partially identical. 
   It is likewise feasible for the detection beam path and the reference beam path to be physically separated from one another. In addition, a filter wheel may be provided in the reference beam path. It is likewise feasible to provide a mask in the reference beam path in order to illuminate selected light-sensitive elements on the photosensor chip. The selected illumination of light-sensitive elements on the photosensor chip can likewise be achieved by an array of microelectronic mirrors (MEMS). 
   The reference light source may be in the form of a point light source. It is likewise feasible for the reference light source to be followed by a perforated shutter in order in this way to produce a point light source. The perforated shutter may be in the form of a fixed shutter or a variable shutter. 
   The reference light source can be switched on and off in such a manner that either only light from the detection beam path or only light from the reference beam path strikes the photosensor chip. The reference light source can be switched on and off via the open-loop or closed-loop control unit. A switchable filter can be provided, and can be used to adjust the light intensity of the reference light via the open-loop or closed-loop control unit. The switchable filter is an acousto-optical element, an electro-optical element or a liquid-crystal display. 
   The open-loop or closed-loop control unit corrects the different gains of the light-sensitive elements on the photosensor chip. This correction is carried out by amplification of the signal by the light-sensitive elements on the photosensor chip. 
   It is likewise feasible for the open-loop or closed-loop control unit to correct the different gains of the light-sensitive elements on the photosensor chip by individual closed-loop control of the gain of the signal from the light-sensitive elements on the photosensor chip. 
   A microscope is provided, having at least one light source which emits an illumination light beam which illuminates a sample point-by-point, in the form of a point grid or line-by-line through the optics and via a scanning device. A dispersive element is provided, which spatially splits a detection light beam which originates from the sample, and directs it at a light-sensitive area on a photosensor chip. In addition, a reference light source is provided in the microscope and directs light at at least one part of the photosensor chip. An open-loop or closed-loop control unit is likewise provided, and determines and corrects variances between the individual light-sensitive elements. 
   A method for calibration of a photosensor chip which has a multiplicity of light-sensitive elements is also advantageous. A reference light source emits a reference light beam. The reference light beam is directed at at least one part of the photosensor chip. Furthermore, an open-loop or closed-loop control unit is provided, and is used to determine and correct variances between the individual light-sensitive elements. 
   Further advantageous refinements of the invention can be found in the dependent claims. 
   Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying figures of drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is illustrated schematically in the drawing, and will be described in the following text with reference to the figures, in which: 
       FIG. 1  shows a schematic illustration of a scanning microscope having a photosensor chip for recording of the signals of the detection light; 
       FIG. 2  shows a schematic illustration of the beam path downstream from the detection light shutter, with the light intensity of the reference light being adjusted by means of a filter wheel; 
       FIG. 3  shows a schematic illustration of the beam path downstream from the detection light shutter, with the detection beam path and the reference beam path running partially identically; 
       FIG. 4  shows a schematic illustration of a detector array, as is used in a photosensor chip according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the schematic design of a laser scanning microscope  1 . The illumination light beam  3  coming from at least one illumination system  2  is passed from a beam splitter or a suitable deflection means  5  to a scanning device  7 . Before the illumination light beam  3  strikes the deflection means  5 , it passes through an illumination pinhole  6 . The scanning device  7  has at least one scanning mirror  9 , which passes the illumination light beam  3  through scanning optics  12  and microscope optics  13  over or through an object  15  the illumination light beam  3  is passed over the object surface, if the objects  15  are not transparent. In the case of biological objects  15  (preparations) or transparent objects, the illumination light beam  3  can also be passed through the object  15 . For this purpose, non-fluorescent preparations are marked, if required, with a suitable dye (not shown, since this is established prior art). The dyes in the object  15  are excited by the illumination light beam  3 , and emit light in their own characteristic range of the spectrum. This light which originates from the object  15  defines a detection light beam  17 , which is passed through the microscope optics  13 , the scanning optics  12  and via the scanning device  7  to the deflection means  5 , through which it passes before passing through a detection pinhole  18  to at least one photosensor chip  19 , which is in the form of a CCD chip, EMCCD chip, APD array or PMT array. The detection light beam  17  originating from or defined by the object is illustrated as a dashed line in  FIG. 1 . Electrical detection signals, which are largely proportional to the power of the light originating from the object  15 , are produced in the photosensor chip  19 . Since, as already mentioned above, light is emitted from the object  15  not just at one wavelength, it is worthwhile arranging a dispersive element  20  upstream of the photosensor chip  19 . The dispersive element  20  splits the detection light beam spectrally, so that the individual wavelengths in the detection light are spatially spectrally separated. The dispersive element  20  is preceded by a lens  21 , which converts the detection light beam  17  to a parallel form. The dispersive element  20  is followed by a further lens  22 , which focuses the spectrally separated beams  24 ,  25  of the detection light beam  17  onto the photosensor chip  19 . The spectrally separated beams  24 ,  25  are at different wavelengths, and thus strike different areas on the photosensor chip  19 . 
   In order to achieve an optimum EMCCD chip response, the gain of the register must be matched to the illumination intensity and the illumination duration on the EMCCD chip. The amplification voltage at the various registers in the photosensor chip  19  can be varied for this purpose. The amplification of the charges on passing through the amplifier path is, however, dependent not only on the amplified voltage, but also on the temperature. Furthermore, for example, manufacturing tolerances result in variations in the gain in the various amplifier registers. Particular difficulties result from the fact that the temperature dependency of the gain is different in different amplifiers. The amplification registers on the photosensor chip  19  may behave differently, and the reasons for this may be as follows: with the same basic setting, the different amplification registers may have a different gain because of manufacturing tolerances, and the amplification voltage may likewise fluctuate from one amplification register to another. A further influence may be a fluctuation in the temperature along the photosensor chip. Changes in the local temperature on the photosensor chip (temperature gradient over the photosensor chip) likewise lead to different gain. Ageing effects in the photosensor chip can likewise influence the gain of the individual amplification registers, and changes in the other environmental conditions likewise have a negative effect on homogenous amplification by the individual amplification registers. The fluctuations and dependencies to be expected are considerably greater than, for example, in the case of photomultipliers. A quantitative comparison between the output signals from two different registers, or from one and the same register for two successive measurements at, for example, different temperatures, is very difficult. Problems occur in particular if one EMCCD chip is equipped with a plurality of amplification registers, and the application demands that the gain of all of the signals be identical. Similar problems, as mentioned above, also occur in the case of APD arrays or photomultiplier arrays. All of these negative influences are detected by means of the open-loop or closed-loop control unit  19   a , and the various gains of the amplification registers are matched. 
     FIG. 2  shows an embodiment of the beam path in a laser scanning microscope downstream from the detection perforated shutter  18 . As already mentioned above, the detection light from the detection perforated shutter  18  is imaged by an optical element  21  on a dispersive element  20 . In this case, the dispersive element  20  is a prism. The dispersive element  20  spectrally splits the detection light. The detection light is imaged by a further optical element  22  on a photosensor chip  19 , with the different spectral components of the detection light being imaged on different areas of the photosensor chip  19 . For example, the photosensor chip  19  may be an EMCCD chip, an APD array or a PMT array. The dispersive element  20  may be a prism, a grating, a holographic element, etc. As already mentioned, the optical element  22  spatially splits the various spectral components of the detection light, so that they strike the photosensor chip  19  at different points. The spectrum of the illumination light is then obtained directly by reading the photosensor chip  19 . However, the light intensity in specific, selected spectral bands can also be determined by suitable evaluation of the data obtained. However, this gives sufficiently good results only when there is no change in the gain of the photosensor chip  19  over time. A further condition is that different pixels or elements of the photosensor chip  19  behave in the same way or in a similar way. In order to determine the unknown gain of an individual amplification register, or of a plurality of different amplification registers, in the photosensor chip  19 , and to the match them, it is sufficient to carry out a reference measurement with known illumination intensity. For this purpose, a plurality of pixels or elements of the photosensor chip  19  which are associated with different amplification registers must have the same illumination intensity applied to them, preferably at the same time. A different illumination intensity is also possible if the differences are either known or do not vary over time. In the embodiment illustrated in  FIG. 2 , a reference light source  30  is provided for application of light to the pixels or elements of the photosensor chip  19 . The reference light source  30  may be provided, for example, by means of an LED, a laser diode, an incandescent bulb, a discharge lamp, etc. In particular, a reference light source  30  which can be switched rapidly, such as an LED or a laser diode, is advantageous, since this allows the illumination for calibration to be restricted to the small number of times that are relevant for calibration. The reference light source  30  is followed by an optical element  31 ,  32 , which images the reference light  40  on the photo sensor chips  19 . A switchable filter  33  is provided between the photosensor chip  19  and the optical system  31 ,  32 , and can be used to adjust the light intensity of the reference light  40 . By way of example, the switchable filter  33  may be an acousto-optical element, and/or an electro-optical element or an LCD, etc. In the embodiment illustrated in  FIG. 2 , the detection light beam  17  and the reference light beam are physically separated from one another. By way of example, the photosensor chip  19  may be illuminated for the reference measurement either before or after the actual measurement. It is likewise feasible for the reference measurement to be carried out during the return, or at the reversal points of the scanner or of a scanning mirror  9 . If the reference measurement is carried out before or after the actual measurement, it is thus possible to also illuminate larger areas of the photosensor chip  19 . In this case, it is also possible to illuminate areas which are required for the detection light during the actual measurement. As already mentioned above, it must be possible to switch the light from the reference light source  30  on and off for the reference measurement. It is likewise feasible for the reference measurement also to be carried out during the actual measurement. In this case, in consequence, only areas of the photosensor chip  19  which are not required for the actual measurement are then illuminated. In this case, care should be taken to ensure that areas associated with each amplification register are illuminated for the reference measurement for each amplification register. 
     FIG. 3  shows a further embodiment of the present invention, in which the detection beam path  17  and the reference beam path  40  are partially identical. The dispersive element  20  is followed by an input element  50 , by means of which the reference beam path  40  is input into the detection beam path  17 . In this case, the reference beam path  40  and the beam paths  24  and  25  which have been spectrally split by the dispersive element then strike different areas of the photosensitive element  19 . The reference light beam originating from the reference light source is converted to a parallel form by a lens  45 . Alternatively, the lens  45  may be a cylindrical lens. With the arrangement proposed in  FIG. 3 , a limited number of pixels are illuminated on the photosensor chip  19 . Pixels which are not required for the actual measurement are preferred for illumination with the reference light beam. 
     FIG. 4  shows a schematic view of the photosensor chip  19 . The photosensor chip  19  has an optically sensitive surface  60 , which is composed of a plurality of lines  601 ,  602 , . . . ,  60   n . In the embodiment illustrated here, at least one line  62  which can be used for the reference measurement is adjacent to the optically sensitive surface  60 . A plurality of addition elements  64  are connected to the optically sensitive surface  60 . A plurality of addition elements  66  are likewise connected to the line for the reference measurement  62 . The content of the addition elements  64  is supplied to appropriate amplifier registers  68 . The contents of the addition elements  66  are likewise supplied to a plurality of amplification registers  68 . The output of each of the amplification registers  68  is passed to the appropriate output amplifiers  70 . As already mentioned, there are two alternative methods for the reference measurement. 
   In the first method, the reference measurement is carried out before or after the actual measurement. In this case, larger areas of the photosensor chip can also be illuminated. For example, this may even be the entire optically sensitive surface  60  of the photosensor chip  19 . In consequence, the light from the light source can be switched on and off appropriately for this measurement. The data from the reference measurement is supplied to the appropriate amplification registers  68  via the appropriate addition elements  64  and  66 . 
   In a second possible method, the reference measurement is carried out during the actual measurement. In this case, of course, only areas of the optically sensitive surface  60  of the photosensor chip which are not required for the actual measurement are illuminated. In this case, for example, a line  62  is illuminated for the reference measurement. In this case, care must be taken to ensure that the areas of the photosensor chip  19  which must be illuminated are those which are associated with those amplification registers for which the reference measurement is intended to be carried out. 
   The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible and/or would be apparent in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and that the claims encompass all embodiments of the invention, including the disclosed embodiments and their equivalents.