ILLUMINATION FILTER FOR AN ILLUMINATION FILTER SYSTEM OF A SURGICAL MULTISPECTRAL FLUORESCENCE MICROSCOPE HAVING A SPATIAL FILTER PATTERN

Abstract

The invention relates to an illumination filter (36, 44) for an illumination filter system (2) for medical imaging, in particular multispectral fluorescence imaging, as performed e.g. in a microscope (1) or endoscope, in particular a multispectral fluorescence microscope. The present invention provides an illumination filter for medical imaging, in particular a multispectral fluorescence imaging, that is capable of capturing simultaneously more than one fluorescence signal, and allow a homogeneous illumination for obtaining different images from the object illuminated by comprising a spatial filter pattern (43) masking a defined filtering fraction of a first illumination path (47) on the filter and masking a defined filtering fraction of a second illumination path (48, 49, 50) on the filter, wherein the filtering fraction of the first and the second illumination paths (47, 48, 49, 50) are different. The invention further relates to an illumination filter system (2) for medical imaging, in particular multispectral fluorescence imaging, as performed e.g. in a microscope (1) or endoscope, in particular a multispectral fluorescence microscope, comprising such illumination filter (36, 44).

Claims

1. An illumination filter (36, 44) for an illumination filter system (2) for medical imaging (1), in particular multispectral fluorescence imaging, comprising a spatial filter pattern (43) masking a defined filtering fraction of a first illumination path (47) on the filter and masking a defined filtering fraction of a second illumination path (48, 49, 50) on the filter, wherein the filtering fraction of the first and the second illumination paths (47, 48, 49, 50) are different.

2. The illumination filter (36, 44) of claim 1, wherein a filter coating (42) is applied as the spatial filter pattern (43) on a substrate (41), said coating (42) defining the filtering fraction of the first and the second illumination paths (47, 48, 49, 50) on the substrate (41).

3. The illumination filter (36, 44) of claim 2, wherein the spatial filter pattern (43) is a band-stop filter pattern (43a) or a band-pass filter pattern (43b).

4. The illumination filter (36, 44) of claim 3, wherein a band-stop filter coating (42) or a band-pass filter coating (46) is applied as the spatial filter pattern (43) on the substrate (41).

5. The illumination filter (36, 44) of claim 1, wherein the spatial filter pattern (43) extends over a plurality of illumination paths (47-50) and the filtering fraction is different in each illumination path (47-50).

6. The illumination filter (36, 44) of claim 5, wherein the plurality of illumination paths (47-50) is arranged along an axis of the illumination filter (36, 44).

7. The illumination filter (36, 44) of claim 1, wherein the spatial filter pattern (43) comprises a plurality of filter patches (51).

8. The illumination filter (36, 44) of claim 7, wherein respective centers (53) of adjacent filter patches (51) are essentially spaced equidistantly and respective areas (A) of the filter patches (51) vary.

9. The illumination filter (36, 44) of claim 7, wherein the filter patches (51) are filter squares (52).

10. The illumination filter (36, 44) of claim 9, wherein respective diagonal lengths (1) of the filter squares (52) varies in different illumination paths (47-50).

11. The illumination filter (36, 44) of claim 10, wherein the diagonal length (l) of the filter squares (52) varies along an axis of the illumination filter (36, 44).

12. An illumination filter system (2) for a microscope (1), in particular a multispectral fluorescence microscope, comprising an illumination filter (36, 44) having a spatial filter pattern (43) masking a defined filtering fraction of a first illumination path (47) on the filter and masking a defined filtering fraction of a second illumination path (48, 49, 50) on the filter, wherein the filtering fraction of the first and the second illumination paths (47, 48, 49, 50) are different.

13. The illumination filter system (2) of claim 12, wherein the illumination filter (36, 44) is configured to be moved from a first operation position (37), in which the first illumination path (47) is in optical communication with a light source (4), to a second operation position (38), in which the second illumination path (48-50) is in optical communication with a light source (4).

14. The illumination filter system (2) of claim 12, comprising a first illumination filter (36) having a spatial band-stop filter pattern (43a) and a second illumination filter (44) having a spatial band-pass filter pattern (43b), wherein each of the first and second illumination filters (36, 44) is configured to be moved from a first operation position (37), in which the first illumination path (47) is in optical communication with a light source (4), to a second operation position (38), in which the second illumination path (48-50) is in optical communication with a light source (4).

15. The illumination filter system (2) of claim 12, further comprising a blocking filter (35) adapted to quench light of a fluorescence emission band (Em.1, Em.2).

Description

BRIEF DESCRIPTION OF THE DRAWING VIEWS

[0035] In the drawings, the same reference numeral is used for elements which correspond to each other with respect to their design and/or their function.

[0036] In the drawings:

[0037] FIG. 1 shows a schematic block diagram of a medical imaging apparatus in which the illumination filter or the illumination filter system of the invention can be applied;

[0038] FIG. 2A shows the excitation and emission bands and spectra of 5-ALA/ppIX;

[0039] FIG. 2B shows the excitation and emission spectral and bands of Indocyanine Green (ICG);

[0040] FIG. 2C shows how the spectral portions of the light used in an exemplary embodiment of the present invention is distributed over a spectrum of visible and near infrared (NIR) light;

[0041] FIG. 3A shows spectral characteristics (transmittance) of an illumination filter used in an illumination filter system according to a first embodiment;

[0042] FIG. 3B shows an illumination system of a first embodiment comprising the filters having the spectral characteristics of FIG. 3A;

[0043] FIG. 3C shows the illumination filter of the illumination filter system of FIG. 3B in more detail together with the spectral characteristics of the illumination filter in the respective illumination path;

[0044] FIG. 4A shows an illumination filter system according to a second embodiment comprising an illumination filter in another exemplary embodiment;

[0045] FIG. 4B shows the spectral characteristics of the individual illumination filters encompassed by the illumination filter system as shown in FIG. 4A as well as the spectra characteristics of the whole illumination filter system;

[0046] FIG. 5A shows the spectral characteristics of the illumination filter of FIG. 5B in the respective illumination path;

[0047] FIG. 5B shows an illumination filter in an exemplary embodiment;

[0048] FIG. 5C shows another illumination filter of another embodiment;

[0049] FIG. 5D shows the spectral characteristics of the illumination filter of FIG. 5C in the respective illumination path;

[0050] FIG. 6A shows the spectral characteristics (transmittance and reflectance) of filters and the beam splitter of the observation system of FIG. 1;

[0051] FIG. 6B shows an observation system comprising filters and a beam splitter having the spectra characteristics shown in FIG. 6A; and

[0052] FIG. 7 shows an illumination filter according a further embodiment having a disc-shaped substrate.

DETAILED DESCRIPTION OF THE INVENTION

[0053] First, the design and function of a medical imaging apparatus 1, such as a microscope 1 or endoscope comprising an illumination filter system 2 as well as an observation system 3 is explained with reference to FIG. 1. The microscope 1 comprises a light source 4 that emits an illumination light 5 onto an object 6 to be observed. The illumination filter system 2 is in optical communication with the light source 4 as well as the object 6, that is, the illumination system 2 is in the light path of the illumination light 5 from the light source 4 to the object 6.

[0054] The illumination filter system 2 filters and spectrally modifies the illumination light 5. It adjusts the intensity of specific portions within the illumination light 5 relative to each other using an illumination filter of the present invention, as will be explained in more detail below with respect to preferred embodiments of the illumination filters and the illumination filter system 2 of the present invention. Thus, spectrally modified illumination light 7 exits the illumination filter system 2 and is directed onto the object 6. The spectrally modified illumination light 7 is specifically adapted in order to improve a multi-spectral fluorescence microscopying method. In the shown embodiment, the spectrally modified illumination light 7, provided by the illumination filter system 2, is adapted in order to capture a reflected visible image as well as two fluorescence signals simultaneously, which will also be described in more detail below.

[0055] The light source 4 as well as the illumination filter system 2 are both regulated by a controller 8. The controller 8 is connected with the light source 4 via a bi-directional signal line 9, via which the controller 8 may regulate, for example, the intensity of the illumination light 5 or, in case of a light unit having two different light sources, selects the respective light source for emitting the illumination light 5. Via another bi-directional signal line 10, the controller 8 also regulates the illumination filter system 2, e.g. by setting the filters for adjusting the degree of attenuation of certain filters in order to adjust the ratio of light intensities of certain spectral bands included in the spectrally modified illumination light 7. Using bi-directional signal lines 9, 10 allows a loop-control of the settings of the light source 4 and the filter system 2.

[0056] The controller 8 itself is coupled via a further bi-directional signal line 11 with a controller interface 12 for inputting settings of the microscope.

[0057] A light image 13 is sent from the object 6 to the observation system 3 at defined microscope settings 14. In FIG. 1, the microscope observation settings 14, such as the working distance, magnification, elements used the observation system 3 are represented as a box. The controller 8 may adjust the observation parameters of the microscope via a further signal line 15. The light image 13 sent from object 6 to observation system 3, is split into a first light portion 16, 17 along the first light path 18 and the second light portion 20 along a second light path 19 in a beam splitter 21 of the observation system 3. The first light portion 16, 17 comprises two fluorescence emission bands. The second light portion comprises reflected visible light (VISR), i.e. the visible light reflected from the object. The first light portion 16, 17 passes a band-pass filter 22. The first emission band and the second fluorescence emission band of the first light portion 16′, 17′ exiting the band-pass filter 22 are captured by the fluorescence sensor 23. The fluorescence sensor 23 may for example be a fluorescence camera, for example, an NIR camera if the fluorescent emission bands are in the near infrared range.

[0058] The second light portion 20 passes through the band-stop filter 24. The reflected visible light of the second light portion 20′ exiting the band-stop filter 24 is captured by a second sensor 25. The second sensor 25 may be a visible camera such as for example a charge coupled device (CCD).

[0059] The first sensor 23, sends, via a signal line 26 a first image read-out I.sub.1 comprising information on the captured fluorescence emission bands to a processing unit 28. A second image readout I.sub.2 is sent via a signal line 27 from a second sensor 25 to the processing unit 28. The image readout I.sub.2 contains the image data of the reflected visible light 20′ captured by the sensor 25.

[0060] The processing unit 28 is further connected to each of the sensor 23 and the sensor 25 by a bi-directional signal lines 29 and 30, respectively. Via these bi-directional signal lines 29, 30, the processing unit 28 controls the sensors 23 and 25 and reads out the settings of said sensors 23, 25 allowing a loop-control of the sensors 23, 25 by the processing unit 28. The processing unit 28 itself may receive the settings from a user of the microscope by inputting the corresponding parameters into the controller interface 12 and sending the settings via a signal line 31.

[0061] The processing unit 28 may process the image readouts I.sub.1 and I.sub.2. In a preferred embodiment, a pseudo-image P may be generated by the processing unit 28 and sent from the processing unit 28 via signal line 32 to a display device 33, such as for example, a monitor. Even though it is not shown in FIG. 1, the pseudo-image P may be stored in a documentation system. The pseudo-image P may be a merger of the image readout I.sub.1 from the fluorescence (FL) sensor 23 and the VISR image readout I.sub.2 from the visible camera 25. It is to be noted that the merged pseudo-image P is not merely an overlay of the image readouts I.sub.1 and I.sub.2. The pseudo-image P does not obscure any image readout information (which would be the case by overlaying the two image readouts I.sub.1 and I.sub.2), but rather presents the fluorescence image readout within the VISR image readout I.sub.2 in a natural way, resembling the injection of a bright dye. The pseudo-image P may be generated in real time allowing the user of the microscope 1 to capture a combination of the white light image and fluorescence light signals in one merged image.

[0062] In order to improve the quality of the pseudo-image P, the image readouts I.sub.1 and I.sub.2 may be homogenized. The homogenization may correct inhomogeneities in illumination and vignetting of the image optics which would otherwise result in an uneven brightness across the field of view as the periphery of the field of view may be significantly darker at the periphery than in the center. Further, the homogenized image readouts I.sub.1 and I.sub.2 may be aligned with each other before merging. For example, a spatial correction transformation may be performed to correct alignment errors in the relative position of the two sensors 23 and 25 and digital filters may be applied taking into consideration translation, rotational and magnification mismatches between the sensors 23 and 25. Further, a threshold may be set on an image readout, in particular the image readout I.sub.1 received from the fluorescence sensor 23 in order to remove a dark current from the fluorescence sensor 23, thus avoiding a false contribution in measurement of the fluorescence signals.

[0063] The controller 8 may provide the processing unit 28, via a signal line 34, with data of the microscope settings 14 that may be inputted by the user via the controller interface 12, such as for example, the working distance, magnification, as well as settings of the illumination filter system 2, the light source 1.

[0064] FIGS. 2A to 2C illustrate the excitation and emission spectra of the fluorophores 5-amino levolinic acid-induced protoporphyrin IX (5-ALA/ppIX) (FIG. 2A) and indocyanine green (ICG) (FIG. 2B). Ex.1, ppIX indicates the excitation band of 5-ALA/ppIX, Em.1, ppIX indicates the fluorescence emission band of protoporphyrin IX, Ex.2, ICG indicates the fluorescent excitation band of ICG, and Em.2, ICG indicates the fluorescence emission band of ICG.

[0065] The graph of FIG. 2C shows the fluorescence excitation and emission bands as well as the visible spectrum, in particular the visible reflected (VISR) light over the wavelengths. As will be explained in more detail below, in the shown exemplary embodiments, the VISR spectrum defined in FIG. 2C is directed onto the second sensor 25, the VISR-sensor. The fluorescence emission bands Em.1, ppIX and Em.2, ICG are directed to the sensor 23, i.e. the fluorescence sensor. Thus, the spectrum of the visible reflected light indicated in FIG. 2C corresponds to the second light portion 20′ in FIG. 1. The two fluorescence emission spectra Em.1, ppIX and Em.2, ICG indicated in FIG. 2C correspond to the first light portions 16′ and 17′, respectively.

[0066] In order to clearly distinguish the spectral bands, in particular, exclude the fluorescence excitation band of photoporphyrin IX and avoid an overlapping of the fluorescence excitation bands with the visible spectrum, in particular the visible reflected light, the illumination filter system 2 and the observation system 3 of the present invention are used, as will be explained in the following.

[0067] First, the design and function of an illumination filter system 2 according to a first embodiment is explained with reference to FIGS. 3A, 3B and 3C. FIG. 3B shows an exemplary embodiment of an illumination filter system 2 in optical communication with a light source 4. The illumination filter system 2 is arranged in the light path of the illumination light 5. The illumination filter system 2 comprises a first optical filter 35. The first optical filter 35 may be a band-stop filter. The first optical filter 35 is adapted to quench light of the fluorescence emission bands. In the shown embodiment, the fluorescence emission bands Em.1 and Em.2 for ppIX and ICG are quenched, respectively. The first optical filter 35 is always in optical communication with the light source 4, thus, the fluorescence emission bands to be detected by sensors 23 are always eliminated from the illumination light 5 by the illumination filter system 2.

[0068] The illumination filter system 2 furthermore comprises an illumination filter 36 of the present invention in an exemplary embodiment as a second optical filter 36. The second optical filter 36 may be a band-stop filter. In the following, this illumination filter 36 is referred to as the second optical filter 36. The second optical filter 36 is configured to be moved from a first operation position 37, indicated in dashed lines in FIG. 3B. In the first operation position 37, a first illumination path 47 of the second optical filter 36 is in optical communication with the light source 4 and first optical filter 35, that is, the illumination light 5 passes both the first optical filter 35 and the first illumination path 47 of the second optical filter 36 when the latter is in its first operation position 37.

[0069] The second optical filter 36 is configured to be moved from the first operation position 37 to a second operation position 38, in which another, second illumination path 48 of the second optical filter 36 is in optical communication with the light source 4 and the first optical filter 35. In the second operation position 38, the second optical filter 36 is arranged in the light path of the illumination light 5, so the illumination light 5 passes the first optical filter 35 and the second illumination path 48 of the second optical filter 36 if the latter is in its second operation position 38.

[0070] The arrow indicates the transition 39 of the second optical filter 36 from its first operation position 37 to its second operation position 38. This transition 39 may be performed, e.g. by displacing the filter 36 between its first 47 and second illumination path 48.

[0071] As can be seen in FIG. 3A, the first optical filter 35 is a band-stop filter adapted to transmit light of the fluorescence excitation bands Ex.1 of ppIX and Ex. 2, ICG as well as the whole spectrum of visible light and a portion of NIR light adjacent to the visible light, except for the quenched fluorescence emission band Em.1 of ppIX.

[0072] In one embodiment, the first optical filter 35 may be a dual-notch filter quenching the excitation bands of Em.1 ppIX and Em.2 ICG.

[0073] The illumination filter, here the second optical filter 36 is adapted to transmit the visible reflected light, except for the fluorescence excitation bands Ex.1 and Ex.2 attenuated by said second optical filter 36.

[0074] This way, the intensity of the fluorescence excitation bands may be adjusted, as will be described with more detail below referring to FIG. 3C.

[0075] The illumination filter system 2 of the present invention allows to adjust the intensity of fluorescence excitation relative to white light and/or the intensity of difference fluorescence excitation bands. Such relative intensity adjustment is useful when for example the maximum excitation power is needed, while maximum white light illumination is too bright for the use of the eyepiece of a microscope.

[0076] In FIG. 3C, the illumination filter 36 of FIG. 3B is shown in more detail. The illumination filter 36 comprises a substrate 41, which is a glass slide having a longitudinal direction L defining the axis 40 of the filter, i.e. the axis of movement. The filter 36 is divided into four sectors 36a-36d, which are adjacent to one another on the slide along the axis 40 of the slide 41. In each sector 36a-36d, the filter 36 is masked with a different defined filtering fraction for a first to fourth illumination path 47-50 of the illumination filter 36. In the shown embodiment, a filtering compound, indicated as dots, is embedded in the material of the filter substrate 41. Different concentrations of filtering material are embedded in the individual sectors 36a-36d. In sector 36a, no filtering material at all is embedded, meaning that, in the second illumination path 48, which lies in the sector 36a, the filtering fraction is zero, i.e. the adjustable illumination filter 36 has no filtering effect here. In the sector 36d, which is on the opposite end of the slide 41, relative to sector 36a, a high concentration of filtering material is embedded in the substrate 41, resulting in 100% coverage, i.e. in a filtering fraction of 100% in sector 36d, encompassing the first illumination path 47. In between these extremes, two further sectors 36b and 36c are arranged, encompassing a third 49 and fourth illumination path 50. In the sector 36b, the filtering fraction is about 25%, i.e. about 25% of light having the wavelength of the excitation bands Ex. 1, Ex. 2 is filtered by the band-stop filtering material embedded in the substrate. In sector 36c, the filtering fraction is about 75%, meaning that only 25% light having the wavelength of the fluorescence excitation bands Ex.1 and Ex. 2 may transmit the illumination filter 36 in its fourth illumination path 50.

[0077] Using the illumination filters 36 of the embodiment shown in FIG. 3, one may choose between four discreet filtering fractions to be applied for adjusting the intensity of the fluorescence excitation bands Ex. 1 and Ex. 2, thereby stepwise adjusting the intensity of the excitation light.

[0078] The illumination filter system 2 of the present invention may be used to adjust the intensity ratio of the excitation light and the white (or visible) light, for example using an illumination filter system 2 according to a second embodiment. The design and function of the illumination filter system 2 of the second embodiment is explained with reference to FIGS. 4 and 5 in the following.

[0079] The design and function of another embodiment of an illumination filter system 2 will be explained with reference to FIGS. 4 and 5. The embodiment of the illumination filter system 2 shown in FIGS. 4 and 5 comprises the first optical filter 35, which may be a dual-notch filter to eliminate any light at the fluorescence emission bands Em.1, ppIX and Em.2, ICG. The illumination filter system 2 of the embodiment shown in FIGS. 4 and 5 furthermore comprises a (first) illumination filter as a second optical filter 36 adapted to attenuate light of the fluorescence excitation bands Ex.1, ppIX and Ex.2, ICG. The second optical filter 36 is configured to be moved from a first operation position 37, in which its first illumination path 47 is in optical communication with the light source 4 and the first optical filter 35, to a second operation position 38, in which its second illumination path 48 is in optical communication with the light source 4. Transition 39 between the first operation position 37 and the second operation position 38 is achieved by moving the second optical filter 36 along an axis of movement 40.

[0080] The second optical filter 36 may comprise a rectangular substrate in the shown embodiment a glass slide, 41 and the axis of movement 40 corresponds to the longitudinal direction L of the substrate 41.

[0081] A band-stop filter coating 42 is applied in a spatial pattern 43 of the substrate 41, which pattern 43 will be described in more detail below.

[0082] The illumination filter system 2 of the embodiment shown in FIGS. 4 and 5 furthermore comprises another (second) illumination filter 44 as a third optical filter 44 adapted to transmit light of the fluorescence excitation bands Ex.1, ppIX and Ex.2, ICG only. The second illumination filter (i.e. the third optical filter) 44 may be a band-pass filter 44. Like the second optical filter 36, the third optical filter 44 is configured to be moved from a first operation position 37, in which the first illumination path 47 of the third optical filter 44 is in optical communication with the light source 4, to a second operation position 38 in which the third optical filter 44 is in optical communication with the first optical filter 35 and the light source 4.

[0083] The third optical filter 44 is likewise comprised in a substrate 45, similar to the substrate 41, which is also composed of a clean glass slide having a rectangular shape, and which can be moved along its longitudinal axis L, which is identical to the axis of movement 40, during transition 39 from the first operation position 37 into the second operation position 38.

[0084] The substrate 45 of the third optical filter 44 comprises a band-pass filter coating 46 which is applied in a spatial band-pass filter pattern 43b similar to the spatial band stop filter pattern 43a of the second optical filter 36 on the substrate 41.

[0085] The spatial pattern 43 allows gradual attenuation of the intensity of fluorescence excitation bands Ex.1, ppIX and Ex.2, ICG from 100% to 0% by means of the second optical filter 36 as well as the intensity of the white light from 0 to 100% transmittance by means of the third optical filter 44.

[0086] Filters 36 and 44 are variable filters allowing adjustment of transmittance of the fluorescence excitation bands and the white light intensity, respectively, depending on their position in the path of illumination light 5 along the longitudinal direction L/the axis of movement 40. This is achieved by the spatial pattern 43 of coating 42, 46 which is identical in the shown in embodiment for both, the second optical filter 36 and the third optical filter 44.

[0087] The spatial pattern 43 masks a filtering fraction of 100%, e.g. has coverage of 100% of a first illumination path 47. Coverage here means the ratio of coated areas with respect to the total area of an illumination path which corresponds to the area of light passing through the respective filters 44, 36 of the illumination filter system 2.

[0088] The spatial pattern 43 has coverage of less than 100% of a second illumination path 48 on the substrate 41, 45. In the shown embodiment, the filtering fraction masks 0% in the exemplary second illumination path 48 meaning that no coating 46 at all is applied to the substrate 41, 45 on the position corresponding to the second illumination path 48. In between the first illumination path 47, which is on one end of substrate 41, 45 and the second illumination path 48, which is on the opposite end along the axis 40, i.e. longitudinal direction L of the substrate 41, 45, a plurality of illumination paths are provided from which as an example, two further illumination paths 49 and 50 are shown in FIGS. 5B and 5C.

[0089] The plurality of illumination paths 48, 49, 50, 47 are arranged along the axis of movement 40 on the substrate 41, 45. The spatial pattern 43 comprises a plurality of coating patches 51. In the shown embodiment, the coating patches 51 are coating squares 52. The center 53 of adjacent patches 51/squares 52 are spaced apart equidistantly, i.e. at the same distance d from one another. However, the area A of the patches 51 varies along the axis of movement 40. In the shown example, the length l of the patches 51, corresponds to the diagonal length l of the coating squares 52 that varies along the axis of movement 40. In detail, i.e. the diagonal of the coating squares 52 increases gradually from an area adjacent to the second illumination path 48 having no coating in direction along the axis of movement 40 to the first illumination path 47 having complete, i.e. 100% coating. The spatial pattern 43 starts from a filtering fraction of 100% spatial coverage in the first illumination path 47 shown on the left in FIG. 5, and the filtering fraction/coverage drops gradually along the axis of movement 40 until it is completely absent in the second illumination path 48 on the opposite side, the right side shown in FIG. 5.

[0090] The coverage, i.e. the ratio of coated versus total area of an illumination path determines the percentage of transmittance of the fluorescence excitation bands in case of a second band-stop filter 36 as well as the transmittance and thus intensity of the white light having a wavelength of about 400-750 nm in the shown embodiment by the band-pass filter 44. This can be seen for the four exemplary illumination path 47 to 50 in FIGS. 5A and 5D.

[0091] The intensities of the fluorescence excitation bands as well as the white light potion in the spectrally-modified illumination light 7 can thus be individually adjusted. The combination of all three filters results in a spectrum of the spectrally-modified illumination light 7 with the desired ratio between white light and excitation intensities as it is shown for one example in FIG. 4B. FIG. 4B shows, from left to right, the quenching of the fluorescence emission bands by the dual-notch filter 35, the attenuation of the white light portion by the band-pass filter 44, the attenuation of the fluorescence excitation light by the second band-stop filter 36. All three of these filters are in optic communication and result in the spectrally-modified illumination light 7 shown on the right side of FIG. 4B.

[0092] In order to obtain a nearly gradual changing coverage of the coating 42, 46, the distance d between the centers 53 of the coating patches 51/coating squares 52 should be significantly shorter than the diameter 54 of an illumination path. This way, filtering becomes more homogeneous. Significantly shorter in this respect means a magnitude of at least 10.

[0093] An exemplary embodiment of an observation system 3 is explained with reference to FIG. 6. FIG. 6B shows a schematic design of the observation system 3 and FIG. 6A shows the spectral characteristic (transmittance and reflection) of the components of the observation system 3.

[0094] The observation system 3 comprises a beam splitter 21 adapted to split the light image 13 of the illuminated object 6 into a first light portion 16, 17 along a first light path 18 and into a second light portion 20 along a second light path 19. The first light portion 16, 17 comprises the fluorescence emission bands Em.1 of ppIX and Em.2 of ICG. The second portion 20 comprises reflected visible light. In the shown embodiment, the beam splitter 21 is a polychroic mirror 55 that reflects light having a wavelength in the fluorescence emission bands Em.1 and Em.2 and transmits all light of the visible spectrum, except for the fluorescence emission band Em.1 falling into the white light spectrum. The observation system 3 furthermore comprises the two filters 22 and 24 as well as the two senses 23 and 25 already explained with respect to FIG. 1 above.

[0095] Further, filter 22, through which the first light portion 16, 17 passes before reaching the sensor 23, may be a band-pass filter adapted to transmit light of fluorescence emission bands Em.1 and Em.2 only. The filter 24, through which the second light portion 20 passes before reaching a sensor 25 may be a band-stop filter adapted to quench light of fluorescence emission bands Em.1, Em.2 as well as the fluorescence excitation bands Ex.1, Ex. 2.

[0096] Finally, FIG. 7 shows a further embodiment of an illumination filter 36. The illumination filter 36 of FIG. 7 has a disc-shape substrate 41. Said substrate 41 may be rotated about its center C along its axis of movement 40, which is a rotational movement indicated by the arrow. A series of eight windows 56a-56h is arranged along the axis of movement 40 in the disc-shaped substrate 41. Each window 56a-56h defines an different illumination path of the illumination filter 36 of the embodiment shown in FIG. 7. Each window 56a-56h is coated with a different band-stop filter coating 42, the different band-stop filter coatings 42 differing in the concentration of the band-pass filter material included therein, this way providing different filtering fractions for each window 56a-56h.

TABLE-US-00001 REFERENCE NUMERALS  1 Microscope/Medical Imaging Apparatus  2 Illumination Filter System  3 Observation System  4 Light Source  5 Illumination Light  6 Object  7 Spectrally Modified Illumination Light  8 Controller  9 Signal Line 10 Signal Line 11 Signal Line 12 Controller Interface 13 Light Image 14 Microscope Settings 15 Signal Line 16, 16′ 1.sup.st Light Portion 17, 17′ 1.sup.st Light Portion 18 1.sup.st Light Path 19 2.sup.nd Light Path 20 2.sup.nd Light Portion/VISR 21 Beam Splitter 22 Band-Pass Filter 23 1.sup.st Sensor/FL-Sensor 24 Band-Stop Filter 25 2.sup.nd Sensor/VISR-Sensor 26 Signal Line 27 Signal Line 28 Processing Unit 29 Signal Line 30 Signal Line 31 Signal Line 32 Signal Line 33 Display Device 34 Signal Line 35 Blocking (Band-stop) Filter 36 1.sup.st Illumination Filter/2.sup.nd Optical (Band-stop) Filter 36a-36d Sector 37 1.sup.st Operation Position 38 2.sup.nd Operation Position 39 Transition 40 Axis of Movement 41 Substrate 42 Band-Stop Filter Coating 43 Spatial Pattern 43a Band-stop filter pattern 43b Band-pass filter pattern 44 2.sup.nd Illumination Filter/3.sup.rd optical (Band-pass) filter 45 Substrate 46 Band-Pass Filter Coating 47 1.sup.st Illumination Path 48 2.sup.nd Illumination Path 49 3.sup.rd Illumination Path 50 4.sup.th Illumination Path 51 Filter Patch/Coating Patch 52 Coating Square 53 Center of Patch/Square 54 Diameter of Illumination Path 55 Polychroic Mirror 56a-56h Window A Area of Coating Patch/Square d Distance Between Adjacent Centers C Center Ex. 1 Excitation Band of ppIX Em. 1 Emission Band of ppIX Ex. 2 Excitation Band of ICG Em. 2 Emission Band of ICG I.sub.1 Image Readout (FL) I.sub.2 Image Readout (VISR) L Longitudinal Direction I Length of Patch/Diagonal of Square P Pseudo-image VISR Visible Reflected Light