Patent Publication Number: US-2018049644-A1

Title: Observation apparatus and method for visual enhancement of an observed object

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of European patent application number 16184311.5 filed Aug. 16, 2016, the entire disclosure of which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to an observation apparatus and method for visual enhancement of an observed object, in particular a medical observation apparatus and method. 
     BACKGROUND OF THE INVENTION 
     Imaging devices are known which utilize projectors to enhance visually the veins on the body of a patient by projecting an image directly on to the patient. These devices are used to increase visibility of the veins for clinical assistance, such as facilitating the setting of a syringe. The operating principle is based on an NIR camera which images the tissue using an NIR spectrum in which veins exhibit a higher contrast to the surrounding tissue than in the visible-light range. The visible-light range designates electromagnetic radiation with wavelengths from about 390 to 700 nm. The contrast of the NIR image is enhanced and displayed as an image in the visible-light range on the skin by means of the projector. The veins of the image are projected onto the veins of the body which are thus made more visible. 
     This visualization method allows easy and direct visual inspection of the patient but suffers from several drawbacks. 
     First, this technology relies on using an NIR camera which does not have sensitivity in the visible-light range of the image projected onto the patient. Thus, the camera input is not influenced by the projection of visible-light images onto the area which is observed by the NIR camera. The technology cannot be expanded to enhance visibility of features that need to be captured in the visible-light range. 
     Another drawback is that the projected image, which is based on an image in the non-visible-light range, does not necessarily reflect the actual visual appearance of the vein. The extent and boundaries of the veins in the non-visible light range may not coincide with the extent and boundaries in the visible-light range. 
     Finally, the contrast of the image projected onto the patient depends on the ambient light conditions. In very bright surroundings, the brightness of the projector may not be sufficient to produce high contrasts on the patient. Further, visibility of the projected image depends on the reflectance of the surface onto which the image is projected. For example, wet surfaces, such as bloody surfaces or surfaces treated with an ointment may obscure the projected image. 
     SUMMARY OF THE INVENTION 
     It is the aim of the invention to provide an observation apparatus and method for visual enhancement of an observed object which does not have the above-mentioned drawbacks and thus can be used for a wider range of imaging applications, such as for example direct visualization of tumors, lymph nodes, and that can also be used in combination with fluorescent tissue markers. 
     According to the invention, this aim is achieved by providing an observation apparatus for visual enhancement of an observed object, comprising an output color image projector for projecting a time sequence of output color images onto a projection area on the object, having an output spectrum including, at least in the visible-light range, a set of output spectral bands, a multispectral camera system for capturing a time sequence of input color images from the projection area in at least two input spectral bands, the input spectral bands being different from the output spectral bands at least in the visible-light range, wherein the observation apparatus is configured to compute the output color images based on the input color images and to project the output color images onto the projection area in real time. 
     Further, the above aim is achieved by a method for visually enhancing the observation of an object, the method comprising the steps of capturing input color images at least in the visible-light range from an observation area on the object with an input spectrum of input spectral bands, both computing output color images based on the input color images and projecting the output color images onto the observation area at least in the visible-light range using an output spectrum of output spectral bands, which differ from the input spectral bands, in real time. 
     The apparatus and method according to the invention allows to visualize features of the object right on the object where these features are observed. The input multispectral camera records the input color images using a first set of at least two spectral bands. Thus, the input images correspond to what an observer would see by looking at the object. The output color image projector projects onto the projection zone output color images using a different set of spectral bands, but also in the visible-light range. The projection of the output color images does not influence the capturing of the input color images, as both are composed of different spectral bands, which do not interfere. Although the observer sees a superposition of the output color image and the reflections from the ambient light on the projection area, the input camera system does not capture the reflections from the output color images. 
     By modifying colors, contrast or brightness in the output color image, and/or restricting these changes to certain patterns recognized by the apparatus in the input color image, specific features of the object can be visually enhanced right on the object to increase visibility for an observer or facilitate recognition of certain features. Further, the quality of the output color images can be improved by performing noise reduction or spatial deconvolution algorithms. 
     If the input camera system is also sensitive to input spectral bands in the NIR or IR-range, the output color images may be used to represent IR- or NIR-image data in the visible-light range. This allows e.g. to display areas of different temperature and/or areas, which are marked by a fluorescent tissue marker, in the visible-light range directly on the object without impairing the capturing of the input color images. 
     The above solution according to the invention can be improved by the following features which can be combined independent of one another. 
     According to one aspect of the invention, the output spectral bands have a different color appearance. The output spectral bands may comprise predetermined narrow-band spectral bands such as e.g. generated by lasers of different colors. In particular, the output spectral bands may be narrower and/or less in number than the output spectral bands. Thus, more and/or wider spectral bands are available for the camera system, which allows to capture a greater variety of features of the object, which may be present in only one spectral band. 
     In order to be able to display all colors using the output spectral bands, it is preferred that the output spectral bands form a preferably additive color space. In particular, a tri-band output spectrum, such as an RGB spectrum can be used. This approach only excludes three spectral bands from the input spectrum and thus allows most information gathered from the object in the input image for being captured by the camera system. 
     The input color image and output color image should be spatially congruent and of the same orientation in the projection area. This makes sure that features in the projected output color image are congruent and aligned with, or mapped identically onto, the corresponding features of the object. 
     It is further preferred that the projection area of the output color image projector is identical to a field of view or observation area of the camera system. Thus, the output color images cover all that is seen by the camera system. 
     Preferably, there is no overlap between the input spectral bands and the output spectral bands. This makes sure that the output color images cannot be captured by the camera system. 
     If a projector and/or a camera is used which is not limited to specific spectral bands, spectral separation of the input color images from the output color images can be achieved if the output color image projector comprises a band-pass output filter system which blocks the input spectral bands and/or in that the camera system comprises a band-pass input filter system which blocks the output spectral bands. 
     If the projector uses narrow-band and discrete output spectral bands, as in particular generated by one or more lasers of different colors, simple notch filters may be used to block the output spectral bands. A projector may e.g. use at least one of a blue, red and green laser to project color images onto the object. 
     In the context of this description, the expression “blocking spectral bands” comprises both eliminating and attenuating the spectral bands. However, elimination is preferred. 
     The observation apparatus may further comprise an image processor which is connected to the camera system for receiving the input color images and connected to the output color image projector for outputting the output color images. The input color images may contain input pixels and the output color images may contain output pixels. The image processor may be a hardware device which is dedicated to a specific task, e.g. image processing, by its hardware layout. The image processor may additionally or alternatively comprise a general-purpose computer which is configured to execute image processing software. 
     The image processor may be configured to compute the output color images automatically based on the input color images. This computation may include conversion of the data formats of the input color image and the output color image such as transforming the images between different color spaces and/or color representations and/or resolutions both with regard to spatial resolution and/or color resolution. The computation of the output color images may further be dependent on the reflective distribution across the projection area to compensate differences in reflectance across the projection area. 
     Further, the image processor may be configured to perform automatic pattern recognition and/or automatic image enhancement. For example, at least one area of the output color image may have at least one of a modified color, brightness and contrast compared to the at least one spatially corresponding area in the input image. For color modification, a color in the input color image may be replaced by a pseudo-color, in particular a neon color, which does not occur in nature. The image processor is preferably configured to compute this color modification. 
     The processing of the image processor is, according to one aspect of the invention, preferably done in real-time. Thus, an output color image may be projected onto the projection area based on an input color image before the next input color image in the time sequence is captured by the camera system. 
     The time sequence of output color images is projected preferably at a frame rate which is higher than the flicker fusion rate, to reduce tiring effects for the observer and to provide smooth transition between subsequent input color images. The capture rate in which the camera system captures subsequent input color images is preferably larger than the flicker fusion rate. The frame rate of the output color images may be lower than the capture rate of the input color images. In particular, an output color image may be based on a number of subsequent input images. For example, subpixel-shifting of subsequent input images may be used to increase spatial resolution in a single output color image. Basing the output color image on a number of input color images may also be used to compute output color images having an increased dynamic range, such as HDR images. 
     According to a further aspect of the invention, the observation apparatus may comprise a light-transmitting viewer, such as an ocular or an observation screen, for use by an observer. The viewer may be directed onto the projection area and comprise a viewing band-pass filter system which restricts light transmission to the output spectral bands. Thus, the viewing band-pass filter system only lets pass the output spectral bands which constitute the output color image. Reflections at other spectral bands are blocked and thus contrast is increased while at the same time allowing to view the object and the output color images at the output spectral bands. 
     The input spectral bands in which the multispectral camera is sensitive may include at least one of the primary colors, such as red, green and blue. The input spectral bands may be overlapping or discrete. The output spectral bands may be overlapping or discrete. Discrete output spectral bands may e.g. be generated by lasers of different color. 
     Preferably, at least one of the at least two spectral bands of the multispectral camera system is in the visible light range. The multispectral camera system may in particular be a RGB camera. 
     According to another advantageous aspect, the camera system may be a multispectral camera having an input spectrum which includes more than four input spectral bands. In particular, the camera system may comprise at least one imaging spectrometer and/or a hyperspectral camera. This camera allows to simultaneously capture image data in a large number of spectral bands. An imaging spectrometer does not necessarily need an input filter system for blocking the output spectral bands, as instead, only the image data at the input spectral bands may be read out of the camera system while data at the input spectral bands are dismissed. Alternatively, the input spectrum may be selected to not overlap with the output spectrum. 
     To further reduce reflections from the object, at least one of the output color image projector and the viewer may comprise a polarization filter system. For example, the output color image projector may comprise a linear polarizing filter and the viewer may comprise an adjustable polarization filter system comprising e.g. one rotatable linear polarizing filter. This allows to adjust the attenuation of intensity and of reflections according to the observer&#39;s needs. 
     According to another aspect of the invention, an illumination system may be comprised. Although a simple illumination system such as a light source with a continuous illumination spectrum which may include spectral bands which stimulate fluorescence in tissue-marking fluorophores, may be used, it is preferred that the illumination system comprises an illumination color image projector. Thus, instead of merely providing a diffuse illumination by a light source such as an LED or light bulb assembly, illumination is performed by projecting an illumination color image onto the object. Consequently, the illumination of the projection area or of the observation area of the camera system can be controlled to the accuracy of a pixel in the projected illumination color image. 
     In particular, the illumination color image projector may be provided for projecting a time sequence of illumination color images onto the projection area. Preferably, the observation apparatus, in particular its image processor, is configured to compute the illumination color images based on the input color images, in particular in real time, and project them, in particular in real time, onto the projection area. Computation and projection of the illumination color images based on input color images is preferably performed by the image processor before the next input color image is captured. 
     Using an illumination color image for illumination allows to exactly match the illumination to the spatial distribution of at least one of the reflectance, absorption, fluorescence and transmission of light across the projection or observation area. The observation apparatus, in particular its image processor is preferably adapted to compute a distribution of the reflectance of the object across the projection area. This distribution may then be used for computation of the output color images to obtain optimum contrast. In particular, the distribution which is obtained for the illumination spectral bands may simply be interpolated by the image processor to cover the output spectral bands between the illumination spectral bands. 
     In particular, the illumination color image projector may be controlled to adapt locally the intensity of the illumination in spectral bands, in which fluorescence is excited, to the intensity of the fluorescence. Areas with a high fluorescence intensity may be illuminated in the excitation spectral bands with a smaller intensity than areas, in which the fluorescence intensity is lower. 
     In order to avoid interference between the illumination color image and the output color image, the illumination color image projector has preferably an illumination spectrum in which the output spectral bands are at least partly blocked. For this, an illumination filter system can be used which comprises band-stop filters for the output spectral bands. In particular, the illumination spectrum may correspond to the input spectrum. If, however, the input spectrum contains fluorescence spectral bands, in which e.g. fluorophores emit fluorescence, as input spectral bands, it is preferred that the illumination spectrum does not contain the fluorescence bands. This avoids decreasing the contrast in the input color images of the fluorescence bands. 
     To avoid deterioration of the contrast of the output color images, in particular if the output color images are observed through a viewer as described above, the illumination spectrum does not overlap with the output spectrum at least in the visible-light range. 
     The illumination color image compensates the light-reflecting, absorbing, transmitting and/or fluorescence characteristics of the object surface across the observation area. It allows to lighten-up specific areas, which would otherwise be too dark for a full color resolution of the camera system. 
     In order to make full use of the dynamic capabilities of the camera system, the illumination color image may be at least in sections, preferably as a whole, a negative color image of the input color image. The observation apparatus, in particular its image processor, may be configured to compute the illumination color image as the negative color image of the input color image in real-time. 
     An observation apparatus comprising an illumination system in one of the above embodiments, a camera system in one of the above embodiments, an output color image projector in one of the above embodiments, and/or a viewer in one of the above embodiments in general uses two distinct imaging systems which operate on the same area, i.e. the observation or projector area on the object, and are functionally separated from each other by using the different input and output spectral bands. The output color image is used for viewing and represents the result of the image enhancement process. The camera system and the illumination system are used to optimize the input color image, which forms the basis of the image enhancement process and the output color images. The functional separation allows to optimize the input image captured independently of the result projection, although both use the same projection area. 
     It is preferred that the input color images and the output color images and preferably also the illumination color images are spatially congruent and have the same orientation. Congruence and orientation of these images ensures that the output images give an exact rendition of what is observed by the camera system and what is present on the object. 
     The observation apparatus, in particular its image processor, is preferably configured to transform and rectify the input color images, illumination color images and/or output color images preferably using the input color images as the base for adjusting the output and/or illumination color images. 
     According to another aspect of the invention, a common optical system is provided for at least two of the output color image projector, the illumination system and the camera system. The common optical system may comprise a common lens system. The common lens system ensures alignment of the optical axes of the output color image projector, the illumination system and the camera system. It also ensures congruence of the observation area of the camera system and the projection area onto which the illumination system and the output color image projector project their images. The common optical system may also comprise fibre optics if at least one of the camera system, output color image projector, viewer and illumination system is located remote from the object. 
     A beam splitter arrangement may be used to separate the light from the illumination system and/or the output color image projector into the optical system from the light from the projection or observation area. 
     The method according to the invention can be further improved by comprising the step of illuminating the observation area by projecting thereon an illumination color image, and basing the illumination color image on the input color image. 
     The method may further comprise the step of illuminating the observation area with an illumination spectrum including the input spectral bands. 
     According to another embodiment, the method may comprise the step of providing a viewer for observation area at the output spectrum by blocking input spectrum. 
     The method may also comprise the step of adapting the illumination color images to compensate differences in at least one of light reflectance, fluorescence, transmission and absorption across the observation area, at least in the visible-light range. 
     The method may further comprise the step of computing the illuminating color images as the color negative of the input color images. 
     The step of computing the output color images based on the input color images may comprise the step of replacing a color in the input color image in at least one coherent area by another color, preferably a pseudo-color, in the output color image. 
     The step of computing the output color image based on the input color images may comprise the steps of changing at least one of color, intensity and contrast in the output color images compared to the input color images. 
     In the following, the invention is described in detail with reference to the drawings using an exemplarily embodiment. 
     In the figures, elements, which correspond to each other with respect to function and/or structure are marked with the same reference numeral. 
     Further, it is to be understood that the combination of features which is exemplarily shown in the embodiment of the accompanying drawings, one or more features can be omitted if their technical effect as described above is not needed for a specific application. Conversely, one or more figures may be added to the embodiment if their technical effect as described above is beneficial for a specific application of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       In the figures: 
         FIG. 1  shows a schematic representation of an observation apparatus according to the invention; 
         FIG. 2  shows a schematic representation of a usage of the observation apparatus according to the invention; 
         FIG. 3  shows a schematic representation of the observation apparatus shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First, the structure and function of an observation apparatus  1  for visual enhancement of an observed object  2  is described with reference to  FIG. 1 . The observed object  2  may in particular be image tissue such as the body of a human, an animal and/or a plant. 
     The observation apparatus  1  comprises a camera system  4 , an illumination system  6 , an output color image projector  8 , a viewer  10 , a lens system  12 , an image processor  13  and a beam splitter system  14 . 
     The light path  16  of the camera system  4 , the light path  18  of the illumination system  6  and the light path  20  of the output color image projector  8  are all directed via the beam splitter system  14  through an optical system  11  onto the object  2 . The optical system  11  may comprise a lens system  11  for aligning the light paths  16 ,  18 ,  20 . 
     The camera system  4  is configured to capture a time sequence  21  of color input images  22  from an observation area  24  on the object  2  at a frame rate f I . The camera system may comprise at least one imaging spectroscope  25  such as a spectral camera or hyperspectral camera. 
     The illumination system  6  may comprise an illumination color image projector  26  which projects a time sequence  27  of illumination color images  28  on a projection area  30  on the object  2  at a frame rate f L . The projection area  30  is preferably spatially congruent to the observation area  24 . Alternatively a conventional illumination device (not shown) or ambient illumination may be used. 
     The illumination color images  28  in the time sequence  21  are computed by the observation apparatus  1 , in particular its image processor  13 , based on the input color image  22 . 
     The illumination color images  28  in the time sequence  27  are preferably congruent and of the same orientation as the corresponding color input images  22  in the time sequence  21  captured by the camera system  4  on the observation area  24 . 
     The output color image projector  8  projects a time sequence  31  of output color images  32  at a frame rate f o  onto a projection area  34  of the output color image projector  8 . The frame rate f o  may be independent of the frame rate f I  of the camera system  4 . The projection area  34  is preferably spatially congruent with at least one of the projection area  30  and the observation area  24 . The output color images  32  have, in the projection area  34 , features  36  that are spatially congruent and of the same orientation as corresponding features in the illumination color image  28  and/or the color input image  22  and/or on the object  2  for each image in the time sequence. Thus, the images  28  and  32  are congruent to each other on the object  2  and with the object  2 . 
     The output color images  32  are computed by the observation apparatus  1 , in particular its image processor  13 , based on the input color images  22 . The viewer  10  may be provided to an observer for viewing the common projection area  40  which corresponds to the projection areas  34 ,  30  and the observation area  24 . 
     The camera system  4  and the illumination system  6  on the one hand and the output color image projector  8  and the viewer  10  on the other form two separate imaging subassemblies of the observation apparatus  1 . Both subassemblies work on the same part of the object  2 , namely the projection area  40  but are decoupled from each other by using different spectra operating at preferably non-overlapping spectral bands and being linked solely through the image processing performed by the observation apparatus  1  or its image processor  13 , respectively. This separation results in a visual enhancement of arbitrarily selectable features of the object  2  without impairing image capture by the camera system  4 . The subassembly comprising the camera system  4  and the illumination system  6  is used for providing color input images  22  with optimum quality. The subassembly comprising the output color image projector  8  and the viewer  10  is for viewing processed color input images  22  in the form of the color output images  32  directly on the object  2 . This is explained in the following. 
     The camera system  4  is preferably a multispectral camera system that performs imaging at multiple, i.e. at least two, preferably more than four input spectral bands  42 . Preferably at least one input spectral band  42  is located in the visible-light range. At least one input spectral band may also be located in the NIR and/or IR range. The input spectral bands  42  may comprise fluorescent spectral bands  44  which correspond to fluorescence emission spectral bands of fluorescent materials on or in the object  2 , such as fluorescent tissue markers or other fluorophores. 
     The combination of input spectral bands  42  defines the input spectrum  46  of the camera system  4 . 
     The illumination system  6  has an illumination spectrum  48  including preferably non-overlapping illumination spectral bands  50 . The illumination spectrum  48  corresponds to the input spectrum  46  preferably excluding any fluorescence spectral bands  44  if such are present. Thus, the illumination spectral bands  50  correspond to the input spectral bands  42  in case there is no fluorescence emission from the object in the respective spectral band. 
     The illumination color images  28  are superposed onto the object  2  in the projection area  40  and reflected by the object  2  depending on the distribution of reflectance across the projection area  40 . Thus, the input color images  22  are directly influenced by the illumination color images  28  projected onto the projection area  40 . In addition, any fluorescence triggered by the illumination spectrum  50  is also captured by the camera system  4 . 
     Preferably, the illumination color images  28  are computed to be the color negatives of the respective input color images  22  for each of the images in a time sequence  21 ,  27 . This maximizes the utilization of the dynamic range of the camera system  4 . The computation is performed by the observation apparatus  1 , in particular its image processor  13 , in real time. 
     The illumination color images  28  may be computed to compensate other effects. For example, the intensity of illumination in a fluorescence excitation spectral band may be increased in an area, in which the fluorescence excited by this particular spectral band is low and vice versa. 
     Thus, a new illumination color image  28  is computed and projected onto the projection area  40  preferably within the time 1/f I  between successive input color images  22  in a time sequence  21 . 
     The illumination color image projector  26  may include an illumination filter system  52  which comprises band-pass or band-stop illumination filters  54  which block, i.e. attenuate or preferably eliminate, light outside the illumination spectral bands  50 . 
     The camera system  4  may comprise an input filter system  56  which may be part of an internal beam splitter  58  which may direct light to different cameras which together form the camera system  4 . If an imaging spectroscope is used, an input filter system  56  may not be necessary. In this case, input color images are put together simply by discarding any image data outside a input spectral band  42  from the imaging spectroscope. 
     The output color image projector  8  emits an output spectrum  60  including output spectral bands  62 . The output spectral bands  62  are separate from and do not overlap with the input spectral bands  42  and the illumination spectral bands  50 . For a human observer it is sufficient for the output spectrum  60  to contain only three output spectral bands  62  which together form a color space such as an RGB space and which may overlap. The output color image projector  8  may comprise an output filter system  64  which restricts the output spectrum  60  to the output spectral bands  62 . The output filter system  64  may towards this end comprise band-pass or band-stop output filters  66  which block in particular the input spectral bands  42 . 
     Alternatively, the output color image projector  8  may be a laser projector using at least one laser, preferably a set of lasers of different color, which may form an additive color system. Using color lasers has the advantage that the spectral bands in the different colors do not overlap and may be blocked easily by using a notch filter. 
     As the output spectral bands  62  do not overlap the input spectral bands  42 , the camera system  4  cannot detect the output color images  32  even though they are projected onto the common projection area  40 . Moreover, the output color images  32  projected onto the projection area  40  are not influenced by the illumination color images  28  as they do not share common spectral bands. The output spectral bands  62  are preferably narrower than the input spectral bands  42 . The number of output spectral bands  62  is preferably larger than the number of input spectral bands  42 . 
     The viewer  10  comprises a viewer filter system  68  which is designed to block the illumination spectral bands  50  and preferably only lets pass the output spectral bands  62 . The viewer filter system  68  may correspond to the output filter system  64  in its band-pass or band-stop characteristics. Thus, an observer looking through the viewer  10  only observes the output images  32  but does not see the illumination color images  28  as they are built using spectral bands  50  which are blocked by the viewing filter system  68 . This greatly enhances the contrast of the output color images  32 . 
     The image processor  13  may perform any image processing on the input color images  22  to generate the output color images  32 , such as noise reduction, spatial deconvolution to compensate the light scattering effects of the object, replacing a color in the input color images  22  by a pseudo-color or any naturally occurring color, adding marks such as symbols and letters, and/or enhancing contrast and/or color and/or brightness distributions. Furthermore, the color output images  32  may be computed from captured fluorescence images, e.g. by displaying a ratio of the fluorescence intensities in false colors. The output color images  32  containing any of these modifications of the input color images  22  are projected directly onto the object  2  at exactly the position where the visually modified features are present on the object  2 . 
     Using the multispectral illumination color image projector  26  and the multispectral camera system  4  allows to capture images of optimum quality as the illumination may be adjusted on a pixel basis to the reflectance and fluorescence characteristics of the object  2  and the characteristics of the camera system  4 . A real time computational comparison of the illumination color images  32  and the input color images, e.g. done by the image processor  13  yields the distribution of reflectance across the projection area  40  in the output spectrum  48 . This distribution may be interpolated by the image processor in real time to also cover the spectral bands  62  of the output spectrum  60  and for compensation of the output color images  32 . 
     Reflections in the output spectral bands  62  may be reduced by providing a polarization filter system  70  with the viewer  10  which may be combined with an optional polarization filter  72  in the light path  20 . The polarization filter  72  may be part of the output color image projector  8 . For example, linear polarization may be used in the polarization filter system  70  and the polarization filter  72  and the polarization filter system  70  may be adjustable by e.g. providing a rotatable linear polarization filter. 
     The observation apparatus  1  as shown in  FIG. 1  may be simplified. For example, it may be sufficient to use an illumination system  6  providing a continuous illumination spectrum  48  instead of illumination spectral bands  50 , or to use ambient light for illumination of the object. Such an illumination may result in a comparatively lower quality of the input color images  22 . It is, however, still possible to compute output color images  32  based on the input color images  22  and to project them onto the projection area  40  without the output color images  32  being captured by the camera system  4  as long as the input spectral bands  42  and the output spectral bands  62  do not overlap. 
     The viewer  10  does not need to be part of the observation apparatus  1 . It may be sufficient to project the output color image  32  onto the projection zone  40  so that an observer is able to view the superposition of the output color images  32  and the illumination color images  28  on the projection zone  40 . 
     The output spectrum  60  contains preferably output spectral bands  62  exclusively in the visible-light range. 
     The illumination color image projector  52  and the output color image projector  8  are shown to comprise each a digital light processor  74  and a rotating filter wheel  75 . Of course, any other type of color image projector may be used for any of the projectors  8 ,  26 . 
       FIG. 2  shows an example of an observation apparatus  1  which is used for observing an observation area  40  on a patient  76  as the object  2 . The observation area  40  can be on the skin of the patient  76  or be a part of the patient  76  undergoing surgery. An observer  78 , such as a surgeon or a medical assistant, may inspect the observation area  40  through the viewer  10  which in this case is a transparent screen  80  which preferably incorporates the viewer filter system  68  and optionally also a polarization filter system  70 . 
     The observation apparatus of  FIG. 2  again shown in  FIG. 3 , where the viewing screen  80  in which the viewer filter system  68  and optionally the polarization filter system  70  are incorporated. The viewer is held by a housing  82  which is supported by an adjustable stand  84 . In the housing  82 , at least the optical system  11  is received. The housing  82  may further also contain at least one of the camera system  4 , and the output color image projector  8 . 
     Alternatively, these devices may be arranged remote from the viewer  10  and fiber optics  86  as part of the optical system  11  may be employed to carry the light to and from the projection zone  40 . The fiber optics may be received in the stand  84 . 
     As described with reference to  FIG. 1 , the observation apparatus  1  may also comprise an illumination system  6  as shown above, which is either received in the housing  82  or connected via the fiber optics  86 . 
     REFERENCE NUMERAL LIST 
     
         
         
           
               1 . Observation apparatus 
               2 . Object 
               4 . Camera system 
               6 . Illumination system 
               8 . Output color image projector 
               10 . Viewer 
               11 . Optical system 
               12 . Lens system 
               13 . Image processor 
               14 . Beam splitter system 
               16  Light path of the camera system 
               18 . Light path of the illumination system 
               20 . Light path of the outward color image projector 
               21 . Time sequence of color input images 
               22 . Color input image 
               24 . Observation area of camera system 
               25 . Imaging spectroscope 
               26 . Illumination color image projector 
               27 . Time sequence of illumination color images 
               28 . Illumination color image 
               30 . Projection area of illumination color image projector 
               31 . Time sequence of output color images 
               32 . Output color images 
               34 . Projection area of output color image projector 
               36 . Feature in output color images 
               38 . Feature of illumination color image 
               40 . Common projection and observation area 
               42 . Input spectral bands 
               44 . Fluorescence spectral bands 
               46 . Input spectrum 
               48 . Illumination spectrum 
               50 . Illumination spectral bands 
               52 . Illumination filter system 
               54 . Illumination filter 
               56 . Input filter system 
               58 . Internal beam splitter 
               60 . Output spectrum 
               62 . Output spectral bands 
               64 . Output filter system 
               66 . Output filter 
               68 . Viewer filter system 
               70 . Polarization filter system of viewer 
               72 . Polarization filter of output color image projector 
               74 . Digital light processor 
               75 . Rotating filter wheel 
               76 . Patient 
               78 . Observer 
               80 . Screen 
               82 . Housing 
               84 . Stand 
               86 . Fibre optics 
             f I  Frame rate of camera system 
             f L  Frame rate of illumination color image projector 
             f O  Frame rate of output color image projector