Abstract:
There is provided an optical imaging device ( 18 ) for splitting an initial image into at least two images with different optical characteristics. The device comprises a dichroic mirror ( 32 ) to create first and second optical pathways respectively incident on first and second mirrors ( 41, 41 ′) carried on a centrally pivoted rotatable arm, characterised in that the first and second reflective means are moveable along the arm ( 42 ) whilst held in fixed relationship to each other, thereby to adjust separation of the first and second optical pathways. A third mirror ( 46 ) in fixed relationship to the beam splitter ( 32 ) is positioned adjacent where the first and second optical pathways intersect, or just before the intersection of the first and second optical pathways, or just after the point of intersection.

Description:
This invention relates to an optical imaging device, and in particular to a device capable of splitting a single optical image into further images. 
   BACKGROUND TO THE INVENTION 
   Within complex optical systems, optical imaging devices are used for manipulation of an optical image. Devices which allow an image to be split into a plurality of images with different optical characteristics are often used within spectroscopes or spectral imagers. The different images have, for example, different wavelengths or different polarisations, and can be viewed simultaneously to give information about a sample being examined under a microscope. 
   Prior art devices used for producing the images with different optical characteristics are often complex, and can introduce optical aberrations which complicate assessment of the plurality of images. 
   It is an aim of the present invention to provide a simplified optical imaging device which essentially reduces optical aberrations. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, there is provided an optical imaging device for splitting an initial image into at least two images with different optical characteristics, wherein the device comprises a beam splitter to create first and second optical pathways respectively incident on first and second reflective means carried on a centrally pivoted rotatable arm, characterised in that the first and second reflective means are moveable along the arm whilst held in fixed relationship to each other, thereby to adjust separation of the first and second optical pathways. By having the first and second reflective means moveable along the arm, but yet the first and second reflective means remaining fixed relative to one another, the characteristics of the optics preceding the optical imaging device, for example a microscope in which the optical imaging device is placed so as to allow spectroscopic analysis, are no longer relevant to adjustment of the optical pathways. 
   The beam splitter will typically be a dichroic mirror which reflects a proportion of incident light whilst transmitting the remainder of the incident light. Other types of beam splitters which may be used include polarising beam splitters. The dichroic mirror may be coated so as to alter the optical characteristics of both, or either of, the reflected and transmitted light. Additional optical elements, such as filters, may be placed in the optical pathways. 
   Preferably the first and second reflective means are mirrors, which again may be provided with coatings or modified in other ways so as to alter the optical characteristics along the optical pathways. Movement of the first and second reflective means along the arm allows the position of the first optical pathway to be adjusted relative to the second optical pathway prior to recombination at an output. This allows adjustment of the optical pathways to match the preceding optics, and allows the first and second optical pathways to be placed as close together as possible whilst avoiding overlap. 
   Preferably a third reflective means, such as another mirror, in fixed relationship to the beam splitter is positioned adjacent where the first and second optical pathways intersect so as to deflect one, or the first, optical pathway to a first focussing element, such as a lens, for recombination with the other, or the second, optical pathway. 
   The third reflective means may be positioned just before the intersection of the first and second optical pathways, as shown in  FIGS. 3 and 7 , which partially obstructs the undeflected pathway. However this allows the focussing element to be positioned closer to the first and second reflective means. In such an embodiment an edge of the third reflective means can be cut away at an angle to avoid any obstruction to the undeflected optical pathway. 
   Alternatively the third reflective means may be positioned just after the point of intersection, see  FIGS. 5 and 6 , so as to avoid obstruction of the undeflected pathway. 
   The orientation of the third reflective means may be adjusted dependent on whether an input beam to the device is to be parallel or at right angles to an output beam. 
   If desired the third reflective means may be omitted and first and second focussing elements used to focus the first and second optical pathways respectively, the pathways intersecting, but not recombining, before reaching their respective focussing elements. 

   
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  shows a schematic diagram of optical imaging apparatus incorporating an optical imaging device in accordance with the present invention; 
       FIG. 2  shows a prior art optical imaging device. 
       FIG. 3  shows a schematic diagram of one embodiment of an optical imaging device in accordance with the present invention; 
       FIG. 4  shows a mechanism for use in the optical imaging device; and 
       FIGS. 5 to 7  show further embodiments of the optical imaging device. 
   

   DESCRIPTION 
     FIG. 1  shows a schematic diagram of an optical system  10  used for obtaining an image from a sample  12 , for example as in fluorescence and spectroscopic analysis, ratiometric ion imaging, dual probe imaging, fluorescence resonance energy transfer or total internal reflection fluorescence. 
   The optical system comprises a microscope  14  which includes an objective lens  16  for receiving light from the sample  12  so as to create an image, an optical imaging device  18  mounted on a microscope port and for splitting the image into two or more further images with different optical characteristics, and an imaging lens  20  for focussing these images on output to an image receiving surface  22  such as a solid state imaging device or a detector array. 
   As is known in the art, the transmission of light through any optical system is limited by the size of the various apertures within it. Some of these apertures are there by physical necessity, for example the finite diameters of various lenses and other optical components, and others are selected in order to give the system specific characteristics. An aperture may be used to limit a field of view, for example of a specimen that is being imaged. In order to do this, the aperture must be in focus when the image of the object is in focus: this is known as a field iris. An aperture may also be used to control the amount of light transmitted by the system and in microscopy this is known as a condenser iris on the illumination side, and an exit pupil on the light collection side. The condenser iris must not be in or near focus with the object. Whereas a field iris sets the lateral displacement from the optical axis (i.e. centre of field) over which the object can be seen, a condenser iris or exit pupil sets the range of angles over which light can be collected from the object and re-imaged. 
   In the system shown in  FIG. 1 , typically an adjustable rectangular aperture is placed at an image plane prior to the input to the splitter  18 , so as to provide a field iris to limit the extent of the image seen by the camera  22 . This allows multiple images to be focussed onto the camera by the splitter  18  without the images overlapping. 
   The images obtained by the microscope  14  are usually viewed at high magnification, with the effect of magnifying an image being to correspondingly reduce the range of angles that the imaging light subtends to the optical axis. Thus the greater the magnification, the more closely the image-forming light approximates to a parallel beam, and therefore the beam divergence becomes less with magnification. By keeping the overall path length through the splitter  18  as short as possible, and by using optical components of reasonably large diameter relative to the beam, the beam divergence remains below a diameter at which significant optical aberration occurs. 
   A prior art image splitter is shown in  FIG. 2 . The image splitter comprises a lens  30  which acts as a collimator for light coming from an input image plane  31 , and a dichroic mirror or polariser  32  which transmits a proportion of the light incident upon it, and reflects the remainder so as to split the original image into two successive images which follow different optical pathways  33 ,  34  as defined by mirrors  35 ,  35 ′ and  35 ″. The two images which follow the two optical pathways defined within the image splitter are focussed by lens  36  before simultaneous output to an output image plane  37 , such as that defined at a detector which allows recording and analysis of the two images that have been derived from the original image. The optical distance between lenses  30  and  36  is close to the sum of their focal lengths. 
   Thus light from the image plane is collimated by lens  30 , and then split into two essentially equal pathways by dichroic mirror  32  which is oriented slightly clockwise with respect to an exact 45° angle. The beam transmitted by dichroic mirror  32  is redirected by 45° mirror  35  and reflected by mirror  35 ″ towards the output focussing lens  36 . The beam reflected by mirror  32  is reflected by 45° mirror  35 ′ and passes by the side of mirror  35 ″ to be focussed by lens  36 . Separation of the two images at the output is effected by anticlockwise rotation of mirrors  35  and  35 ′. To achieve equal rotation, mirrors  35  and  35 ′ are mounted on a common carrier which rotates around a pivot  38 . The rotation also physically separates the two beams at mirror  35 ″ so that the beam reflected from mirror  35  falls entirely on mirror  35 ″, whereas the beam reflected from mirror  35 ′ passes completely to the right of  35 ″. 
   The light reflected by dichroic mirror  32  arrives at mirror  35 ′ somewhat lower and to the right than the transmitted light arriving at mirror  35 ″, and mirrors  35  and  35 ′ are biased slightly clockwise to compensate. Mirror  35 ″ is also biased slightly clockwise to match the rotation of mirror  35 . The effect of this is that even at zero image separation the two beams may not overlap at the position of mirror  35 ″. However the two beams should not be separated by more than necessary at mirror  35 ″, otherwise their centres will be further away from the optical axis of lens  36  than they need to be, which will require lens  36  to operate at a faster focal ratio, which will in turn increase the aberrations from lens  36 . Image splitters are usually sold as add-on devices, and there is no way of knowing the characteristics of the optical system to which the splitter is going to be connected, even though the characteristics of the optical system will influence the beam diameters at the location of mirror  35 ″, and thus the required centre spacing. For example, one cannot predict where the exit pupil of an unknown optical device will be. 
   Where a microscope is operating at high magnifications, there are very long focal ratios and the collimated beams between lenses  30  and  36  will generally not diverge very much so reducing the possible problems with beam separation at mirror  35 ″. 
   However where magnification is not particularly high, the greater beam divergence between lenses  30  and  36  requires a greater beam separation within the splitter, so that optical pathway  33  is still entirely unobstructed by mirror  35 ″ and optical pathway  34  is still entirely reflected by it. In order to deal with worst-case conditions the beam separation at lens  36  may therefore be large enough for this lens to produce significant aberration, as the light pathways are further off-axis. An image splitter in accordance with the present invention, see  FIG. 3 , allows the beam separation to be adjusted without affecting the separation of the focussed images, so that it is no greater than required for the above criterion to be met in that particular case. The aberrations produced by lens  36  will therefore be correspondingly reduced, and the device can be used at both high and low magnifications. 
   In the image splitter  40  of  FIG. 3 , mirrors  41  and  41 ′ are carried on an arm or hub  42  rotatable about pivot point or central axle  44 . The arm  42  has a central track or slot  43  running along its length and in which a carriage  45  bearing both 45° mirrors  41  and  41 ′ is moveable. Mirrors  41  and  41 ′ are carried on the same rectangular carriage so as to be held in a fixed relationship to each other. Sliding movement of the carriage  45  along the arm  42  by a distance x moves mirror  41  closer to axis point  44  by x and moves mirror  41 ′ away from axis point  44  by x. Thus mirrors  41  and  41 ′ always remain the same distance from each other but their distance from point  44  can be adjusted. Typically the carriage  45  will be moved manually and then locked in position by a locking screw. 
   Alternatively an arrangement as shown in  FIG. 4  can be used. The mirror adjustment mechanism of  FIG. 4  operates a vertically moveable pushrod  50  that comes up through the centre of the axle  44  which pushes against a 90 degree crank, which is aligned along the mirror bar or carriage  45 . Both the top end of the pushrod  50  and the pivot  52  for the crank are some way above the mirror bar  45  with the pivot being horizontal and at right angles to the axis of the bar, and fixed to the hub so that it rotates with it. The other end of the crank  51  passes down into the bar  45 , so that as the pushrod goes up and down, the crank  51  converts this to the appropriate sliding movement of the bar  45 . Operation will be against a return spring to prevent backlash. There is no interaction with the rotation of the bar, since this would merely cause the point of contact of the pushrod with the crank to rotate. 
   Two positions of mirrors  41  and  41 ′ are shown in  FIG. 3 . At the position shown in solid lines, mirrors  41  and  41 ′ are equidistant from point  44 . At the position shown in dashed lines, mirrors  41  and  41 ′ have been slid along the groove  43  on their common carriage  45 , so that mirror  41  moves towards point  44  by the same amount that mirror  41 ′ moves away from point  44 . This increases separation of the two beams at mirror  46 , the adjustment being made such that according to their actual diameters at this point, the beam following pathway  33  is entirely unobstructed by mirror  46 , while the beam following pathway  34  is entirely reflected by it. This adjustment, being made in an infinity space for the imaging light, does not affect the separation of the images produced by lens  36 , but it does affect the combined diameter of the two beams at lens  36 , and hence also the aberrations introduced by this lens. The movement of the mirrors fixed to the carriage along the arm allows a user to redefine the geometry of the reflecting space to match the preceding optics. So whilst the splitter is typically set up in a square geometry, as shown by the solid lines, movement of the carriage alters where the optical pathways impinge upon the mirrors  41 ,  41 ′ and  46  to define a rectangular geometry, with, in particular, the point at which optical pathway  34  impinges upon mirror  46  altering. 
   As the position of the carriage is adjustable to allow the optical pathways of the splitter to be altered to match the preceding optics, whose characteristics will always be unknown at point of sale, the dichroic mirror  32  is positioned at exactly 45°. 
   Ideally, the two beams are only separated by the minimum distance necessary to prevent overlap, which is simple to do given the ease of sliding the carriage relative to the axis  44 . Also mirror  46  can have its reflective surface angled back at 45° at the edge closest to beam  33  so as to avoid any obstruction to beam  33 . Too much separation will require lens  36  to operate at a faster focal ratio, so increasing aberrations from the lens, which is not desirable. 
   The splitter of  FIG. 3 , thus provides a way of adjusting the beam separation at mirror  46 , independently of the separation of the two images, i.e. independently of the original imaging aperture. 
     FIGS. 5 ,  6  and  7  show further embodiments of the basic splitter shown in  FIG. 3 . 
   In  FIG. 5 , mirror  46 ′ is positioned just after the point of intersection of the two beams, in fixed relation to splitter  32 . The finite thickness of the half-length mirror  46 ′ does not obstruct beam  33 . 
   The direction of rotation is selected so as to increase the separation of the two beams at the half-length mirror  46 ,  46 ′, so as to require less linear displacement of mirrors  41 ,  41 ′. Whilst the preferred direction of rotation for the embodiment shown in  FIG. 3  is anticlockwise, for the embodiment shown in  FIG. 5 , the preferred direction of rotation is clockwise. Thus for the configurations shown in  FIGS. 3 and 6 , ideally rotation is anticlockwise, and for  FIGS. 5 and 7  rotation is clockwise. However the implementations shown would still work if rotation was in the opposite direction to that which is preferred. 
   In the embodiments shown in  FIGS. 6 and 7 , the exit beam is at right angles to the input beam instead of being parallel with it as for the embodiments shown in  FIGS. 3 and 5 . This is achieved by reversing the orientation of the half-length mirror  46 ,  46 ′. The embodiment shown in  FIG. 6  shares the advantage of the embodiment shown in  FIG. 5  that the thickness of the substrate of mirror  46 ′ does not affect the functioning of the splitter. The embodiment shown in  FIG. 7  is able to have the refocussing lens  36  positioned closer to the rotating mirror pair  41 ,  41 ′, as can the embodiment shown in  FIG. 3 , thereby slightly reducing the length of the infinity space between lenses  30  and  36 . 
   If required, adjustable iris diaphragms  48  are placed in the pathways from the beam splitter  32  to obstruct the beams and so allow the intensities of beams  33  and  34  to be adjusted independently. This is useful if one beam is significantly brighter than the other and allows the intensities of the beams to be adjusted to be approximately the same. This is particularly useful if both beams are being imaged at the same projector. The diaphragms  48  are ideally placed one focal length before the refocussing lens  36 , so that they are completely out of focus at the final image. 
   Two or more of the embodiments shown may be combined, for example those shown in  FIGS. 5 and 6 , or those shown in  FIGS. 3 and 7 , so as to provide two refocussing lenses at 90° to each other. This allows either one or two cameras to be used at the output. If no half-mirror  46 ,  46 ′ is fitted, then light paths  33  and  34  intersect to cross over and are focussed independently onto two detectors. In such a case, the rotating mirror pair  41 ,  41 ′ would be left in a neutral position giving equal path lengths within the image splitter for beams  33  and  34 . 
   As will be apparent to one skilled in the art, other optical elements can be used instead of mirrors, for example gratings, prisms, or polarisers. Optical elements such as filters, polarisers etc. can also be used in combination with the mirrors. For the arrangement shown in  FIG. 3 , typically one filter is disposed between the splitter  32  and the mirror  41 . A second filter can be placed between splitter  32  and mirror  41 ′, in which case the dichroic mirror and filters can be mounted together in a readily interchangeable assembly. Alternatively the second filter can be placed between mirror  41 ′ and mirror  46  which allows the filters to be mounted in a common carrier which can be interchanged independently of the dichroic mirror. 
   The preceding optics, especially if working at high magnification such as a microscope, may introduce significant chromatic aberration. When image separation is carried out on the basis of wavelength, it may no longer be possible for both images to be in sharp focus. However, this situation can be alleviated by including corrector optics in the split pathways. In the configuration shown in  FIG. 3 , they should ideally be placed as close as possible to mirror  46 . Typically, either a single lens of the minimum power to perform the correction can be disposed in either pathway  33  or  34  as appropriate, or more powerful lenses can be placed in both pathways, with chromatic correction being performed by varying the relative distance of these two lenses from mirror  46 . If these lenses are of appropriately higher power, then lens  36  can be omitted. Although mirror  46  is no longer in infinity space as far as the imaging light is concerned, the operation of the splitter is essentially unchanged as long as these lenses are placed relatively close to mirror  46 . This alternative can be useful even if chromatic correction is not required, as it is more compact than those in which there is a single refocussing lens  36 .