Stereo microscopy enables us to view microstructures in 3D with the help of two separate beam paths – which, in principle, work like an extension of our two eyes. Ever since their invention by Horatio S. Greenough, stereo microscopes have worked according to the optical principles based primarily on Ernst Abbe’s work. For over a century, optics designers have worked to push magnification, resolution and image quality to the limit permitted by optics. These limits are determined by the correlation between resolution, convergence angle and working distance. The higher the microscope resolution, the higher the convergence angle between the left and right beam paths and the lower the available working distance. However, increasing the distance between the optical axes would cause the three-dimensional image seen by the observer to become distorted; a cube in the object would then appear as a tall tower. A greater zoom range alone is of little use, since with increasing magnification, there is not an attendant increase in optical resolution. The result is what is known as empty magnification.
Scientific studies about visual perception and vision problems have shown that the brain can selectively process information from individual eyes and that it is very capable of compensating for differences in the visual acuity of the two eyes. This gave the development engineers at Leica Microsystems a simple but ingenious idea. Why not make use of this ability of the brain and use each beam path of the microscope for different information? One image channel provides high resolution, the other depth of field. The two very different images are merged to a single, optimal spatial image by the brain. The completely new optical approach FusionOptics™ brings with it two distinct advantages. Compared to existing stereo microscopes, the resolution can be increased drastically and the focus depth can be improved significantly. Furthermore, the resolution can be increased without increasing the convergence angle between the two beam paths.
However, the feasibility of this design first had to be reviewed in the light of neurophysiology – whether the brain can process signals that differ greatly between the two eyes into correct three-dimensional images. Earlier studies were primarily concerned with two-dimensional images. Leica Microsystems presented the idea to Dr. Daniel Kiper of the Institute of Neuroinformatics at the University of Zurich and Swiss Federal Institute of Technology, who specialises in researching signal processing in primate brains and agreed to carry out corresponding studies. Kiper, along with Graduate Assistant Cornelia Schulthess and Dr. Harald Schnitzler of Leica Microsystems, designed a study. 36 test subjects with normal visual acuity underwent psychophysical tests that investigated the binocular combination of visual signals. Of particular interest was whether an interocular signal suppression takes place when both eyes are exposed to different stimuli. The result of this would be that the image of the suppressed eye would be perceived only partially or not at all.
During the experiments, the test subjects observed patches arranged around a central fixation point. The fields either had grating or were uniform (Figure 4). To create differences in the spatial perception of both eyes, binocular disparity is required – both eyes must be exposed to different stimuli. This is done using special stereo goggles with which separate test images can be projected to each eye. In a series of trials, the test subjects saw changing arrangements of the grid patches in various depth planes. After each image that was visible for 1,000 msec, the subjects reported where they saw the grid patches, and whether they appeared in front of or behind the central fixation point.
The evaluation of the correct/incorrect answers for the position of the grid patches and the spatial resolution in various spatial planes showed no significant differences. No evidence of signal suppression was observed in any of the tests. This means that the human brain is capable of using the best information from both eyes in order to compose an optimal spatial image. This is true regardless of whether the images are acquired using both eyes or each eye provides entirely different information. The results prove once again how adaptable and powerful our brain is in processing visual impressions.
Fig. 4: Schematic depiction of the visual stimuli. A: The four possible perceptions of the test images. The test subjects specified where the grids appeared and whether they appeared in front of or behind the fixation point. B: An example from the test series for different binocular stimulation (corresponds to perceived image 3 from A). The grids were presented to either the same eye or different eyes. A few patches were also shown shifted in one eye (white arrows).
On the theoretical basis provided by the study, Leica Microsystems was able to implement the Fusion Optics™ concept in a completely new stereomicroscope, which is the world’s first one with a zoom range of 20.5:1 and a resolution of up to 525 lp/mm. This corresponds to a resolved structure size of 952 nm. When appropriately configured, this can be increased to up to 1,050 lp/mm (structure size of 476 nm). Up until now, optical attachments could only achieve a maximum zoom range of 16:1 or a magnification increase without an increase in resolution (empty magnification).
The significant performance increase attained by FusionOptics™ is highly valuable for everyday work at the microscope. The large working distance of the new objective generation allows convenient freedom of movement for examining specimens on the microscope stage. Whether in semiconductor technology, plastics development, materials testing, criminology, natural sciences or earth sciences – the new stereo microscope opens up frontiers that had previously been unattainable in conventional stereomicroscopy.