In microscopy, depth of field is determined by the correlation between numerical aperture and magnification. For the best possible visual impression, the adjustment facilities of modern microscopes produce an optimum balance between depth of field and resolution – two parameters which in theory are inversely correlated. Particularly at low magnifications, the depth of field can be significantly increased by stopping down, i.e. reducing the numerical aperture. However, the smaller the numerical aperture, the lower the lateral resolution. Therefore, it has always been a challenge to find the optimum balance of resolution and depth of field.
A sophisticated optical approach that resolves the inverse correlation between resolution and depth of field is FusionOptics. The technology was developed by Leica Microsystems for industrial stereomicroscopy and is now also available for a neurosurgical and ophthalmic microscope. FusionOptics technology uses two different-sized apertures in the optical pathways: A large aperture provides high resolution, and a smaller one provides greater depth of field. The human brain combines the two separate images into a single impression that features both high resolution and high depth of field. Thus, the depth of field can be increased by over 30%.
In the process of developing this new technology, Leica Microsystems worked with Dr. Daniel Kiper of the Institute of Neuroinformatics at the University of Zurich and Swiss Federal Institute of Technology. A study  was conducted to investigate the binocular combination of visual signals. The results showed 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. Today, approximately 8000 stereomicroscopes from Leica Microsystems with FusionOptics technology are in use in demanding production and research applications worldwide. The M530 OH6 microscope for neurosurgery was the first surgical microscope on the market that features this unique technology for an optimal view with high resolution and increased depth of field. This has recently been joined by the Proveo 8, M530 OHX and PROvido ophthalmology microscope.
During Kiper's study, the test subjects observed patches arranged around a central fixation point. The fields either had grating or were uniform. 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 illustration shows a 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).
For surgeons, FusionOptics can help to streamline the surgical workflow, as with a greater area in focus less refocusing is required. In combination with advanced illumination and apochromatic optics, the surgeon is able to see more anatomical details clearly, especially at the bottom of a deep cavity, helping him to maneuver the instruments accurately and be confident in his surgical actions.
- Kiper et al.: unpublished data.