Interactive Tutorials - Spinning Disk Fundamentals

Interactive Tutorials

Optical Sectioning

Structured Illumination: ZEISS ApoTome Basics

Imaging of thick specimens in fluorescence microscopy is compromised by signal originating from regions that are removed from the focal plane. The result is that sharp image information originating from the focal plane is overlaid with blurred image information arising from distant areas, thus reducing contrast and resolution in the axial (Z) dimension. Furthermore, three-dimensional reconstruction of the specimen is not possible under these conditions. Optical sections of the specimen, which only extract information from the region that corresponds to the objective depth of field, can alleviate blurring and enable volume rendering of stacks to generate three-dimensional images. Aside from using confocal techniques, optical sections can also be obtained in widefield fluorescence microscopy using structured illumination, as has been implemented in the ApoTome attachment manufactured by ZEISS.

The principle behind the ApoTome implementation of structured illumination is the following. A grid created by evaporating metal on the surface of an optical-grade glass window is inserted into the field diaphragm plane in the illumination pathway of a fluorescence microscope and projected onto the specimen. The projected grid image is translated over the specimen using a plane-parallel glass plate that is tilted back and forth in the light path. At least three raw images of the specimen are acquired with the grid structure superimposed in different positions. These images are subsequently processed in real time using the microscope software to create an optical section. The underlying principle is that the projected grid becomes visible in the focal plane as a result of specimen features being excited by the structured light. In regions where no light reaches the specimen (in effect, the dark grid lines), no fluorescence is generated. The software determines grid contrast as a function of location and removes the out-of-focus image information before collating the three images into a final optical section.

Contributing Authors

Tony B. Gines and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.