Focus depth discrimination

Fig. 2. Depth discrimination (z-axis resolution) properties of a confocal microscope. The illumination and detection images in a confocal microscope are diffraction-limited and confined to a small region of the specimen (1). Only light emitted in the plane of focus and on the optical axis will pass the detector pinhole and form an image. Light emitted from other areas of the specimen does not enter the detector pinhole.

Laser scanning confocal microscope A fluorescence microscope achieving improved depth discrimination and contrast by blocking fluorescence that originates outside the plane of focus by use of a confocal pinhole. 

As a result of extreme depth discrimination (optical sectioning), the resolution is considerably improved (by up to 40% compared to optical microscopy). The CLSM technique acquires images by laser scanning or uses computer software to subtract out-of-focus details from the in-focus image. The images are stored as the sample is advanced through the focal plane in increments as small as 50 nm. Three-dimensional images can be constructed to show the shape of the particles. 

One way to measure the axial response of a confocal microscope is to record light intensity from a thin layer with infinite lateral extension. In a conventional microscope, no difference—and therefore no depth discrimination—are obtained. However, in confocal microscopy, a strong falloff occurs if the object is out of focus. The free width at half maximum (FWHM) of such a function is propoitional to the optical section thickness [64], [67]. The optical section thickness also depends on the diameter of the pinhole. The optimum pinhole size D with a maximum deterioration of 10% in reflection is given by [63]... 

Several factors may account for large resuspension rates. The retrieval and deployment of the trap at the sediment surface may resuspend some particulate matter. Natural resuspension may result from storms and sediment-focusing mechanisms. Postdepositional remobilization may increase the sedimentation rate of210 Pb at the deepest point of Lake Sempach (41). Because we cannot discriminate among different resuspension processes, we assumed that the Mn concentration in the resuspended material is equal to that in the sedimenting particles at a depth of 86 m. Particulate MnO, is rapidly reduced at the sediment surface therefore, this procedure tends to overestimate the resuspension term. 

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