Uniform density filters

Uniform density filters provide the means of varying the intensity of photochemical radiation. They are made by the vacuum deposition of thin metal films on quartz plates. 

Figure 10 illustrates schematic drawing of objective-type TIRFM. A high numerical aperture objective lens is mounted on an inverted microscope. A laser beam is passed through a neutral density filter (ND) and a beam expander (BE) to adjust its power and diameter. When the laser polarized linearly is used, the polarization of the laser beam is converted from linear to circular by a quarter-wave plate (A/4). The laser beam is focused by a lens (L) on the back focal plane of the objective, so that specimens are illuminated uniformly with Koehler illumination. By shifting the position of the mirror (M) located between the lens (L) and dichroic mirror (DM), the path of the incident laser light is shifted from the center to the edge of the objective. At the center position, the microscope can be used as a standard epi-fluorescence microscope (Fig. 10b). 

Neutral density filters. These are commercially available glasses able to reduce the intensity of light in an almost uniform and wavelength-independent way. They are therefore used when a reduction of the intensity of the exciting radiation is required. 

Diatomaceous Silica Filter aids of diatomaceous silica have a dry bulk density of 128 to 320 kg/m (8 to 20 Ib/fU), contain paiiicies mostly smaller than 50 [Lm, and produce a cake with porosity in the range of 0.9 (volume of voids/total filter-cake volume). The high porosity (compared with a porosity of 0.38 for randomly packed uniform spheres and 0.2 to 0.3 for a typical filter cake) is indicative of its filter-aid ability Different methods of processing the crude diatomite result in a series of filter aids having a wide range of permeability. 

The ideal filter would be one in which the size of the media particles would decrease uniformly in the direction of flow. The coarsest materials would be at the top the finest at the bottom. Fig. 3 illustrates the ideal arrangement. Materials of different densities and sizes would be required to provide the tapered void... 

Example 1 Substance of known triple-point in a triple-point cell Example 2 Glass of known optical density in a transmission filter holder Example 3 Spheres of uniform size mounted on a microscope slide. 

Nucleopore filters from the same lot should be used throughout the experiments. While pore diameter is uniform, there might be considerable variability in pore density. Therefore, filters should be checked before use by passing a certain amount of medium (20-50 ml), and measuring the time it takes to pass through this is proportional to the total pore area. Only filters giving similar flow values should be used in the experiments. 

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