As a rule, the diameter of the pinholes should be roughly equal to the 3-dB width of the Airy pattern projected onto the disk by the objective lens and the center-to-center spacing should be great enough to eliminate the light reflected from out-of-focus planes. 7,46,47,48 The dimensions required are discussed in detail in Chapter 3. Pinhole Size and Layout The optimum size and spacing of the pinholes depend on the focal length of the tube lens and the diameter of the objective pupil. Devices based on this principle operate at frame rates of several hundred per second, with several thousand lines in the image. By illuminating many pinholes at once, Petran and Hadravsky were able to increase both the light efficiency and scanning speed of the microscope. Several hundred pinholes at a time were simultaneously illuminated. Their disk contained many thousands of pinholes arranged in multiple interleaved spirals. For the microscopy application, because they had an extended light source and a CCD camera or the human eye as the receiver, Petran and Hadravsky could use parallel processing. A single modulated light source was used in the original application of the Nipkow disk, so that only one pinhole of a set arranged along a rotating single spiral was illuminated at a time. Although the disk is conceptually based on Nipkow's original design for a mechanically scanned television, it was modified by Petran and Hadravsky to improve the light efficiency and increase the number of lines in the image. The rotating Nipkow disk is the key technology that makes the direct view microscopes possible. Kino, in Confocal Scanning Optical Microscopy and Related Imaging Systems, 1996 2.3.2 The Nipkow Disk

Nipkow disk confocal microscopy