The City Frame Rule, also known as the Serelo Rule, is a specialized optical ruler designed to take precise measurements of frames and multifocal lenses. Developed at City University, London, by Campbell and Obstfeld, this tool is essential for optometrists and optical technicians who need to ensure accurate fitting and alignment of eyewear
The City Frame Rule is crafted to measure various aspects of frame and lens parameters. It includes scales and markings that allow users to measure the boxed lens size and centres, bridge width, crest height, and distance between lenses
Additionally, it can measure the angle of crest, angle of let-back, angle of sides, downward angle of drop, frontal angle of pad, inward angle of drop, splay angle of pad, and vertical angle of pad
These measurements are crucial for ensuring that frames fit comfortably and lenses are positioned correctly.
This is used to measure the geometrical centre of segments, monocular centration distance, fitting height, segment bottom position, segment depth, segment diameter, segment height, segment size, and segment top position
These measurements are particularly important for multifocal lenses, where precise alignment is necessary to ensure optimal vision correction. The rule also helps in determining the projection of the bridge, lens insets, progression heights, and segment positions
To use the City Frame Rule, the frame is positioned so that the lower edge of the crest is on the horizontal centre line, and the nasal edge of the right eye is at the 10mm mark on the vertical scale
This setup allows for accurate measurement of the distance between rims, pad centres, and other critical parameters.
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