C55465: Progressive power lenses - part 7

In the seventh part of our comprehensive series discussing progressive spectacle lens design, Professor Mo Jalie looks at the fitting of the lenses

So far in this series we have considered the evolution of lens designs. In this article, we will discuss the correct fitting of the lenses to ensure maximum benefit from their design.

Fitting progressive lenses

BS EN ISO 13666:2012, Ophthalmic optics – Spectacle lenses – Vocabulary, gives the following definitions (figure 1):

Note that the fitting point position is measured from the boxed centre (ie the horizontal centre line in the case of the vertical fitting distance), whereas the fitting point height is specified, just as for a bifocal segment, from the lower horizontal tangent to the lens periphery. Horizontally, the lens will be mounted at half the boxed centre distance minus the monocular centration distance (CD) for each eye. It is important to realise that the monocular centration distances are measured from the centre of the bridge of the spectacle frame to the pupil centres, rather than the centre of the subject’s nose.

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