OPTICAL CORRECTION

Trifocus

The trifocal lens for the near vision and the distant vision , and for intermediate distance, for the  PRESBYOPIC eye.

The intermediate and reading portions of fused trifocals are constructed from a button, which is normally made up from three different glasses, a crown glass of exactly the same material as the main lens and two high-refractive index glasses, chosen to provide the correct IP/NP ratio. The button is fused at high temperature to a depression curve worked on the main lens made from crown glass. After completion of the segment side, the segment surface is continuous over the DP and NP so the dividing line cannot be felt. The reading additions, A¹ and A n, which are obtained depend upon the power of the DP surface, F¹ the power of the original depression curve, Fc and the fused bifocal blank ratios, for the two glasses used.

If the refractive index of the crown glass main lens is denoted by n¹ the refractive index of the light flint glass used for the intermediate portion by n² and the higher refractive index glass used for the near portion by n³ then the IP/NP ratio is given by:

IP/NP ratio = (n² - n¹)/(n³- n¹) x 100%

Typical segment glasses used in Europe are n t = 1.523,. n² = 1.606 and n³ = 1.654.

Since the IP/NP ratios of these designs depend upon the glasses used they are fixed by the manufacturer, usually at 50%, but some designs are available with the IP addition 40%. 60% or 70% of the near addition.

Fused trifocals are currently available in the following segment diameters and inter- mediate depths (25 x 7 indicates that the segment diameter is 25mm and the depth of the intermediate portion is 7mm):

Trifocal fitting

Trifocal fitting must always be a compromise in that when the height and inset of the inter- mediate portion have been determined, this position also fixes the height and inset of the reading portion. In the case of flat- and curved-top trifocals, which are the most popular in general use, the inset of the intermediate and near portions is exactly the same; it is not possible to inset these two portions separately. The position of one portion of the lens in front of the eye predetermines the position of the other portion.

In the case of trifocals for general-purpose use, the intermediate portion lies above the reading portion and the eye must pass through the IP before it reaches the near portion. The near portion is therefore lower than in the case of a bifocal design and the eye will need to rotate downwards somewhat further before it obtains the benefit of the near addition. A good starting position for the IP top is at or just below the lower pupil margin and it is sensible, whenever possible, to dispense a frame which allows some vertical adjustment of the segment height at, or subsequent to, the final fitting. This enables the segment top to be raised or lowered if this is found to be necessary, to speed adaptation to the new lenses.