Wednesday, July 6, 2011



Precal is a lens calculation process which predetermines the thickness of a glazed lens, based on lens and frame shape, prescription and centration. Precal calculates the minimum centre thickness to which a lens can be surfaced to provide the thickness appropriate to the lens/mount combination. The Precal calculation will only be applied when specified by the customer or when stock lenses cannot be used due to the frame type e.g. supra or rimless frames.

1. Availability

Establish that the prescription is within range in the diameter required.

2. Provide full prescription
Lens type
Tints and/ or coatings
Frame type or edge thickness. Quote the minimum distance between lenses (measured at the nasal points, NOT HCL)

3. Shape outline

Draw around former or glazed lens with point of pen inclined inwards to ensure outline is true size and shape. Indicate the A, B measurements of the lens shape on the order. This eliminates any errors caused by drawing or faxes being stretched.

4. Centration
Quote right and left monocular PDs.
Quote right and left fitting heights*

5. Frame details

State frame type or quote the lens edge thickness required at the thinnest point, no less than 1mm. If no edge thickness is quoted for a rimless frame we will automatically aim for 1.5mm at thinnest edge (depending on frame) ensuring 2mm or 2.5mm at drill holes depending on lens material. Essilor give a minimum edge thickness of 2mm for Nylor frames.

You may also order Precal by telephone by quoting the lens type, prescription and coatings etc. accompanied by:

A standard shape number
The A box measurement
The B box measurement
The boxed minimum distance between lenses
The thinnest glazed edge requirement or Frame type e.g. metal, plastic, supra, rimless

Tip - When it is difficult to clearly define the shape by number, SHAPE No.7, (a rectangular shape with rounded corners) or SHAPE No.5, (an oval shape) have been found to give consistently good results.

* Fitting height may be quoted in relation to the horizontal centre line (HCL) or as distance to the tangent of the lowest edge of the lens

Plus factors for using Precal
Mid or high plus prescription
Plus cylinder axis of a toric lens running along the horizontal ± 30° (e.g. +200/+250 X 165)
Varifocal lens
Lens with prism base up, down, out or in
Shallow eye shape
High decentration

Precal may also be used:
To ensure the correct edging for rimless glazing, supra glazing and faceting
To balance thickness or weight of right or left lenses for both minus and plus prescriptions

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