Generally speaking, labs would prefer to use a finished lens blank whenever possible, as long as the blank is large enough for the prescription, and, in the case of plus lenses, gives a thickness at both edge and center that is acceptable to the lab. In the case of flat top and progressives (both of which are available as finished lenses, although with limited selection), the prescription must also be respected. While there are similarities between them, each of these types of finished lenses is described separately. The common element, that of determining thickness, is described first.
Is the finished lens of the right thickness?
When surfacing a lens, the lab will grind the lens to a specific thickness, based on rules for both minus and plus lenses. These rules are described in the section Default Thicknesses.
First, for purposes of thickness, a lens is consider “plus” if it is plus along ANY axis. This is because, along any axis where the lens is plus, the edge will be thinner than the center, and edge thickness rules come into play.
The default thickness rules define what the “target” thickness is for a job. For example, a lab may stipulate that a +2.00 spherical lens should have an edge thickness of 1.8mm (in some arbitrary material). When grinding a semi-finished blank, Rx-Universe will calculate the center thickness such that the thinnest edge will be 2.0mm, respecting the lab’s preference.
When using a finished lens, however, the center thickness of the lens is already determined, and the edge thickness of the lens at the thinnest point will be determined by the center thickness and the distance from the center of the blank (the OC or MRP) to the thinnest edge.
To illustrate using an example, the thickness controls are set to target a thinnest edge of 2.0mm on this material at a +2.00 power. To simplify the illustration, we’ll use a spherical Rx (no cylinder), but the principal is exactly the same for a compound lens; the power is calculated along each axis of the finished lens. The order appears as follows:
The Butterfly shape (a standard shape in Rx-Universe), with the specified PD, results in a lens which fits on the frame as follows (this is a simple work ticket diagram):
The OC (MRP) is indicated on the diagram by the circle; the center of the blank is the cross inside the OC, since the OC is fixed on a finished lens and the blank must be decentered to the correct location (for comparison, on a semi-finished lens the blank can be centered on the frame and prism used to move the OC, resulting in a smaller minimum blanksize).
In the case of this particular lens, the CT of the blank is 4.0, and the thinnest edge will be 1.99mm, which is almost identical to the 2.0mm target.
However, if the decentration were slightly different, the ET at the thinnest edge will change. If the PD were a little narrower, the thinnest edge will be thinner, since the distance from the OC (and the blank center) to the thinnest edge, will be longer. For example, if we change the PD from 31 to 28, the blank is moved over more, resulting in something like this:
The thinnest edge is now around 1.54mm, instead of 2.0mm, because the center of the blank (with the fixed 4mm center) is further away from the thinnest edge. The question is, is this acceptable?
The lab can set how much thinner a finished lens can be and still be acceptable.
First, there is a SYSTEM setting for this, found on the “Blank Selection” tab under “System Settings”:
Tolerances can also be set on the material and on individual lens styles:
If a tolerance is specified on any combination of system default, material, and lens style, for a particular order, the SMALLEST value of the three is the one that will be used (excluding 0 in the case of the material and lens style, as that indicates the material/lens style has no specific tolerance). In the above example, the tolerance is 0.3mm, the smallest of the 3 values, indicating the finished lens could be up to 0.3mm thinner than the target thickness and still be acceptable.