Deciding Whether To Use a Finished Lens Blank
- Tony LeBlanc
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.
In the specific example we have been looking at, with these settings, Rx-Universe would select the finished lens when the PD=31.0, but would NOT select the same finished lens when the PD=28.0. This is because in the first case, the resulting ET (1.99) is within tolerance (within 0.3mm of the 2.0mm target), but in the second case, the resulting ET (1.54) is NOT within the 0.3mm tolerance of the 2.0mm target ET.
Different Tolerances by Edge Type
In addition to the tolerances that can be set system wide, and by material and lens style, specific tolerances can be set by edge (or frame) type. For example, a lab might allow a tolerance of 0.5mm for most jobs, but for groove frames (where the edge must be thick enough to groove), the lab may desire a tighter tolerance. These tolerances can be set on the Frame Type table (under “Setup”):
In the example above, Polycarbonate lenses that are to be grooved have two settings; one to make the minimum ET 2.2mm regardless of power, and the second is to stipulate that a finished lens can be no more than 0.2mm thinner than the target (in this case, no thinner than 2.0mm). The handling of tolerance at a frame type level is a little different than for materials and lens styles; while a thinner value in system, material, or lens style would still take precedence over the frame type value, a value of 0 for the frame type is interpreted as 0 - not allowing the ET of a finished lens to be any thinner than the target thickness - this would be typical in the case of safety jobs, for example. (Note that there is a separate system setting for finish lens thinness tolerance for safety jobs, on the System, Blank Selection tab).
Single Vision Lenses
Finished single vision lenses come in wide selection of sphere and cylinder combinations; typically both are available in increments of 0.25D (meaning an Rx of +1.37, for example, would typically not use a finished lens, since a finished lens in that power is rarely, if ever, available).
The three “checks” or restrictions on using a finished SV lens then, are:
The Rx must be available in a finished lens (remember that jobs entered in plus cylinder can be transposed and a finish lens of the transposed power used)
The blank must be large enough to be decentered (put the center of the blank at the OC relative to the frame)
The thickness of the lens must meet the thickness rules and tolerances specified by the lab
Flat Top Lenses
Finished flat top lenses tend to be available in far fewer combinations than SV lenses; many labs do not stock nor even order them. Finished flat top lenses are available in combinations of sphere and add power only; they are not available with cylinder powers, because one cylinder power would have to be available in many axes, since the lens cannot be rotated because of the segment (unlike a SV lens, which can be rotated to put the cylinder on the desired axis).
The second restriction on a finished flat top is that the location of the distance OC (or MRP) of the lens is fixed relative to the segment, since the lab cannot grind prism to move the OC relative to the segment, as can be done with a semi-finished flat top. This means that the difference between the distance and near PD of the Rx, must match the horizontal segment inset of the finished blank. As an example, if the inset of the lens was 3.0mm (the difference between the OC location and the center of the top of the segment), and the difference in the patient’s PD was 1.5mm, the lens could only be used if the 1.5mm difference (between the inset of 3mm on the blank and the patient’s inset of 1.5mm) resulted in less than 0.25D of prism (based on the power of the lens in the distance). In practice, many vendors manufactured these lenses with an inset of 2.0mm, which is similar to most patients' PD difference.
Progressive Lenses
The same restrictions that are described regarding finished flat top lenses also apply to finished progressives. With the rise in popularity of digital progressives, it’s very rare to find traditional progressives manufactured as finished lenses today.