Digital Freeform Lenses, Part II

By John T. Fried, and Christian Laurent
In part one, which appeared in the March/April issue of LabTalk, the importance of ensuring proper power measurement within the zone of interest, was discussed and suggestions were offered on how to achieve success in this important function. In Part II, determining the faithful reproduction of the lens design and ensuring control of the digital surfacing process will be discussed.

Measuring the Progressive Design

There are a number of approaches available for measuring the faithful reproduction of the optical designer’s intention for the progressive lens produced by digital surfacing. Some instruments use light reflection measurement for an estimation of the concave surface of the blocked lens being produced and therefore give an estimation of the surfacing process. Other systems measure the refraction effect, or ‘through power’ of the lens by light transmission over a large area. Even others use plungers to plot the contours of the surface along a single, defined axis. When taken individually each of these methods suffer from at least one of the following limitations and/or drawbacks:

1.The lens is often damaged or very specific treatment for the front surface is necessary to avoid back reflected light.

2.They can not capture enough of the potential manufacturing errors with one measurement methodology:

a. Blocking deformations.

b. Lathing marks.

c. Errors that are only apparent when evaluating both surfaces together.

d. Misalignment of front and back surfaces according to the semi-visible micro-engravings.

e. Errors in placement of the micro-engravings.

f. Effect of the measurement geometry on the through power and particularly lens surface tilt in relation to the plane of me surement.

With contact measuring systems the drawbacks include:

a. Multiple measurements must be taken along each axis of interest in order to “compose” a topographical map of the surface.

b. Surface scratches can result from the probe.

c. Multiple measurements take a long time.

d. Limited accuracy for local errors detection.

The approach discussed in this article is a method combining the use of light reflection off the front surface and light that is transmitted through the lens—measurements by reflection and transmission.

A well-defined evaluation of the lens can be done with multiple measurement types performed simultaneously with a single device by mapping the front surface on a loose lens by reflection, and using transmission light for detecting the semi-visible engravings position and then measuring the through power on the lens surface. In this way the lens is free from the influence of the block, takes advantage of both reflection and transmission methodologies and represents a real world result.

The measurement done by reflection must use a method that avoids the effect of light bouncing off the back surface without the requirement for special surface treatments like paint, soap, tape, or abrasive opacity. Preferably, the multiple measurements are performed with a single optical system ensuring a perfect match between the various results. As mentioned above, the refraction effect depends on the exact optical configuration. So, the precise lens geometry (including tilt) as it is placed in the measurement device must be known and is a necessary condition for the exacting analysis of the through power map. The geometry is defined via the precise engraving position measurement and convex surface measurement by reflection.

The resultant measurement map can then be compared with the data supplied from the optical design of the lens and a confirmation of the design can be achieved via an “error map” analysis. It is also then possible to identify areas on the lens that do not comply with the desired design. This feed-back to the digital surfacing process can pinpoint areas in need of correction. Such a through power mapping analysis identifies errors induced by the global manufacturing process, including both front and back surfaces, blocking errors, engravings or surface misalignment.

Moreover, this approach of checking the lens and manufacturing process is valid, whatever the lens type, however it may have been produced (i.e. both lens surfaces progressive).

Patient lenses can be checked prior to shipment and the production process can not only be monitored but provided with quantitative information helpful for taking immediate corrective steps and taking actions for improving and mastering the digital surfacing process.

Ensuring Surface Quality

It is recognized that even though lenses can be measured to be within tolerance via conventional local power checking techniques, they can still be strongly defective. By using the method described above, mapping the loose lens with reflection and transmission and comparing the measured values against the data with the expected design, numerous key digital surfacing manufacturing defects may be identified that would not be recognized by conventional inspection techniques.

Surface defects like the lathing dimple in the center of the lens that can develop as a result of a worn tool or misaligned spindle may escape detection without careful lens mapping analysis. The polishing process for free form is intended to remove very little stock. When unexpected variables are introduced into the digital generating and polishing processes such small stock removal may not be sufficient to remove all these lathing marks. Unfortunately the defect may not be apparent with conventional measuring techniques and the system we have described above is needed.

Tools for Control

The challenge to set up, improve and maintain an effective digital surfacing process is a reality for many labs. Their efforts to manage the process is now supported with the availability of these new measurement and control tools. By measuring the power of the lens aligned in the proper zone with an accurate system that exchanges data with the host and by evaluating the lens using the reflection and transmission lens mapping, the free form process can be further controlled by identifying defects earlier and leading to a proactive preventative program that will stop problems and unnecessary scrap before they begin. Lens mapping and accurate power measurement will also result in reduced returns from the eye care provider and an improved acceptance rate from their patients.

The primary goal of measuring the resultant lens from the digital surface process is to ensure that the lenses provided to the patient address the visual correction needs as identified by the eye care provider and as conceived by the lens designer. The consistency of doing this with success is dependent upon feedback to the lab so the process of making these lenses is under control.


CURRENT ISSUE


May/June LabTalk 2017