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Some prescriptions require special intervention
Lenses

Getting the right fit part 2

07/09/2018
By Bruce Wain
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No matter the type of optical appliance, the fitting of lenses is a vital part of the overall dispensing process. BRUCE WAIN explores the most important aspects in his special two-part series.

Accuracy is crucial for any good optical dispenser. After all, it is both the information and measurements initiated at a practice – in combination with the manufacturing facility – that result in the production of high quality optical appliances. Any unwanted variable, at any stage along the way, will reduce the quality of the product and cause unnecessary delays.

In the first article, we discussed how to take PDs and heights, measure the patient’s frames, and complete calculations for centration and heights – all of which, of course, need to be accurate.

The next step

By now, the lenses are ready for blocking and while this is highly automated in manufacturing facilities, I find it is also helpful for students to learn how to manually ‘block’ lenses by using their calculations for centration and heights.

This is because while accuracy is emphasised, quality issues may arise when lenses rotate or move as the fitting block is applied, and students are able to quickly learn about machine and operator induced errors.

Once blocking has been finished, the lenses are now ready to be edged to shape. Modern edgers rarely cause too many quality issues but then there are operators!

"I find gaining firsthand experience or visiting a fitting workshop enhances the overall dispensing process"

Unfortunately, more operator errors occur than machine – for example, wrong shapes, wrong eyes (right for left – left for right) lens upside down and in cases at the workshop, round lenses are edged because the template was not held securely on the edger.

Another variable in the process is to decide whether to edge the lenses with a ‘vee’ or ‘flat’ edge.

This will depend on the type of frame that needs completing – metal and shell will have a vee bevel edged, while lenses for the nyl-tag and rimless mounts will have a flat edge. Safety chamfers are then completed and the lenses fitted into their respective frames.

It is important to note that quality control (QC) needs to be completed before optical appliances are handed over to the customer. The QC process ensures fitted jobs comply with current optical standards.

Common issues

Some of the frustrations experienced by students in our workshops during the fitting process include:

  • inaccurate dotting – 90 off axis

  • calculation errors for centration and heights leading to poor position of OCs, segments or fitting cross

  • blocking errors – lenses with incorrect insets – that is, decentering a right eye as a left and visa versa, not so good when this is not realised until final checking

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  • edging errors – lenses wrong eyes or upside down

Note: these errors are not machine related but due to human errors, and may go some way to explain why remakes occur.

In our workshop, and almost certainly in manufacturing facilities as well, we emphasise the need to take accurate measurements, double check calculations, and stop and inspect at each stage of the process to ensure the job is proceeding without errors.

This fitting process need not be difficult but there are various tasks that need to be ‘checked’ along the way in order to eliminate unwanted errors that can delay the job.

While a good percentage of prescriptions come under the ‘normal’ run of production there are others that require some special intervention to ensure they are completed within optical standards. For example, the higher the cylinder, the more accurate the process needs to be to ensure the correct axis is attained.

Furthermore, higher powers along the horizontal meridian need some attention to prevent inducing unwanted horizontal prism, and higher minus powers require special ‘edging’ techniques to ensure the best fit of lenses.

Additionally, certain lens types, for example progressives (as-worn, degressive, enhanced readers), require very accurate measurements.

I find gaining firsthand experience or visiting a fitting workshop enhances the overall dispensing process, as it allows for a better understanding of how some irregular (atypical) prescriptions may need a little more intervention than the ‘normal’.

Students going through our workshop certainly experience all the intricate variables involved in the fitting process.

Among the 10 jobs they fit, they complete astigmatic single-vision with prescribed prism into various frame materials, progressive lenses and multifocal lenses into different frame materials, and they fit lenses into both nyl-tag frames and rimless mounts.

Most students relish the challenge – some are daunted by the process (not having previously been involved in workshops) but overall, they all come away with additional skills and greater understanding of an important aspect of the dispensing process – getting the right fit for their customers.

More reading: Part 1: The importance of getting the right fit
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