Free-Form : the basics

Free-form vs Traditional Lens Surfacing
It is important to note that the concept free-form is based on two fundamentals:

  • design (during this process the correct geometrics for the optimal optical surfaces must be found – this had long been limited by production processes) and
  • production ( most intricate surface designs can be produced with Free-form equipment).

Free-form – the advantages
Free-form is definitely not a passing phenomenon. In an ideal world all lenses would have been made this way.
Traditional surfacing simply cannot create the advanced surfaces required for optically superior free-form lenses.
Free-form is not an optional extra – it should be viewed as the norm. No practitioner should run the risk of being
blamed for not introducing a patient to this latest technology.

Improved Optical Performance

  • minimised oblique astigmatism & reduction of power error in the whole field of view;
  • correct power experienced by the wearer over a much larger part of the lens surface;
  • exact lens power is accurately computed at any point on the lens (more than 100,000 points on a varifocal lens);
  • easier adaptation for new varifocal wearers – reduced rejection rate; and
  • varifocal lens design can be individualised for each patient, taking into consideration:
  1.   the frame selected,
  2.   individual traits,
  3.   hobbies & use of spectacles,
  4.   eye movement,
  5.   vertex distance
  6.   pantoscopic angle.

Production advantages

  •   production accuracy considerably better (error variance down to .05D as opposed to +/- 0.12D);
  •   wide range of base curves available to produce lenses for extra flat or extra curved frames;
  •   proper wrap lenses (including varifocals) with optimal optics in largest part of lens, not merely a lens that consists of a large blank with the optical centre decentred to accommodate the lens in a specific frame;
  •   centre and edge thickness limits defined by dispenser/lab technician to meet fitting and or frame requirements, e.g. Trivex lens, reduce lens thickness to ensure thinnest edges, or manipulate e.t. to ensure ample material for three-piece fitting;
  •   lenses in almost any material possible – not limited by blank availability

The ultimate Single Vision lenses
By producing a single vision lens according to the Free-form-method, optimal optics is ensured over a much larger area of the lens as in the case of traditionally produced lenses, including stock aspheric lenses. As Free-form
production is not limited by lens blank-size, thinner lenses can be surfaced as the lens is designed and surfaced to best fit into a specific frame (the equipment can remodel a blank to a desired diameter before starting the surfacing process – also remember that surfacing is much more precise than with traditionally surfaced or cast lenses). No wrap-around lens can match the optical performance of a Free-form wrap-around lens.
Verifying Free-form Lens Powers
Conventional progressive lenses are typically designed to read correctly in a standard vertometer/ lensmeter. Devices of this type measure the “vertex” power of the lens, not the power of the fitted lens as perceived by the actual wearer.
Since lens tilt and vertex distance can produce significant changes to the desired sphere, cylinder and add powers of the prescription, free-form lenses that have been optimised for the position of wear will often necessitate small
changes to the original prescription. These power changes are sent to the dispenser as a compensated Rx for power verification, which represents the vertex power (for measurement) necessary to produce the desired prescription
once the lenses are worn. For many free-form progressive lenses, back vertex power measurements are often used for add power verification as well. Due to the complexities of measuring the add power of progressive lenses, however, use of the semi-visible engravings for add power verification is preferable.
Most conventional progressive lenses are designed to provide the intended distance and add powers when measured by a vertometer/lensmeter, not by the actual wearer. However, many free-form progressive lenses are optically optimised for the as-worn/used position.
This process uses computer ray-tracing to refine the powers of the lens as perceived by the wearer with the lens in its intended position of wear, taking into account factors such as vertex distance, near working distance, and lens tilt.
User Power is the power that the patient really “sees” when looking through each point of the lens. User power depends on the shape of each surface of the lens at each point, and also on the lens position and orientation with respect to the eye. User power is important because the final satisfaction of the patient depends on it. For progressive lenses, when looking through the far region of the lens, user power should be equal to the user prescription, and when looking through the near part of the lens, user power should be equal to the prescription plus the addition.  Measured power is the power calculated by a vertometer or a mapper. When we measure the power on a lens point, or when we see the power map of a lens with a lens mapper device, we observe an approximation to user power, but not the exact value.
The vertometer or the lens mapper are based on an optical system that is not equal to the eye, and that is why there are differences between the power observed on an instrument and the real user power. These differences are bigger for points of the lens that are far from the centre, due to the bigger obliquity of the rays. These differences depend on the prescription, the addition, the base curve, the material, and the pantoscopic and wrapping angles. For progressive lenses, these differences will be significant in the near region, and there can be differences as high as 0.5D between measured power and user power.