There’s a clear and growing trend towards Freeform progressive lenses. That’s because more lens manufacturers are able to offer varying priced Freeform, progressive lenses thanks to greater access to designs and production technologies.
The key to Freeform is well researched lens design combined with precisely managed freeform manufacturing techniques.
“Research conducted recently by Hoya clearly indicates that Optical Professionals perceive Freeform lenses as the new premium category, although many are unclear about what defines Freeform and find it difficult to distinguish between different products and offers,” says Brad Bairstow, Sales and Marketing Director of Hoya Lens Australia and New Zealand.
Not all ‘Freeform’ is the Same
Some manufacturers produce lenses which they refer to as ‘Freeform’. These lenses employ only a standard progressive design which is pre-cast into the front surface and combined with a standard back surface produced using a Computer Numeric Control (CNC) generator. This technique can produce a very accurate back surface however because there is no optimisation of the design, I do not believe it can be truly defined as Freeform. There is also typically little demonstrable improvement in performance for the wearer. These lenses are often called Freeform and sold at a lower price point.
I define Freeform lens technology as the manipulated of the lens surface in order to optimise the design for the individual wearer during the production process in the laboratory. This design optimisation is what delivers the true benefits to the wearer. This can be achieved in two ways:
1. By utilising a specially designed lens blank that has part of the progressive design pre-cast into the front surface, and using very sophisticated design software and, a CNC generator to produce a back surface that optimizes the design for the wearer.
2. By utilising propriety lens design software to design the entire lens at the point of production in the laboratory using the Rx frame, fitting and other parameters to produce a 100 per cent unique lens design for the individual wearer. This technique does not use any pre-cast lens design, the entire progressive zone is cut into a single vision lens blank.
Over the last decade we have applied research to the interaction between the wearer and the lens. The result of this research has meant that our Freeform lenses are now as close to optimal design as possible for every combination of Rx, fitting and frame. Therefore, each pair of Freeform lenses is 100 per cent unique to the individual wearer and produces superior performance.
Currently, Hoya has developed two of the leading Freeform progressive lenses on the market – Hoyalux FD and Hoyalux ID.
Hoyalux FD lenses use an advanced design pre-cast front surface, which holds part of the progressive design focusing on the near Rx. Combined with its patented optimised back surface, it incorporates the remainder of the design, focusing on the distance and intermediate components to create the complete design.
Hoyalux FD lenses occupy a unique position within the advanced progressive market segment particularly for new progressive lens wearers to ensure successful adaption and a positive wearing experience.
Freeform to the Next Level
Hoyalux ID begins life as a single vision lens blank. Hoyalux ID takes this advanced design to the next level by incorporating Integrated Double Surface Freeform design.
Hoya’s proprietary lens design software is used to develop a fully optimised front and back surfaces allowing for greater customisation and refinement of design during the production process in the laboratory. Splitting the design between the front and back surface provides detailed aberration control, producing wider fields of vision, particularly in the intermediate segment.
The result for wearers is enhanced comfort in the interaction between the eye and the lens at all distances and a consistent visual performance for each prescribed addition value. The unique interaction between front and back surface of Hoyalux ID ensures presbyopes in every age group who want the latest lens, will enjoy a natural equilibrium between sight and touch. They will literally and metaphorically feel ‘in balance’, enjoy greater freedom of movement and feel more confident.
The accessibility of these advanced lenses will appeal to the broad target group of younger, as well as more experienced presbyopes. While they have varied lifestyles, they both become increasingly more demanding in what they expect from their lenses.
Tailor-Made Quality and Performance
It is virtually impossible to achieve a useful evaluation of Freeform progressive lens performance purely by means of mathematical analysis. Even when differential ray tracing is used to quantify the aberrations of a progressive lens, interpretation of the results is still fraught with difficulty, since wearer adaptation cannot be taken into account.
Contour plots showing iso-cylinder and mean sphere power variations may provide a fingerprint of the lens, but wearer acceptance of the design is the most important factor and is not easy to predict from the plots.
In order to quantify progressive lens performance, the designers at Hoya oyaHoyhave used three lens performance indicators; the Clearness Index, the Static Deformation Index and the Dynamic Deformation Index, which when combined, help predict the overall performance of the lens from the wearers point of view.
Collectively, these form what the designers refer to as ‘Balanced View Control’. The Clearness Index relates to the blurring of the retinal image, the Static Deformation Index quantifies the change in shape of the retinal image, and the Dynamic Deformation Index indicates the morphing of the perceived shape of the object.
The Clearness Index is designed to indicate the quality of the image formed on the retina. When mapped across the lens, it reveals how the visual acuity is expected to vary in different zones of the lens. The object is defined in space by three-dimensional coordinates (x,y,z) and rays of light traced from each object point through the spectacle lens/ eye system to the retina. Since the rays are traced through both the lens and the eye, the resulting retinal image of each point on the object is generally not a point, but rather, a diffuse patch of light.
Examination of the patch shows that its shape and light distribution is a combination of both aberrations in the refracted pencils and diffraction effects. The patch is referred to as a point spread function (PSF). From knowledge of the angular subtense of the PSF, the degree of blur in the image and the likely resulting visual acuity can be readily predicted.
The Clearness Index is calculated by transforming each PSF into a simulated value for the visual acuity, while topographic mapping of the Clearness Index is then carried out by computer to produce the Clearness Index Maps.
Static Deformation Index
If the magnification produced by the lens in the peripheral field is irregular from rapid changes in the shape of the surface, objects viewed through peripheral zones of the lens will appear to change direction. To the new wearer of progressive lenses, this often presents as a swimming effect, as images viewed through peripheral portions of the right and left lenses move at different speeds and in unexpected directions.
The effect is a combination of distortion, produced by the increasing power between the distance and near portions, and changes that occur in surface topography when trying to achieve the correct blend between the various lens zones. This is particularly troublesome to the new progressive lens wearer, especially when looking at the floor, or walking down stairs, when the visual system is attempting to process information, which is arriving from oblique sight directions.
Dynamic Deformation Index
To describe the rate of change in the size and shape of the perceived image, Hoya has introduced a further indicator known as the Dynamic Deformation Index.
The concept of Dynamic Deformation Index describes how the shape of the image alters through various zones of a progressive lens, however it does not indicate the speed at which the shape of the image changes, which has a significant impact on wearer comfort. The faster the rate of change in the size and shape of the retinal image, the more unpleasant the effect upon the wearer will be.
Hoya’s research into progressive lens design is particularly focused on the interaction between the eye, the lens, the wearer and their surroundings to ensure the wearer perceives a visual experience as close to natural as possible.
Dr. Akira Kitani is Head of Hoya Lens Technology Development. He is a Doctor of Optometry and is named as an inventor of a number of Hoya patents relating to lens technology.