Lately, the practice has been looking at new technology to evaluate the health and test the prescription of our patient’s eyes. One of the new technologies that caught our eye (pun intended) was the i.Scription by Carl Zeiss.
Traditionally, the final refraction that serves as the basis for the spectacle prescription is the result of a two-part process: The refractive errors of the eye are first determined objectively, using either retinoscopy or an autorefractor. The prescription is then subjectively refined by comparing the vision of the patient through various trial lenses, using either a Phoropter (refractor head) or trial frame. For those familiar with eye examinations that’s the ‘Which one is better, number one or number two?”
Now, with the recent advent of commercially-available aberrometers, which measure the “wavefront” aberrations of the eye, new enabling technologies have arrived that can raise the standard of refractive eye tests for the first time in decades. Evaluating the wavefront aberrations of an optical system is already commonplace in high-performance optical fields such as astronomy. There has been increasing interest in the ophthalmic applications of this technology, driven by advances in laser refractive surgery and orthokeratology.
Ok, I can hear some of you say, ‘What was that paragraph all about?” So for those not in the field of optics and feeling a bit lost at the moment, let’s quickly look at the meaning of the words wavefront and aberration.
The propagation of “light” is typically described as a rapid movement of energy particles—or photons—that travel in a wave-like manner through time and space. (It sounds like the opening lines of a Star Trek Movie). The propagation of light from any given object point can be represented conceptually using either rays or waves emanating outward from the light source. Just as rays of light diverge from an object point, waves of light spread out like ripples of water traveling away from a stone that has been dropped into a pond. At any given distance from the original object point, a wavefront exists that represents the envelope bounding waves of light that have travelled an equal distance from the object.
An aberration is essentially an error in focus. In a perfect optical system, wavefronts of light from a distant object should converge to a single point focus on the back of the eye, called the retina, after refraction through the eye’s optical system. In the presence of focusing errors or aberrations, however, these wavefronts become either too steep, too flat, or distorted from their ideal shape. Accordingly, the rays of light corresponding to these wavefronts are spread out at the plane of the desired focus, instead of intersecting at a single, sharp point. (Stay with me here.)
At any point across the aperture of the optical system, such as the pupil of the eye, the wavefront error is the effective optical separation between the actual wavefront and the ideal spherical wavefront centred on the desired focal point.
Wavefront Guided Subjective Eye Tests
An aberrometer measures the wavefront aberrations of the eye. Just as topographers now provide more detail regarding the surface characteristics of the cornea than conventional keratometers, aberrometers now capture more detail regarding the refractive features of the eye than conventional autorefractors.
Aberrometry is raising the standard of refractive eye care by allowing Eye Care Professionals to evaluate the individual optical characteristics of the eye more thoroughly, including both low-order and high-order wavefront aberrations. Determining the endpoint of refraction by taking into account the effects of high-order aberrations on retinal image quality can provide improved vision corrections that deliver optimum visual performance over a broader range of luminance levels, even under demanding viewing conditions like night driving.
The i.Scription® by Carl Zeiss represents a “wavefront-guided” vision correction derived from aberration data captured by the i.Profilerplus® aberrometer. Each i.Scription is then combined with a precisely fabricated, fully customized lens design in order to deliver the ultimate visual experience for wearers.
Precision Customised Spectacle Lenses
Once an i.Scription vision correction has been determined, a spectacle lens must be fabricated to the desired prescription powers. Unfortunately, traditional spectacle lenses often introduce additional wavefront aberrations that can compromise optical performance and vision quality for the wearer compared to the vision achieved during the eye exam:
- Residual low-order aberrations occur in traditional spectacle lenses due to oblique astigmatism as a result of either the tilt of the fitted lens (that is, position of wear) or the angle that the line of sight makes with the lens during peripheral vision.
- Residual low-order aberrations also occur in spectacle lenses due to the power rounding errors inherent in traditional lens surfacing, which typically relies on hard “lap” tools that are stocked in only 0.12 Dioptre increments of surface power.
Normally, semi-finished lens blanks are usually available in only a handful of unique base curves or lens designs, which are factory moulded in mass quantity. Changes to the basic design of traditional spectacle lenses are limited to subtle variations in optical design across a small number of base curves that must work sufficiently well for relatively broad prescription ranges. Regular spectacle lenses are therefore specifically designed for a few “average” prescription powers, using either “average” fitting parameters for progressive lenses or assuming a fitting condition free of lens tilt for single vision lenses. The use of average assumptions for each lens design results in uncorrected low-order aberrations for many wearers that can restrict, distort and blur the fields of clear vision.
Fortunately, the advent of free-form or digital surfacing technology has freed many lens designers from the constraints of traditional, mass-produced spectacle lenses. This modern manufacturing platform relies on computer-controlled generators that can precisely grind surface curves of extremely high complexity, including aspheric and progressive designs, directly onto a semi-finished lens blank.
When combined with advanced optical design software, free-form lens designs can be calculated in “real time,” using the wearer’s particular prescription and fitting parameters, immediately before fabrication. Progressive and single vision lens designed with i.Scription by Carl Zeiss can, therefore, be fully customized to the unique visual requirements of the individual wearer. Customized lenses preserve the intended optical performance of the lens design by minimizing residual low-order wavefront aberrations while ensuring that every wearer enjoys the visual benefits of the i.Scription vision correction, regardless of his or her prescription requirements or position of wear. Additionally, free-form surfacing is not subject to the power rounding errors of traditional lens surfacing.
In the picture above the contour plots of ray-traced astigmatism demonstrate that the optics of traditional progressive lens designs may be significantly influenced by the position of wear, resulting in residual low-order aberrations that can restrict, distort, and blur the zones of clear vision, whereas the fully-customized lens designs utilized for i.Scription lenses maintain the desired optical performance, regardless of the prescription or position of wear.
As you can see we are all understandably excited about this new technology that will be available at the practice in the near future. We will keep you updated as the story unfolds.