Focusing in milliseconds without translational mechanics involved is possible with electrically tunable lenses. Fast
shape-changing lenses enable fast imaging systems which can focus at distances from infinity to a few centimeters with a
high optical quality. Furthermore, rapid laser processing in three dimensions is realized without mechanical translation of
the focusing lens or the sample. With tunable lenses the entire optics can be made compact, robust and abrasion-free.
Different configurations are discussed, how to integrate the tunable lens in the optical path. For machine vision
applications, the achievable optical quality depends on the chosen combination of the tunable lens with a fixed focal
length lens and a camera. It is recommended to use a fixed focus lens with a short distance between the stop position and
the front of the lens. Furthermore, important points are presented how to achieve optimal performance in laser processing
applications such as orientation and position of the tunable lens and the diameter of the beam incident on the lens.
Additionally, different approaches will be discussed for monitoring the focal length of the tunable lens. The focal length
of the tunable lens is sensitive to temperature changes, as the lens material is a fluid. However, in contrast to
conventional lenses, the focal length of the tunable lens can be corrected electrically. For that purpose, the tunable lens
exhibits an integrated temperature sensor for temperature compensation. Also optical feedback solutions will be
presented for applications requiring highest precision and tracking of the absolute focal length value.
Based on selected liquid and elastic polymers, Optotune has developed adaptive optical components, such as focus
tunable lenses and laser speckle reducers.
The lenses range from 2 to 55mm in aperture, are mechanically or electrically actuated and offer a continuous range of
focal powers of several 10 diopters. This additional degree of freedom enables the design of compact optical systems,
typically with less mechanics. We show how tunable lenses can be used to improve optical designs for imaging and
illumination systems in terms of size, quality and speed.
The speckle reducers are based on electroactive polymers and offer an extremely compact and low cost solution for
removing speckles, which is a key benefit for laser projectors and illumination systems.
Several approaches have been demonstrated to build focus tunable lenses. The additional degree of freedom enables the
design of elegant, compact optical systems, typically with less mechanics. We present a new range of electrically and
mechanically focus tunable lenses of different sizes and tuning ranges and discuss their characteristics. We show how
tunable lenses can be used to improve optical design for auto-focus and zoom in terms of size, quality and speed.
Furthermore, we present an LED-based spot light with variable illumination angle, which shows optimal performance in
terms of spot quality and optical efficiency.
Nowadays UV-cross-linking is an established method for the treatment of keraectasia. Currently a standardized
protocol is used for the cross-linking treatment. We will now present a theoretical model which predicts the
number of induced crosslinks in the corneal tissue, in dependence of the Riboflavin concentration, the radiation
intensity, the pre-treatment time and the treatment time. The model is developed by merging the difussion
equation, the equation for the light distribution in dependence on the absorbers in the tissue and a rate equation
for the polymerization process. A higher concentration of Riboflavin solution as well as a higher irradiation
intensity will increase the number of induced crosslinks. However, performed stress-strain experiments which
support the model showed that higher Riboflavin concentrations (> 0.125%) do not result in a further increase in
stability of the corneal tissue. This is caused by the inhomogeneous distribution of induced crosslinks throughout
the cornea due to the uneven absorption of the UV-light. The new model offers the possibility to optimize the
treatment individually for every patient depending on their corneal thickness in terms of efficiency, saftey and
treatment time.
Cross-linking of the cornea is a new curative approach to re-increase the mechanical stability of corneal tissue that is
progressively decreasing as a result of a corneal disease such as keratoconus or pellucid marginal degeneration. The
new procedure might have the potential to reduce the need for invasive corneal transplantation. The aim of the treatment is to create additional chemical bonds inside the corneal stroma by means of a photopolymerizer and UV light at 365 nm. Two different potential damage mechanisms must be considered: the UV-irradiation alone and the action of the photochemically induced free radicals (photochemical damage). In this study damage thresholds from the literature were compared to the treatment parameters currently used in corneal cross-linking and aspects of the design of a UV illumination system for corneal cross-linking were discussed with respect to the safety of the procedure. It was shown that the currently used UVA dose density of 5.4 mJ/cm2 is below the known damage thresholds of UVA for the corneal endothelium, lens, and retina. All these safety considerations assume an optically homogeneous irradiation. Optical inhomogeneities such as hot spots may lead to localized supra-threshold irradiation with consecutive damage to the corneal endothelium which represents the most endangered structure. Some authors have used direct illumination of the cornea by means of UV-LEDs, which bares the risk of creating too high intensities. Therefore, clinically used light sources must guarantee a perfect homogeneity of the irradiance across the beam area. The illumination system presented here provides good homogeneity and shows a very high tolerance towards variations in treatment distance which was shown to cause dangerous hot spots when direct LED illumination is used.
The effectiveness of the corneal ablation process in refractive surgery is mostly evaluated by indirect measures of vision or optical quality such as post-operative refraction or wavefront aberrometry. Yet, the effective amount of corneal tissue removed in the treatment can only be determined by correctly overlapping a pre- and a post-operative topography measurement. However such an overlap is not trivial due to the discrepancy in the centration axes used in the measurement and the treatment, as well as due to the shift of ocular axes through the treatment or tilt between the two surfaces. We therefore present two methods for overlapping pre- and post-operative topographies for the purpose of extracting an effective corneal ablation profile. Method one uses a 3-dimensional profile matching algorithm and cross-correlation analysis on surface rings outside the optical zone of the topographies. Method two employs a surface normal matching routine to align the two surfaces along their common ablation axis. The profile matching method implies the problem that it requires measurement data outside of the optical zone which was found to be uncertain with placido-disk-based topographers. Method number two is more simple and implies the advantage of using measurement data within the optical zone. For regular profiles the extracted ablation profiles showed a very good match with the planned ones. Surprisingly, even for highly irregular profiles of topography-guided laser treatments the method delivered reasonable overlaps when being compared to the planned profiles. Analysis of the effective tissue removal yields valuable information on the quality of the ablation process.
Based on eye movement data, we present a study on the effect of various laser and eye-tracking parameters on the optical outcome after scanning spot refractive surgery. Numerical simulations of the entire ablation process were performed on a schematic model eye under variation of the following parameters: ablation depth per pulse, laser spot size, eye tracker latency and magnitude of refractive correction. Three-dimensional ray tracing through an analytical model eye featuring the ablated corneal front surface enabled evaluation of the resulting optical quality. The modulation transfer function (MTF) was calculated to rate the difference in optical quality between an ideal (movement-free) treatment, and treatments performed with an eye-tracker working with a certain latency. For all the calculations it was assumed, that the laser repetition rate remains constant at 250 Hz. It was shown, that the contrast transfer can decrease significantly with increasing latency of the eye-tracker. For constant laser and tracking parameters, this decrease was found to be more significant for higher myopic corrections. It was further shown, that treatments performed with smaller spot sizes and smaller ablation depths per pulse are more sensitive to tracking latency. Assuming a certain eye tracker latency, the most stable results are obtained for large beam diameters and high central ablation depths per pulse. Latencies below 10 ms would allow for a reduction of the beam diameter to 0.50 mm as well as for ablation depths as small as 0.50 microns.
Purpose: Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality at the retina depends on all optical elements of the eye such as the human lens. Thus, the aim of this study was to investigate the correlations between the corneal and total wavefront aberrations and to discuss the importance of corneal aberrations for representing corneal laser surgery results. Methods: Thirty three eyes of 22 myopic subjects were measured with a corneal topography system and a Tschernig-type wavefront analyzer after the pupils were dilated to at least 6 mm in diameter. All measurements were centered with respect to the line of sight. Corneal and total wavefront aberrations were calculated up to the 6th Zernike order in the same reference plane. Results: Statistically significant correlations (p < 0.05) between the corneal and total wavefront aberrations were found for the astigmatism (C3,C5) and all 3rd Zernike order coefficients such as coma (C7,C8). No statistically significant correlations were found for all 4th to 6th order Zernike coefficients except for the 5th order horizontal coma C18 (p equals 0.003). On average, all Zernike coefficients for the corneal aberrations were found to be larger compared to Zernike coefficients for the total wavefront aberrations. Conclusions: Corneal aberrations are only of limited use for representing the optical quality of the human eye after corneal laser surgery. This is due to the lack of correlation between corneal and total wavefront aberrations in most of the higher order aberrations. Besides this, the data present in this study yield towards an aberration balancing between corneal aberrations and the optical elements within the eye that reduces the aberration from the cornea by a certain degree. Consequently, ideal customized ablations have to take both, corneal and total wavefront aberrations, into consideration.
The effect of lateral and torsional misalignments of the ablation on the postoperative optical outcome was theoretically investigated based on measured wavefront aberration data from 130 normal eyes. Simulations included lateral decentrations and rotations around the longitudinal axis of the eye (torsion). The optical quality of the simulated refractive correction was rated by means of the root-mean-squared residual wavefront error. The accuracy for lateral centration in order to achieve the diffraction limit at a pupil size of 8 mm in 95% of the investigated eyes should be 50 microns or better. However, an accuracy of 450 microns was found to be enough to guarantee that none of the investigated eyes would suffer from a decreased optical performance after surgery. Alignment would have to be performed with a torsional precision of approximately 1 deg or better in order to achieve the diffraction limit in 95% of the measured normal eyes for an 8-mm pupil, whereas an accuracy of 15 deg is required to obtain at least some improvement of the optical quality in all the examined eyes. The accuracy needed for torsional alignment increases compared to pure sphero-cylindrical treatments when additional correction of the higher-order aberrations is aspired.
The human eyes are not made to detect disease, however visual perception is the most common screening method for early cancer detection. With optimal illumination and observation configuration there is significant improvement of optical contrast between normal and pre-cancerous tissue in the oral cavity, both for reflected and fluorescent light.
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