Modified point diffraction interferometer for inspection and evaluation of ophthalmic components

Ultrahigh-dynamic-range wavefront sensor based on absolute double-slit interferometry

This Letter reports a new, to the best of our knowledge, technique for the quality testing of steep optical samples by introducing an absolute interferometry method based on a double-slit interference experiment. We determine the quality of the sample with an ultrahigh-dynamic-range wavefront sensor by determining the deformation of the central fringe of the double-slit interferometer recorded for two different separations of the slits. The transmission function of the double slit is implemented on an amplitude spatial light modulator. Therefore, the slits' location can be easily displaced over the entire area of the sample's wavefront. We applied the proposed method on two samples: a microscope slide and a conventional ophthalmic lens, and maximum absolute phase variations of 0.33 and 26.7 rad were measured, respectively. Our estimation shows that an absolute phase variation of about 700 rad can be measured with this method.

Shadowfocimetry: adapting the holographic principle to a manual focimeter for visualization/marking of permanent engravings in progressive addition lenses

Focimeters, especially manual versions, are the most used ophthalmic devices for dioptric power measurement in optometric clinical care. In the particular case of progressive addition lenses (PALs), they are used to determine far/near vision correction powers, but the user/clinician needs to know at which part of the PAL the measurement must be taken. For this reason, PALs have permanent engravings acting as reference marks to define the far/near vision areas for every PAL design. However, for several reasons these engravings are often difficult to localize and identify, making an accurate dioptric power determination difficult. In this Letter, we present an adaptation of the Gabor holographic principle to a manual focimeter and describe the methodology for the correct localization, visualization, and marking process of the reference engravings in PALs. Experimental results considering different types of PALs are included and the main limitations of the technique are also discussed.

Optical-component-only adaptive optics

This Letter introduces a technique for performing binary adaptive optics, which is carried out by optical components only, without the help of any electronic or optoelectronic device. In this technique, the interferogram produced by a point diffraction interferometer modulates a light-driven crystal. The modulated light-driven crystal may produce pupil-plane only-phase or only-amplitude binary masks to mitigate phase aberrations. The capability of working unsupported makes it suitable for application in hard-to-reach or hazardous locations such as satellites, underwater, or contaminated places. The Letter includes an experimental validation where the ability of the technique to produce pupil amplitude masking is confirmed.

Quaternary adaptive optics

We present a new Point Diffraction Interferometer (PDI). Binary adaptive optics (BAO) and Quaternary Adaptive Optics (QAO) can be performed with the help of this PDI as a wavefront sensor. The PDI interferogram, once binarized, is used in two consecutive steps to produce a quaternary mask with phase values 0, π/2, π and 3π/2. The addition of the quaternary mask compensates for the aberrated wavefront and allows us to reach a Strehl ratio of about 0.81. We have verified through computer simulations that the use of QAO depends on the number of actuators of the compensating device to achieve effective compensation. The technique was successfully validated through an experiment.

Pupil size stability of the cubic phase mask solution for presbyopia

Presbyopia correction involves different types of studies such as lens design, clinical study, and the development of objective metrics, such as the visual Strehl ratio. Different contact lens designs have been proposed for presbyopia correction, but performance depends on pupil diameter. We will analyze the potential use of a nonsymmetrical element, a cubic phase mask (CPM) solution, to develop a contact or intraocular lens whose performance is nearly insensitive to changes in pupil diameter. We will show the through focus optical transfer function of the proposed element for different pupil diameters ranging from 3 to 7 mm. Additionally, we will show the images obtained through computation and experiment for a group of eye charts with different visual acuities. Our results show that a CPM shaped as 7.07  μm*(Z33-Z3-3)-0.9  μmZ20 is a good solution for a range of clear vision with a visual acuity of at least 0.1 logMar from 0.4 to 6 m for pupil diameters in the 3- to 7-mm range. Our results appear to be a good starting point for further development and study of this kind of CPM solution for presbyopia.

Interferometric Local Measurements of High-Order Aberrations in Progressive Addition Lenses

PURPOSE

The main goal of this work is to show the principles, design, and performance of a modified point diffraction interferometer (PDI) for evaluating the local higher-order aberrations of progressive addition lenses (PALs). Because aberrations have different impacts in visual perception, we propose a device to provide a customized solution with the best permissible amounts and combinations of aberrations for improving a subject's vision.

METHODS

A PDI has been adapted to measure local high-order aberrations in PALs.

RESULTS

High-order aberrations in three different PALs within four relevant circular regions of interest (ROIs) with a radius ranging from 0.4 to 2.4 mm were measured with an accuracy of λ/10 within the ROI using the tailored PDI. The interferometer also allows for easily choosing the position and number of the ROIs.

CONCLUSIONS

The interferometric device is compact, robust, and accurate. The operational principle is very simple, and it provides the local high-order aberrations directly without adding additional parts to the interferometer. As expected, the amount of high-order aberrations depends on the chosen ROI of the PAL; the corridor is the more critical region. We found, in accordance with the literature, that, in the corridor, second- and third-order aberrations are dominant and spherical aberration is negligible.

Measurement of spherical and cylindrical power in ophthalmic lenses based in the change of lateral amplification

In this article, we present a new technique to measure spherical and cylindrical power in ophthalmic lenses. This method is based in the change of lateral amplification produced by an optical system when introducing an ophthalmic lens. Ophthalmic lens power is calculated by considering the change in image size from a reference object and its own image seen through the ophthalmic lens. Mathematical analysis is presented along with the experimental setup and the obtained results. Several algorithms were applied to the obtained results as a method to compensate the error in order to fit into ISO 8598 specifications.

Mathematical description and measurement of refractive parameters of freeform spectacle lenses

Refractive parameters are the main design and evaluation parameters of freeform spectacle lenses. In this paper, the mathematical model of refractive parameters is established, and the refractive power distribution on the whole surface is drawn with a radial basis function. The measurement methods are analyzed, and typical freeform spectacle lenses are measured with a freeform verifier. The refractive power distribution on the whole surface, the cylinder view, and the refractive power curve along the progressive corridor are drawn up. There are no evident image changes on the whole surface. The refractive power smoothly varies along the progressive corridor. In comparing the results with the analysis, measurement results are in agreement with the calculation.

Wavefront-coding technique for inexpensive and robust retinal imaging

We propose a hybrid optical-digital imaging system that can provide high-resolution retinal images without wavefront sensing or correction of the spatial and dynamic variations of eye aberrations. A methodology based on wavefront coding is implemented in a fundus camera in order to obtain a high-quality image of retinal detail. Wavefront-coded systems rely simply on the use of a cubic-phase plate in the pupil of the optical system. The phase element is intended to blur images in such a way that invariance to optical aberrations is achieved. The blur is then removed by image postprocessing. Thus, the system can provide high-resolution retinal images, avoiding all the optics needed to sense and correct ocular aberration, i.e., wavefront sensors and deformable mirrors.

Inspection of freeform intraocular lens topography by phase measuring deflectometric methods

Manufacturing spherical, aspheric, and freeform surfaces requires testing throughout the development and production process. State-of-the-art topography measurement is limited in applicability for intraocular lenses (IOLs), and there is no dedicated commercial surface measurement system available for freeform IOLs. The purpose of this work was to validate a deflectometric setup for surface measurement, detection of defects, and shape fidelity analysis for the development and production of IOLs. The setup is based on a phase measuring deflectometer with a field-of-view of 80 mm×80 mm and a mean repetition accuracy of 1.6·10(-3) D. The technique is suitable for detection of global and local surface errors, extracted from geometry and topography analysis. For validation according to DIN ISO 5725:2002, spherical IOLs with radii of curvature of 10 and 20 mm, a commercial aspheric IOL, and single-sided freeform IOL samples were used.

Optical pressure sensor based on the combined system of a variable liquid lens and a point diffraction interferometer

We propose an experimental efficient optical pressure sensor based on a variable liquid lens and a modified point diffraction interferometer. The working principle of the sensor is based on the fact that a variation in pressure induces a change in lens curvature and hence in its focal length, which can be tracked and measured with the interferometer. The pressure is then measured by recording and processing the interferometric images. The sensor in this proposal can change its dynamic range by the simple axial movement of one of the components of the optical system. In this work we show the performance of the system within three working ranges: from 0 to 1 kPa with accuracy of approximately 0.01 kPa, from 0 to 7 kPa with 0.05 kPa accuracy, and from 0 to 30 kPa with 0.3 kPa accuracy.

Wavefront sensing with an axicon

The use of a large apex-angle axicon for common-path interferometric wavefront sensing is proposed. The approach is a variant of point-diffraction interferometry bearing similarities to pyramidal wavefront sensing. A theoretical basis for wavefront sensing with an axicon is developed, and the outcomes of numerical simulations are compared to experimental results obtained with spherical and cylindrical ophthalmic trial lenses. It is confirmed that the axicon can be used for wavefront sensing, although its refraction may ultimately complicate and limit its operational range.

Wavefront measurements of phase plates combining a point-diffraction interferometer and a Hartmann-Shack sensor

A dual setup composed of a point diffraction interferometer (PDI) and a Hartmann-Shack (HS) wavefront sensor was built to compare the estimates of wavefront aberrations provided by the two different and complementary techniques when applied to different phase plates. Results show that under the same experimental and fitting conditions both techniques provide similar information concerning the wavefront aberration map. When taking into account all Zernike terms up to 6th order, the maximum difference in root-mean-square wavefront error was 0.08 microm, and this reduced up to 0.03 microm when excluding lower-order terms. The effects of the pupil size and the order of the Zernike expansion used to reconstruct the wavefront were evaluated. The combination of the two techniques can accurately measure complicated phase profiles, combining the robustness of the HS and the higher resolution and dynamic range of the PDI.

Assessing the optical performance of multifocal (diffractive) intraocular lenses

BACKGROUND AND PURPOSE

While the optical performance of monofocal refractive lenses can be measured quite easily, more efforts are required to assess the performance of multifocal lenses, due to imaging to several foci. The purpose of this study was to develop a rugged test setup for evaluation of the optical properties of intraocular lenses (IOLs) in conformity to the International Standard ISO 11979.

METHODS

We built a test setup in order to measure the point spread function and to determine the modulation transfer function (MTF) of IOLs. We measured three multifocal IOLs with (1) a diffraction limited model cornea and (2) an individualized aspheric model cornea which shows a significant amount of spherical aberration.

RESULTS

All the three IOL samples tested showed a different impact of spherical aberration on the MTF. The bispheric lenses (Alcon and Dr Schmidt) showed a degradation of the MTF and Strehl ratio with model (2) compared to model (1). In contrast, the MTF of the Tecnis lens, which has a prolate aspheric front surface, improved dramatically in combination with model (2).

CONCLUSION

We demonstrated the functional capability of our measurement system by presentation of a set of working examples. Two different model corneas were used to study the influence of spherical aberration on the MTF of our working examples. An aspheric model cornea, which had already been shown to be suitable for testing monofocal aspherics, was shown to be well suited for testing aspheric multifocal lenses.

Single dispersive gradient-index profile for the aging human lens

We provide a single gradient-index (GRIN) profile for the crystalline lens in an updated age-dependent emmetropic-eye model. The parameters defining the GRIN profile include their variation with age and the dispersion of the refractive index in order to account for the increase in the positive-wave spherical aberration, for the constant chromatic difference in the refraction of the human eye, as well as for the decrease in the retinal-image quality with aging. In accounting for these ocular properties, the results show that first, the value of the dispersion parameters are invariant with age. Second, those parameters defining the distribution of the lens index cause the lens-center-index value to decrease slightly, and its position along the lens axis changes with age. Furthermore, these findings are in agreement with the lens paradox.

Phase-shifting point-diffraction interferometer developed by using the electro-optic effect in ferroelectric crystals

A novel and simple phase-shifting point-diffraction interferometer using a z-cut lithium niobate wafer is proposed. The pinhole is realized by an optical lithography process, aluminum deposition, and subsequent lift-off on the surface of the wafer. The phase shifting is obtained by inducing the electro-optic effect along the z crystal axis. We demonstrate experimentally the possibility of retrieving an aberrated wavefront.