Reducing/cancelling the effects of vitreous floaters using a phase retrieval method based on coded diffraction patterns (2017 - 2020)

Project leader Varis Karitans

Agreement No 1.1.1.2/16/I/001

Research application No 1.1.1.2/VIAA/1/16/199

The project goal is to develop a non-invasive optical system and method for reduction/cancellation of the undesired effects of the vitreous floaters. The vitreous floaters manifest themselves as chains, bubbles and other structures floating in the field of view. These structures are actually transparent phase diffraction gratings. This problem causing the visual discomfort is experienced by about 80 % of all people. The problem will be solved using the method of coded diffraction patterns to measure the structure of these objects and modulate the light so that the effects of the floaters are reduced. The method of coded diffraction patterns is one of the most popular numerical methods for measuring the phase and structure of objects. This method is suited for measuring the optical aberrations, determining the structure of complexes of proteins, structure of crystals and other purposes. During the project the applicability of this method for designing a new type of wavefront sensors in astronomy and determining the structure of objects floating in microfluidic systems will be evaluated. Such studies are essential for Latvian industry by facilitating development of high-tech products and companies.

The project will be realized at the Institute of Solid State Physics (ISSP) in collaboration with the optics laboratiry at the Dublin University College. The project will be multidisciplinary and will be realized in the field of innovative materials and technologies – natural sciences (1.1 Mathematics, 1.3 Physical sciences, 1.4 Chemical sciences) and engineering sciences and technologies (2.5 Materials engineering). The project activities are related to RIS3 as its goal corresponds to the specified directions of transformation of national economy and the 1st, 2nd, 3rd and 6th priority of the economic growth. The project matches the following specialization areas: 1) advanced materials, technologies and engineering systems; 2) Information and communication technologies and key technologies identified by EC (nanotechnologies, micro- and nano-electronics, photonics, advanced materials and manufacturing systems, biotechnologies).

The total duration of the project is 36 months, the total cost is 133,805.88 EUR.


Project progress

August 07, 2020

The model eye has been improved so that type II collagen fibers cam be passed through a microfluidics channel included in the  model eye. The fluid is pumped through the channel using a microfluidics pump with syringes attached to it. Light scattering media will also be attached to the model eye to simulate the optical system of a living eye. Currently, phase retrieval in presence of light scattering is discussed with the Optics laboratory of the University College Dublin. An optical system for practical phase retrieval and compensation in light scattering media is also developed. The phase of vitreous floaters will be retrieved and compensated using spatial light modulators already incorporated in the optical system. The optical system will also be used for observing the 2nd order motion of vitreous floaters. Besides the mentioned activities, possibilities to compensate eye movements in the system of the model eye are also discussed with the partner institution.

A manuscript entitled “The effect of noise, a constant background, and bit depth on the phase retrieval of pure phase objects” has been revised and resubmitted to the journal “Optica Applicata”. A manuscript entitled “Phase retrieval of a Kolmogorov phase screen from very sparse data using four binary masks” and submitted to the journal “Applied Optics” is being currently revised. The manuscript needs minor revisions and has been provisionary accepted for publication.


Project progress

May 05, 2020

Using the method of coded diffraction patterns, the phase maps of vitreous floaters have been obtained. A Kolmogorov phase screen has been designed using grayscale lithography. The influence of of oversampling, bit depth and the number of masks on the quality of phase retrieval of vitreous floaters and a Kolmogorov phase screen has been studied. A reconstruction error of the algorithm PhaseLift is significantly reduced as the oversampling ratio is increased several times. The reconstruction error also is reduced as the bit depth is lowered thereby increasing the sparsity of the input data and minimizing the influence of noise and background on the phase retrieval. The influence of the non-linearity of the camera on the measurements has also been studied.

An abstract for the conference “Visual and Physiological Optics” (VPO) to be held in August, 2020 has been prepared, however, the conference was postponed until the year 2021 due to the crisis caused by the coronavirus. A manuscript entitled “Influence of noise, background and misalignment on the phase retrieval of micron-sized phase objects” was submitted to the journal “Optica Applicata”. The manuscript addresses the question how various factors influence the phase retrieval of vitreous floaters. Another manuscript about the influence of various factors on the phase retrieval of Kolmogorov phase screen is being prepared. A popular sceintific article has been accpeted for publication in the journal “Starry Sky” and another popular scientific article has been prepared for the journal “Alma Mater”. Phase retrieval in noise caused by laser speckles has been discussed with the foreign partner.


Project progress

February 07, 2020

In the timeframe from the 1st October 2019 to 31th January 2020 the applicability of the method of coded diffraction patterns for retrieving the phase of vitreous floaters has been successfully validated in simulations. The optical system for the phase retrieval has also been completed. Using tunable optical elements, the optical system has been updated based on Nyquist conditions. Using the designed eye model and microoptical elements reported in the technical documents, measurements of diffraction patterns have been obtained and their analysis is being continued. The eye model has been improved using SU-8 and PDMS microstructures. In the specified timeframe the diffraction patterns of a coherent extended source have been analyzed, evaluated various illumination schemes of the retina of the eye model. The influence of the optical noise and background on the measurements in the optical system has been assessed.

The work on development of the optical system for measuring the atmospheric turbulence in various spectral regions has been continued to prove the efficiency of the method of coded diffraction patterns in astronomy.

A popular scientific article has been prepared and submitted to the journal “The starry Sky”. A lecture was presented at the doctoral school of vision science.

An abstract has been submitted to the annual conference of the ISSP. An abstract is being prepared for the conference “Visual and Physiological Optics” (VPO) in August, 2020.


Project progress

November 13, 2019

A research article “Optical phase retrieval using four rotated versions of a single binary amplitude modulating mask” has been published in a journal “Journal of Astronomical Telescopes, Instruments, and Systems” (2018. gadā SNIP = 1,827). As the method of coded diffraction patterns has been validated in a real, physical system it can now be used to measure the phase of vitreous floaters and compensate them in a real physical system. Until now the applicability of the aforementioned method for measuring the phase of vitreous floaters has been demonstrated in MATLAB and the initital optical system for measuring the phase of vitreous floaters has already been completed.

A presentation about the technologies of adaptive and diffractive optics and the activities of the postdoctoral project is being prepared as a part of the pupils’ excellence school. In the doctoral school of vision science, the listeners were introduced to current achievements of the postdoctoral project and future plans.


Project progress

May 07, 2019

The applicability of the Gerchberg-Saxton algorithm and PhaseLift algorithm for phase retrieval depending on the optical and geometrical properties of wavefront modulators has been analyzed. An optical system for measuring the structure of a wavefront modulator using modulated information in several wavelengths has been planned. In March 2019, A revised manuscript discussing the results of the aforementioned activities has been submitted to the journal “Journal of Astronomical Telescopes, Instruments, and Systems”.

The Institute of Chemical Physics was attended where experience in modifying the surface of polydimethylsiloxane (PDMS) has been gained to improve the hydrophilicity of the surface and create strong bonding between PDMS and plastics. An eye model for simulating static vitreous floaters has been designed. The static floaters have been simulated using microstructures etched in glass. The microstructures have been formed using photolithography methods and etching the glass in a buffered fluoric acid. These microstructures have been measured and analyzed using the white light profiler Zygo NewView 7100. A manuscript discussing the results of the aforementioned activities is to be submitted to the journal “Optics Letters”.

An abstract has been prepared and submitted to the congress “Imaging and Applied Optics” organized by the Optical Society of America (OSA). The abstract was accepted in April, and the results will be presented as a poster in the congress. Posters have also been presented in the annual conference of the Institute of Solid State Physics and in the conference VISPEP in Vilnius. A popular scientific lecture about the activities of the postdoctoral project is being prepared for pupils of the Riga 94th secondary school. This lecture will be given as a part of a campaign “Back to school/Back to University 2019”. A list of optical/optomechanical/chemical materials necessary for the project has also been summarized.


Project progress

November 05, 2018

Thesis “Optical lithography for studying vitreous floaters” has been submitted to the International Symposium on Visual Physiology, Environment, and Perception “VISPEP 2018”. An oral presentation “Optical lithography for filtering images” about designing microstructures and filtering images has been presented to pupils in a seminar “Beta”. The method of phase retrieval to be used in the project has been discussed in an exhibition “Environment and Energy 2018”, Kipsala.

The Gerchberg-Saxton method for phase retrieval has been studied using differently defocused diffraction patterns. Parameters of copper (II) oxide CuO thin films have been optimized for reducing the coefficient of reflection and increasing the coefficient of absorption in the optical system. The technical document about designing wavefront modulators has been prepared and a poster “Wavefront Recovery From Intensity Measurements Using a Single Amplitude Modulating Mask” has been presented in an international conference “Functional Materials and Nanotechnologies 2018”.


Project progress

June 15, 2018

Studies on the method of coded diffraction patterns and aberrometry have been continued. Structures of various size and shape to be used for phase retrieval of the vitreous floaters in a model eye have been designed. Applicability of Zernike polynomials for filtering out the optical noise in the calculated wavefront has been studied. Results on analysis of aberrations have been presented in the 34th scientific conference of ISSP on 21.02.2018. and the scientific seminar of ISSP on 29.03.2018.

A research paper has been submitted to the proceedings of a SPIE conference “Optical Systems Design” (Frankfurt, Germany from 14.05.2018. to 17.05.2018.). The paper was published on 28.05.2018.

Varis Karitans; Edgars Nitiss; Andrejs Tokmakovs; Kaspars Pudzs. “Optical phase retrieval using four rotated versions of a single binary mask – simulation results”. Proc. SPIE 10694. DOI: 10.1117/12.2311861

Results on a study investigating detection of the vitreous floaters have been presented in the conference “Developments in Optics and Communications” (an invited talk: Simulation of Vitreous Floaters using an Eye Model with Microfluidics System).


Project progress

March 15, 2018

During the initial stages of the project the method of coded diffraction patterns has been customized to measure the structure of microoptical elements designed in a photoresistive layer. A good relation between the method of coded diffraction patterns and the non-contact profilometry has been proven. The method of coded diffraction patterns is realized using the photolithography method while the non-contact profilometry employs MATLAB tool LightPipes providing possibility to simulate propagation of a wavefront in a free space. In very near future a manuscript will be submitted to a journal of the Optical Society of America (OSA). In the manuscript, both methods will be compared.

Up to now, applicability of different plastics (PET, PVC etc.) for forming microfluidics contacts has also been analyzed. The strength of contacts between different plastics and polydimethylsiloxane (PDMS) has been determined. It has been observed that a good contact can be achieved using a non-modified polycarbonate (PC). Polyurethane bonds are formed on the PC surface and after being inserted in an ozone cleaner -OH groups are formed. In order to reduce aberrations in the microfluidics system, the eye model incorporating the microfluidics system has also been improved.

An optical system using 4 CMOS cameras are being developed so that 4 coded diffractions patterns can be measured at the same time thus reducing the measurement time. The MATLAB code calculating the structure of the object based on the intensity measurements has also been optimized. After optimization the computational time has been reduced very significantly.

The results obtained have been presented at the 34th annual conference of the Institute of Solid State Physics (ISSP). Thesis has also been submitted to a conference “Optical Systems Design” being held in May 2018 in Frankfurt, Germany. The thesis has been approved and the results will be reported in an oral presentation.