The impact of the intraocular straylight (IOS) on the visual performance and retinal imaging is still a challenging topic. Direct optical methods to measure IOS avoid psychophysical approaches and interaction with the patient. In this work, we developed an optical instrument providing direct imaging measurement of IOS based on the double-pass technology. The system was tested in an artificial eye IOS model constructed with holographic diffusers and validated with theoretical simulations

Ares, Jorge

Collados, María Victoria

Remón, Laura

Ávila, Francisco

English

Repositorio Universidad de Zaragoza

Research Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 11237Wide-field direct ocular straylight meterFRANCISCO ÁVILA,* MARÍA VICTORIA COLLADOS, JORGE ARES,AND LAURA REMÓNDepartamento de Física Aplicada, Universidad de Zaragoza, Zaragoza, 50009, Spain*avila@unizar.esAbstract: The impact of the intraocular straylight (IOS) on the visual performance and retinalimaging is still a challenging topic. Direct optical methods to measure IOS avoid psychophysicalapproaches and interaction with the patient. In this work, we developed an optical instrumentproviding direct imaging measurement of IOS based on the double-pass technology. The systemwas tested in an artificial eye IOS model constructed with holographic diffusers and validatedwith theoretical simulations.© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement1. IntroductionWhen the eye is exposed to an intense glare source, the contrast sensitivity decreases limitingthe ability to discriminate details due to the scattering effects that spreads the light out of thedesire focus. If the intensity of the light is high enough, the eye experiences a temporal lossof visibility due to a veiling effect covering the whole retina. This phenomenon is known asdisability glare [1] or intra-ocular straylight (IOS). The main contributions to IOS in the eyecorrespond to the cornea [2], iris, crystalline lens [3] and the retina [4]. Those contributionsincrease significantly with age [5] due to physiological changes in the crystalline lens [6], irispigmentation or loss of optical transparency [7]. IOS is also associated to pathological processessuch as corneal edema, keratoconus [7], cataracts [8], retinal diseases and high myopia [9]. Otherfactors including refractive surgery have been reported to increase the IOS due to the alterationof the stromal morphology and corneal density [10].At the beginning of the 20th century, Percy Cobb [11] studied for the first time the effect of anintense light source on the visual acuity introducing the concept of “disability glare”. Since then,psychophysical and optical techniques have been developed to estimate the IOS. The contrastsensitivity function has the potential of assessing the visual system over a spatial frequencydistribution, in this sense some clinical instruments have been proposed to test the disability glarebased on the relationship between straylight and glare perception [12,13]. Other methods basedon the equivalent luminance assume that the effect of projecting a bright source at a given retinaleccentricity is equivalent to superimpose a veiling with the same illuminance over the observedobject [14,15]. The need of expanding the angular field of the illumination tests brought thedevelopment of the direct compensation method which consists of the estimation of the veilingthat produces a glare source using the retina as the detector itself [16,17].However, the main disadvantage of those psychophysics methods is the indirect nature of themeasurement as require the participation of the patient. On the other hand, the optical methodsbased on objective measurements are wide in technology and converging into the double-pass(DP) technique. DP technology was first introduced by Flamant in 1955 [18] and consist ofacquiring the image of a punctual source after retinal reflection [19]. The acquired image containsintegrated information about aberrations, diffraction and scattering. The DP commercial systems(HD Analyzer, Visiometrics, CO) have been successfully employed to assess the retinal imagequality or the optical quality of the eye, however its capabilities to measure IOS are scarce due tothe limitation of angular range of analysis to a few tens of arc minutes [20], considering that theangular domain of the retinal straylight ranges between 1° and 90°. This limitation was overpassed#387940 https://doi.org/10.1364/OE.387940Journal © 2020 Received 14 Jan 2020; revised 20 Feb 2020; accepted 27 Feb 2020; published 1 Apr 2020Research Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 11238by Ginis and collaborators [21] with the development of a new optical method to reconstructthe wide-angle point spread function (PSF) of the eye up to 8° of eccentricity. Employing DPtechnology, the punctual illumination was replaced by an extended source projecting uniformdisks on the retina.To date, there are no reported optical methods providing objective measurements of the IOS atlarge peripheral eccentricities and direct visualization of the generated veiling effect. This isparticularly relevant due to the significant differences between the central and peripheral retinaat both embryologic, anatomic and functional levels. In addition, some retinal injuries have itsorigin at the periphery which signs and manifestations are quite different from the central retina.In this work, a new methodology based on the DP technique is proposed to provide directmeasurement of the IOS at a maximum eccentricity of 22°, visualization of the fundus and theveiling produced by scattering effects. Increasing the measurement range for scattering it is astep further in a more complete description of the image quality of the eye. For this purpose,a new instrument incorporating a wide-field circular pattern illumination will be designed andtested with holographic diffusers in an artificial eye.2. Methods2.1. Optical layout and operating principle of the instrumentThis section describes the optical instrument developed for the measurement of the IOS and theoperating procedure. The illumination source is composed of a high-power green LED followedby a plastic diffuser (aperture angle of 20°) and limited by a field-stop mask composed of anannulus and a central occluder as shown in Fig. 1. The source is collimated and conjugated withthe pupil plane by means of a relay telescope (composed of a couple of achromatic doublets). Toensure diffraction-limited illumination, the entrance pupil diameter was chosen to be 2 mm.Fig. 1. Optical layout of the direct straylight meter.After reflection at the retinal plane of an artificial eye (see Section 2.2), the light pass-throughthe same optical pathway and is deflected by a 50:50 beam splitter (BS) to a CMOS camera (DMK42BUC03, the Imaging Source). The exit pupil (4-mm diameter) is conjugated in the double-passwith the pupil plane trough the relay telescope. A camera objective ensures conjugation of theretinal plane with the CMOS sensor.The optical layout was design to project a circular annular pattern of 22° of angular field onthe retina. This pattern also includes a central occluder to avoid illuminating the central 1.6° ofvisual field that acts as the testing area.If the eye has no straylight contributions, the measured intensity at the occluder of acquired DPimage must be zero (or equivalently the offset minimum gray level of the camera). Conversely,Research Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 11239the presence of scattering particles or straylight contributions produce a veil of light spreadthroughout the retina [22] as illustrated in Figs. 1(b) and 1(c).2.2. Artificial eye and straylight modelThe artificial eye employed in the experimental set up was composed of a 22-mm focal-length meniscus and a white screen static diffuser. A straylight eye model was constructed byincorporating light shaping diffusers (Luminit, LLC) at the retinal plane. The diffusers arebased on holographic patterns imbedded in acrylic sheets that change the direction of the lightenergy according to a symmetric radial distribution. To generate different levels of straylight,different aperture angles of the diffusers were chosen: 0° (control), 5°, 20°, 30°, 60° and 80°.The proximity of the light diffusers to the image plane was chosen in order to obtain a reliableretinal image for all the elements of this particular set of light shaping diffusers.To simulate the performance of the IOS effects, Zemax optical design software (ZemaxOpticStudio, LCC) was used to model the diffusers incorporating the Luminit´s optical modelto Zemax Dynamic Link Library (DLL) provided by the manufacturer. Figure 2 shows thesimulation of the diffusers performed in non-sequential component (NSC). The left panels depict auniform disk projected onto the retinal plane without [Fig. 2(a)] and with different angle aperturesof the diffuser [Figs. 2(b) and 2(c)]. The right panels show the cross-sectional intensity profiles:The higher the aperture angle the lower the peak intensity and wider the angular distribution.Fig. 2. Simulation of the diffusers in Zemax OpticStudio for different aperture angles (Ap.);control (a); 30° (b); and 80° (c), respectively. Right panels show the cross-sectional intensityprofiles.Once the diffusers were modeled, the optical layout shown in Fig. 1 was incorporated to a NSCZemax file including the diffusers at the retinal plane of the eye model. To generate a realisticsimulation of the illumination pattern, both LED source and fundus camera specifications wereincorporated into the Zemax file.2.3. Straylight index calculationThe aim of this work is to provide a direct measurement of the intraocular straylight by projectinga wide-field circular pattern on the retinal fundus using DP technology. The proposed algorithmis applied directly on the DP acquired image since the testing area corresponding to the centraloccluder contain the straylight information. The sequential algorithm is depicted in Fig. 3. Afterimage acquisition, the angular field and the centroid of the occluder are detected (step 1), wherea, is the radius of the occluder in degrees and θ, the maximum visual angle (22°). Once “a”and “θ” are stablished, a radial profile is computed to obtain the spatial intensity distributioncorresponding to both the testing area and the annulus (step 3). Finally, the straylight index (SI)is calculated (step 4) by means of Eq. (1). To avoid undesired signal background and spuriousreflections, a reference image was acquired before the imaging session and then every final imagewas the result of subtracting the reference from the original. A set of DP images were acquired forevery aperture angle of the light diffusers placed at the retinal plane of an artificial eye (describedin Sec. 2.2). Image processing was performed using MATLAB (The Mathworks Inc., Natick,Research Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 11240MA).SI(θ) =[∫a0 IDp(θ)dθ]/(L · a2)[∫ϑ0 IDp(θ).dθ]/(L · θ2)(1)Fig. 3. Algorithm to compute the straylight index from wide-field DP circular patterns.3. ResultsFigure 4 compares the simulated and experimental DP images of the projected circular patternfor a straylight-free artificial eye and for different angles of aperture of the diffusers placed at theretinal plane. Clearly evident is the veiling effect induced by the diffusers and the scattered lightcontribution at the testing area. A visual inspection reveals that the higher the aperture of thediffuser, the stronger the straylight contribution at the testing area.Fig. 4. DP images of the circular pattern (described in Fig. 1) as a function of the apertureangle of the retinal diffuser. For a better visualization of the central testing area, imageshave been cropped to a visual field of 6°. Images are shown at the same gray-scale levelwithout normalization.To quantify the straylight contribution shown in the DP images of Fig. 4, the algorithmdescribed in Sec. 2.3 was applied to both simulated and experimental DP images. Figure 5shows the average radial profiles obtained from the images corresponding to 30° [Fig. 4(b)], 60°[Fig. 4(c)] and 80° [Fig. 4(d)] aperture angles of the diffusers. No significant differences werefound between experimental and simulated profiles.Finally, the SI parameter was computed for the maximum visual field of 22° through theapplication of Eq. (1) in both experimental and simulated DP images. Figure 5(d) comparesResearch Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 11241Fig. 5. DP images of the circular pattern (described in Fig. 1) as a function of the apertureangle of the retinal diffuser. For a better visualization of the central testing area, imageshave been cropped to a visual field of 6°.the experimental and theoretical SI values as a function of the increasing aperture angle of theretinal diffusers. No significant differences were found between the theoretical and experimentalvalues (Student´s t-test, p=0.981). These results allow to assume the capacity of the proposedmethodology to directly quantify the IOS due to retinal diffusion.4. Discussion and conclusionsIn the present work, a new instrument to measure the IOS based on direct imaging measurementmethodology has been presented. The performance of the optical instrument has been evaluatedin an artificial eye and tested with holographic diffusers and compared with optical simulationcarried out with Zemax optical design software.IOS has been measured in the clinical practice through psychophysical methods over thepast years [12,16,23,24]. According to the Commission of International de L’Eclairage (CIE),the disability glare due to straylight must be assessed by measuring the PSF of the eye [25],in particular paying attention to larger radial visual angles. There is a wide battery of opticalmethods that provide direct measurement of the central part of the PSF (see for instance Ref.[13]), however only an optical method exploring the peripheral areas of the PSF (up to 8° ofvisual field) providing objective IOS assessment of weak scattering media has been reported[26] so far. Increasing the field range of measurement, the proposed device opens the possibilityto measure the IOS generated by clinically relevant scattering conditions, such as cataracts forinstance.In conclusions, we have implemented a DP-based instrument to measure intra-ocular scatteringat large eccentricities (22° of visual field). The measurement methodology allows computingthe IOS directly from the acquired image avoiding complex imaging post-processing or Fourieranalysis [26], visualizing the fundus and the veiling effect at the testing area. The implementationof the instrument in a clinical prototype will allow the potential of exploring the wide-field IOSin the living human eye up to 22° of visual field by direct measurement of the DP images.FundingMinisterio de Economía y Competitividad (Grant DPI2017-84047-R); Department of Industryand Innovation (Government of Aragon) (research group E24-17R, research group R44-17R).DisclosuresThe authors declare no conflict of interest.References1. T. J. van Den Berg, L. Franssen, and J. E. Coppens, “Straylight in the human eye: testing objectivity and opticalcharacter of the psychophysical measurement,” Ophthal. Physl. Opt. 29(3), 345–350 (2009).2. J. J. Vos and J. Boogaard, “Contribution of the cornea to to entopic scatter,” J. Opt. Soc. Am. 53(7), 869–873 (1963).Research Article Vol. 28, No. 8 / 13 April 2020 / Optics Express 112423. F. A. Bettelheim and S. Ali, “Light scattering of normal human lens. III. Relationship between forward and backscatter of whole excised lenses,” Exp. Eye Res. 41(1), 1–9 (1985).4. F. C.Delori, “Autofluorescencemethod tomeasuremacular pigment optical densities fluorometry and autofluorescenceimaging,” Arch. Biochem. Biophys. 430(2), 156–162 (2004).5. M. J. Allen and J. J. Vos, “Ocular scattered light and visual performance as a function of age,” Am. J. Optom. Arch.Am. Acad. Optom. 44(11), 717–727 (1967).6. G. T. Smith, N. A. Brown, and G. A. Shun-Shin, “Light scatter from the central human cornea,” Eye (London, U. K.)4(4), 584–588 (1990).7. K. M. Meek, S. J. Tuft, Y. Huang, P. S. Gill, S. Hayes, R. H. Newton, and A. J. Bron, “Changes in collagen orientationand distribution in keratoconus corneas,” Invest. Ophthalmol. Visual Sci. 46(6), 1948–1956 (2005).8. S. George, “Scatter in the eye,” The eye and the light (2005).9. G. L. van der Heijde, J. Weber, and R. Boukes, “Effects of straylight on visual acuity in pseudophakia,” Doc.Ophthalmol. 59(1), 81–84 (1985).10. A. Nieto-Bona, A. Lorente-Velázquez, C. V. Collar, P. Nieto-Bona, and A. G. Mesa, “Intraocular straylight andcorneal morphology six months after LASIK,” Curr. Eye Res. 35(3), 212–219 (2010).11. P. W. Cobb, “The influcence of illumination of the eye on visual acuity,” Am. J. Physiol. 29(1), 76–99 (1911).12. D. B. Elliott and M. A. Bullimor, “Assessing the reliability, discriminative ability, and validity of disability glaretests,” Invest. Ophhalmol. Vis. Sci. 34(1), 108–119 (1993).13. I. L. Bailey and M. A. Bullimore, “A new test for the evaluation of disability glare,” Optom. Vis. Sci. 68(12), 911–917(1991).14. J. J. Vos, “On the cause of disability glare and its dependence on glare angle, age and ocular pigmentation,” Clin.Exp. Optom. 86(6), 363–370 (2003).15. J. T. Holladay, T. C. Prager, J. Trujillo, and R. S. Ruiz, “Brightness acuity test and outdoor visual acuity in cataractpatients,” J. Cataract Refractive Surg. 13(1), 67–69 (1987).16. T. M. Aslam, D. Haider, and J. Murray, “Principles of disability glare measurement: an ophthalmological perspective,”Acta Ophthalmol. Scand. 85(4), 354–360 (2007).17. T. J. van der Berg, “On the relation between glare and straylight,” Doc. Ophthalmol. 78(3-4), 177–181 (1991).18. M. F. Flamant, “Étude de la repartition de lumière dans l’image retinienne d’une fente,” Revue d’Optique Theoriqueet Instrumentale 34, 433–459 (1955).19. P. Artal, S. Marcos, R. Navarro, and D. R. Williams, “Odd aberrations and double-pass measurements of retinalimage quality,” J. Opt. Soc. Am. A 12(2), 195–201 (1995).20. P. Artal, A. Benito, G. M. Perez, E. Alcón, A. De Casas, J. Pujol, and J. M. Marín, “An Objective scatter index basedon double-pass retinal images of a point source to classify cataracts,” PLoS One 6(2), e16823 (2011).21. H. Ginis, G.M. Perez, J. M. Bueno, and P. Artal, “The wide-angle point spread function of the human eye reconstructedby a new optical method,” J. Vis. 12(3), 20 (2012).22. M. A. Mainster and P. L. Turner, “Glare’s causes, consequences, and clinical challenges after a century of ophthalmicstudy,” Am. J. Ophthalmol. 153(4), 587–593 (2012).23. T. J. van der Berg, L. Franssen, and J. Coppens, “Straylight in the human eye: testing objetivity and optical characterof the psychophysical measurement,” Ophthal. Physl. Opt. 29(3), 345–350 (2009).24. T. J. van der Berg, L. Franssen, and J. Coppens, “Ocular media clarity and straylight,” Encyclopedia of Eye 3,173–183 (2010).25. J. J. Vos, “Disability glare - a state of the art report,” Commission International de l’Eclairage Journal 3, 39–53(1984).26. H. Ginis, O. Sahin, A. Pennos, and P. Artal, “Compact optical integration instrument to measure intraocular straylight,”Biomed. Opt. Express 5(9), 3036–3041 (2014).

Wide-field direct ocular straylight meter

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Wide-field direct ocular straylight meter

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