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Abstract
Light emitting diodes (LEDs) have become a major source of lighting conditions. The increased prevalence of LED light sources introduces new concerns for the spectral effects of positive dysphotopsia (PD) or glare type photic phenomena for pseudo-phakic patients with intraocular lenses (IOLs). A significant amount of work has been published in the area of spectral discomfort and sensitivity of LEDs as well as automotive lighting. The wavelength dependence or spectral properties of PD due to LEDs for IOLs has not been reported. This study, to our knowledge, is the first one to assess the glare characteristics of four commercially available IOL models with different material types and design features using an optical bench and non-sequential ray trace simulations with LEDs of different wavelengths. A novel approach of representing the reflected and transmitted IOL glare utilizing Fresnel coefficients is found to be in close agreement with the measurements.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The study of glare or its impact on the human vision has been an interest of active research since the last century [1–3]. Glare or positive dysphotopsia (PD) is defined as the sensation caused by the illumination within the visual field that is enough to cause annoyance, discomfort, degradation of visibility, and loss of vision. Glare is classified into two types: disability glare and discomfort glare [4,5]. Disability glare, also known as ‘physiological glare,’ impairs vision quality by reducing photoreceptors sensitivity. The discomfort glare, also known as ‘psychological glare,’ does not necessarily impair visual performance, but may cause stress. Light emitting diode (LED) sources are strong enough to create both discomfort and disability glare in a healthy eye. Modern cataract surgery has is a refractive procedure and patient expectations are high; therefore, manufacturers and surgeons are increasing aware of the importance of reducing all forms of dysphotopsia.
With many advantages of LEDs such as higher reliability, compact size, lower power consumption, and superior efficiency compared to traditional tungsten halogen light sources, there is a pragmatic shift from halogen lamps to LED sources. The development of white LED has increased the lumen output from device and is being employed in automobile headlights, streetlights, and many other applications [6]. Further, there is high usage of LEDs in decorative lighting, general illumination such as room LEDs, retail display lighting, entertainment LED lightning, and many more. Despite enormous advantages, many groups have reported discomfort glare and the spectral sensitivity characteristics caused by LED sources [7–9]. In particular, Borns et al. explored discomfort glare from outdoor lighting [10] and Schmidt-Clausen & Bindels assessed discomfort glare from motor vehicle lighting [11].
In 2004, Sivak et al. reported the usage of LED sources in vehicle headlamps resulted in higher discomfort glare than current high intensity discharge lamps and tungsten halogen lamps during night driving [12]. Apart from the automobile headlights, Kasahara et al. reported discomfort glare characteristics from room LED sources also at off axis angles of illumination [13]. A qualitative relationship between discomfort glare and LED sources has been investigated by Tashiro et al. with experimental analysis [14]. Lin et al. proposed a model to assess discomfort glare from LED street/road lighting based on four key variables including source luminance, background luminance, solid angle of the glare source, and angle between the glare source and line of sight. This model reveals that discomfort glare increases with increasing eye illuminance and matches accurately with the well-known deBoer rating scale [15]. A more recent glare study by Fotios used a 9-point scale with “not perceptible glare” option to reduce bias [16].
There have been substantial studies published in the area of discomfort glare for the human eye. Glare sensitivity would be less of a concern for a younger adult and non-cataract population. On the other hand, glare-type photic phenomenon may be more severe for a pseudo-phakic eye where an artificial intraocular lens (IOL) is implanted after removing the opacified human crystalline lens for cataract treatment [17]. The incoming off-axis illumination can interact with the IOL and may cause unwanted visual disturbances within the visual field of the patient. Thus, the edge design and peripheral features of the IOL contribute to glare. This condition was first reported by Masket et al. in 1993 after the implantation of posterior chamber IOL [18]. In 2000, Tester et al. coined the term dysphotopsia [19], and Davison reported positive dysphotopsia and negative dysphotopsia-type visual conditions [20]. Positive dysphotopsia includes unwanted bright images such as glare, flashes, arcs, and so forth within the patient’s visual field [21,22]. Negative dysphotopsia is a dark shadow within patient’s visual field [23]. The importance of understating overall dysphotopsia for the cataract patients has been reiterated by Werner in a recent editorial [24].
When off-axis light falls on the edge and periphery of an IOL, it can be reflected or transmitted as shown in Fig. 1. The modeling is based on a well referenced schematic eye model by Liou & Brennan [25]. A ZCB00 lens with a light source at 555 nm for an off-axis angle of 55 degrees was used to demonstrate both reflected and transmitted glare. The reflected and transmitted light can be observed as arc of light depending on the edge, peripheral design and profile, and radius of curvature [26–32].
Fig. 1. (a) A schematic eye model with ZCB00 IOL showing the potential impact of off-axis incoming beams of light (green) onto the retina of the schematic model eye with (b) transmitted rays from the IOL edge and peripheral feature (purple) and (c) reflected rays (red) from the IOL.
To our knowledge, this is the first study to assess the spectral properties of glare as might occur in pseudophakic eyes. The glare characteristics of four commercially available IOL models with different material types and design features were evaluated using optical bench and non-sequential ray trace simulations with LEDs of different wavelengths. Glare characteristics from three different LED sources corresponding to central wavelengths of 480 nm, 555 nm, and 640 nm were measured on the four different IOL designs, and their respective glare spectra were compared. The measured glare spectra are found to be in close agreement with the simulation results using non-sequential FRED model (Photon Engineering, LLC, Tucson, AZ) and also with the calculated values from Fresnel’s coefficients.
2. Materials and methods
Four different monofocal IOL designs - AcrySof SN60WF (refractive index 1.55) (Alcon Vision, LLC), Clareon CNA0T0 (refractive index 1.54) (Alcon Vision, LLC), Vivinex XY1-SP (refractive index 1.52) (Hoya Surgical Optics, Inc.) and Tecnis ZCB00 (refractive index 1.47) (Johnson & Johnson, Inc.) were used to evaluate the spectral properties of glare or PD. Five 20 diopter (D) samples of each IOL type were tested. The glare for these lenses was measured on an in-vitro optical test bench model with blue (480 nm), green (555 nm), and red (640 nm) collimated LED light sources at similar intensities and off-axis angles of illumination from 25 to 55 degrees with increments of 5 degrees. We modeled and measured the glare with respect to the optical axis. The schematic of the experimental setup is shown in Fig. 2. The collimated beam of light was illuminated through a model cornea with a 5.0 mm aperture. We used a model eye with a cornea that is very close to a physiological model (as described in ISO 11979-2). The cornea did not have any asphericity. The cornea was made of a PMMA material with a curvature of 7.8 mm and a thickness of 0.5 mm. The test IOLs were mounted on a 3D adjustable lens holder immersed in a wet cell with deionized water and placed 4.05 mm away from the cornea. A curved fiber-optic taper (source: Schott AG) that mimics a human retina with a radius of curvature of 11.35 mm was placed on a linear translation stage to adjust the position for the best focal point. The theory and use of the fiber-optic taper as a model for a retina has been published by Brodie et al. [33]. The optimum distance was determined by achieving the highest intensity within the smallest area, indicating in-plane alignment and the best focal point. The curved fiber-optic taper transferred the images to an orthogonal plane where a 12-bit high resolution charge-coupled device (CCD) array was placed to obtain the glare from the test IOL. Fig. 3. further illustrates the experimental setup with additional details of how the fiber taper retina was positioned to work as an artificial retina and the detector to capture images. To ensure consistency for all the lenses for the comparative analysis, the intensity was calibrated at on-axis to ensure that the detector was not saturated. Then we obtained the glare ratio by normalizing the intensity of glare at a particular angle with respect to the intensity of the main beam (the IOL image).
Fig. 2. Schematic of the experimental set-up used for objective glare measurement of the IOLs with multiple LED sources.
Fig. 3. Illustration of the experimental set-up detailing the cornea, pupil, IOL, fiber taper retina, and detector (CCD array).
The CCD array captures several images and combines them to form a single high dynamic range (HDR) image. The glare bench software was used to determine the correct exposure steps that were needed to test the glare performance of an IOL model. If the step side was incorrect, then the intensity plot would have stair step features and not a smooth transition. Testing was then performed using appropriate exposure step size to ensure measurement accuracy.
Typically, the intensity of glare features was several orders of magnitude lower than the bright main focused images formed by the clear optic of the lens. Therefore, we needed HDR imaging to precisely capture the phenomena. The left image in Fig. 4(a) illustrates the various features including the transmitted and reflected glare from the optical bench for the same ZCB00 IOL as used in Fig. 1. The transmitted glare is the part of glare light that transmits through the edge or peripheral non-optical feature of the lens. The reflected glare is quite low and therefore it looks invisible. Figure 4(b) represents the simulated image using the FRED non-sequential ray-trace program in a schematic model of Fig. 1(a). Both images are in pseudo-color in a logarithmic scale. The bench image had some background noise, while the nonsequential ray-trace model image clearly shows the main image of both reflected and transmitted glare. We also show the rays that missed the IOL, that is, the rays that escaped the edge of the IOL.
Fig. 4. (a) Captured image of an ZCB00 IOL by the HDR CCD array from the incident light on curved fiber-optic taper showing transmitted glare, reflected glare, main image, and rays that missed the IOL; and (b) Simulated image from an IOL using the FRED non-sequential ray trace model in a schematic model eye.
3. Results and discussion
The four IOL designs SN60WF, CNA0T0, XY1-SP, and ZCB00 were initially illuminated by white light; the combination of blue (480 nm), green (555 nm) and red (640 nm) LED collimated sources with similar intensities. The intensity distribution falling on the curved fiber-optic taper were captured using the high-resolution CCD array at various off-axis angles of illumination. Figures 5(a-d) show the representative images for SN60WF, CNA0T0, XY1-SP, and ZCB00 IOLs at a 55-degree angle of illumination. The intensity distribution for glare was normalized relative to the highest intensity of the main focused image (the IOL), which we defined as the glare ratio. This was represented in the pseudo-color, ranging between red and blue corresponding to the highest and lowest intensities of light respectively. Figure 5(e) compares the glare ratio for all the test lenses as a function of the off-axis illumination angle.
Fig. 5. In-vitro retina HDR images showing transmitted and reflected glare in pseudo-color at 55-degree off-axis angle of incidence with white LED source with a 5 mm pupil (a) SN60WF; (b) CNA0T0; (c) XY1-SP; (d) ZCB00; and (e) Glare ratio of all four IOLs at various off-axis angles of illumination (25-55 degrees) with a white light source.
In Fig. 6, the in vitro measured glare ratio in the same logarithmic scale for all IOLs were compared at various LED wavelengths including white light. As observed in Figs. 6(a) and (b), SN60WF and CNA0T0 had edge-reflected glare located away from the main beam image with an intensity range of -2.68 dB in log scale, normalized to the main focused image of the IOL. XY1-SP (Fig. 6(c)) had glare with intensity of -1.34 dB relative to the main image, which was higher than SN60WF and CNA0T0. On the other hand, ZCB00 had dominant edge-transmitted glare, closer to the main beam image and relative intensity of -0.67 dB. Figure 6(e) shows the normalized glare ratio at different angles of illumination starting from 25 to 55 degrees for all four IOL types. SN60WF and CNA0T0 had the lowest glare, predominantly edge-reflected glare, followed by XY1-SP. ZCB00 had very high edge-transmitted glare component.
Fig. 6. In-vitro measurement of glare-type dysphotopsia at different off-axis angles of incidence using white light, 480 nm, 555 nm, and 640 nm LED sources for different IOL models (a) SN60WF, (b) CNA0T0, (c) XY1-SP, and (d) ZCB00.
The spectral characteristics of the glare for each IOL design were measured by using only one light-emitting diode emission beam on the IOL with the same optics bench. The intensity profile for the glare component was normalized relative to the highest intensity of the main beam image during each test. The 480 nm blue LED source followed by 555 nm and 640 nm LED sources were illuminated separately and glare was measured from 25 to 55 degrees off-axis angles of illumination. The resulting glare characteristics for the four IOL designs are shown in Fig. 6. For an appropriate comparison, the measured glare values are shown in the same vertical scale for all lenses.
The spectral characteristics for glare-type photic phenomenon at 55-degree angle of illumination of light for all IOL models in the visible spectrum are shown in Fig. 7. In addition to the previously reported results [20] that the glare from SN60WF and CNA0T0 was lower compared to XY1-SP and ZCB00 IOLs at 550 nm wavelength, this study reveals that the glare was lower for SN60WF and CNA0T0 over the measured wavelength spectrum including white light. We modeled the glare component for CNA0T0 and ZCB00 by using Fresnel equations. We found increasing glare for ZCB00 as shown in Fig. 7, which coincides with the measured data. The IOL light transmission properties and its relationship to dysphotopsia is not clearly known. A study by Hammond et al. found that blue-filtering IOLs provided a lower level of disability glare and can reduce glare disability [34].
Fig. 7. Spectral characteristics of glare of SN60WF, CNA0T0, XY1-SP, and ZCB00 IOLs at 55-degree angle of incidence.
For any given off-axis angle of incidence, the reflection and transmission properties of IOL glare may change as a function of the wavelength. These two properties for two lens types are demonstrated in Fig. 8. The reflected glare for CNA0T0 reduced with the wavelength (Fig. 8(a)). The transmitted glare for ZCB00 increased with increasing wavelength (Fig. 8(b)). This can be explained by the optical properties of the Fresnel reflection and transmission coefficients. The E-field magnitudes of transmitted |Et|2 and reflected |Er|2 glare were simulated at different wavelengths for fixed off-axis angle of illumination. The trend was verified using Fresnel’s equation model developed to evaluate the reflected and transmitted glare characteristics of IOLs based on their geometry. Figure 8 shows the comparison for CNA0T0 and ZCB00 IOLs at a 55-degree off-axis angle of illumination. A good agreement was found between the measured and calculated values of glare.
Fig. 8. Comparison of (a) CNA0T0 and (b) ZCB00 glare ratio between Fresnel equation model calculation and measurement at 55-degrees off-axis angle of illumination.
4. Conclusion
Despite the increased use of LEDs in everyday lighting, including automobile and traffic lighting, we have not found any study that assessed the impact of LED sources with various wavelengths on pseudophakic patients with IOLs. Hence, this is the first reported study to evaluate the wavelength dependence of PD or glare-type photic phenomenon on IOLs. The glare in ZCB00, which was an order of magnitude higher than the glare for CNA0T0 and SN60WF, increased with wavelength. The measured glare spectral components were observed to be in close agreement with Fresnel’s equations. Both the transmitted and reflected glare for SN60WF and CNA0T0 resulting from white light, 480 nm, 555 nm, and 640 nm LEDs were lower than ZCB00 and XY1-SP IOLs. A future study can assess the IOL light transmission for blue light filtering IOLs and its impact (or lack thereof) on the spectral properties of glare or PD type photic phenomena.
Acknowledgements
AK acknowledges Christopher Joseph for demonstrating the glare bench. We acknowledge the helpful comments of Steve Van Noy and Ceyhun Akcay on the manuscript, and Heather S. Oliff (Science Consulting Group, LLC) for editorial assistance.
Part of this work was presented at the ARVO Annual Meeting, Online, May 2021.
Disclosures
KD is an employee of Alcon Research, LLC and a senior member of Optica. AK received summer support from Alcon Research, LLC. He has no other conflict of interest to declare.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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