Exposure Geometry and Spectral Environment Determine Photobiological Effects on the Human Eye¶ †

Current Insights on the Photoprotective Mechanism of the Macular Carotenoids, Lutein and Zeaxanthin: Safety, Efficacy and Bio-Delivery

Ocular health has emerged as one of the major issues of global health concern with a decline in quality of life in an aging population, in particular and rise in the number of associated morbidities and mortalities. One of the chief reasons for vision impairment is oxidative damage inflicted to photoreceptors in rods and cone cells by blue light as well as UV radiation. The scenario has been aggravated by unprecedented rise in screen-time during the COVID and post-COVID era. Lutein and Zeaxanthin are oxygenated carotenoids with proven roles in augmentation of ocular health largely by virtue of their antioxidant properties and protective effects against photobleaching of retinal pigments, age-linked macular degeneration, cataract, and retinitis pigmentosa. These molecules are characterized by their characteristic yellow-orange colored pigmentation and are found in significant amounts in vegetables such as corn, spinach, broccoli, carrots as well as fish and eggs. Unique structural signatures including tetraterpenoid skeleton with extensive conjugation and the presence of hydroxyl groups at the end rings have made these molecules evolutionarily adapted to localize in the membrane of the photoreceptor cells and prevent their free radical induced peroxidation. Apart from the benefits imparted to ocular health, lutein and zeaxanthin are also known to improve cognitive function, cardiovascular physiology, and arrest the development of malignancy. Although abundant in many natural sources, bioavailability of these compounds is low owing to their long aliphatic backbones. Under the circumstances, there has been a concerted effort to develop vegetable oil-based carriers such as lipid nano-emulsions for therapeutic administration of carotenoids. This review presents a comprehensive update of the therapeutic potential of the carotenoids along with the challenges in achieving an optimized delivery tool for maximizing their effectiveness inside the body.

Blue Light Exposure: Ocular Hazards and Prevention-A Narrative Review

INTRODUCTION

Exposure to blue light has seriously increased in our environment since the arrival of light emitting diodes (LEDs) and, in recent years, the proliferation of digital devices rich in blue light. This raises some questions about its potential deleterious effects on eye health. The aim of this narrative review is to provide an update on the ocular effects of blue light and to discuss the efficiency of methods of protection and prevention against potential blue light-induced ocular injury.

METHODS

The search of relevant English articles was conducted in PubMed, Medline, and Google Scholar databases until December 2022.

RESULTS

Blue light exposure provokes photochemical reactions in most eye tissues, in particular the cornea, the lens, and the retina. In vitro and in vivo studies have shown that certain exposures to blue light (depending on the wavelength or intensity) can cause temporary or permanent damage to some structures of the eye, especially the retina. However, currently, there is no evidence that screen use and LEDs in normal use are deleterious to the human retina. Regarding protection, there is currently no evidence of a beneficial effect of blue blocking lenses for the prevention of eye diseases, in particular age-related macular degeneration (AMD). In humans, macular pigments (composed of lutein and zeaxanthin) represent a natural protection by filtering blue light, and can be increased through increased intake from foods or food supplements. These nutrients are associated with lower risk for AMD and cataract. Antioxidants such as vitamins C, E, or zinc might also contribute to the prevention of photochemical ocular damage by preventing oxidative stress.

CONCLUSION

Currently, there is no evidence that LEDs in normal use at domestic intensity levels or in screen devices are retinotoxic to the human eye. However, the potential toxicity of long-term cumulative exposure and the dose-response effect are currently unknown.

Magnesium: properties and rich chemistry for new material synthesis and energy applications

Magnesium (Mg) has many unique properties suitable for applications in the fields of energy conversion and storage. These fields presently rely on noble metals for efficient performance. However, among other challenges, noble metals have low natural abundance, which undermines their sustainability. Mg has a high negative standard reduction potential and a unique crystal structure, and its low melting point at 650 °C makes it a good candidate to replace or supplement numerous other metals in various energy applications. These attractive features are particularly helpful for improving the properties and limits of materials in energy systems. However, knowledge of Mg and its practical uses is still limited, despite recent studies which have reported Mg's key roles in synthesizing new structures and modifying the chemical properties of materials. At present, information about Mg chemistry has been rather scattered without any organized report. The present review highlights the chemistry of Mg and its uses in energy applications such as electrocatalysis, photocatalysis, and secondary batteries, among others. Future perspectives on the development of Mg-based materials are further discussed to identify the challenges that need to be addressed.

A dataset of global variations in directional solar radiation exposure for ocular research using the libRadtran radiative transfer model

The libRadtran radiative transfer model was used to calculate examples of the amount of spectral radiation (250-2500 nm) incident on the eye. Simulations were run for every hour of four individual days (representing spring, summer, autumn and winter) and at three latitudes (from southern Spain to central Finland), in order to demonstrate diurnal and seasonal variations in directional photon flux density due to solar angle. The dataset also includes outputs under strong and weak aerosol optical density, three bidirectional reflectance distribution functions (corresponding to a forested, urban and snowy ground surfaces), eight cardinal directions, and two tilt angles (either looking towards the horizon or 15° downward). All simulations were parametrized according to local meteorological conditions (elevation, pressure, temperature) and atmospheric condition on the simulated day (aerosol optical density, water column, O₃ and NO₂ concentrations), at 170 cm above the ground (representing the average human height). Example data are presented for a 17° field of view relevant to exposure of the macula (without correction for spectral transmission of ocular media). For each simulation, a file in ".csv" format is available containing the radiance at each wavelength. The simulations were performed in batches via R software, from a template input parameter file modified for each simulation from a summary input table. The R code and input files are also available. By describing the amount and wavelength composition of directional radiation incident on the eye, this dataset and future simulations will help parameterize research aimed at understanding and mitigating eye-related diseases.

Ocular and Facial Far-UVC Doses from Ceiling-Mounted 222 nm Far-UVC Fixtures

Far-UVC radiation, defined in this paper as ultraviolet (UV) radiation with wavelengths from 200 to 235 nm, is a promising tool to help prevent the spread of disease. The unique advantage of far-UVC technology over traditional UV germicidal irradiation lies in the potential for direct application of far-UVC into occupied spaces since antimicrobial doses of far-UVC are significantly below the recommended daily safe exposure limits. This study used a ceiling-mounted far-UVC fixture emitting at 222 nm to directly irradiate an indoor space and then evaluated the doses received upon a manikin. Radiation-sensitive film was affixed to the head, nose, lip and eyes of the manikin, and the 8-h equivalent exposure dose was determined. Variables examined included manikin height (sitting or standing position), manikin offset from directly below the fixture, tilt of the manikin, the addition of glasses, the addition of hair and different anatomical feature sizes. Importantly, at the manikin position with the highest dose to eyes, the average eye dose was only 5.8% of the maximum directly measured dose. These results provide the first experimental analysis of possible exposure doses a human would experience from an indoor far-UVC installation.

Verification, analytical validation and clinical validation (V3) of wearable dosimeters and light loggers

Background

Light exposure is an important driver and modulator of human physiology, behavior and overall health, including the biological clock, sleep-wake cycles, mood and alertness. Light can also be used as a directed intervention, e.g., in the form of light therapy in seasonal affective disorder (SAD), jetlag prevention and treatment, or to treat circadian disorders. Recently, a system of quantities and units related to the physiological effects of light was standardized by the International Commission on Illumination (CIE S 026/E:2018). At the same time, biometric monitoring technologies (BioMeTs) to capture personalized light exposure were developed. However, because there are currently no standard approaches to evaluate the digital dosimeters, the need to provide a firm framework for the characterization, calibration, and reporting for these digital sensors is urgent.

Objective

This article provides such a framework by applying the principles of verification, analytic validation and clinical validation (V3) as a state-of-the-art approach for tools and standards in digital medicine to light dosimetry.

Results

This article describes opportunities for the use of digital dosimeters for basic research, for monitoring light exposure, and for measuring adherence in both clinical and non-clinical populations to light-based interventions in clinical trials.

Prevalence of Solar Keratopathy, Pterygium and Cataract in the Islands of Northern Red Sea Zone, Eritrea: Cross-Sectional Study, 2021

Background

Everybody is exposed to ultraviolet radiation and the natural source of ultraviolet radiation is sunlight. The objective of this study was to determine the prevalence of solar keratopathy, pterygium and cataract and further to evaluate other factors that cause blindness to the inhabitants of the Northern Red Sea Zone islands of Eritrea.

Methods

It was a community-based cross-sectional study of all people aged 40 years and above, with a census sampling method. Study participants were screened for presence of solar keratopathy and other ocular diseases, and data were collected by a checklist from March 20 to April 20, 2021. Data were entered on an Excel sheet and transported to SPSS version 21. Results were presented with frequencies and percentages and chi-squared bivariable analysis was carried. P-value less than 0.05 was considered significant.

Results

The prevalence of solar keratopathy and cataract was 19.6% and 15.8%, respectively. Pterygium (40%) and pinguecula (32.1%) were also commonly found in the community. Participants aged 70 years and above (P<0.001), fishermen (P<0.001), housewife (P<0.001) and females (P<0.001) had a higher rate of blindness. The prevalence of solar keratopathy was higher with increased age and work experience (P<0.001). Fishermen and housewives had higher rates of solar keratopathy (44.6%) and cataract (30.2%), respectively, (P<0.001). Respondents aged 40 to 49 years had the highest prevalence of pterygium (44.1%) and pinguecula (39.1%) (P<0.001).

Conclusion

The prevalence of solar keratopathy, cataract, pterygium and pinguecula was high in the community. Fishermen with increased work experience were having the highest prevalence of blindness and solar keratopathy. Age, occupation, work experience, sex, vision problems and history of eye operations had showed significant association with visual acuity, solar keratopathy, cataract, pterygium, and pinguecula. Community awareness about the preventive aspects of these diseases is highly recommended.

Activation of NF-κB signaling via cytosolic mitochondrial RNA sensing in kerotocytes with mitochondrial DNA common deletion

Scar formation as a result of corneal wound healing is a leading cause of blindness. It is a challenge to understand why scar formation is more likely to occur in the central part of the cornea as compared to the peripheral part. The purpose of this study was to unravel the underlying mechanisms. We applied RNA-seq to uncover the differences of expression profile in keratocytes in the central/peripheral part of the cornea. The relative quantity of mitochondrial RNA was measured by multiplex qPCR. The characterization of mitochondrial RNA in the cytoplasm was confirmed by immunofluoresence microscope and biochemical approach. Gene expression was analyzed by western blot and RT qPCR. We demonstrate that the occurrence of mitochondrial DNA common deletion is greater in keratocytes from the central cornea as compared to those of the peripheral part. The keratocytes with CD have elevated oxidative stress levels, which leads to the leakage of mitochondrial double-stranded RNA into the cytoplasm. The cytoplasmic mitochondrial double-stranded RNA is sensed by MDA5, which induces NF-κB activation. The NF-κB activation thereafter induces fibrosis-like extracellular matrix expressions and IL-8 mRNA transcription. These results provide a novel explanation of the different clinical outcome in different regions of the cornea during wound healing.

Iris Colour and the Risk of Developing Uveal Melanoma

Uveal melanoma (UM) is a global disease which especially occurs in elderly people. Its incidence varies widely between populations, with the highest incidence among Caucasians, and a South-to-North increase in Europe. As northern Europeans often have blond hair and light eyes, we wondered whether iris colour may be a predisposing factor for UM and if so, why. We compared the distribution of iris colour between Dutch UM patients and healthy Dutch controls, using data from the Rotterdam Study (RS), and reviewed the literature regarding iris colour. We describe molecular mechanisms that might explain the observed associations. When comparing a group of Dutch UM patients with controls, we observed that individuals from Caucasian ancestry with a green/hazel iris colour (Odds Ratio (OR) = 3.64, 95% Confidence Interval (CI) 2.57-5.14) and individuals with a blue/grey iris colour (OR = 1.38, 95% CI 1.04-1.82) had a significantly higher crude risk of UM than those with brown eyes. According to the literature, this may be due to a difference in the function of pheomelanin (associated with a light iris colour) and eumelanin (associated with a brown iris colour). The combination of light-induced stress and aging may affect pheomelanin-carrying melanocytes in a different way than eumelanin-carrying melanocytes, increasing the risk of developing a malignancy.

Phototoxic damage to cone photoreceptors can be independent of the visual pigment: the porphyrin hypothesis

Lighting is rapidly changing with the introduction of light-emitting diodes (LEDs) in our homes, workplaces, and cities. This evolution of our optical landscape raises major concerns regarding phototoxicity to the retina since light exposure is an identified risk factor for the development of age-related macular degeneration (AMD). In this disease, cone photoreceptors degenerate while the retinal pigment epithelium (RPE) is accumulating lipofuscin containing phototoxic compounds such as A2E. Therefore, it remains unclear if the light-elicited degenerative process is initiated in cones or in the RPE. Using purified cone photoreceptors from pig retina, we here investigated the effect of light on cone survival from 390 to 510 nm in 10 nm steps, plus the 630 nm band. If at a given intensity (0.2 mW/cm²), the most toxic wavelengths are comprised in the visible-to-near-UV range, they shift to blue-violet light (425–445 nm) when exposing cells to a solar source filtered by the eye optics. In contrast to previous rodent studies, this cone photoreceptor phototoxicity is not related to light absorption by the visual pigment. Despite bright flavin autofluorescence of cone inner segment, excitation–emission matrix of this inner segment suggested that cone phototoxicity was instead caused by porphyrin. Toxic light intensities were lower than those previously defined for A2E-loaded RPE cells indicating cones are the first cells at risk for a direct light insult. These results are essential to normative regulations of new lighting but also for the prevention of human retinal pathologies since toxic solar light intensities are encountered even at high latitudes.

Characterization of an Electronic Corneal Prosthesis System

PURPOSE

Corneal opacity is a leading cause of reversible blindness worldwide. An electronic corneal prosthesis, or intraocular projector, could potentially restore high-quality vision without need for corneal clarity.

MATERIALS AND METHODS

Four intraocular projection systems were constructed from commercially available electronic components and encased in biocompatible plastic housing. They were tested for optical properties, biocompatibility, heat dissipation, waterproofing, and accelerated wear. A surgical implantation technique was developed.

RESULTS

Intraocular projectors were produced of a size that can fit within the eye. Their optics produce better than 20/200 equivalent visual acuity. MTT assay demonstrated no cytotoxicity of devices in vitro. Temperature testing demonstrated less than 2°C increase in temperature after 1 h. Three devices lasted over 12 weeks under accelerated wear conditions. Implantation surgery was demonstrated via corneal trephination insertion in a cadaver eye.

CONCLUSION

This is the first study to demonstrate and characterize fully functional intraocular projection systems. This technology has the potential to be an important new tool in the treatment of intractable corneal blindness.

Solar Blue Light Radiation Enhancement during Mid to Low Solar Elevation Periods under Cloud Affected Skies

Solar blue-violet wavelengths (380-455 nm) are at the high energy end of the visible spectrum; referred to as "high energy visible" (HEV). Both chronic and acute exposure to these wavelengths has been often highlighted as a cause for concern with respect to ocular health. The sun is the source of HEV which reaches the Earth's surface either directly or after scattering by the atmosphere and clouds. This research has investigated the effect of clouds on HEV for low solar elevation (solar zenith angles between 60° and 80°), simulating time periods when the opportunity for ocular exposure in global populations with office jobs is high during the early morning and late afternoon. The enhancement of "bluing" of the sky due to the influence of clouds was found to increase significantly with the amount of cloud. A method is presented for calculating HEV irradiance at sub-tropical latitudes from the more commonly measured global solar radiation (300-3000 nm) for all cases when clouds do and do not obscure the sun. The method; when applied to global solar radiation data correlates well with measured HEV within the solar zenith angle range 60° and 80° (R2 = 0.82; mean bias error (MBE) = -1.62%, mean absolute bias error (MABE) = 10.3% and root mean square error (RMSE) = 14.6%). The technique can be used to develop repeatable HEV hazard evaluations for human ocular health applications.

Sun exposure to the eyes: predicted UV protection effectiveness of various sunglasses

The aim of this study was to assess solar ultraviolet radiation (UVR) doses received by the eyes in different exposure situations, and to predict the sun protection effectiveness provided by various styles of sunglasses at facial, periorbital, and ocular skin zones including the cornea and accounting for different head positions. A 3D numeric model was optimized to predict direct, diffuse and reflected erythemally weighted UVR doses received at various skin zones. Precisely defined facial, periorbital, and ocular skin zones, sunglasses (goggles, medium-, and large-sized sunglasses) and three head positions were modeled to simulate daily (08:00-17:00) and midday (12:00-14:00) UVR doses. The shading from sunglasses' frame and lenses' UVR transmission were used to calculate a predictive protection factor (PPF [%]). Highest ocular daily UVR doses were estimated at the uncovered cornea (1718.4 J/m²). Least sun protection was provided by middle-sized sunglasses with highest midday dose at the white lateral (290.8 J/m²) and lateral periorbital zones (390.9 J/m²). Goggles reached almost 100% protection at all skin zones. Large-sized sunglasses were highly effective in winter; however, their effectiveness depended on diffuse UVR doses received. In "looking-up" head positions highest midday UVR doses were received at the unprotected cornea (908.1 J/m²), totally protected when large-sized sunglasses are used. All tested sunglass lenses fully blocked UVR. Sunglasses' protection effectiveness is strongly influenced by geometry, wearing position, head positions, and exposure conditions. Sunglasses do not totally block UVR and should be combined with additional protection means. 3D modeling allows estimating UVR exposure of highly sensitive small skin zones, chronically exposed and rarely assessed.

How to Report Light Exposure in Human Chronobiology and Sleep Research Experiments

Exposure to light has short- and long-term impacts on non-visual responses in humans. While many aspects related to non-visual light sensitivity have been characterised (such as the action spectrum for melatonin suppression), much remains to be elucidated. Here, we provide a set of minimum reporting guidelines for reporting the stimulus conditions involving light as an intervention in chronobiology, sleep research and environmental psychology experiments. Corresponding to the current state-of-the-art knowledge (June 2019), these are (i) measure and report the spectral power distribution of the acute stimulus from the observer's point of view; (ii) measure and report the spectral power distribution of the background light environment from the observer's point of view; (iii), make spectra available in tabulated form, (iv) report α-opic (ir)radiances and illuminance; (v) describe the timing properties of stimulus (duration and pattern); (vi) describe the spatial properties of stimulus (spatial arrangement and extent), and (vii) report measurement conditions and equipment. We supplement the minimum reporting guidelines with optional reporting suggestions and discuss limitations of the reporting scheme.

Dimerization and oxidation of tryptophan in UV-A photolysis sensitized by kynurenic acid

Photoinduced generation of radicals in the eye lens may play an important role in the modification of proteins leading to their coloration, aggregation, and insolubilization. The radicals can be formed via the reactions of photoexcited endogenous chromophores of the human lens with lens proteins, in particular with tryptophan residues. In the present work we studied the reactions induced by UV-A (315-400nm) light between kynurenic acid (KNA), an effective photosensitizer present in the human lens, and N-acetyl-L-tryptophan (NTrpH) under aerobic and anaerobic conditions. Our results show that the reaction mechanism strongly depends on the presence of oxygen in solution. Under aerobic conditions, the generation of singlet oxygen is the major channel of the effective NTrpH oxidation. In argon-bubbled solutions, the quenching of triplet KNA by NTrpH results in the formation of KNA^•- and NTrp^• radicals. Under laser pulse irradiation, when the radical concentration is high, the main pathway of the radical decay is the back electron transfer with the restoration of initial reagents. Other reactions include (i) the radical combination yielding NTrp dimers and (ii) the oxygen atom transfer from KNA^•- to NTrp^• with the formation of oxidized NTrp species and deoxygenated KNA products. In continuous-wave photolysis, even trace amounts of molecular oxygen are sufficient to oxidize the majority of KNA^•- radicals with the rate constant of (2.0 ± 0.2) × 10⁹M^-1s^-1, leading to the restoration of KNA and the formation of superoxide radical O₂^•-. The latter reacts with NTrp^• via either the radical combination to form oxidized NTrp (minor pathway), or the electron transfer to restore NTrpH in the ground state (major pathway). As the result, the quantum yields of the starting compound decomposition under continuous-wave anaerobic photolysis are rather low: 1.6% for NTrpH and 0.02% for KNA. The photolysis of KNA with alpha-crystallin yields the same deoxygenated KNA products as the photolysis of KNA with NTrpH, indicating the similarity of the photolysis mechanisms. Thus, inside the eye lens KNA can sensitize both protein photooxidation and protein covalent cross-linking with the minor self-degradation. This may play an important role in the lens protein modifications during the normal aging and cataract development.

UV-B induced fibrillization of crystallin protein mixtures

Environmental factors, mainly oxidative stress and exposure to sunlight, induce the oxidation, cross-linking, cleavage, and deamination of crystallin proteins, resulting in their aggregation and, ultimately, cataract formation. Various denaturants have been used to initiate the aggregation of crystallin proteins in vitro. All of these regimens, however, are obviously far from replicating conditions that exist in vivo that lead to cataract formation. In fact, it is our supposition that only UV-B radiation may mimic the observed in vivo cause of crystallin alteration leading to cataract formation. This means of inducing cataract formation may provide the most appropriate in vitro platform for in-depth study of the fundamental cataractous fibril properties and allow for testing of possible treatment strategies. Herein, we showed that cataractous fibrils can be formed using UV-B radiation from α:β:γ crystallin protein mixtures. Characterization of the properties of formed aggregates confirmed the development of amyloid-like fibrils, which are in cross-β-pattern and possibly in anti-parallel β-sheet arrangement. Furthermore, we were also able to confirm that the presence of the molecular chaperone, α-crystallin, was able to inhibit fibril formation, as observed for ‘naturally’ occurring fibrils. Finally, the time-dependent fibrillation profile was found to be similar to the gradual formation of age-related nuclear cataracts. This data provided evidence for the initiation of fibril formation from physiologically relevant crystallin mixtures using UV-B radiation, and that the formed fibrils had several traits similar to that expected from cataracts developing in vivo.

Light-induced retinal damage using different light sources, protocols and rat strains reveals LED phototoxicity

To save energy, the European directives from the Eco-design of Energy Using Products (2005/32/CE) have recommended the replacement of incandescent lamps by more economic devices such as Light Emitting Diodes (LEDs). However, the emission spectrum of these devices is enriched in blue radiations, known to be potentially dangerous to the retina. Recent studies showed that light exposure contributes to the onset of early stages of age-related macular degeneration (AMD). Here, we investigate, in albinos and pigmented rats, the effects of different exposure protocols. Twenty-four hours exposure at high luminance was compared to a cyclic (dark/light) exposure at domestic levels for 1week and 1month, using different LEDs (Cold-white, blue and green), as well as fluorocompact bulbs and fluorescent tubes. The data suggest that the blue component of the white-LED may cause retinal toxicity at occupational domestic illuminance and not only in extreme experimental conditions, as previously reported. It is important to note that the current regulations and standards have been established on the basis of acute light exposure and do not take into account the effects of repeated exposure.

Protective Effect of Proanthocyanidins from Sea Buckthorn (Hippophae Rhamnoides L.) Seed against Visible Light-Induced Retinal Degeneration in Vivo

Dietary proanthocyanidins (PACs) as health-protective agents have become an important area of human nutrition research because of their potent bioactivities. We investigated the retinoprotective effects of PACs from sea buckthorn (Hippophae rhamnoides L.) seed against visible light-induced retinal degeneration in vivo. Pigmented rabbits were orally administered sea buckthorn seed PACs (50 and 100 mg/kg/day) for 14 consecutive days of pre-illumination and seven consecutive days of post-illumination. Retinal function was quantified via electroretinography 7 days after light exposure. Retinal damage was evaluated by measuring the thickness of the full-thickness retina and outer nuclear layer 7 days after light exposure. Sea buckthorn seed PACs significantly attenuated the destruction of electroretinograms and maintained the retinal structure. Increased retinal photooxidative damage was expressed by the depletion of glutathione peroxidase and catalase activities, the decrease of total antioxidant capacity level and the increase of malondialdehyde level. Light exposure induced a significant increase of inflammatory cytokines (IL-1β, TNF-α and IL-6) and angiogenesis (VEGF) levels in retina. Light exposure upregulated the expression of pro-apoptotic proteins Bax and caspase-3 and downregulated the expression of anti-apoptotic protein Bcl-2. However, sea buckthorn seed PACs ameliorated these changes induced by light exposure. Sea buckthorn seed PACs mediated the protective effect against light-induced retinal degeneration via antioxidant, anti-inflammatory and antiapoptotic mechanisms.

Maintaining ocular safety with light exposure, focusing on devices for optogenetic stimulation

Optogenetics methods are rapidly being developed as therapeutic tools for treating neurological diseases, in particular, retinal degenerative diseases. A critical component of the development is testing the safety of the light stimulation used to activate the optogenetic proteins. While the stimulation needs to be sufficient to produce neural responses in the targeted retinal cell class, it also needs to be below photochemical and photothermal limits known to cause ocular damage. The maximal permissible exposure is determined by a variety of factors, including wavelength, exposure duration, visual angle, pupil size, pulse width, pulse pattern, and repetition frequency. In this paper, we develop utilities to systematically and efficiently assess the contributions of these parameters in relation to the limits, following directly from the 2014 American National Standards Institute (ANSI). We also provide an array of stimulus protocols that fall within the bounds of both safety and effectiveness. Additional verification of safety is provided with a case study in rats using one of these protocols.

Does infrared or ultraviolet light damage the lens?

In daylight, the human eye is exposed to long wavelength ultraviolet radiation (UVR), visible radiation and short wavelength infrared radiation (IRR). Almost all the UVR and a fraction of the IRR waveband, respectively, left over after attenuation in the cornea, is absorbed in the lens. The time delay between exposure and onset of biological response in the lens varies from immediate-to-short-to-late. After exposure to sunlight or artificial sources, generating irradiances of the same order of magnitude or slightly higher, biological damage may occur photochemically or thermally. Epidemiological studies suggest a dose-dependent association between short wavelength UVR and cortical cataract. Experimental data infer that repeated daily in vivo exposures to short wavelength UVR generate photochemically induced damage in the lens, and that short delay onset cataract after UVR exposure is photochemically induced. Epidemiology suggests that daily high-intensity short wavelength IRR exposure of workers, is associated with a higher prevalence of age-related cataract. It cannot be excluded that this effect is owing to a thermally induced higher denaturation rate. Recent experimental data rule out a photochemical effect of 1090 nm in the lens but other wavelengths in the near IRR should be investigated.

What is light? The visible spectrum and beyond

In this International Year of Light, it is particularly appropriate to review the historical concept of what is light and the controversies surrounding the extent of the visible spectrum. Today we recognize that light possesses both a wave and particle nature. It is also clear that the limits of visibility really extend from about 310 nm in the ultraviolet (in youth) to about 1100 nm in the near-infrared, but depend very much on the radiance, that is, 'brightness' of the light source. The spectral content of artificial lighting are undergoing very significant changes in our lifetime, and the full biological implications of the spectral content of newer lighting technologies remain to be fully explored.

The Photobiology of Lutein and Zeaxanthin in the Eye

Lutein and zeaxanthin are antioxidants found in the human retina and macula. Recent clinical trials have determined that age- and diet-related loss of lutein and zeaxanthin enhances phototoxic damage to the human eye and that supplementation of these carotenoids has a protective effect against photoinduced damage to the lens and the retina. Two of the major mechanisms of protection offered by lutein and zeaxanthin against age-related blue light damage are the quenching of singlet oxygen and other reactive oxygen species and the absorption of blue light. Determining the specific reactive intermediate(s) produced by a particular phototoxic ocular chromophore not only defines the mechanism of toxicity but can also later be used as a tool to prevent damage.

Broadband activation by white-opsin lowers intensity threshold for cellular stimulation

Photoreceptors, which initiate the conversion of ambient light to action potentials via retinal circuitry, degenerate in retinal diseases such as retinitis pigmentosa and age related macular degeneration leading to loss of vision. Current prosthetic devices using arrays consisting of electrodes or LEDs (for optogenetic activation of conventional narrow-band opsins) have limited spatial resolution and can cause damage to retinal circuits by mechanical or photochemical (by absorption of intense narrow band light) means. Here, we describe a broad-band light activatable white-opsin for generating significant photocurrent at white light intensity levels close to ambient daylight conditions. White-opsin produced an order of magnitude higher photocurrent in response to white light as compared to narrow-band opsin channelrhodopsin-2, while maintaining the ms-channel kinetics. High fidelity of peak-photocurrent (both amplitude and latency) of white-opsin in response to repetitive white light stimulation of varying pulse width was observed. The significantly lower intensity stimulation required for activating white-opsin sensitized cells may facilitate ambient white light-based restoration of vision for patients with widespread photoreceptor degeneration.

Human retinal transmitochondrial cybrids with J or H mtDNA haplogroups respond differently to ultraviolet radiation: implications for retinal diseases

Background

It has been recognized that cells do not respond equally to ultraviolet (UV) radiation but it is not clear whether this is due to genetic, biochemical or structural differences of the cells. We have a novel cybrid (cytoplasmic hybrids) model that allows us to analyze the contribution of mitochondrial DNA (mtDNA) to cellular response after exposure to sub-lethal dose of UV. mtDNA can be classified into haplogroups as defined by accumulations of specific single nucleotide polymorphisms (SNPs). Recent studies have shown that J haplogroup is high risk for age-related macular degeneration while the H haplogroup is protective. This study investigates gene expression responses in J cybrids versus H cybrids after exposure to sub-lethal doses of UV-radiation.

Methodology/Principal Findings

Cybrids were created by fusing platelets isolated from subjects with either H (n = 3) or J (n = 3) haplogroups with mitochondria-free (Rho⁰) ARPE-19 cells. The H and J cybrids were cultured for 24 hours, treated with 10 mJ of UV-radiation and cultured for an additional 120 hours. Untreated and treated cybrids were analyzed for growth rates and gene expression profiles. The UV-treated and untreated J cybrids had higher growth rates compared to H cybrids. Before treatment, J cybrids showed lower expression levels for CFH, CD55, IL-33, TGF-A, EFEMP-1, RARA, BCL2L13 and BBC3. At 120 hours after UV-treatment, the J cybrids had decreased CFH, RARA and BBC3 levels but increased CD55, IL-33 and EFEMP-1 compared to UV-treated H cybrids.

Conclusion/Significance

In cells with identical nuclei, the cellular response to sub-lethal UV-radiation is mediated in part by the mtDNA haplogroup. This supports the hypothesis that differences in growth rates and expression levels of complement, inflammation and apoptosis genes may result from population-specific, hereditary SNP variations in mtDNA. Therefore, when analyzing UV-induced damage in tissues, the mtDNA haplogroup background may be important to consider.

A polymer optoelectronic interface restores light sensitivity in blind rat retinas

Interfacing organic electronics with biological substrates offers new possibilities for biotechnology due to the beneficial properties exhibited by organic conducting polymers. These polymers have been used for cellular interfaces in several fashions, including cellular scaffolds, neural probes, biosensors and actuators for drug release. Recently, an organic photovoltaic blend has been exploited for neuronal stimulation via a photo-excitation process. Here, we document the use of a single-component organic film of poly(3-hexylthiophene) (P3HT) to trigger neuronal firing upon illumination. Moreover, we demonstrate that this bio-organic interface restored light sensitivity in explants of rat retinas with light-induced photoreceptor degeneration. These findings suggest that all-organic devices may play an important future role in sub-retinal prosthetic implants.

Solar UV exposure at eye is different from environmental UV: diurnal monitoring at different rotation angles using a manikin

Solar ultraviolet radiation (UVR) promotes pterygium and cataract development in the human eye. When outdoors, people are subject to varying ocular UVR exposure intensity depending on time of day and orientation to the sun. To assess this variability, a manikin eye was exposed to solar UVR at 12 rotation angles relative to the sun with a solar elevation angle (SEA) ranging from 24.6° to 88.2°. At rotation angles of 0°, 30°, and 330°, the diurnal variation of ocular UVR exposure intensity showed a bimodal distribution that peaked at a SEA of about 40°, which was 3 to 4 hr both before and after noon. This timing differed from peak environmental UVR exposure intensity. At the other rotation angles, diurnal variations in ocular UV exposure exhibited unimodal curves, with maximum intensity around noon, the same as for environmental UVR. Thus, the idea that UVR exposure is most intense at midday is true for skin surfaces positioned somewhat horizontally but not for the eyes in a 60° arc with a centerline toward the sun (i.e., ranging 30° clockwise or counter-clockwise from the centerline). Maintaining certain orientations relative to the sun's position (for example, being clockwise or counter-clockwise by 30° from the sun) should effectively reduce ocular UVR exposure, especially at times when the SEA is 40°.

Modulation of photochemical damage in normal and malignant cells by naturally occurring compounds

Certain phytochemicals, such as the stilbene, resveratrol (RES, found in red grapes and berries), and the triterpenoid, ursolic acid (UA, found in waxy berries and herbs such as rosemary and oregano), have antioxidant, anti-inflammatory and antiproliferative effects. Two human-derived cell lines, hTERT-RPE with a nonmalignant phenotype derived from retinal pigment epithelium, and ATCC CRL-11147 derived from a malignant skin melanoma, were used as in vitro models of photooxidative stress produced by exposure to the broadband output of a 150 W Hg vapor arc lamp at an irradiance of 19-26 mW cm(-2). In untreated cells, UV-VIS broadband light exposure produced a loss of proliferative ability, an activation of NF-κB and an increase in protein carbonyl adducts at 24 h postexposure. Pretreatment of the cells with RES or UA at 1-2 μmsignificantly reduced the amount of phosphorylated NF-κB at 24 h postexposure. RES pretreatment reduced the burden of light-induced protein carbonyl adducts by up to 25% in exposed cells. UA treatment markedly increased the sensitivity of melanoma cells to UV radiation, while conferring some photoprotection to RPE cells. These observations indicate that phytochemicals modulate the cellular response to photochemical stress by interacting with specific cell-signaling pathways.

Concentration dependence of vitamin C in combinations with vitamin E and zeaxanthin on light-induced toxicity to retinal pigment epithelial cells

The purpose of this study was to determine the effects of increasing concentration of ascorbate alone and in combinations with α-tocopherol and zeaxanthin on phototoxicity to the retinal pigment epithelium. ARPE-19 cells were exposed to rose bengal and visible light in the presence and absence of antioxidants. Toxicity was quantified by an assay of cell-reductive activity. A 20 min exposure to visible light and photosensitizer decreased cell viability to ca 42%. Lipophilic antioxidants increased viabilities to ca 70%, 61% and 75% for α-tocopherol, zeaxanthin and their combination, respectively. Cell viabilities were ca 70%, 56% and 5% after exposures in the presence of 0.35, 0.7 and 1.4 mm ascorbate, respectively. A 45 min exposure increased cell death to ca 74% and >95% in the absence and presence of ascorbate, respectively. In the presence of ascorbate, zeaxanthin did not significantly affect phototoxicity. α-Tocopherol and its combination with zeaxanthin enhanced protective effects of ascorbate, but did not prevent from ascorbate-mediated deleterious effects. In conclusion, there is a narrow range of concentrations and exposure times where ascorbate exerts photoprotective effects, exceeding which leads to ascorbate-mediated increase in photocytotoxicity. Vitamin E and its combination with zeaxanthin can enhance protective effects of ascorbate, but do not ameliorate its deleterious effects.

Retinal photodamage by endogenous and xenobiotic agents

The human eye is constantly exposed to sunlight and artificial lighting. Light transmission through the eye is fundamental to its unique biological functions of directing vision and circadian rhythm and therefore light absorbed by the eye must be benign. However, exposure to the very intense ambient radiation can pose a hazard particularly if the recipient is over 40 years of age. There are age-related changes in the endogenous (natural) chromophores (lipofuscin, A2E and all-trans-retinal derivatives) in the human retina that makes it more susceptible to visible light damage. Intense visible light sources that do not filter short blue visible light (400-440 nm) used for phototherapy of circadian imbalance (i.e. seasonal affective disorder) increase the risk for age-related light damage to the retina. Moreover, many drugs, dietary supplements, nanoparticles and diagnostic dyes (xenobiotics) absorb ocular light and have the potential to induce photodamage to the retina, leading to transient or permanent blinding disorders. This article will review the underlying reasons why visible light in general and short blue visible light in particular dramatically raises the risk of photodamage to the human retina.

Central corneal thickness considered an index of corneal hydration of the UVB irradiated rabbit cornea as influenced by UVB absorber

UVB radiation from sunlight induces an acute corneal inflammation, photokeratitis, accompanied by changes in corneal hydration. We employed a method of ultrasonic pachymetry for daily examination of central corneal thickness as an index of corneal hydration of the rabbit cornea repeatedly irradiated by UVB radiation (312 nm, daily dose of 0.25 J/cm(2) during three or four days) as influenced by UVB absorber (actinoquinol combined with hyaluronic acid) dropped on the ocular surface during irradiation. One day after the third irradiation procedure the animals were sacrificed and corneas examined immuno-histochemically for peroxynitrite formation, a marker of oxidative damage, the antioxidant aldehyde dehydrogenase 3A1 and endothelial nitric oxide synthase, an enzyme generated nitric oxide. Results show that UV absorber combined with hyaluronic acid protected the cornea against UVB-induced changes in corneal thickness and microscopical disturbances to the cornea (both seen after buffered saline application) until the fourth experimental day. These UVB doses are equivalent to a daily exposure of 2.5 hrs of the human cornea to solar UVB radiation for three consecutive days. It is suggested that actinoquinol/ hyaluronic acid drops might be helpful for the human eye in the defence against photooxidative and other oxidative processes.

The susceptibility of the retina to photochemical damage from visible light

The photoreceptor/RPE complex must maintain a delicate balance between maximizing the absorption of photons for vision and retinal image quality while simultaneously minimizing the risk of photodamage when exposed to bright light. We review the recent discovery of two new effects of light exposure on the photoreceptor/RPE complex in the context of current thinking about the causes of retinal phototoxicity. These effects are autofluorescence photobleaching in which exposure to bright light reduces lipofuscin autofluorescence and, at higher light levels, RPE disruption in which the pattern of autofluorescence is permanently altered following light exposure. Both effects occur following exposure to visible light at irradiances that were previously thought to be safe. Photopigment, retinoids involved in the visual cycle, and bisretinoids in lipofuscin have been implicated as possible photosensitizers for photochemical damage. The mechanism of RPE disruption may follow either of these paths. On the other hand, autofluorescence photobleaching is likely an indicator of photooxidation of lipofuscin. The permanent changes inherent in RPE disruption might require modification of the light safety standards. AF photobleaching recovers after several hours although the mechanisms by which this occurs are not yet clear. Understanding the mechanisms of phototoxicity is all the more important given the potential for increased susceptibility in the presence of ocular diseases that affect either the visual cycle and/or lipofuscin accumulation. In addition, knowledge of photochemical mechanisms can improve our understanding of some disease processes that may be influenced by light exposure, such as some forms of Leber's congenital amaurosis, and aid in the development of new therapies. Such treatment prior to intentional light exposures, as in ophthalmic examinations or surgeries, could provide an effective preventative strategy.

Optical effects of exposing intact human lenses to ultraviolet radiation and visible light

BACKGROUND

The human lens is continuously exposed to high levels of light. Ultraviolet radiation is believed to play a causative role in the development of cataract. In vivo, however, the lens is mainly exposed to visible light and the ageing lens absorbs a great part of the short wavelength region of incoming visible light. The aim of the present study was to examine the optical effects on human lenses of short wavelength visible light and ultraviolet radiation.

METHODS

Naturally aged human donor lenses were irradiated with UVA (355 nm), violet (400 and 405 nm) and green (532 nm) lasers. The effect of irradiation was evaluated qualitatively by photography and quantitatively by measuring the direct transmission before and after irradiation. Furthermore, the effect of pulsed and continuous laser systems was compared as was the effect of short, intermediate and prolonged exposures.

RESULTS

Irradiation with high intensity lasers caused scattering lesions in the human lenses. These effects were more likely to be seen when using pulsed lasers because of the high pulse intensity. Prolonged irradiation with UVA led to photodarkening whereas no detrimental effects were observed after irradiation with visible light.

CONCLUSIONS

Irradiation with visible light does not seem to be harmful to the human lens except if the lens is exposed to laser irradiances that are high enough to warrant thermal protein denaturation that is more readily seen using pulsed laser systems.

UV-B exposure to the eye depending on solar altitude

PURPOSE

To assess the validity of the solar ultraviolet index (UVI) as a determiner of eye risk under different conditions of facial profiles and orientation, and reflected light.

METHODS

Ocular UV radiation (UVR) exposure was measured as a function of the time of the day (solar altitude) using a two-dummy-type mannequin dosimetry system with embedded UVR (260-310 nm) sensors, in September and November in Kanazawa, Japan, on a motorized sun-tracking mount with one dummy face directed toward the sun and the other away from the sun.

RESULTS

A bimodal distribution of UV-B exposure was found in September for the face directed toward the sun, which differed dramatically from the pattern of ambient UVR exposure and measurements taken on the top of the head and those for the eye taken later in the year. Although the overall level was lower, a higher solar altitude is associated with higher UVR exposure in the condition facing away from the sun.

CONCLUSIONS

The UVI is based on ambient solar radiation on an unobstructed horizontal plane similar to our measures taken on the top of the head, which differed so much from our measures of ocular exposure that UVI as a determiner of eye risk is deemed invalid. The use of the UVI as an indicator for the need for eye protection can be seriously misleading. Doctors should caution patients with regard to this problem, and eye protection may be warranted throughout the year.

Ultraviolet radiation as a risk factor for cataract and macular degeneration

The human eye is constantly exposed to sunlight and artificial lighting. Light transmission through the eye is fundamental to its unique biological functions of directing vision and circadian rhythm, and therefore, light absorbed by the eye must be benign. However, exposure to the intense ambient radiation can pose a hazard particularly if the recipient is over 40 years of age. This radiation exposure can lead to impaired vision and transient or permanent blindness.Both ultraviolet-A (UV-A) and UV-B induce cataract formation and are not necessary for sight. Ultraviolet radiation is also a risk factor for damage to the retinas of children. The removal of these wavelengths from ocular exposure will greatly reduce the risk of early cataract and retinal damage. One way this may be easily done is by wearing sunglasses that block wavelengths below 400 nm (marked 400 on the glasses). However, because of the geometry of the eye, these glasses must be wraparound sunglasses to prevent reflective UV radiation from reaching the eye. Additional protection may be offered by contact lenses that absorb significant amounts of UV radiation.In addition to UV radiation, short blue visible light (400-440 nm) is a risk factor for the adult human retina. This wavelength of light is not essential for sight and not necessary for a circadian rhythm response. For those over 50 years old, it would be of value to remove these wavelengths of light with specially designed sunglasses or contact lenses to reduce the risk of age-related macular degeneration.

Optical radiation and the eyes with special emphasis on children

The Sun is the most abundant source of optical radiation for the child eye. New hand-held visible lasers are a threat to the child eye. Some scientific data suggest that near infrared radiation may cause cumulative damage in the ocular lens. The child eye usually is exposed to ambient solar radiation, gazing at the horizon. Ambient Sun ultraviolet radiation (UVR) exposure to the child is complex due to atmospheric scattering and strong dependence on background reflection. Solar exposure causes biological damage, only by photochemical mechanisms. UVR exposure to a child eye is mainly a threat to the anterior segment of the eye, but also age dependently to the retina. Above threshold exposure to UVR, for short delay onset of damage, causes a toxic reaction on the surface of the eye, snow blindness, and cataract. Sub-threshold daily exposure to UVR over decades is associated with several ocular surface pathologies and eye lid cancer. Visible radiation is a threat to the retina. A single above threshold exposure, for short delay onset of damage to the retina causes immediate photochemical Type II retinal damage, Sun blindness. A single exposure of the retina to a very high intensity laser beam may cause thermal or thermo-mechanical damage in the retina. In environments with high irradiance of optical radiation, the child eye should be protected. Legislation and public information is required for avoidance of damage from high intensity laser systems. More research is urgently needed to exclude the potential hazard of near infrared radiation.

Hydration and transparency of the rabbit cornea irradiated with UVB-doses of 0.25 J/cm(2) and 0.5 J/cm(2) compared with equivalent UVB radiation exposure reaching the human cornea from sunlight

PURPOSE

Exposure of the cornea to UV radiation from sunlight evokes intraocular inflammation, photokeratitis. Photokeratitis is caused by UVB radiation. It is accompanied by changes of corneal hydration and light absorption. The aim of this study was to examine the effect of two UVB doses on corneal optics in rabbits and to compare these UVB doses with the equivalent exposure of UVB radiation reaching the human cornea from sunlight.

MATERIALS AND METHODS

Rabbit corneas were irradiated with a daily UVB dose of 0.25 J/cm(2) or 0.5 J/cm(2) for 4 days. One day after finishing the irradiations the rabbits were sacrificed and corneal light absorption measured using our spectrophotometrical method. Corneal hydration was examined using an ultrasonic Pachymeter every experimental day before the irradiation procedure and the last day before sacrificing the animals.

RESULTS

Changes in corneal optics appeared after the repeated exposure of the cornea to a UVB dose of 0.25 J/ cm(2) and massively increased after the repeated exposure of the cornea to a UVB dose of 0.5 J/cm(2). The first significant changes in corneal hydration appeared after a single exposure of the cornea to a UVB dose of 0.25 J/cm(2).

CONCLUSIONS

Changes in corneal hydration appeared after the exposure of the rabbit cornea to a single UVB dose equivalent to 2.6 hours of solar UVB radiation reaching the human cornea, as measured by UVB sensors embedded in the eyes of mannequin heads facing the sun on a beach at noon in July. Repeated exposure of the rabbit cornea to the same UVB dose evoked profound changes in corneal optics. Although comparison of experimental and outdoor conditions are only approximate, the results in rabbits point to the danger for the human eye from UVB radiation when short stays in sunlight are repeated for several consecutive days without UV protection.

Light-emitting diodes (LED) for domestic lighting: any risks for the eye?

Light-emitting diodes (LEDs) are taking an increasing place in the market of domestic lighting because they produce light with low energy consumption. In the EU, by 2016, no traditional incandescent light sources will be available and LEDs may become the major domestic light sources. Due to specific spectral and energetic characteristics of white LEDs as compared to other domestic light sources, some concerns have been raised regarding their safety for human health and particularly potential harmful risks for the eye. To conduct a health risk assessment on systems using LEDs, the French Agency for Food, Environmental and Occupational Health & Safety (ANSES), a public body reporting to the French Ministers for ecology, for health and for employment, has organized a task group. This group consisted physicists, lighting and metrology specialists, retinal biologist and ophthalmologist who have worked together for a year. Part of this work has comprised the evaluation of group risks of different white LEDs commercialized on the French market, according to the standards and found that some of these lights belonged to the group risk 1 or 2. This paper gives a comprehensive analysis of the potential risks of white LEDs, taking into account pre-clinical knowledge as well as epidemiologic studies and reports the French Agency's recommendations to avoid potential retinal hazards.

Retinal light toxicity

The ability of light to enact damage on the neurosensory retina and underlying structures has been well understood for hundreds of years. While the eye has adapted several mechanisms to protect itself from such damage, certain exposures to light can still result in temporal or permanent damage. Both clinical observations and laboratory studies have enabled us to understand the various ways by which the eye can protect itself from such damage. Light or electromagnetic radiation can result in damage through photothermal, photomechanical, and photochemical mechanisms. The following review seeks to describe these various processes of injury and many of the variables, which can mitigate these modes of injury.

Lens autofluorescence is not increased at high altitude

PURPOSE

To study the relation between ambient environmental ultraviolet radiation exposure and lens fluorescence.

METHODS

Non-invasive lens fluorometry measurements were compared in healthy Bolivian and Danish subjects. Background ultraviolet radiation was 4.5 times higher in Bolivia than in Denmark.

RESULTS

No significant differences in lens fluorescence or transmittance were found between Bolivian and Danish volunteers.

CONCLUSION

Age-corrected lens fluorescence and transmittance were comparable for healthy participants living at high altitude near the equator and healthy volunteers living at sea level at 55 degrees northern latitude. These results suggest that lens ageing, as assessed by lens autofluorometry, is independent of exposure to ultraviolet radiation.

Potential of wind turbines to elicit seizures under various meteorological conditions

PURPOSE

To determine the potential risk of epileptic seizures from wind turbine shadow flicker under various meteorologic conditions.

METHODS

We extend a previous model to include attenuation of sunlight by the atmosphere using the libradtran radiative transfer code.

RESULTS

Under conditions in which observers look toward the horizon with their eyes open we find that there is risk when the observer is closer than 1.2 times the total turbine height when on land, and 2.8 times the total turbine height in marine environments, the risk limited by the size of the image of the sun's disc on the retina. When looking at the ground, where the shadow of the blade is cast, observers are at risk only when at a distance <36 times the blade width, the risk limited by image contrast. If the observer views the horizon and closes their eyes, however, the stimulus size and contrast ratio are epileptogenic for solar elevation angles down to approximately 5 degrees.

DISCUSSION

Large turbines rotate at a rate below that at which the flicker is likely to present a risk, although there is a risk from smaller turbines that interrupt sunlight more than three times per second. For the scenarios considered, we find the risk is negligible at a distance more than about nine times the maximum height reached by the turbine blade, a distance similar to that in guidance from the United Kingdom planning authorities.

Blue light irradiation inhibits the production of HGF by human retinal pigment epithelium cells in vitro

Blue visible light damage to retinal pigment epithelial cells occurs through a photooxidative mechanism and the resultant damage is hypothesized to induce or exacerbate age-related macular degeneration. The purpose of the present study was to identify changes in the cell growth and the expression of hepatocyte growth factor (HGF) in cultured human retinal pigment epithelium (RPE) cells as a result of both blue and red light irradiation. HGF is a growth factor and neurotrophic factor that stimulates growth of various ocular cells and promotes the survival of RPE and retinal neurons. Early passages of human RPE cells were exposed to blue light (460 nm) and red light (640 nm). Nonirradiated cells were used as controls. After 24 and 48 h, conditioned medium was collected and the amount of HGF was measured by ELISA. Cells were detached from the well and counted. Cell viability was evaluated by trypan-blue exclusion study. Blue light at dosage of 63 J/cm(2) significantly inhibited the growth of RPE cells without affecting of cell viability. Amounts of HGF in the culture medium were significantly inhibited by blue-light irradiation at the dosage from 32 to 63 J/cm(2). Red light at a dose of 174 J/cm(2) causes a nonsignificant inhibition of growth of RPE cells and a slight decrease of secretion of HGF. As HGF promotes survival of RPE cells and retinal neurons, the inhibition of production of HGF by visible light, especially by blue light, may enhance the phototoxic effects of visible light on the RPE and retinal neurons.