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Even though users of light therapy are often advised not to look directly at the light source, the mechanisms of the eye focus incoming light onto the macula, the small region of the retina where vision takes place, and where age-related macular degeneration occurs. Since blue light wavelength make up only a small percentage of the light in white light, any form of light therapy using a high proportion of blue light therefore risks subverting a variety of defensive mechanisms that protect the retina against blue light hazard. These defensive mechanisms include the anatomical positioning and structure of eye and its surrounding features, as well as human posture, which makes it awkward for humans to gaze upwards for long periods of time. Sunnex Biotechnologies, 2008)
The work of David H. Sliney entitled: "Ocular Hazards of Light" presented at the International Lighting in Controlled Environments Workshop states the following risks and hazards to the eye due to lighting: (1) Ultraviolet photochemical injury to cornea (photokeratitis) and lens (cataract) stated at 180mn to 400 nm; (2) thermal injury to the retina of the eye (400nm to 1400nm); (3) blue-light photochemical injury to the retina of the eye (principally 400nm to 550 nm; unless aphakic, 310 to 550 nm); (4) near infrared thermal hazards to the eye lens (approximately 800 nm to 3000 nm); and (5) thermal injury to the eye cornea (approximately 1400 nm to 1mm). (1994) Sliney states that the primary "retinal hazard" due to bright light sources is "photoretinitis, e.g., solar retinitis with an accompanying scotoma which results from staring at the sun." (1994) Sliny states that it is only recently that it has become clear that "photoretinitis results from "a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, i.e., violet and blue light."(1994) Sliny states that it has been show conclusively that an intense exposure to short-wavelength light, or 'blue light' can cause retinal injury. Sliny specifically states: "The product of the dose-rate and the exposure duration always must result in the same exposure dose (in joules-per-square centimeter at the retina) to produce a threshold injury. Blue-light retinal injury (photoretinitis) can result from viewing either an extremely bright light for a short time, or a less bright light for longer exposure periods. This characteristic of photochemical injury mechanisms is termed reciprocity and helps to distinguish these effects from thermal burns, where heat conduction requires a very intense exposure within seconds to cause a retinal coagulation; otherwise, surrounding tissue conducts the heat away from the retinal image. Injury thresholds for acute injury in experimental animals for both corneal and retinal effects have been corroborated for the human eye from accident data. Occupational safety limits for exposure to UVR and bright light are based upon this knowledge. As with any photochemical injury mechanism, one must consider the action spectrum, which describes the relative effectiveness of different wavelengths in causing a photobiological effect. The action spectrum for photochemical retinal injury peaks at approximately 440 nm." (1994)
Calculation of retinal exposure is also addressed in Sliny's work who states that the knowledge "of the optical parameters of the human eye and from radiometric parameters of a light source" enables the calculation of "irradiances (dose rates) at the retina. (1994) Sliny states that there are two sets of light-measurement quantities and units in the endeavor to define light exposure of the retina: (1) radiometric; and (2) photometric. (1994) Specifically, Sliny states: "Radiometric quantities such as radiance -- used to describe the "brightness" of a source [in W/cm2 sr] and irradiance -- used to describe the irradiance level on a surface [in W/cm2] are particularly useful for hazard analysis. Radiance and luminance are particularly valuable because these quantities describe the source and do not vary with distance. Photometric quantities such as luminance (brightness in cd/cm2 as perceived by a human "standard observer") and illuminance in lux (the "light" falling on a surface) indicate light levels spectrally weighted by the standard photometric visibility curve which peaks at 550 nm for the human eye (Figure 1). To quantify a photochemical effect it is not sufficient to specify the number of photons-per-square-centimeter (photon flux) or the irradiance (W/cm2) since the efficiency of the effect will be highly dependent on wavelength. Generally, shorter-wavelength, higher-energy photons are more efficient." (1994) Sliny goes on to state: "Unfortunately, since the spectral distributions of different light sources vary widely, there is no simple conversion factor between photometric (either photopic or scotopic) and radiometric quantities. This conversion may vary from 15 to 50 lumens/watt (1m/W) for an incandescent source to about 100 1m/W for the sun or a xenon arc, to perhaps 300 to 400 lm/W for a fluorescent source (Sliney and Wolbarsht, 1980; as cited in Sliny, 1994)
The work of Wu (2004) entitled: "Blue Light Induced Retinal Damage" states that there is a high damage potential for "visible, non-coherent blue light" and that green light "in sharp contrast does not induce lesions." The work of Wenzel, Grimm and Iseli (2003) relate that photochemical retinal damage "are exceed within one minute for nine out of ten combinations" when the spectrum and energy of light are measured. The work of Biesen, et al. entitled: "Endoiollumination During Vitrectomy and Phototoxicity Thresholds" relates is has been shown that "intense exposure to shorter wavelengths in the visible spectrum, i.e. violet and blue light...can cause retinal injury." This work related that it has been shown that reduction of the risk of photochemical injury to the retina can be greatly reduced through filtering out the short-wavelengths of light.
Sunnex Biotechnologies has produced a 'GreenLIGHT' System which uses a low-intensity narrow spectrum technology. Sunnex reports having conducted studies that show that "intensities between 5,000 and 10,000 lux of white light are most successful in shifting the human body clock to align it with a night work schedule." (2008) it is stated however, that "this high intensity of white light is not practical in the workplace, and may be harmful to the eye." (Sunnex Biotechnologies GreenLIGHT System, 2008) the GreenLIGHT system is stated to be safe for the individual's eye and makes provision of a "non-UV light source that filters out the harmful wavelengths of visible light, the blue rays, contained in white and full-spectrum lighting." (Sunnex Biotechnologies GreenLIGHT System, 2008) the elimination of the 'blue light hazard' "eliminates the risk of eye damage inherent from repetitious exposure to bright or blue light." (Sunnex Biotechnologies GreenLIGHT System, 2008) GreenLIGHT is a system that has been patented by Sunnex Biotechnologies.
Okudaira, Kripe, and Webster (1983) conducted a study and reported it in the work entitled: "Naturalistic Studies of Human Light Exposure." In this study which included 10 volunteers, who, for twenty-four hours "wore an apparatus that recorded their exposure to light at eye level and at the wrist" with activity being recorded at the heard, both wrists and one ankle findings are stated which relate that "Illuminations at the eye level and wrist were correlated 0.76 while eye-level illumination was correlated 0.25, 0.44, 0.39, and 0.44 with head wrists and ankles respectively,: Conclusions of the study state: "Because human biologic rhythms are probably well synchronized only by illumination approaching daylight intensities, inadequate illumination could be a source of sleep disturbance, chronobiologic disorders, or depression." (Okudaira, Kripe, and Webster, 1983)
The U.S. Department of Transportation and Polytechnic Institute reported in 2007 in relation to the debate about how much light is enough the fact: "Most visual processing is adequately supported by quite low light levels as evidenced by the lighting recommendations for exterior locations published by the Illuminating Engineering Society of North America. Considering that the illuminances from the sun and sky found outdoors during the solar day regularly exceed 10,000 lx, and that this cycle has been largely responsible for maintaining circadian entrainment, it is perhaps not surprising that the human circadian system requires much higher quantities of light than the visual system for maintaining entrainment or shifting circadian rhythms. In comparison, most light levels experienced indoors away from windows are relatively low, adequate for visual function but near threshold levels for activating the human circadian system.,"
The USDOT and Polytechnic Institute report states importantly, in the endeavor to understand what might be optimal lighting for the human being, and the individual dependent upon the settings in which the daily life is carried out as follows: "The circadian system, on the other hand, is maximally sensitive to the short-wavelength (blue) portion of the visible spectrum; in simple words, it can be said that the circadian system is a "blue sky detector." A combination of classical photoreceptors (rods and cones) and a recently discovered novel photoreceptor in the retina, intrinsically-photosensitive retinal ganglion cells (maximally sensitive to blue light near 480 nm), participate in circadian phototransduction, the process whereby retinal photopigments absorb light signals and convert them into neural signals. Moreover, it has been shown that the human circadian system seems to respond to light in such a way…[continue]
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