High-energy visible light
In ophthalmology, high-energy visible light (HEV light) is high-frequency, high-energy light in the violet/blue band from 400 to 450 nm in the visible spectrum. Despite a lack of concurring scientific evidence, HEV light has sometimes been claimed to be a cause of age-related macular degeneration. Some sunglasses and beauty creams specifically block HEV, for added marketing value.
Blue-light hazard is defined as the potential for a photochemically-induced retinal injury resulting from electromagnetic radiation exposure at wavelengths primarily between 400 and 450 nm. This study has not been done in humans, only inconclusively in some rodent, primate, and in vitro studies. The mechanisms for photochemically-induced retinal injury are caused by the absorption of light by photoreceptors in the eye. Under normal conditions when light hits a photoreceptor, the cell bleaches and becomes useless until it has recovered through a metabolic process called the visual cycle.
Absorption of blue light, however, has been shown in rats and a susceptible strain of mice to cause a reversal of the process where cells become unbleached and responsive again to light before they are ready. At wavelengths of blue light below 430 nm this greatly increases the potential for oxidative damage. For blue-light circadian therapy, harm is minimized by employing blue light at the near-green end of the blue spectrum. "1-2 min of 408 nm and 25 minutes of 430 nm are sufficient to cause irreversible death of photoreceptors and lesions of the retinal pigment epithelium. ... The action spectrum of light-sensitive retinal ganglion cells was found to peak at approximately 450 nm, a range with lower damage potential, yet not completely outside the damaging range." A 2014 study found that LEDs cause retinal damage even in settings where they are used indirectly, such as household light bulbs.
An unpublished and non peer-reviewed 2013 in vitro study financed by skin care company Lipo Chemicals using shorter blue band spectrum LED lights claims that prolonged exposure may permanently damage the pigment epithelial cells of the retina. However, according to a specialist, the test conditions were the equivalent of staring at a blue light equivalent to a 100 watt incandescent source from 20 cm (8 in) for 12 hours, which is not deemed to be a realistic light exposure.. Sébastien Point and colleagues discussed in some recent peer-reviewed publications the validity of rodent models and conclude that LEDs are not a problem of public health in normal use but that caution must be taken for newborn infants, as their eye collect more light  and for specific occupational situations for which observers can look at high power leds for hours (for example LEDs quality control or alternative light therapies).
A recent study has given more insight about the Blue light hazard: permanent damage to the eye cells, as reported by a research made by a team from Toledo University , especially for children, who are big users of LED screen (smartphones,tablets...). However, the conclusion that LED technology would be more dangerous for retina than Tungsten lamp and fluorescent lamp is debated, as the LED spectrum does not contain potentially retinotoxic violet light (to which a child's lens is highly transparent) in contrary to spectra of fluorescent and incandescent lamps which are rich in violet light.
Blue light within the range 400-450 nm has been reported in a number of studies to be effective as local treatment of eczema and psoriasis, as it purportedly helps dampen the immune response. Recent studies have also shown improvement of facial acne upon exposure to a LED emitting at 414 nm. A combination of exposure to red and blue lights is used more and more in clinical dermatologic therapies. Constructors such as Philips currently develop devices and techniques emitting in the blue visible spectrum to be used in dermatologic therapy.
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