Light is defined as any electromagnetic radiation that can create a visual sensation by directly stimulating the retinal photoreceptors of the visual system. In addition to enabling vision, these photoreceptors also drive biological effects that powerfully regulate human health, performance and well-being. Light is the main synchronizer of the human biological clock. It can shift the phase of the circadian rhythm and can regulate the timing and quality of our sleep. Light in the evening and at night can disrupt sleep and can cause acute suppression of the nocturnal release of the hormone melatonin. There are also reports that light can increase heart rate, improve alertness, alleviate seasonal and non-seasonal depression, influence thermoregulation, and affect brain activity as measured with the electroencephalogram (EEG). Exposure to light elicits fast responses (in the range of milliseconds and seconds) in the pupillary reflex or in brain activity. To reflect their distinction from perceptual vision, these effects are often referred to as non-image-forming (NIF) or non-visual (NV) responses to light. In recent years various marketing terms, such as “human-centric lighting” (HCL), “circadian lighting”, and “biodynamic lighting”, have come to describe lighting solutions that target such effects. In the upcoming 2nd edition of the CIE International Lighting Vocabulary, currently available as DIS (CIE 2016), “integrative lighting” is the official term for lighting that is specifically intended to integrate visual and non-visual effects, producing physiological and psychological effects on humans that are reflected in scientific evidence.
The above-mentioned biological effects of light are elicited by stimulation of ocular photoreceptors. The classical receptors for vision, the rods and cones, are relatively well understood and characterized by existing CIE publications. Pioneering work over the last 25 years revealed that the eye has another kind of photoreceptor. These photoreceptors play an important role in non-visual effects of light and have a peak sensitivity in the shorter wavelength part of the visible spectrum. Such photoreceptors are known as intrinsically-photosensitive retinal ganglion cells (ipRGCs), and their intrinsic photosensitivity is based on the photopigment melanopsin.
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