Open flames and incandescent sources are described technically as having low-color temperatures, typically ⩽2800 Kelvins (K)-rich in longer visible (orange, red) wavelengths and infrared–near-infrared radiation. Since the 1820s-1830s gas lamps and (later) incandescent (red-rich) lamps have dominated our indoor environment at night. But today-and over the past century-electrically powered lamps have dominated our nighttime environments in the developed countries. One might think of sunsets or even the nighttime sky! Throughout almost all of humankind’s evolution, there was only natural sunlight-or fire (including, candles, flame torches, and later oil lamps). 2Īlthough ‘light’ refers to visible radiant energy, it may refer to sources of illumination, such as sunlight or artificial sources such as a lamp and luminaires (ie, lamp fixtures). When necessary to avoid confusion between these 2 meanings the term ‘ perceived light’ may be used in the first sense’ and ‘NOTE 2-Light is normally, but not always, perceived as a result of the action of a light stimulus on the visual system.’ [This latter note was added even before the photoreceptive retinal–ganglion cells (pRGCs) were known, and of course the pRGCs also influence vision when controlling pupil size and lid position 1 and may even have a type of retinal image role in vestibular function. The CIE provides two interesting notes to this formal definition of light: ‘NOTE 1-This term has 2 meanings that should be clearly distinguished. radiation that is considered from the point of view of its ability to excite the human visual system.’ characteristic of all sensations and perceptions that is specific to vision Ģ. The definition found in the ILV has a number (#17–659) and reads:ġ. The typical dictionary definitions of ‘light’ (noun) are: ‘The natural agent that stimulates sight and makes things visible also, a source of illumination, especially an electric lamp.’ Finally, ‘a device that makes something start burning, as a match, lighter, or flame.’ However, the more technical, scientific definition can be obtained from the ‘International Lighting Vocabulary (ILV) published by the International Commission on Illumination (the CIE).
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