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Lighting and Daylighting
Published in T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl, Heating and Cooling of Buildings, 2016
T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl
The quantity of light emitted, per unit time, by a source of radiation is called the luminous flux, and it is measured in units of lumen (abbreviated lm). The lumen is defined in terms of radiative power weighted by the spectral sensitivity of the human eye. For energy-conscious design, a quantity of special importance is the luminous efficacy of a light source, defined as the ratio of its light output (in lumens) to the necessary power input (in watts). The theoretical upper limit is 683 lm/W corresponding to lossless conversion of the input to monochromatic light at the wavelength of 555 nm, where the eye is most sensitive. For other spectral distributions, the efficacy can only be lower. For instance, much of the solar spectrum is outside the visible range, and the luminous efficacy of daylight is only around 100–120 lm/W (relative to its radiative power). The luminous efficacy of incandescent lamps is around 10–20 lm/W, and that of fluorescent lamps is around 50–90 lm/W. The relatively high efficacy of daylight explains its interest for energy efficiency.
Introduction
Published in M. Nisa Khan, Understanding LED Illumination, 2013
Luminous efficacy is a conventional measure of how well an electrical lighting element produces visible light. It is a measure of how much luminous flux is produced per unit of electrical input power and therefore is measured in “lumen per watt” (lm/W). It should not be confused with luminous efficiency, which is given in percentages where both power units must be either in lumens or watts, depending on what efficiency one is interested in describing. For most traditional lamps and luminaires, such as those comprising incandescent and fluorescent lamps, luminous efficacy provides the lamp efficacy and luminous efficiency provides the luminaire efficiency. Luminaire efficiency is of interest when we ask, for example, how much light is still available when a lamp is placed in a fixture where some light is being hindered by the fixture. Consequently, it is determined as the following, from the ratio of two light powers measured in lumens: () luminaireefficiency(%)=luminaire output(lm)lampoutput(lm)×100
Illumination and Lighting System Design
Published in S. Bobby Rauf, Electrical Engineering for Non-Electrical Engineers, 2021
Luminous efficacy is also referred to as luminous efficiency or lamp efficacy. Luminous efficacy is defined as the ratio of luminous flux of a light source to the amount of power required by the light source. Since the units for luminous flux are lumens and the electrical power is measured in watts, the units for luminous efficacy are lm/W. Luminous efficacy or efficiency is denoted by the symbols “n” or “K,” and is defined, mathematically, as: η=K=ΦP
A common type of commercially available LED light source allows for colour discrimination performance at a level comparable to halogen lighting
Published in Ergonomics, 2019
Sara Königs, Susanne Mayr, Axel Buchner
Light sources based on light emitting diodes (LEDs) have become superior to other conventional light sources such as incandescent, halogen, or fluorescent lamps in terms of lifetime and efficiency (Chang et al. 2012). The efficiency of a light source is characterised by its luminous efficacy which is defined as the ratio of the luminous flux to the electrical power consumption (Boyce 2014), measured in lumens per watt (lm/W). Apart from luminous efficacy and lifetime, LED-based light sources differ from other conventional light sources in their spectral power distribution. The spectral power distribution reflects the intensity of emitted radiation at each wavelength (Houser et al. 2016). Radiation in the wavelength range from 380 to 780 nm (the visible spectrum) leads to a response of the human visual system (Boyce 2014) and thus triggers the perception of colour; for normal observers short wavelengths appear as blue light, medium wavelengths as green light, and long wavelengths as red light (Houser et al. 2016).
The Problem with Luminous Efficacy
Published in LEUKOS, 2020
Luminous efficacy is defined as a ratio of lumens per watt and is applied to light-emitting devices such as LED emitters, lamps, and luminaires. Implicit is that lumens are a suitable proxy for lighting’s benefit and watts are a suitable proxy for lighting’s cost. Uncomfortably, this is often not true when light-emitting devices are employed in lighting applications.