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Solar radiation incident at the top of the atmosphere
Published in Lucien Wald, Fundamentals of Solar Radiation, 2021
The irradiance is a density of radiant flux, or in other words, the power received per unit area. It is expressed in watt per square meter (W m−2). The spectral irradiance E(λ) is the irradiance at a given wavelength and is expressed in W m−2 nm−1. The total irradiance is the irradiance integrated over the whole spectrum of the solar radiation.
Vision and Illumination
Published in Stephan Konz, Steven Johnson, Work Design, 2018
Radiant power (radiant flux) is the time rate of flow of radiant energy; it is measured in watts. Luminous power, the visible part of radiant power from a source, is measured in lumens (lm). For example, a lamp may give 1,200 lumens. Luminous intensity, I, the luminous flux in a specified direction, is given in candelas (cd). A sphere subtends an angle in space of 4 π steradians; thus the total flux emitted by a 1-candela uniform light source is 12.57 1m. The illuminance on a 1-m sphere by a 1-candela source is 1 lm/m2 = 1 lux. In the SI system, lumens/m2 is called lux (lx); in the U. S. system, lumens/ft2 is called foot-candles (fc). Illuminance transmitted through a surface is called transmittance, t, unitless, with no transmittal = 0 and perfect transmittal = 1.
Radiometry, Photometry, and Radiation Heat Transfer
Published in Julio Chaves, Introduction to Nonimaging Optics, 2017
The human eye has differing sensitivity to different wavelengths (colors) of light. We must, therefore, distinguish two concepts. Radiant flux is the power of the radiation, measured in watts. Luminous flux is the measure of the perceived power of light by the human eye, measured in lumens. These quantities are related by the luminous efficacy function shown in Figure 20.1.1 This function tells us how many lumens are there for each 1 W of power at a given wavelength. It has a maximum of 683 lm/W at 555 nm wavelength. For example, for 1 W power of radiation with a wavelength of 555 nm, we have 683 lm of visual sensation. For 1 W power of radiation of other wavelengths, the corresponding visual sensation in lumens is given by the luminous efficacy function. For example, radiation of wavelength 900 nm (infrared) will not be visible, so its luminous efficacy is zero.
Understanding multi-domain compact modeling of light-emitting diodes
Published in Cogent Engineering, 2021
Efficient dissipation of waste heat from the junction is limited by , thermal resistance along the heat conduction path from the junction-to-ambient. Improper dissipation of waste heat results in increase of junction temperature , junction temperature build-up beyond the safety limits specified by the manufacturer will limit the useful life of the LED (Tsai et al., 2007). White LEDs used for general illumination applications, emit electromagnetic radiation along a wide spectral band, known as spectral power distribution (SPD), it is a representation of the radiant power as a function of wavelength. Luminous flux is a measure of the perceived power of radiant flux .
Tutorial: Theoretical Considerations When Planning Research on Human Factors in Lighting
Published in LEUKOS, 2019
Under dim conditions, we rely mostly on rod receptors for vision, which are more sensitive to light, whereas under brighter conditions, vision is driven most by the cone receptors. Rod receptors and the three types of cone receptors—S, M, and L, for short, medium, and longer wavelengths—each have a slightly different peak sensitivity in terms of the wavelength of light (see Fig. 2a). In bright conditions, the combined (differential) input of the three types of cones provides us color information, in addition to a lightness sensation. The human spectral sensitivity curve V(λ) describes the combined (S, M, and L) sensitivity for cone vision in bright (>3 cd/m2, termed photopic) conditions as a function of the wavelength of light (see Fig. 2b). All photometric measures (luminous flux, luminous intensity, luminance, illuminance) are basically derived from their radiometric counterparts (radiant flux, intensity, radiance, and irradiance) by weighting them according to this curve. Similarly, V′(λ) is the established sensitivity for rod vision in dim (<0.03 cd/m2, termed scotopic) conditions. The ranges of the luminous conditions in which rods and cones are sensitive overlap. Importantly, in this intermediate (mesopic) range, both types of photoreceptors are active, and both sensitivity curves should be considered.
Methods to Describe and Measure Lighting Conditions in Experiments on Non-Image-Forming Aspects
Published in LEUKOS, 2019
Martine Knoop, Kai Broszio, Aicha Diakite, Carolin Liedtke, Mathias Niedling, Inga Rothert, Frederic Rudawski, Nils Weber
Using this principle in radiometric terms means describing the incident spectral flux or incident radiant flux within a specific solid angle. This (spectral) radiant incidence can be weighted with various spectral sensitivity functions, which is the base for all methods addressed in this article. As soon as relevant retinal regions and their respective spectral sensitivity weightings are known, one could work with simplified sensors using a small number of spectroradiometers as, for example, presented by Khademagha et al. (2018) with predefined solid angles as already proposed for other purposes by Van Derlofske et al. (2000, 2002). Nonetheless, the number of studies is as yet too small to define areas with different ipRGC sensitivities; in particular, daytime responses are largely unknown. Therefore, it is proposed to scan the field of view of an observer (in a single hemisphere, with its zenith in the viewing direction) and measure the incoming luminous flux or (spectral) radiant flux within a fixed solid angle at predefined points in high angular resolution.