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Optical and Video Principles
Published in John Watkinson, The Art of Digital Video, 2013
The lumen (lm) is a weighted value based on the luminous efficiency function of the HVS. Thus the same numerical value in lumens will appear equally bright to the HVS whatever the colour. If three sources of light, red, green, and blue, each of one lumen, are added, the total luminous flux will be three lumen but the result will not appear white. It is worthwhile discussing this in some detail. Figure 2.12a shows three monochromatic light sources of variable intensity that are weighted by the luminous efficiency function of the HVS to measure the luminous flux correctly. To obtain one lumen from each source, the red and blue sources must be set to produce more luminous flux than the green source. This means that the spectral distribution of the source is no longer uniform and so it will not appear white. In contrast, Figure 2.12b shows three sources that have the same luminous flux. After being weighted by the luminous efficiency function, each source produces a different number of lumens, but the eye perceives the effect as white. Essentially the eye has a nonuniform response, but in judging colour it appears to compensate for that so that a spectrum that is physically white, i.e., having equal luminous flux at all visible wavelengths, also appears white to the eye. As a consequence it is more convenient to have a set of units in which equal values result in white. These are known as tristimulus units and are obtained by weighting the value in lumens by a factor that depends on the response of the eye to each of the three wavelengths. The weighting factors add up to unity so that three tristimulus units, one of each colour, when added together produce one lumen.
Basic Construction of Safety Helmets and Eye and Face Protectors
Published in Katarzyna Majchrzycka, Head, Eye, and Face Personal Protective Equipment, 2020
Spectral luminous efficiency function of the average human eye is a relationship determining the amount of energy that reaches an eye in a wavelength function. Vision is different in daylight conditions (photopic) than it is in night-time conditions (scotopic). Figure 2.15 presents spectral luminous efficiency function of the average human eye in daylight conditions (V(λ)day) [CIE 1926] and in night-time conditions (V(λ)night) [CIE 1951; Crawford 1949; Wald 1945].
Radiometry, Photometry, and Radiation Heat Transfer
Published in Julio Chaves, Introduction to Nonimaging Optics, 2017
We may now define the luminosity function V(λ) (or photopic luminous efficiency function) the same way as the luminous efficacy, but normalized to its maximum value of 683, which occurs at 555 nm. We then have V(555) = 1. The luminous efficacy function can then be given by 683V(λ) where V(λ) is the luminosity function.2 Note that V(λ) is dimensionless, but is multiplied by 683 lm/W to give the luminous efficacy.
Comparison of Static and Ambulatory Measurements of Illuminance and Spectral Composition That Can Be Used for Assessing Light Exposure in Real Working Environments
Published in LEUKOS, 2019
Mathias Adamsson, Thorbjörn Laike, Takeshi Morita
Thorne et al. (2009) measured light exposure in daily life with a wrist-worn Actiwatch-RGB monitor. This instrument differentiates between short, middle, and long wavelength light radiation. The short wavelength band register light radiation between 400 and 580 nm, with a peak sensitivity at 470 nm; the middle wavelength band includes wavelengths between 500 and 650 nm, with a peak sensitivity at 565 nm; and the long wavelength band records wavelengths between 620 and 720 nm, with a peak sensitivity at 640 nm. Figuiero and Rea (2010, 2016] used a portable instrument worn on the head and with two sensors placed near the plane of the cornea [Bierman et al. 2005). One of the sensors measured illuminance according to the CIE photopic luminous efficiency function and the other sensor measured light radiation in relation to a sensitivity function with a peak in the short wavelength part of the spectrum. The parameter circadian light was then calculated by postprocessing of the two types of light radiation data. In a Swiss study, Hubalek et al. (2010) used an instrument measuring both illuminance with respect to the spectral sensitivity function for photopic vision and irradiance according to a spectral sensitivity function for non-image-forming effects of light. The instrument was worn on the head with the two sensors attached to spectacle frames.