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Chapter 3 Physics of the Senses
Published in B H Brown, R H Smallwood, D C Barber, P V Lawford, D R Hose, Medical Physics and Biomedical Engineering, 2017
There are about 7 million cones. Their density is highest at the fovea centralis, and diminishes rapidly across the macula lutae: outside this region the density of cones is insufficient for useful function. Within the fovea there are no rods and the cones are closely packed together, at a density of up to 140 000 cones per mm2. The diameter of a single cone is about 2 µm. The peak sensitivity of the cones is to light with a wavelength of about 550 nm, in the yellow-green region. The sensitivity of the cones is low relative to that of the rods, perhaps by four orders of magnitude. However, within the fovea there is almost a one-to-one correspondence between cones and ganglions, and resolution is very high. The cones are responsible for our colour vision, although the actual mechanism by which colour is perceived is still unknown. It is seductive to assume that the cones function in a similar manner to the rods, and that there are chemical equivalents to the rhodopsin cycle occurring within the cones. Many of the phenomena associated with our colour vision can be explained by the existence of three types of cone each containing a different photopigment, responding optimally to light of one particular wavelength. Unfortunately we have yet to isolate any of them. Nevertheless the so-called trichromatic theories are most valuable in furthering the understanding of our perception of colour.
Lighting
Published in Sue Reed, Dino Pisaniello, Geza Benke, Principles of Occupational Health & Hygiene, 2020
Budnick, Lerman and Nicolich (1995) demonstrated that exposure to high levels of bright light (i.e. 6000 to 12,000 lux) on at least half of a worker’s night shifts over three months was effective in altering the worker’s circadian rhythm pacemakers. Additionally, the effectiveness of light in setting a diurnal rhythm is a function of the light’s wavelength (colour), with peak sensitivity between 460 nm and 484 nm in all vertebrates studied so far, including humans. Circadian photoreception is mediated primarily by melanopsin (a vitamin-A photopigment) contained in intrinsically photosensitive retinal ganglion cells (ipRGCs) distributed in a network across the inner retina (SCENIHR, 2012).
Perception of Objects in the World
Published in Robert W. Proctor, Van Zandt Trisha, Human Factors in Simple and Complex Systems, 2018
Robert W. Proctor, Van Zandt Trisha
As trichromatic theory predicted, there are three types of cones with distinct photopigments. Color information is coded by the cones in terms of the relative sensitivities of the pigments. For example, a light source of 500 nm will affect all three cone types, with the middle-wavelength cones being affected the most, the short-wavelength cones the least, and the long-wavelength cones an intermediate amount (see Figure 5.11). Because each color is signaled by the relative levels of activity in the three cone systems, any spectral color can be matched with a combination of three primary colors.
Validation of spectral simulation tools in the context of ipRGC-influenced light responses of building occupants
Published in Journal of Building Performance Simulation, 2023
Clotilde Pierson, Mariëlle P. J. Aarts, Marilyne Andersen
More specifically, the human retina contains a population of ipRGCs, which, in the presence of light, express the photopigment melanopsin (or ‘OPN4’) (Foster 2021). Through their own neural connections, the ipRGCs send light-induced signals to the suprachiasmatic nucleus (SCN)—the site of the central pacemaker driving circadian rhythms—and other areas in the brain—such as those implicated in the regulation of arousal (Cajochen 2007; Zhang et al. 2021). There seem to be two main pathways through which a signal sent by the ipRGCs induces IIL responses (Amundadottir 2016; Soto Magán 2021): the indirect pathway, through which light can shift the timing of our circadian phase, i.e. our body internal clock, and for which the effect is not immediate;the direct pathway, through which light can have more immediate effects on our alertness, melatonin level, or pupil size for instance.