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Story of the Human Brain
Published in Junichi Takeno, Self-Aware Robots, 2022
An image entering through the lenses of the eyes is inverted when reaching the retinas. There are two types of photoreceptor cells in the retina: cones and rods. The cone cells detect intensity and the rod cells color information. The fovea in the retina is where photoreceptors are most densely concentrated. Signals from the photoreceptors, already image-processed on the retina to some extent, leave the eyes and go to the brain via the part called the blind spot and through a bundle of neural pathways. The blind spot consists of a dense bundle of neural pathways and there are no photoreceptors. The blind spot is therefore “invisible,” or cannot respond to light stimulation. It is very interesting to note that the photoreceptors are viewing light that has penetrated the layers of nerve fiber bundles (from the back side of the nerve cables) since the photoreceptors are located at the deepest part or bottom of the retina (Fig. 3.5).
Lighting
Published in Sue Reed, Dino Pisaniello, Geza Benke, Kerrie Burton, Principles of Occupational Health & Hygiene, 2020
The retina contains two sets of photoreceptor cells, the rods and the cones, so called because of their shape. The cones are concerned with colour vision and fine detail (photopic vision); the rods are much more sensitive to low levels of illumination than the cones, but have less visual acuity. The human eye has three types of cones to sense light in three respective bands of colour. The biological pigments of the cones have maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green) and 564 nm (yellowish-green). Their sensitivity ranges overlap to provide vision throughout the visible spectrum, with maximum efficacy at 555 nm (yellow-green). The part of the retina termed the fovea is a region where only cones are found, and these are particularly densely packed. This is the region of the highest visual acuity or most detailed vision—about 40 times as sharp as that at the retinal border. The photoreceptor cells convert light energy into nerve impulses that travel via the optic nerve to the visual area of the brain where the image is formed.
Computer and Human Vision Systems
Published in Sheila Anand, L. Priya, A Guide for Machine Vision in Quality Control, 2019
The rods and cones are elongated retinal cells that collect the light that hits the retina. Rod photoreceptors work well in low light, provide black-and-white vision, and detect movements. Cones are responsible for color vision and work best in medium and bright light. There are three types of color-sensitive cones in the retina of the human eye, corresponding roughly to red, green, and blue detectors. The different colors are produced by various combinations of these three types of cones and their photopigments. White light is produced if the three types of cones are stimulated equally. Rods are located throughout the retina, while cones are concentrated in a small central area of the retina called the macula. The macula is yellow in color and absorbs excess blue and ultraviolet light that enters the eye. The macula is responsible for high resolution and therefore gives the eye the capability to discern details. At the center of the macula is a small depression called the fovea, which has the largest concentration of cones. The fovea is responsible for maximum visual activity and color vision. Photoreceptor cells take light focused by the cornea and lens and convert it into chemical and nervous signals which are transported to visual centers in the brain by way of the optic nerve.
Colour forecasting
Published in Textile Progress, 2019
The light sensitive area of the retina in the eye is composed two types of photoreceptor cell, rods and cones. The rods respond to low levels of illumination but do not enable colour vision. It is the cones, which respond to high levels of illumination which enable us to perceive colour. Because there are three types of cone: S, M and L. The shorter colour wavelengths we see (blues/violets) are detected by the S cone type, long wavelengths (reds) are detected by cone type L and cone type M is sensitive to the mid-range wavelengths (greens). With overlap of sensations between the three cone types, humans also perceive the full range of intermediary colours with which we are familiar, and this is known as trichromacy [269]. If one of the cone types is faulty certain wavelengths cannot be detected, and this will affect the way an individual will perceive colour. For example, if cone type S is deficient blue-violet colours are affected, if cone type L is deficient red colours are affected, and if the M cones are deficient green colours are affected. Around 8% of males (around the globe) are believed to have a deficiency of colour. Faulty or missing M cones are most common affecting the experience of reds and greens. Defective L cones can also impact on an individuals’ ability to see red and green colours. In more-rare cases, the S cones are faulty or missing affecting the experience of blues and yellows. Colour deficiency is often incorrectly referred to as colour blindness. Colour blindness only occurs when two cone types are faulty or missing which is extremely rare. Colour experts, often with the exception of forecasters and fashion designers, are commonly tested for colour deficiency (as this negative impact on dyeing and colour matching) and most often the Ishihara test is used [270]. It involves a system of 38 plates wherein on each plate a number or a randomly formed shape appears as a series of coloured dots embedded within a further series of background dots [271]. An example of one of the plates is shown in Figure 7 showing the number 74.