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Physiology of the eye

The eye as an optical system

When describing human perception, it is inadequate to portray the eye as an optical
system. The process of perception is not a matter of how an image of our environment is transferred to the retina, but how the image is interpreted and how we differentiate between objects with constant properties in a changing environment.

Optical system

Optical system

Spherical aberration. Projected images are distorted due to the curvature of the retina.

Optical system

Chromatic aberration. Images are blurred due to the various degrees of refraction of spectral colours.

Eye and camera

The process of perception is frequently explained by comparing the eye with a camera. In the case of the camera, an adjustable system of lenses projects the reversed image of an object onto a film. The amount of light is controlled by a diaphragm. After developing the film and reversing the image during the enlarging process, a visible, two-dimensional image of the object becomes apparent. Similarly, in the eye, a reversed image is projected onto the retina of the eye via a deformable lens. The iris takes on the function of the diaphragm, the light-sensitive retina the role of the film. The image is then transported via the optic nerve from the retina to the brain, where it is adapted in the visual cortex and made available to the conscious mind.
In regard to the eye, however, there are considerable differences between what is actually perceived and the image on the retina. The image is spatially distorted through its projection onto the curved surface of the retina. Through chromatic aberration - light of various wavelengths is refracted to varying degrees, which produces coloured rings around the objects viewed. These defects, however, are eliminated when the image is being processed in the brain.

Optical system

Perceptual constancy: perception of a shape in spite of the fact that the image on the retina is changing with the changing perspective.

Perspective

If we perceive objects that are arranged within a
space, the perspectives of the images produced on the retina are distorted. A square perceived at an angle, for example, will produce a trapezoidal image on the retina. This image may, however, also have been produced by a trapezoidal surface viewed front on. The only thing that is perceived is one single shape – the square that this image has actually produced. This perception of a square shape remains consistent, even if the viewer or object move, although the shape of the image projected on the retina is constantly changing due to the changing perspective.

Receptors

Receptors

Receptors

There are two different types of receptor: the rods and the cones, which are not distributed evenly over the retina. At one point, the so-called “blind spot”, there are no receptors at all, as this is the point at which optic nerves enter the retina.

Receptors

Number N of rods and cones on the retina in relation to the angle of sight.

Receptor density

An area of the retina called the fovea is the focal point of the lens. In this area, the concentration of the cones is greatest, whereas the density of the cones reduces rapidly outwards to the periphery. Here we find the greatest concentration of rods, which do not exist in the fovea.

Receptors

Relative spectral luminous efficiency of rods V and cones V’ in relation to the wavelength.

Rods

The older of these two systems, from an evolutionary point of view, is the one consisting of rods. The special attributes_3040 of this system include high light-sensitivity and a great capacity for perceiving movement over the entire field of vision. On the other hand, rods do not allow us to perceive colour; contours are not sharp and it is not possible to concentrate on objects, i.e. to study items clearly even if they are in the centre of our field of vision. The rod system is extremely sensitive and is activated when the illuminance level is less than 1 lux. Our night vision features, particularly the fact that colour is not evident, contours are blurred and poorly lit items in our peripheral field of vision are more visible – can be explained by the properties of the rod system.

Receptors

Spectral colour sensitivity of the cones in relation to the wavelength.

Cones

The cones form a system with very different properties. This is a system which we require to see things under higher luminous intensities, i.e. under daylight or electric light. The cone system has lower light-sensitivity and is concentrated in the central area in and around the fovea. It allows us to see colours and sharper contours of the objects on which we focus, i.e. whose image falls in the fovea area. In contrast to rod vision, we do not perceive the entire field of vision uniformly; the main area of perception is in the
central area. The peripheral field of vision is also significant, if interesting phenomena are perceived in that area; in that case our attention is automatically drawn to these points. This is then received as an image on the fovea to be examined more closely. Apart from noticing sudden movement, striking colours and patterns, the main reason for us to change our direction of view is the presence of high luminances - our eyes and attention are attracted by bright light.

Adaptation

Adaptation

Typical illuminances E and luminances L under daylight and electric lighting.

Adaptation

Day and night

One of the most remarkable properties of the eye is its ability to adapt to different lighting conditions. We can perceive the world around us by moonlight or sunlight, although there is a difference of a factor of 100,000 in the illuminance. The extent of tasks the eye is capable of performing is extremely wide - a faintly glowing star in the night sky can be perceived, although it only produces an illuminance of 10-12 lux on the eye.

Adaptation

Luminance range L of rod vision (1), mesopic vision (2) and cone vision (3). Luminances (4) and preferred luminances (5) in interior spaces. Absolute threshold of vision (6) and threshold of absolute glare (7).

Luminance

This ability to adapt to the illuminance is only influenced to a very small extent by the pupil. Adaptation is performed to a large degree by the retina. The rod and cone system responds to different levels of light intensity. The rod system comes into effect in relation to night vision (scotopic vision), the cones allow us to see during the daytime (photopic vision) and both receptor
systems are activated in the transition times of dawn and dusk (mesopic vision).
Although vision is therefore possible over an extremely wide area of luminances, there are clearly strict limits with regard to contrast perception in each individual lighting situation. The reason for this lies in the fact that the eye cannot cover the entire range of possible luminances at one and the same time. The eye adapts to cover one narrow range in which differentiated perception is possible. Objects that possess too high a luminance for a particular level of adaptation cause glare, that is to say, they appear to be extremely bright. Objects of low luminance, on the other hand, appear to be too dark.

Adaptation

Adaptation time

Adapting from dark to light situations occurs relatively rapidly, whereas adapting from light to darkness requires a considerably longer time. A good example of this is how bright we find it outside having come out of a dark cinema auditorium during the daytime or the transitory
period of night blindness we experience when entering a very dark room. Both the fact that contrast in luminance can only be accommodated by the eye within a certain range and the fact that it takes time to adapt to a new level of lighting, or brightness, have an impact on lighting design. For that reason lighting design requires, for instance, the purposeful planning of different luminance levels within a space or deciding on the adaptation of lighting levels in adjacent spaces.

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