Physiology of the Human Eye

The human eye is not a perfect sphere but rather elliptically shaped with an average size of about 24.2 mm·^{23.8 mm}·24.5 mm transversal, sagittal and axial if measured from sclera, the white, protective outer layer of the eye, to sclera. While the exact reported size and shape of the human eye varies from person to person [12], the overall structure of the eye remains identical for most humans, with some exceptions, due to different diseases, that will not be discussed further. Figure 2.1 shows a cut through the human eye and its most important parts in order to visualize the basic setup and the most important parts of the human eye.

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10 fundamentals of visual perception

(a) (b)

Figure2.1– Anatomy of the human eye with regard to optical and visual axis. The optic disk or optic nerve is seen in both images. Figure(a)shows the schematic anatomy of the human eye. Light falls onto the retina by passing through the cornea, the pupil and the lens.(b)Shows the important difference between optical axis, which is setup through the pupil and the centre of the eyeball, and the visual axis, going from pupil to fovea.

Following figure 2.1a from right to left: the transparent cornea, covering the iris and the pupil and accounts for approximately ⁴/5 of the eyes optical refraction [13]; the iris, which forms the pupil; the lens, focusing objects onto the retina [14]; the sclera, the white of the eye, protecting the eye; muscles to move the eye; the optical disc where nerve endings exit the eye and connect to the brain [15] and of course the retina, consisting of photoreceptor cells, rods and cones, to detect light and therefore enabling humans to see. In total, there are four different types of cones of which only three are relevant for this work. The distribution of rods and cones on the retina is shown in figure2.2. Cones are concentrated in the middle of the retina, the fovea, the only area on the retina where humans can actually see objects sharply. And while it might appear, as if the middle of the pupil, the middle of the eyeball itself and the fovea align, this is not the case. The optical axis of the human eye is tilted by a few degrees away from the centre of the eyeball, as shown in figure2.1b. This misalignment varies similarly to the variation of the eye ball size from person to person.

The rods are located more to the outside. The part of the fovea, where the optical nerve moves through the retina and towards the brain is called blind-spot. Due to the special layering of the human retina, the optical nerve blocks any kind of photoreceptor cell. Therefore, the eye cannot perceive any information on this particular spot.

2.1 the human eye 11

(a) (b)

Figure2.2– The distribution of rods and cones on the human retina.(a)sets up a coordinate system in degrees relative to the fovea. These coordinates are repeated along the bottom of(b). The vertical brown bar near20degrees indicates the optical disc, the blind spot.[13]

As mentioned above, cones can be split into four more categories. Three of them are respon-sible for the perception of colour (S-, M-, L-Cones). Their wavelength sensitivity is shown in figure2.3.S-, M- and L-cones absorb light of short, medium and long wavelength, respectively.

As shown, they have their absorption maximum at about 420 nm, 534 nm and at 564 nm. Rods are sensitive from 400 nm to 600 nm and have their maximum absorption at 498 nm, com-pared to colour information delivered by cones, rods only supply information on brightness.

The fourth cone type, the intrinsically photosensitive Retinal Ganglion Cells (ipRGC), will not be discussed here since their function to suppress melatonin is not relevant for this thesis.

[16]

400 450 500 550 600 650 700 750

0 0.2 0.4 0.6 0.8 1

Wavelength in nm

NormalizedReceptorSensitivityina.u. L-Cones

M-Cones S-Cones Rods

Figure2.3– Wavelength sensitivity for rods (dashed black line) as well as for the S- (blue), M- (green) and L-Cones (red). [16]

Rods and cones do not only differ in their position on the retina and their ability to absorb light of different wavelengths but also in their absolute sensitivity under different brightness

12 fundamentals of visual perception

levels. When the illumination of the environment is changed from bright (photopic) to dark (scotopic), the human eye will adapts to the illumination. This process is shown in figure2.4, where the sensitivity curve for cones (green), rods (blue) and the combination of both (red) is shown over the time spent in a dark environment.

The terms photopic and scotopic are defined by the sensitivity of rods and cones. If only cones are active due to the light situation, this is called a photopic scene. This is valid for a luminance range of 5 cd m⁻² and above. From 0.005 cd m⁻² and lower only rods are active and this luminance range is called scotopic. The range in which both are active is called the mesopic range. [17]

Figure2.4– The red line shows the two-stage dark adaptation curve, with an initial cone branch and a later rod branch. The green line is the cone adaptation curve. The purple curve is the rod adaptation curve. Note that the downward movement of these curves represents an increase in sensitivity. The curves actually begin at the points indicating "light-adapted sensitivity," but there is a slight delay between the time the lights are turned off and when measurement of the curves begins. [13]

This complex sensitivity of the photoreceptor cells results in the unique way humans per-ceive light under different situations, leading to the V(λ)- and the V’(λ)-curve which define the overall sensitivity of the human eye in the photopic and the scotopic range. This will be explained in detail in section 2.3. Furthermore, the Weber-Fechner-Law can be derived from the adaptation curve shown in figure2.4. This law states, that an exponential increase in the stimulating intensity only leads to a linear increase in the perceived sensual stimulus.

After discussing the basic structure of the human eye and how colour perception works on a very basic level, eye movement will be discussed next.