The human eye

The behaviour o f the eye as an optical instrument was first established by Descartes who also explained how its mechanisms obey the laws o f physics and optics. In general, the characteristics o f optical systems are determined from their structure and the physical theory that is used in forming a visual image. In the case o f the human eye, the first is defined from the anatomy and physiology o f the organ while the second is based on the lens theory o f geometrical optics.

3.1.1 Anatomy and physiology

The human eyes are protected by a pair of bony cavities in the skull, called the orbits. Each eye is supported by six extraocular muscles which allow a range o f movement o f 50° to either the left or right o f the resting position and 40° above and 60° below the straight ahead position. The first optical surface of the eye is the cornea; a transparent bulge with no blood supply that obtains its nutrients from the aqueous humour that fills the anterior chamber lying behind it (see Figure 3.2). The latter leads onto the iris, an extremely delicate membrane with a circular opening at the centre, which is called the pupil. The iris dilates or contracts, depending on the amount o f light entering the pupil. Behind the iris lies the crystalline lens, which adjusts the focal length of the eye to bring images into focus on the back face o f the eyeball, the retina. The lens and retina are separated by a large cavity called the posterior chamber, which is filled with vitreous humour. The latter acts as heat absorbent. The retina is a photosensitive assembly of nerve cells, blood vessels and connective tissue that converts electromagnetic radiation into nerve impulses*.

The retina comprises o f ten layers, as shown in Figure 3.3b. The second layer close to the posterior chamber contains the optic nerve fibre, where the spatio-electrical impulses are generated. In-between layers convert electromagnetic radiation to action potentials. The ninth layer is the photosensitive one while the last layer is the pigment layer.

Hu m a n Vi s i o na n d Vi s u a l Di s p l a y s Cornea Posterior Chamber Anterior Lens Chamber Optic Nerve Retina

Figure 3.2: Human eye anatomy.

Light rays are initially refracted at the convex comeal surface. The aqueous humour has almost identical refractive index with the cornea; hence, the rays do not bend much when inside the anterior chamber (Figure 3.3a). When there is excess illumination in the chamber (photopic vision) the iris contracts. The opposite effect occurs when insufficient amount of light {scotopic vision) enters the pupil. The rays are further converged by the crystalline lens before an inverted image of the external world is formed onto the photosensitive layer of the retina. The layer is packed with two groups of photosensitive receptors, rods and cones. Rods are more sensitive to light than cones, but the latter can distinguish different wavelengths of light. Rods are unevenly distributed in the retina. They are absent from the fovea, where the main concentration of cones is found.

Figure 3.3: The human eye: (a) a positive, double lens arrangement that casts a real image on a light-sensitive surface, (b) schematic presentation of the retina layers in a region near the fovea.

Hu m a n Vi s i o n a n d Vi s u a l Di s p l a y s

3.1.2 The visual image

The creation of a visual image in the eye depends on the physiological characteristics mentioned above. Incoming rays are affected by two specific mechanisms in the iris and lens: adaptation and accommodation. Adaptation involves the mechanical opening and closing of the iris that controls the intensity of light entering the pupil. Accommodation is the process by which the lens changes its curvature in order to bring a laminated distant object into focus at the retina. At rest^, the objective focal length of the lens is six metres (see Figure 3.4).

optic axis

visual line

6 m

Figure 3.4: Objective focal length of the resting human eye.

The photo-sensing effect has three perceptual characteristics: brightness, hue and

saturation. The perception of brightness is directly proportional to the intensity of light entering the pupil. However, rays with the same intensity but with different wavelength do not always have the same brightness. Hue is the attribute that distinguishes the different colours of light. It is described by the tristimulus model of colour vision, in which there are three types of photoreceptors, red, green and blue (RGB), with differing spectral sensitivities (see Figure 3.5) [18]. Colours are normally described as mixtures of RGB wavelengths. When two rays with the same spectral composition are observed, they will appear to have the same hue. Nevertheless, it is possible for two or more rays with different spectral compositions to be perceived as having the same hue*^. Saturation describes the “whiteness” of a ray. The less saturated is a colour; the more

^ T he value here assum es that the pupil opening is five mm. Pupil size varies from 2-8 mm, depending on the rays’ intensity.

Such colours are called metamers.

Hu m a n Vis io na n d Vis u a l Dis p l a y s

whitish it appears to be. The degree o f saturation is also known as the colour

chrominance. G (520 nm) R (575 nm) Sensitivity B (430 nm) Wavelength (nm)

Figure 3.5: Human eye response to radiation of different wavelengths.

In photopic vision, visual sensitivity generates further perceptual characteristics: contrast, flicker, and resolution. The ability o f the human eye to discriminate luminance differences in the adjoining fields o f photoreceptors depends on the average intensity of the light rays, their size, and wavelength. This contrast discrimination is not linear; any noticeable difference is a function o f the light intensity. Under normal photopic vision, contrast can be seen in 2% luminance differences, known as the Weber fraction. When the luminance o f a scene fluctuates periodically several times in a second an annoying sensation is perceived, known as flicker. As the frequency o f fluctuation increases, sequential scenes appear disjointed, as frames. The irritation reaches a maximum at around 10 frames per second, but fades out as the frequency rises further. The human eye can detect frames only within the boundaries o f photopic vision. The ability to perceive two individually discernible lightwaves within a frame defines the eye’s limit of spatial resolution.

The study of the human eye as an optical system is necessary in the development o f television and video technologies. Knowledge o f the physiology and psychophysiology helps in understanding the design parameters, as well as the limitations, o f video.