Anatomy And Physiology

Anatomy Physiology



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The human eye is an interesting optical system a sphere roughly 24mm in diameter held in place by the sclera a white fibrous shell (Ackerman 1962). The anterior portion of the eye consists of first the cornea which, with a refractive index of n≈1.376, contributes to a significant portion of the eyes focussing power, it is also specially shaped to reduce spherical aberrations (Hecht 1998). Light passing through the cornea enters the aqueous humour, a water based fluid with a similar refractive index (n≈1.336) to that of the cornea which provides the anterior segment with nutrition and maintains the intraocular pressure (Hecht 1998)

Contained within the aqueous humour is the iris which acts as a diaphragm allowing light into the eye through the pupil. Using muscles attached to the iris the pupil can be varied between 2-8mm in diameter, (Hecht 1998) by blocking peripheral light from entering the eye the iris increases image sharpness and reduces chromatic and spherical aberrations (Beynon 1985). At its smallest size the iris allows the eye to act as a diffraction limited system operating near Rayleigh’s criterion. A remarkable feature of the eye is that its theoretical upper limit for best possible ocular resolution (at Rayleigh’s criterion) is only a factor of two or three better than someone with unimpaired 20/20 vision (Ackerman 1962). 

Behind the iris lies the crystalline eye lens roughly 9mm in diameter and 4mm thick (Hecht 1998), it is an example of gradient index optics with the refractive index decreasing toward the centre. This gradient change of n from roughly 1.42 at the edge to 1.37 in the centre causes light to continuously be refracted, focussing it and reducing spherical aberrations (Beynon 1985). The lens is held in shape by a ring of muscles called ciliary muscles which can contract and change the shape of the lens causing accommodation, the process which controls the eyes focus from close to infinity. A person without a lens can still see from the broad focus already applied at the cornea, but it is the fine tuning of the focus provided by the accommodation within the lens that permits high visual acuity (Ackerman 1962). 

The interior surface of the eye, the fundus, includes many prominent features which the aim of this study is of course to visualise. The chamber within the eye contains vitreous humour a clear gel with a refractive index n≈1.337 and provides support to the eyeball. The retina covers the majority of the top layer of the fundus and is where 108 visual receptors, rod and cone cells are found (Hecht 1998). The fovea is a spot directly aligned with the visual axis and provides sharp focus for tasks such as reading, the foveal centre is about 1.5mm wide and densely packed with cones the visual receptors responsible for sharp and colour vision. Whilst the fovea has the highest concentration of cones it only contains about 1% of the total (D.A. and Smith G 2000). The non-uniformly distributed rods are more sensitive and provide wide peripheral and low light vision (Ackerman 1962). Rods and cones are only receptive to a tiny portion of the electromagnetic spectrum from 390nm to 750nm, consequently we require technology to visualise any other radiation such as Infrared, UV, X-ray. Other notable features include the choroid layer which contains the blood vessels and a pigment which absorbs light to prevent it being reflected and backscattered within the eye and the optic disk, where the optic nerve head joins the retina and contains no photoreceptors leading to a blind spot within the eye which is also highly reflective appearing bright on images. 

References:

Ackerman, Eugene. Biophysical Science. Englewood Cliffs, N. J.: Prentice-Hall, 27-51, 1962.

Beynon, J. “Visual Acuity and the Eye.” Physics Education 20 (1985): 234-237.

D.A., Atchison, and Smith G. Optics of the Human Eye. Edinburgh: Butterworth-Heinemann, 2000.

Hecht, Eugene. Optics, Third Edition. Vol.  Reading, Mass: Addison-Wesley Pub. Co, 118-121, 320-323, 346-349, 1998.

This is an extract adapted from my own masters project “Preliminary Investigation into Non-Mydriatic Fundus Imaging using a Compact Digital Camera”.

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