ASSESSMENT OF VISUAL FUNCTION.
The process of visual perception requires (i) accurate focusing of light by the optical
system of the eye, (ii) photo-chemical processes in the retina which lead to nervous
impulses, and (iii) central interpretation of this information which is correlated with past
experience by the brain. In this experiment attention will be confined to investigation of
the optical system.
THE OPTICAL SYSTEM OF THE EYE
In the normal relaxed eye parallel rays of light (i.e. from object at infinity ) are brought
exactly to focus on the retina. The far point is therefore an infinity. About two-thirds of
the refraction occurs at the corneal/air interface, and one-third at the lens (i.e. 42
diopters by the cornea, 23 diopters by the lens - see below).
In order to focus light on near objects, the total refractive power of the eye must
be increased, which is done by increasing the aveture of the lens by the muscles
of accommodation. The strength of the lens is usually defined in diopters.
Strength in diopters = 100
/Focal length in cm
convex (converging) lenses are assigned the prefix positive, and concave negative.
Thus a diopter diverging lens is called -2D.
In a young person during accommodation the lens should be able to increase its power
by about 10 diopters; substitution in the formula above will therefore give a near point
of about 10 cm. From childhood onwards the lens becomes progressively less
malleable and hence the power of accommodation also decreases. In childhood the
power of accommodation is about 14 diopters, and the near point is 7 cm; at 20 years
the near point is about 10 cm, and by about 45 it has receded to 25 cm, which means a
power of accommodation of only 4D. By 60 only about 1 dioptres of accommodation
remains.
Defects of the Refractory System
1. Myopia (short sight). This common refractive error is due to a relatively too
strong tidal system for the length of the eye. Rays of light are brought into
focus in front of the retina and a blurred image results even when the lens is
relaxed completely. Thus, although near objects are seen clearly, beyond a
certain distance everything is blurred. Unfortunately, even a minor degree of
myopia has relatively severe consequences: for example a myopia of ID
means that the unaccommodated eye will focus objects at 1 metre distant
(nearer by accommodation ) but objects beyond this will be increasingly
blurred as the
, whole range of distances from infinity to 1 metre represents only 1
dioptre. (Apply appropriate figures to lens formula above).
2. Hypermetropia (long sight). Here the eye is too short for the optical system
and even with the eye fully accommodated the rays from near objects are
brought into focus behind the near point, therefore, is further away than
normal. Some accommodation is required even for distant objects.
3. Presbyopia ('eyesight of old age'). This is the state that is reached when the
near point has receded beyond the normal reading or working range at about
30 cm, due to lack of accommodative power (see above).
4. Astigmatism. This is due to abnormalities of the curvature of the cornea or
to a small extent the lens; for instance, the focal length in the vertical and
horizontal planes may be different (i.e. there is a cylindrial component to the
system). Because of this inequality vertical and horizontal objects require
different degrees of accommodation to bring them into focus, and the patient
usually complain of blurring of vision at all distances.
5. Correction of defects. The only effective method is to place a lens in front of
the eye which will cancel out its defects. Myopia, therefore, requires a concave
lens, hypermetropia and presbyopia a convex one. Lenses which fit over the
cornea itself, contact lenses, are being increasingly used for the correction of
visual defects. The space between the lens and the cornea is filled with saline
so that any defects of corneal curvature are abolished. Corneal astigmatism is
treated by the use of cylindrical component in ordinary spectacles or
automatically by the use of contact lenses.
COLOUR VISION
(Note that the following account is a simplified version of the usually accepted
theories and is only intended as a basis for understanding the experiment).
The visible spectrum shows a number of characteristic colours each of which covers a
range of wavelengths (e.g. red, orange, yellow, etc). If two coloured lights are mixed,
the resulting sensation is identical to that produced by a single colour of intermediate
wavelength. This can be shown using a suitable instrument which one half of a screen
can be illuminated by light of a wavelength chosen by the experimentor and the other
half is controlled by the subject who has to match the two halves as accurately as
possible. It is found that, in normal people, any colour (including white) can be
matched by a suitable mixture of three primary spectral wavelengths. The three
primaries are chosen so that any one cannot be produced by other two; thus red, green
and blue are primaries (these are not pigments remember). The total intensity of
illumination is kept constant, but the subject can vary the relative contribution of each
light independently, thus the contribution of a single colour may be varied from zero
to its being the sole source of illumination.
The process of visual perception requires (i) accurate focusing of light by the optical
system of the eye, (ii) photo-chemical processes in the retina which lead to nervous
impulses, and (iii) central interpretation of this information which is correlated with past
experience by the brain. In this experiment attention will be confined to investigation of
the optical system.
THE OPTICAL SYSTEM OF THE EYE
In the normal relaxed eye parallel rays of light (i.e. from object at infinity ) are brought
exactly to focus on the retina. The far point is therefore an infinity. About two-thirds of
the refraction occurs at the corneal/air interface, and one-third at the lens (i.e. 42
diopters by the cornea, 23 diopters by the lens - see below).
In order to focus light on near objects, the total refractive power of the eye must
be increased, which is done by increasing the aveture of the lens by the muscles
of accommodation. The strength of the lens is usually defined in diopters.
Strength in diopters = 100
/Focal length in cm
convex (converging) lenses are assigned the prefix positive, and concave negative.
Thus a diopter diverging lens is called -2D.
In a young person during accommodation the lens should be able to increase its power
by about 10 diopters; substitution in the formula above will therefore give a near point
of about 10 cm. From childhood onwards the lens becomes progressively less
malleable and hence the power of accommodation also decreases. In childhood the
power of accommodation is about 14 diopters, and the near point is 7 cm; at 20 years
the near point is about 10 cm, and by about 45 it has receded to 25 cm, which means a
power of accommodation of only 4D. By 60 only about 1 dioptres of accommodation
remains.
Defects of the Refractory System
1. Myopia (short sight). This common refractive error is due to a relatively too
strong tidal system for the length of the eye. Rays of light are brought into
focus in front of the retina and a blurred image results even when the lens is
relaxed completely. Thus, although near objects are seen clearly, beyond a
certain distance everything is blurred. Unfortunately, even a minor degree of
myopia has relatively severe consequences: for example a myopia of ID
means that the unaccommodated eye will focus objects at 1 metre distant
(nearer by accommodation ) but objects beyond this will be increasingly
blurred as the
, whole range of distances from infinity to 1 metre represents only 1
dioptre. (Apply appropriate figures to lens formula above).
2. Hypermetropia (long sight). Here the eye is too short for the optical system
and even with the eye fully accommodated the rays from near objects are
brought into focus behind the near point, therefore, is further away than
normal. Some accommodation is required even for distant objects.
3. Presbyopia ('eyesight of old age'). This is the state that is reached when the
near point has receded beyond the normal reading or working range at about
30 cm, due to lack of accommodative power (see above).
4. Astigmatism. This is due to abnormalities of the curvature of the cornea or
to a small extent the lens; for instance, the focal length in the vertical and
horizontal planes may be different (i.e. there is a cylindrial component to the
system). Because of this inequality vertical and horizontal objects require
different degrees of accommodation to bring them into focus, and the patient
usually complain of blurring of vision at all distances.
5. Correction of defects. The only effective method is to place a lens in front of
the eye which will cancel out its defects. Myopia, therefore, requires a concave
lens, hypermetropia and presbyopia a convex one. Lenses which fit over the
cornea itself, contact lenses, are being increasingly used for the correction of
visual defects. The space between the lens and the cornea is filled with saline
so that any defects of corneal curvature are abolished. Corneal astigmatism is
treated by the use of cylindrical component in ordinary spectacles or
automatically by the use of contact lenses.
COLOUR VISION
(Note that the following account is a simplified version of the usually accepted
theories and is only intended as a basis for understanding the experiment).
The visible spectrum shows a number of characteristic colours each of which covers a
range of wavelengths (e.g. red, orange, yellow, etc). If two coloured lights are mixed,
the resulting sensation is identical to that produced by a single colour of intermediate
wavelength. This can be shown using a suitable instrument which one half of a screen
can be illuminated by light of a wavelength chosen by the experimentor and the other
half is controlled by the subject who has to match the two halves as accurately as
possible. It is found that, in normal people, any colour (including white) can be
matched by a suitable mixture of three primary spectral wavelengths. The three
primaries are chosen so that any one cannot be produced by other two; thus red, green
and blue are primaries (these are not pigments remember). The total intensity of
illumination is kept constant, but the subject can vary the relative contribution of each
light independently, thus the contribution of a single colour may be varied from zero
to its being the sole source of illumination.
