Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

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Fixation Axis

It is the line joining the fixation point with the centre of rotation of the eyeball (Fig. 7-5).

ANGLES

Angle Alpha (α)

It is the angle formed between the optic axis and the visual axis (Fig. 7-5).

Angle Kappa (κ)

It is the angle formed between the pupillary axis and the visual axis (Fig. 7-5).

Angle Gamma (γ )

It is the angle formed between the optic axis and the fixation axis (Fig. 7-5).

Fig. 7-5: Axes and angles of the eye. OA = Optic axis; α = Angle alpha;

VF = Visual axis; κ = Angle kappa; VC = Fixation axis; γ = Angle gamma and N = Nodal point

These angles are usually positive i.e., the visual axis is nasal to the optic axis. They approximately measure 5°.

OPTICAL ABERRATIONS OF THE EYE

Since the eyeball is not optically perfect, it shows some optical flaws which reduce resolution of the focussed images. They are called

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Fig. 7-8: Coma produced by rays passing obliquely through a lens

show different refractive powers in different meridians only when oblique rays fall upon them.

DISTORTION

Image distortion occurs due to increased prismatic effect of the periphery of the lens resulting in difference in lateral magnification at different points on an object. A classical example of distortion is a grid object distorted as either barrel or pincushion observed in concave lens and convex lens respectively (Fig. 7-9). Image distortion persists even after elimination of all other Seidel aberrations.

Fig. 7-9: Different types of distortion

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CURVATURE OF FIELD

Curvature of field is closely related to oblique/radial astigmatism and means that a plane object gives rise to a curved image. After optical elimination of spherical aberration, coma and radial astigmatism irrespective of parallel/oblique rays a point object will form a point image. These points will be seen to fall on a curved surface (Petzval surface) which phenomenon is called curvature of field (Fig. 7-10). Due to this curvature of field if an extended image is projected on a flat surface some of the points will not be in focus.

Fig. 7-10: Curvature of field

WAVEFRONT ABERRATION

Wavefrontaberrationisdefinedastheamountofdifferencebetween anidealwavefrontandtheactualwavefrontemanatingfromanoptical system. This wavefront aberration is measured by the method “aberrometry”. Clinical applications of aberrometry include:

•Corneal refractive surgery

•Intraocular lenses, etc.

CHROMATIC ABERRATION

White light consists of seven colours. The component colours are refracted differently at spherical surfaces and the image formed have a coloured edge, which is recognised as chromatic aberration. The short wavelength blue rays are refracted the most and come to a focus in front of the retina, i.e. the eye is myopic for blue rays

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(Fig. 7-11). Similarly, the red rays with longer wavelength are refracted the least and come to a focus behind the retina, i.e. the eye is hypermetropic for red rays (Fig. 7-11). The yellow light rays with medium wavelength will come to a focus in the retina, i.e. the eye is emmetropic for yellow rays (Fig. 7-11). However, in human eye visual acuity is surprisingly good although there should be considerable blurring due to uncorrected chromatic aberration. The reasons are:

•The xanthophyll pigments present in the macular area filter out the blue rays, the most offender in chromatic aberrations.

•Since luminous efficiency of the eye is brightest for the yellow rays (in the middle of the visible light spectrum) blue and red coloured rays appear significantly less bright to cause visual blur noticeably.

•Additionally, the lens even in younger subjects filters out UV- A rays.

•Blue rays are filtered out by the lens which turns yellow in nuclear sclerosis with advancing age.

Chromatic aberration is clinically applied in duochrome test (subjective refraction test). Chromatic aberration is corrected by using “achromatic lens”. This is made by combining a convex lens of crown glass of refractive index 1.523 with a concave lens of flint glass (RI = 1.7) of half dioptrical strength. The principle is to neutralise the chromatic aberration by combining the lenses that induce opposite chromatic aberrations.

Fig. 7-11: Chromatic aberration

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PURKINJE IMAGES

These are four in number (Table 7-1) and they are the images of a light reflected from the different refracting surfaces of the eye, described first by JE Purkinje, a Czech scientist.

Table 7-1: Features of Purkinje images

Usually 1st and 2nd images are easily seen. However, in a dark room or in a dilated pupil all the images will be better visualised. If the light source is located 1 m from the eye, 1st, 2nd and 4th images are located between 3.5 to 5 mm behind the anterior corneal surface, while the 3rd image is located 11 mm (approx.) from the anterior corneal surface (Fig. 7-12).

CLINICAL APPLICATION OF PURKINJE IMAGES:

•Presence or absence of lens

•Types of cataract

•Keratometry

•Hirschberg test—estimation of angle of squint.

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Fig. 7-12: Source and location of Purkinje images.

(light source is 1m from the eye)

RETINAL IMAGE

In a reduced eye, the rays passing through the nodal point (N) are not refracted and passes straight to the retina (Fig. 7-13). The retinal image (ab) is formed of the object AB.

Fig. 7-13: Retinal image formation.

AB = Object, ab = Image and N = Nodal point of the eye

The retinal image thus formed is inverted and minified. This image is psychologically again inverted at the cerebral cortex. The true size of the image is also estimated at the cerebral cortex.

C H A P T E R

ACCOMMODATION

Accommodation is defined as the phenomenon/process to focus near objects clearly on the retina by increasing the convergence power of the eye. This is achieved by increasing the refractivity of the crystalline lens through increasing the curvature of it’s anterior surface mainly. At rest, i.e. in unaccommodated state, the radius of curvature of the anterior surface is 10 mm and that of the posterior surface is 6 mm. In accommodation, the radius of curvature of the posterior surface remains almost unchanged. But in strong accommodation the radius of the curvature of the anterior surface becomes 6 mm, i.e. more convex and takes on a more hyperboloid form.

Fig. 8-1: Anatomical changes in the eyeball in cross section during accommodation (upper half) and at rest (lower half), N1 = Front nodal points, N2 = Back nodal points,

P1 = Front principal points and P2 = Back principal points