Physiologic Optics-Human Eyes

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Physiologic Optics

The eye is a system of compound lens consisting principally of the cornea and crystalline lens surfaces.

The axial length and the power of this lens system of the eye determine the refractive error of an eye.

To assist and explain the optical system of an eye, the conventional eye was developed to signal or to know the optical constants. Gull-strand developed the most popular schematic eye.

(It should be understood, however, that very few real eyes would duplicate the measurements noted in the schematic eye.)

The Schematic Eye

Cornea

According to Gull-strand, the human cornea has an index of refraction (IR) of 1.376.

The average radius of curvature of the central anterior surface is 7.7 mm, and the posterior surface is 6.8 mm.

The radius of curvature refers to the measurement from the centre of a circle to its edge; in the case of the cornea, an imaginary circle is created with a curve that is continuous with that of the area being measured.

cornea-physiologic optics
Fig: F= Focal length, P=Principle Plain, N=Nodal Point

The radius of curvature of the cornea translates into a refractive power of +48.83 D on the anterior surface and -5.88 D on the posterior surface.

The posterior surface has a negative value because light travels from a higher IR (the cornea’s IR is 1.376) to a lower IR (aqueous humor in the anterior chamber has a IR of 1.336).

Thus, the total refracting power of the cornea is +42.95 (or +48.83 – 5.88).

The normal cornea is not spherical in shape; it is aspherical.

This aspherical design helps reduce aberration (blurred or distorted image quality) since light passing through the periphery of a spherical lens is bent more than light travelling through the central portion of the same lens.

Pupil

The pupil is analogous to the aperture (diaphragm) of a camera lens. A larger depth of focus is achieved with a smaller aperture (pupil).

Depth of focus refers to the range that is in focus without changing the focus of the lens.

The “ideal” pupil size for good quality vision at any distance is between 2 and 5 mm.

A pupil that is smaller than 2 mm can cause diffraction (formation of light and dark fringes) to occur.

When a pupil expands beyond 5 mm, spherical aberration (blurred image quality) may be noticeable due to the physical optical qualities of the cornea (which tends to bend light more toward its periphery despite its aspheric design discussed earlier).

Crystalline Lens

The lens of the eye consists of two major sections: the cortex, or outer section, surrounds the core (often referred to as the nucleus), or inner section.

The IR of the cortex is 1.386, while the core’s IR is 1.406; the overall index is considered to be 1.42. When the lens is in its unaccommodating state (explained in a moment), its power is +19.11 D.

The maximum power of the lens is +33.06 D with full accommodation.

The anterior radius of curvature is 10.0 mm accommodated and 5.33 during maximum accommodation.

The radius of curvature of the posterior lens surface is 6.0 mm, decreasing to 5.33 mm during maximum accommodation. Accommodation increases the focusing power of the eye, usually to focus on a near target.

When the eye needs to focus on a near target, the ciliary muscle that surrounds the lens receives a signal to contract.

Fine fibres, known as zonules, connect the ciliary muscle to the lens.

When the contraction of ciliary muscle occurs, the tension on the zonules is decreased, and the lens becomes much powerful (optically).

Physiologic Optics Eye Muscles
Fig: Ciliary Muscles Contracted

A closer look will disclose that the front surface of the lens bulges in the centre, while the periphery cadaver less curved to limit spherical aberrance.

The pupil squeezes during accommodation to gain depth of field (the area that is in focus without altering the accommodation) and further trim down spherical aberration.

Unluckily, the ability to change the eye’s focus decrease with age.

The most common theory for this loss of focusing ability is that there is a continual decrease in elasticity of the lens.

This is likely caused by a change in chemical composition when older lens cells are compacted as new lens cells are constantly added throughout a human’s life.

For the majority of people, the gradual loss of accommodation becomes noticeable around the age of 45 years.

This natural ageing phenomenon is known as presbyopia and is treated with reading glasses or bifocals.

These plus-powered lenses help to bring the divergent light from a near object closer to parallel (simulating light originating from a distant object).

This decreases or eliminates the need for the eye to accommodate.

Axial Length

The axial length (measured from the anterior cornea to the macula) of an average emmetropic eye is 24.4 mm.

Assuming that the rest of the eye is average (the lens and cornea are of average power), approximately 3.0 D of refractive error will be induced for every 1.0 mm change in axial length.

If the eye grows larger than normal, myopia (near-sightedness) will be induced.

Hyperopia is created by an eye that is shorter than normal.

Refractive Errors

Objects that are 20 feet or more away emit light with essentially parallel rays. Ideally, these parallel light rays would naturally focus on the retina of the eye without the aid of any lenses or any accommodative effort. This model state is known as emmetropia.

Myopia

Near-sightedness or Myopia, is caused when an eye’s refractive power is very strong for its axial length.

This means that parallel light comes to a focus in front of the retina instead of on the retina, where a clear image would be formed.

Minus-powered spherical lenses are used to cause the parallel light to diverge and, thereby, focus on the retina.

Light from near objects naturally has divergent rays—the closer an object is, the more divergent the light rays emanating from it.

Therefore, the myopic person may simply position the object of interest at the appropriate (near) distance to obtain a clear focus without corrective lenses.

eyedrop

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Hyperopia

Hyperopia, or far-sightedness, is caused when an eye’s accommodated refractive power is insufficient for its axial length.

Parallel light is not focused adequately by the lens system of the eye and fails to come to a focus before it reaches the retina. (Theoretically, the light would come to a focus behind the eye.)

Plus-powered lenses, or the eye’s own accommodation, are used to cause light to focus on the retina. Hyperopia can be divided into three parts: absolute, manifest, and latent hyperopia.

Absolute hyperopia is the amount of hyperopia that cannot be overcome by accommodation; this is measured by the minimum amount of plus sphere that produces best vision. In other words, the patient would require at least this power of lens to see clearly.

Manifest hyperopia is represented by the maximum power of plus sphere lens that the patient would accept without compromising his or her vision.

Latent hyperopia can only be revealed through the use of cycloplegic eyedrops (used to paralyze accommodation); this is the amount of plus sphere measured above the manifest hyperopia.

Example: A hype rope who sees 20/40 uncorrected may require +1.00 sphere to see 20/20 (this means the patient has 1.00 D of absolute hyperopia); this same patient may accept up to +2.50 sphere while still maintaining 20/20 vision (therefore, there are 2.50 D of manifest hyperopia); after cycloplegic drops are used, the patient requires an additional +1.50 sphere to see 20/20, for a total of +4.00 (this represents +1.50 D of latent hyperopia, and 4.00 D of total hyperopia).

Astigmatism

Astigmatism, in most cases, is caused by an irregularly shaped cornea.

If there is regular corneal astigmatism, one meridian of the cornea is the flattest and perpendicular (ie, 90 degrees away) to this is the steepest meridian.

This means that light rays passing through the cornea do not focus at a point; rather, there are two focal points, and the image is not clear.

Regular astigmatism is entirely correctable with cylindrical lenses, which focus light in a linear fashion and can, thus, be aligned to match the meridian of the astigmatism.

Regular astigmatism can be named according to the location of the greatest refractive power of the eye. Astigmatism can be with-the-rule, against-the-rule, or oblique.

With-the-rule astigmatism has the greatest refractive power of the eye in the vertical meridian (ie, minus cylinder axis is at or near 180 degrees, or plus cylinder axis is at or near 90 degrees).

Against-the-rule astigmatism has the greatest refractive power in the horizontal meridian (ie, minus cylinder axis is at or near 90 degrees, and plus cylinder axis is at or near 180 degrees).

Oblique astigmatism indicates that the greatest refractive power is more than 15 degrees from the horizontal or vertical axis (ie, minus or plus cylinder axis is between 15 and 75 degrees or 105 and 165 degrees). Another way of classifying astigmatism is by the location of the focal lines within the eye.

These classifications are simple, compound, and mixed (Figure 1-25). Simple astigmatism indicates that one focal line is on the retina while the other is either in front of the retina (simple myopic astigmatism) or behind the retina (simple hyperopic astigmatism).

Compound astigmatism exists when both focal lines are located either in front of the retina (compound myopic astigmatism) or behind the retina (compound hyperopic astigmatism). When one focal line is in front of the retina and one is behind, mixed astigmatism is present.

astigmatism is where the flat and steep meridians are not 90 degrees away from each other; this is common in eyes that have suffered trauma. This type of astigmatism cannot be fully corrected with cylindrical lenses. The crystalline lens may also have astigmatism (lenticular astigmatism), which may add to or subtract from the astigmatism in the cornea.

If the axis of the lenticular astigmatism is similar to that of the corneal astigmatism, the total astigmatism would be the sum of the lenticular and corneal astigmatism. Conversely, if the axis of the lenticular astigmatism is perpendicular to that of the cornea, the difference between the two would make up the total astigmatism.

Measuring the lenticular astigmatism is done by comparing the corneal astigmatism (from keratometry values) to the refractive amount (the total cylindrical value that provides the sharpest vision).

If the refractive and corneal values match, there is no lenticular astigmatism. If the refractive amount is higher than the keratometry reading indicates, lenticular astigmatism is present in the same axis. When the keratometry readings show a higher amount of astigmatism than that recorded with refractometry, it is assumed that there is lenticular astigmatism in the opposite axis (90 degrees away).

Retinal Image Size

The size of the image on the retina is typically the same in each eye. When wearing spectacles (and to a lesser degree, contact lenses), myopes will see a smaller image than an emmetrope. Conversely, a hyperope tends to see a larger image.

This variation in image size rarely poses a problem unless there is a difference in refractive error between the two eyes (an-iso-metropia).

Small amounts of anisometropia are usually well tolerated. However, when the difference exceeds 3.00 D, there is the potential for a noticeable difference in image size (aniseikonia). Contact lenses are often used in such cases, since the lens is in contact with the eye and there is minimal effect on retinal image size.

If contact lenses cannot be worn, the power of one (or both) of the spectacle lenses may have to be altered to lessen the aniseikonic.

According to Knapp’s rule, no aniseikonic will result if the refractive error is due only to axial length and corrective lenses are positioned 15 mm in front of the eye, regardless of the refractive error.

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