Schematic Eye

APPLYING KNOWLWDGE OF SCHEMATIC EYE IN CALCULATING REFRACTIVE POWER OF THE EYE

CHAPTER TWO

RELATED TASKS

• Define schematic eye
• Describe types of schematic eye
• Describe Gullstrand schematic eye number one
• Describe Gullstrand schematic eye number two
• Describe reduced eye

Demonstrate skills of calculating refractive power.

BASIC OPTICAL PRINCIPLES OF HUMAN EYE

The optics of the eye consist of fluid optical mediums and solid mediums.

Aqueous humour and vitreous humour constitute the fluid mediums, where as the cornea and crystalline lens form the solid mediums.

Practically the refraction of light in the eye takes place at the anterior cornea and the two surfaces of the crystalline lens.

Introduction to schematic eye

Schematic eye is a simplified eye model that is designed to replace the complex optics of the human eye and to facilitate the understanding of the optics of the eye.

• The schematic eye assumes that;
• The eye is homocentric( has a common optical axis)
• The refracting surfaces are spherical

The cornea and lens form the optical refracting elements.

Different types of schematic eyes are designed taking into account various parameters from a simple one to a complex one.

• The schematic eye include;
• Gullstrand schematic eye 1
• Gullstrand Emsley schematic eye 2

Reduced eye

Gullstrand schematic eye 1

This eye provides us with numerical value for the radii of curvature, indices of refraction , distance btn refracting surfaces and location of principal points, nodal points and focal points.

Also known as Gullstrand exact schematic eye , is a hyperopic eye ( about +1.00d) and consist of six refractive surfaces , four which associated with equivalent core lens.

The primary advantage is that all of the optical constants for the eye provide a very good approximation of the dimensions of the “average eye”.

Diagram gullstrand schematic eye 1

Key features of gullstrand exact schematic eye

• Has six refractive surfaces
• Equivalent core lens

Hyperopic ( +1.00)

• Accommodated and unaccommodated version
• Good approximation of the dimension of the average eye

The power of cornea is +43.00D and lens +19.11D.

Gullstrand schematic eye #2

Also known as the Simplified schematic eye, Several schematic models have been developed that consist of 3 refracting surfaces, the Gullstrand is one of them.

Here , the corneal is considered thin and it represent a single spherical refracting surface.

• The crystalline lens is assumed to have a homogenous index of refraction.

The simplified schematic eye is emmetropic.

Reduced schematic eye

Even the simplified eye is too cumbersome for the majority of clinical applications , so Listing and a number of other investigators reduced the optics of the eye to a single spherical refracting surface.

Since there is only one refracting surface, the first and second principal planes, points and nodal points merge to form only one principal plane , principal point and nodal point.

Numerical values of various parameter as follows;

• Refractive index= 1.333
• Dioptric strength= +60.00
• First focal point(f1)= -16.67 infront of the cornea
• Second focal point (f2)= +22.22mm behind the cornea
• Axial length of reduced eye =+22.22mm

Radius of curvature of cornea = 5.55mm

diagram

Clinical application of reduced eye

• Calculation of retinal image size
• Designing of ophthalmic instruments

Calculation of intraocular lens (IOL) power.