VISUAL OPTICS By.

Kausar AliStudent at

Doctor of optometry

* Introduction.

*In this course, the optical principles behind the working of the eye and visual system will be explained .*Further more descriptions of different devices used to measure the properties and functioning of the eye will be described . *Goals are a general understanding of how the visual system works and exposure to an array of different optical instrument .

Emmetropia *Emmetropia is a state in which the eye is relaxed and focused on an object more than 6 meters or 200 feet away . The light rays coming from the object are essentially parallel and the rays are focused on the retina without effort can emmetropic eye does not corrective lenses . *Ammetropic . *An abnormal refractive condition (as myopia, hyperopia, or astigmatism ) of the eye in which images fail to focus upon the retina .

Types of Ametropia. *The word ‘’ametropia’’ can be used interchangeable with refractive error’’ Types of ametropia include myopia hyperopia and astigmatism . They are frequently categorized spherical errors and cylindrical errors . *Spherical error: *Spherical error occur when the optical power of the eye is either too large or two small to focus light on the retina . *Myopia:*When optics are too power full for the length of eye ball one has myopia or nearsightedness . This can arise from a cornea or crystalline lens with too much

*Curvature (refractive myopia ) or an eye ball that is too long (axial myopia) Myopia can easily be corrected with a concave lens which causes the divergence of light rays before they reach the cornea . *Hyperopia: *When the optics are too weak for the length of the eyeball, one has hyperopia of farsightedness . This can arise from the cornea or crystalline lens with no enough curvature ( refractive hyperopia ) or an

eyeball that is too short ( axial hyperopia ). This can be corrected with convex lenses which cause light rays to converge prior to hitting the cornea . *Cylindrical error: *Cylindrical error causes astigmatism when the optical power of the eye is too powerful or too weak across one meridian and different in other meridian, such as if the corneal curvature tends towards a cylindrical shape . The angle between that meridian and the horizontal is know as the

axis of the cylinder . *Presbyopia: *When the flexibility of the lens declines typically due to age . The individual would experience difficulty in near vision often relived by reading glasses , bifocal or progressive lenses .*Astigmatism: *astigmatism is an optical defect in which vision is blurred due to inability of the optics of the eye to focus a point object into a sharp focused image

on the retina . This may be due to an irregular or toric curvature of the cornea of lens.*Types of Astigmatism : *Regular Astigmatism : principal meridian are perpendicular . With the rule Astigmatism : The vertical meridian is steepest ( A rugby ball or American football lying on its side) Against the rule Astigmatism :

The horizontal meridian is steepest ( A rugby ball or American football standing on its end ) Oblique Astigmatism : The steepest cure lies in between 120 and 150 degrees and 30 and 60 . *Irregular Astigmatism : principal meridian are not perpendicular .

*In with the rule astigmatism , a minus cylinder is placed in the horizontal axis to correct the refractive error ( or a plus cylinder in the vertical axis . Adding a minus

Cylinder in the horizontal axis make the horizontal axis steeper. Which makes both the axes equally steep.In against the rule astigmatism a plus cylinder is added in the horizontal axis ( or a minus cylinder in the vertical axis ) *Classification based on the focus of the principal meridian . With accommodation relaxed *Simple Astigmatism . *Simple hyperopic Astigmatism : first focal line on the retina while second is located behind the retina .

*Simple myopic Astigmatism : first focal line is in front of retina while the second is on retina . *Compound Astigmatism : compound hyperopic astigmatism both focal lines are located behind the retina . Compound myopic astigmatism both focal lines located in front of the retina .*Mixed Astigmatism : focal lines are on both sides of the retina .