L13 Flashcards
(18 cards)
What are the three main types of schematic eyes?
Gullstrand’s exact schematic eye (6 refractive surfaces), simplified schematic eye (3 refractive surfaces), and reduced schematic eye (1 refractive surface).
What is Emsley’s reduced eye?
A schematic eye model with one refracting surface, one radius of curvature, one refractive index, and one principal point.
What are the typical parameters of Emsley’s reduced eye?
Radius of curvature: +5.5 mm; refractive index: 4/3 (~1.333, like water); power: +60 D; axial length: +22.22 mm.
What is the axial length in the reduced eye model also known as?
k’ (axial length); the dioptric axial length is known as K’.
In the reduced eye, what does k’ represent?
The axial length of the eye.
In the reduced eye, what does fe’ represent?
The focal length of the eye.
What is emmetropia in the context of the reduced eye?
A state where the axial length (k’) equals the focal length (fe’), so parallel rays focus on the retina without accommodation.
What is the far point (MR) in an emmetropic eye?
At infinity; parallel rays from infinity are focused on the retina.
What is myopia in the reduced eye model?
A condition where the axial length (k’) is greater than the focal length (fe’); the eye is too long, so parallel rays focus in front of the retina.
Where is the far point for a myopic eye?
A real point in front of the eye.
What is the purpose of a spectacle lens in myopia correction?
To create the incident vergence required so that the image is focused on the retina; the lens moves the far point to infinity.
What is hyperopia in the reduced eye model?
A condition where the axial length (k’) is less than the focal length (fe’); the eye is too short, so parallel rays focus behind the retina.
Where is the far point for a hyperopic eye?
A virtual point behind the eye.
What is the purpose of a spectacle lens in hyperopia correction?
To create the incident vergence required so that the image is focused on the retina; the lens moves the far point to infinity.
In the reduced eye model, what does the correcting spectacle lens do for both myopia and hyperopia?
It makes the second focal point of the spectacle lens and the far point of the eye coincident.
What does BVD stand for in spectacle correction?
Back vertex distance (the distance from the back surface of the spectacle lens to the corneal apex/reduced surface).
What is ametropia?
A general term for refractive error; a condition where the eye does not focus light on the retina when unaccommodated (includes myopia and hyperopia).
What is ocular refraction?
The incident vergence required for obtaining a sharp retinal image; it is a measure of ametropia.
