L2 Flashcards
(27 cards)
What are the two main groups of optical aberrations?
Chromatic aberrations (material) and monochromatic aberrations
List the seven aberrations
- Longitudinal chromatic aberration
- Transverse chromatic aberration
- Spherical aberration
- Coma
- Oblique astigmatism
- Curvature of field
- Distortion
What is an aberration in an optical system
Aberrations are light rays from the object that do not form a perfect image in the image plane, causing image imperfections.
Why is it important to minimize aberrations in ophthalmic lens design?
Minimizing aberrations improves image quality and visual performance for the lens wearer.
What is chromatic aberration?
Chromatic aberration occurs because different wavelengths of light are refracted by different amounts, causing different focal points for different colors.
What is longitudinal chromatic aberration?
It is the difference between the focal points for blue and red light along the optical axis of a lens.
How does longitudinal chromatic aberration differ for converging and diverging lenses?
For converging lenses, the blue focus is closer to the lens than the red; for diverging lenses, the virtual blue focal point is closer to the lens than the virtual red focal point.
What determines the level of chromatic aberration in a lens?
The lens material’s dispersive ability (measured by the Abbe number) and the lens power.
How is longitudinal chromatic aberration used clinically?
It is used to assess refractive error in the duochrome test.
What is transverse chromatic aberration?
It occurs when rays from an off-axis point are refracted differently by wavelength, resulting in images of different sizes or positions for different colors.
What is linear transverse chromatic aberration?
The difference in image heights for red and blue light formed by a lens.
What is angular transverse chromatic aberration?
The difference in the angles of refraction between red and blue rays.
What is spherical aberration?
A blurring effect where peripheral rays are focused at a different position than paraxial rays, preventing a single point focus.
How can spherical aberration be corrected?
By changing the lens form, such as using an aspheric lens that becomes flatter in the periphery.
What is the difference between positive and negative spherical aberration?
Positive: paraxial focus is to the right of peripheral focus; Negative: peripheral focus is to the right of paraxial focus.
What is coma?
An off-axis aberration where the image becomes asymmetric and comet-shaped, reducing overall image quality.
What causes coma?
It is produced by oblique light rays and is affected by the lens form.
How does coma differ in the tangential and sagittal planes?
Coma is greatest in the tangential plane (plane with optic axis and chief ray) and least in the sagittal plane (90 degrees to tangential).
What is oblique astigmatism?
An aberration where parallel rays entering a lens obliquely do not form a point image but an astigmatic image.
What are tangential and sagittal foci in oblique astigmatism?
Tangential focus is in the plane containing the optic axis and chief ray; sagittal focus is in the plane perpendicular to the tangential plane.
What is curvature of field?
An aberration where the image is formed on a curved surface (Petzval surface) instead of a plane, due to lens curvature.
Why is curvature of field less noticeable in the human eye?
Because the retina is also curved, compensating for the image curvature.
What is distortion in lens optics?
The inability of a lens to create a rectilinear image, affecting the image’s shape but not its sharpness or color.
What are the two main types of distortion?
Pincushion distortion (aperture behind a positive lens) and barrel distortion (aperture in front of a positive lens).
