Laser Beam Engineering for AM Flashcards
What are lens aberrations?
The inability of a lens to form a perfect image is caused by lens aberrations
What is a spherical aberration?
Imaging error found when a spherical lens focuses on an axial bundle of monochromatic light rays.
Spherical lens are inherenetly imperfect but easier to manufacture
At the paraxial focus, all rays close to the lens axis are focused, whereas rays from zones farther from the axis are focused short of the parasail focus
Whats the relationship between ray distance from axis and focus error?
At the paraxial focus, all rays close to the axis of the lens are accurately focused, whereas rays from zones farther from the axis are focused short of paraxial focus.
The greater the rays are from the axis, the greater the focus error
During sphereical aberration, where is the image quality invariably at its best?
When the lens is focused to a minimum spot size, referred to as the ‘circle of least confusion’.
this is where the combined longitudinal and transverse components of the aberration are a minimum.
What is a coma aberration?
Aberration that afflicts off-axis bundles of light rays (non-paraxial)
Explain what happens to form a coma aberration and how can it be reduced?
Each annulus or zone focuses onto the image plane at a slightly different height and slightly different spot size.
Results in an overall image or spot that is comatic in shape, having a bright central core with a triangular shaped flare extending toward the optical axis of the lens.
Reduced by using symmetrical lens pairs (a relay lens system)
What is a astigmatism aberration?
Arises when a lens has a difference in curvature between the horizontal and vertical surfaces
Causes the ideal circular point image to be blurred into an elliptical patch
How does field curvature form?
Instead of a final image being formed on a flat surface, most optical systems tend to form on a curved surface.
The nominal curvature of the surface is referred to as Petzval, or field curvature, of the lens
What is distortion?
Affects the shape of the image not the quality.
Causes the image to depart from a true, scaled duplicate of the image.
Represents an angular deviation of the wavefront.
This spreads (barrel) or contracts (pincushion) the image, destroying the linear relation between dimensions in object and image.
What are the influnce of aperture stops?
Reduce spherical aberration
Amount of spherical aperture varies as the cube of lens aperture.
If large aperture is necessary, the aberration must either be tolerated or corrected, but image improvement can be achieved by restricting the aperture with a stop.
What are the influnce of correction plates?
removes an advance of wavefront (by the fourth power of the distance ray from the axis)caused by spherical aberration.
Introduce a correction plate that is very nearly planar but thickness varies slightly with radial distance so to provide a wave retardation, exactly compensating for the spherical wave aberration.
This is an example of the Schmidt system
