Optical Systems 1

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Chapter One
Optical systems aberrations
The paraxial formulas developed earlier for image formation by spherical reflecting and refracting surfaces are, of course, only approximately correct. In deriving those equations, it was necessary to assume paraxial rays, that is, rays both near to the optical axis and making small angles with it. The departure from ideal, paraxial imaging may be described quantitatively in several ways. These departures are referred to as aberrations.
The word Aberration in optics refers to a defect in a lens such that light is not focused to a point, but is spread out over some region of space, and hence an image formed by a lens with aberration is blurred or distorted, with the nature of the distortion depending on the type of aberration. More specifically, it can be defined as a departure of the performance of an optical system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.


1- Monochromatic aberrations
Monochromatic aberrations are aberrations that occur in quasi-monochromatic light. These aberrations do not consider the effect that the frequency of light has on its propagation through a system. (Real light is never monochromatic - it always is made of a band of frequencies. However, when this band is narrow, the light is considered to be quasi-monochromatic.). These are aberrations typically related to the paraxial approximation, which assumes that all light is entering the system nearly parallel to each other and perpendicular to the lens. In real systems, this is not the case.
There are five primary monochromatic aberrations (also called Seidel aberrations), which are divided into two subgroups:
- The first subgroup includes aberrations that deteriorate the image:
1- Spherical aberration
2- Coma
3- Astigmatism

- The second subgroup includes aberrations that deform the image:
4- Field curvature (or Petzval field curvature)
5- Distortion