Education Journalist | Study Abroad Lead
Aberration of Lens causes light to be spread over some region of space instead of being focused to a point. Lens takes light from a point on an object and focuses it onto an image's conjugate point. Under most circumstances, lens fails to perform this role due to a flaw in accuracy with which it focuses light. Optical designer's job is to make sure that blur circle is narrow enough to allow for appropriate resolution or image quality.
| Table of Content |
Key Terms: Lens Aberrations, Optical Design, Conjugate Point, Spherical Aberration, Chromatic Aberration, Astigmatism, Distortion
What is Optical Aberration?
[Click Here for Sample Questions]
Light from any point on an object would flow through the lens and converge at a single point on the picture plane in an ideal lens. Real lenses, unlike ideal lenses, do not focus light on a single point. An aberration of the lens is defined as a divergence from the idealized lens performance.
Optical aberration theory is highly complicated, involving several non-trivial geometrical factors as well as a significant amount of mathematics. Regrettably, numerous features, even basic ones, are sometimes presented in incorrect or even contradictory ways. For example, it is inconsistent to characterize aberrations solely in terms of not perfectly sharp pictures and departures from paraxial optics behavior at the same time.
Also Read:
| Related Articles | ||
|---|---|---|
| Uses Of Concave Mirror | Uses Of Convex Mirror | Snell’s law |
| Refractive Index | Total Internal Reflection | Refraction |
Types of Aberration of Lenses
[Click Here for Sample Questions]
Classification of Aberration of lenses is elaborated below:
Spherical Aberration
When a lens focuses on an axial bundle of monochromatic light, the image error known as spherical aberration occurs. Each zone or annulus of the lens aperture has a slightly varied focal length when spherical aberration is present.
The figure depicts the end product. The actual intersection of focussed rays with the picture surface is seen in the expanded view. All rays close to the lens's axis are precisely focused at the paraxial focus A. Rays from zones further out from the axis focus short of the paraxial focus. The larger the focus mistake, the further the rays are from the axis. Spherical aberration is the lack of a consistent focus for all zones of the lens.
Image: Spherical Aberration
Two causes of spherical aberration are:
- Low-quality Lens
- Large aperture Lens
Light rays traveling through the lens do not converge at a common point in this form of aberration. Spherical aberration in lenses is caused by two factors. Let's take a look at each one separately:
- The marginal lines are rays that come from the edge (or from afar). At a focal length of fm, these lines intersect towards the middle of the axis.
- Paraxial rays are those that are closer to the center. At various focus locations along the axis, these paraxial rays of various focal lengths generate pictures.
Chromatic Aberration
White light is made up of seven different hues, each with its focal length and wavelength. When this light passes through the lens, pictures of various focal lengths form at various points along the axis. As a result, the image of the object is blurred. Chromatic aberration is the name for this type of flaw. Chromatic aberrations are divided into two categories:
- Longitudinal: Axial chromatic aberration is also known as longitudinal chromatic aberration. Light beams of different focal lengths cannot be focused on the same focal plane by a lens.
- Lateral Chromatic Aberration (LCA): Color fringing is caused by lateral aberration, which is caused by picture magnification that varies with color wavelength. Lateral chromatic aberration is linked to secondary chromatic aberration. Simultaneous correction of blue, green, and red light rays is difficult due to this additional chromatic distortion.
Coma
The name "Coma" comes from theory that rays from an off-axis object point are photographed by multiple zones of lens, blurring a point picture into a comet appearance. Pictures of an object point that falls on a plane perpendicular to =optical axis are circular and overlaid around a common center in spherical aberration. Pictures of an off-axis object point in a coma are circular, of varied sizes, and displaced from each other.
Image: Blur Caused by Coma
Astigmatism
Astigmatism occurs when a single lens zone fails to focus the image of an off-axis point at a single location. Two planes are perpendicular to each other travel through the optical axis in the illustration. The meridian plane and the sagittal plane are the two planes, with the meridian plane containing the off-axis object point.
Skew rays, or rays that do not lie in the meridian plane, focus farther away from the lens than those that do. The rays do not meet in a point focus in either case, but rather as lines perpendicular to each other. The images are elliptical and are in the middle of these two spots.
Curvature of Field & Distortion
[Click Here for Sample Questions]
The placement of picture points about one another is referred to as the curvature of field and distortion. The first three aberrations listed so far can be remedied by making changes to the lens' design, but these two aberrations may persist.
The image of a flat object perpendicular to the optical axis will sit on a paraboloidal surface known as the Petzval Surface in the curvature of the field. The term "distortion" refers to the deformation of an image. Barrell distortion and pincushion distortion are the two types of deformation.
Things to Remember
- The Petzval, or field curvature of the lens, is the nominal curvature (1/radius) of that surface.
- Except for some metrological systems that collect essential measurements from the image, distortion errors of 5 to 10% are normally considered acceptable.
- The corresponding center ray for off-axis bundles is known as the primary ray or principal ray.
- Transverse chromatic aberration is another name for lateral aberration.
Also Read:
Sample Questions
Ques. What causes optical aberration? (3 Marks)
Ans. Optical aberrations or flaws arise in photography as a result of the way lens surfaces focus the light they capture. When light rays traveling through an optical system fail to converge at a single point, picture formation suffers and image quality suffers.
Reliance of a lens's index of refraction on color causes chromatic distortion (wavelength). Spherical aberration occurs when rays converging from the lens's outer edges converge to a focus closer to the lens, while rays closer to the axis focus further.
Ques. What are monochromatic aberrations? (2 Marks)
Ans. Aberrations that occur in quasimonochromatic light are known as monochromatic aberrations. The effect of light frequency on its propagation through a system is not taken into account in these aberrations. (Real light is never monochromatic; it is always made up of a frequency band.)
Ques. Explain the different types of monochromatic aberrations. (3 Marks)
Ans. There are five monochromatic aberrations when the light of a single wavelength is considered: spherical aberration, coma, astigmatism, the curvature of field, and distortion. Monochromatic aberrations are significantly more numerous than chromatic aberrations.
As a result, in addition to names, they are labeled with wavefront coefficients. The wavefront coefficient of spherical aberration, for example, is W 040. The mathematical summation that yields the real difference between the perfect and aberrated wavefronts yields this wavefront coefficient.
Ques. What is chromatic aberration and why does it occur? (2 Marks)
Ans. When a lens is unable to properly refract all of the wavelengths of color in the same location, chromatic aberration results. It's a frequent photography issue that affects practically all lenses, albeit higher-quality lenses will have less chromatic aberration than lower-quality lenses.
Ques. Why is chromatic aberration important? (2 Marks)
Ans. Light of other visible wavelengths will have a focal length that is similar but not identical to this. During a duo-chrome eye exam, chromatic aberration is performed to check that the correct lens power has been chosen. When the patient is shown red and green images, he or she is asked which is crisper.
Ques. What happens in spherical aberration? (2 Marks)
Ans. When incoming light beams pass through lenses having spherical surfaces, they concentrate at different spots on the camera's sensor, causing spherical aberration. Paraxial rays (light rays passing through spherical surfaces along the horizontal axis) refract less than rays passing closer to the edge (peripheral rays).
Ques. What does spherical aberration depend on? (2 Marks)
Ans. The quantity of spherical aberration in a lens formed of spherical surfaces is determined by the shape of the lens. Coma aberration can be partially corrected by bending the lens. Spherical aberrations are also referred to as non-ideal phase changes for bigger radial positions in a broader sense. Simple optical plates and thermal lensing in laser crystals can also generate such mistakes.
Ques. How is spherical aberration eliminated in a reflecting telescope? (2 Marks)
Ans. Aspheric lenses, in which the curvature of the surfaces is not constant, can be used to eliminate spherical aberration in refractors, although they are difficult and expensive to make. Non-spherical mirrors or corrective lenses are typically used in telescopes with focal ratios less than f/10 due to spherical aberration. Making lenses with an aspheric surface eliminates spherical aberration.
Ques. How is spherical aberration measured? (2 Marks)
Ans. To quantify spherical aberration, an appropriate diaphragm is used to intercept parallel light before it strikes the lens, limiting the transmitting area of the lens to a specified annular zone; the position of the focal plane for this zone is then measured.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments