What is an Optical Aberration?
Aberration: an unwelcome departure from the normal, usual, or expected. Most images are imperfect. Image processing software is able to detect even the slightest distortions in an image that appears ideal to the human eye. This distortion results from optical aberrations, which is the inability of a lens to replicate ideal mathematical performance. Lens designers compensate for optical aberrations with specialized optical coatings and additional lens elements, which add cost, size and weight to the final product. Selecting the best lens for an application means understanding and taking into account issues around optical aberrations.
Imaging systems work by focusing light that passes through a lens (or several lenses) and falls on a sensor or a screen. Optical lenses are bidirectional so this applies just as much to projector systems as to cameras and other imaging systems.
In poorly-designed optical systems it is common to see distortion or poor focus in some or all regions of the image. This is known as an optical aberration. These optical aberrations take many forms. Correction is best addressed during the design process when optical components are being selected. Although in some cases it is possible to correct the distortion or focus after the fact.
In this article we discuss the causes of optical aberrations, the most common types, and offer some ways to avoid them.
Causes of Optical Distortion
Refraction occurs when light passes between two mediums of different density which causes light to change direction. Light passing through an optical lens encounters two interfaces – at its entry point and when it exits from the lens. The thickness of the lens and the curvature of each side determines the extent to which light changes direction.
Lens designers use this behavior to focus light at a focal point and create a mirror image on a sensor. Alternatively, a lens may gather light spreading out from the focal point and disperse it over a wide area. The first setting is inherent in cameras, the second in projectors.
However, lenses don’t exactly perform as predicted by mathematical or theoretical models. In addition, wavelength is a factor in refraction. And, while lenses pass a circle of light, most sensors and screens feature a landscape format. As a result, optical aberrations are more pronounced at the left and right edges of images as these are the farthest from the optical centerline. Designers compensate for these aberrations by adding additional elements or optical coatings. However, this makes the system bigger, heavier and more expensive. Every lens is therefore a compromise between size, weight and cost and the quality of the resulting image.
Chromatic vs. Monochromatic Aberration
Light of a single wavelength is one color, or “monochromatic”. White light is composed of all wavelengths, and is termed “chromatic”. As refraction is proportional to wavelength, white light passing through a lens is subject to chromatic aberration. This particular type of aberration only occurs with light of a range of wavelengths.
A desirable form of chromatic aberration occurs when a prism splits white light into a rainbow. In a camera or projector system the result is less attractive. Here chromatic aberration appears as images of the same object but at different magnifications. Edges show a degree of blurring with colors spanning the rainbow, blue towards the center and red outermost.
The extent to which light changes direction at the interface between media of different optical densities is a function of wavelength. Blue light, (wavelength 470nm) is diverted more than red light (wavelength 660nm.) An optical lens intended for use with a white light illumination system must take this into account or the image will exhibit chromatic aberration.
Chromatic aberration appears as images of the same object but at different magnifications. Edges show a degree of blurring with colors spanning the rainbow, blue towards the center and red outermost.
Where possible, the best way to avoid this problem is by using monochromatic light. Chromatic and monochromatic systems can still suffer from other types of aberration, but monochromatic light eliminates this type of distortion.
Alternatively, applying optical coatings to the surfaces of the lenses can reduce the magnitude of the effect.
Types of Optical Aberrations
- Field Curvature
This very common type of optical aberration is similar to field curvature characterized by the lens being unable to get the outer edges and the image center in focus at the same time. While aspheric lenses or a spherical aberration compensation plate can improve optical performance, the easiest fix is defocussing.
Coma is where a single point of light appears as multiple overlapping points in the image, each increasingly out of focus. The result is a comet-like trace. Coma will usually vary across a lens, disappearing towards the center and more pronounced out at the periphery. Coma can be resolved by adding additional corrective lenses, but as with astigmatism, the best solution is a better lens.
Often thought of as an eye problem, astigmatism results from inconsistent lens curvature. This inconsistency moves the focal point closer to or further away from the lens, depending on the direction in which light enters the lens. In an existing system the only solution is to find an intermediate point of focus between the two points. A better solution is to use a higher quality lens.
This is where the lens changes the position of the outer edges of the image relative to the center. It can be either pincushion or barrel distortion. Pincushion is where the sides of the image are pinched inwards so it takes on the shape of a pincushion as seen from above.
Barrel is the opposite where the sides stretch out along the vertical and horizontal and vertical axes so the image has the shape of a barrel. Barrel distortion is common with wide angle lenses where light is refracted through a large angle. This is exhibited in fisheye lenses and is also seen in a hemispherical mirror. The solution is to use a lens with a longer focal length. Pincushion is the opposite, occurring at long focal lengths.
When projecting light from a lens onto a flat screen or sensor the edges are further from the lens than the center. Unless addressed during design, this results the image being out of focus at the edges. In movie theaters this is handled by curving the screen. In smaller systems the answer is to focus at a point between the optical center and the image edges, in effect putting the whole image slightly out of focus. Buying a lens designed to avoid this type of optical aberration is an option but systems that implement such lenses tend to be more expensive.
Dynasil’s Optics Capabilities
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