Lens, curved mirror, plane mirror, reflecting prism, reticle and other optical parts
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In addition to the relevant issues of Gaussian optics, such as object image conjugate position, magnification, image transformation and turning light path, an optical system also needs to consider a series of issues, such as the size of the imaging range, the size of the aperture angle of the imaging beam, the width of the imaging band, the image clarity and illuminance.The actual optical system that meets a series of requirements is often not a simple combination of several lenses, but consists of a series of lenses, curved mirrorsPlane mirror, reflecting prism, reticle and other optical components, and the required high-quality system can be obtained by means of reasonable aperture setting, fine aberration correction and proper determination of the transverse size of optical components.
Chinese name
Actual optical system
system composition
Lens, curved mirror, plane mirror, reflecting prism, reticle and other optical parts
Each optical part of the optical system is limited by its own frame, and in most cases it is a round hole.Sometimes, fixed or variable special optical holes are added to the system.In all these optical holes, there must be one optical hole to limit the aperture angle of the imaging beam at the point on the axis;In addition, there is an optical hole to limit the imaging range.Such aperture is called diaphragm: the former is called apertureDiaphragmOr effective aperture;The latter is called field aperture.Any optical system must have such two apertures.In the visual optical system, the pupil of the eye must also be considered as an optical aperture of the system.
Aperture diaphragm, entrance pupil and exit pupil are easy to think of because the imaging beam of the point on the axis is limited by the aperture diaphragm. When all the apertures of the system are imaged into the object space through the optical parts in front of them, the aperture corresponding to the image with the smallest opening angle of the object point on the axis or the image with the smallest aperture when the object is at infinity must be the aperture diaphragm.The image of the aperture diaphragm in the object space is called the incident pupil, and its opening angle to the object point is called the beam aperture angle of the object.Similarly, the aperture aperture is formed into an image in the image space by the optical parts behind it, which is called the exit pupil. It must also be an aperture image with the smallest opening angle of the image point on the axis. This opening angle is the beam aperture angle of the image square.The entrance pupil, aperture diaphragm and exit pupil are conjugate.If the aperture aberration is ignored, the incident pupil is the common entrance of the imaging beam at each point on the object plane;The exit pupil is the common exit of the imaging beam.The light passing through the center of the aperture diaphragm is called the main light. Because of the conjugate relationship, it also passes through the center of the entrance pupil and the center of the exit pupil.Therefore, generally speaking, the main ray is the centerline of the imaging beam.
Relevant factors
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The position of the aperture diaphragm in an optical system is related to many factors.There are specific requirements in some systems. For example, the visual optical system must make the exit pupil outside the eyepiece so that the pupil of the eye can coincide with it;In telecentric system, the aperture diaphragm shall be at the focus.In addition, the position of the aperture diaphragm is also related to the aberration correction and the transverse dimensions of the optical components of the system, which should be reasonably determined during the design.
Field aperture
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The field aperture is an optical hole in an optical system that determines its imaging range.In systems with intermediate real image planes (such as Kepler telescopes and microscopes) and systems with real image planes (such as photographic systems), the field stop is set on this image plane.The image of the field stop formed by the optical parts in front of it in the object space is called the incident window. The angle it opens to the center of the incident pupil is the smallest of all the aperture images. This angle is called the field angle.Similarly, the image formed by the optical part behind the field stop in the image space is called the exit window.The entrance window, field aperture and exit window are also conjugate.When the field aperture is set on the real image plane or the middle real image plane, the entrance window and the exit window coincide with the object plane and the image plane respectively, and the field of view has a clear boundary.Where there is no real image or middle real image plane, such as eyes passing through a magnifying glass orGalileo telescopeDuring observation, there is always a part in the system whose light passing aperture plays a role in limiting the field of view. In the above two cases, the aperture of the magnifier itself and the aperture of the telescope objective are the field of view apertures that determine the visible field of view.Obviously, the incident window does not coincide with the object plane at this time, and there is no clear field boundary.
vignetting phenomenon
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In an ideal case, the beams at both on axis and off axis points are limited by aperture apertures and have basically the same beam aperture angle. If the field of view is not too large, the illumination of the image plane in the entire field of view is basically uniform.However, in practical optical systems, the imaging beam of off-axis point is often limited by the through hole of other optical parts, and the result is that the beam aperture angle of off-axis point is much smaller than that of on axis point.This is because if the off-axis point is to be imaged with a beam full of the incident pupil, the lens far away from the aperture aperture needs to have a relatively large diameter, and it is also very difficult to correct the aberration of the full aperture off-axis beam.Therefore, in order to improve the imaging quality of off-axis points and ensure that the transverse size of optical parts is not particularly large, it is often necessary to reduce the diameter of certain lenses to limit the off-axis beam.This phenomenon is called beam vignetting, in which a beam full of the incident pupil emitted from an off-axis point is partially intercepted by some optical parts and cannot pass through the optical system completely.Draw a three piece photographic objective consisting of a positive lens at the front and a negative lens in the middle with a simplified representation. The aperture aperture is set near the negative lens. When the aperture of the aperture changes, the aperture angle 2 of the point on the axis changes accordingly.The part of the off-axis point imaging beam that is intercepted by the front and rear positive lens frames is represented by hatched lines.The farther the off-axis point is away from the optical axis, the more serious the interception phenomenon (i.e. vignetting) is. As a result, the illumination of the image surface outside the field of view is greatly reduced.Of course, most optical systems allow a certain degree of vignetting.
Imaging beam
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The imaging beam of an object point is a spatial light cone with the object point as the vertex and the incident pupil as the bottom.The structure of this beam will change after passing through the optical system. For an axisymmetric optical system (most systems belong to this category), the on axis point beam always has symmetry, but the off axis point beam loses symmetry after passing through the system.In order to understand the structure of this beam, the plane beam on its two characteristic planes is usually used to describe it.
The meridian plane, the plane where the meridian beam contains the off axis object point and the optical axis, is called the meridian plane.Due to the axisymmetric nature of the optical system, the off-axis object points can always be located on the drawing plane, that is, the paper plane is the meridian plane.The beam on the meridian plane is called the meridian beam.Obviously, the main ray must be a ray in the meridian beam.
The sagittal plane and the plane where the sagittal beam contains the main ray and is perpendicular to the meridian plane are called sagittal planes.The beam on the sagittal plane is called sagittal beam.Obviously, the main ray is the intersection line between the meridian plane and the sagittal plane.Since the main ray changes its direction through refraction and reflection of various surfaces of the system, the sagittal plane also changes face by face instead of a unified plane.
Due to the axial symmetry of the optical system, the point beam on the axis does not need to be a molecular meridian beam and a sagittal beam, and the point beam off the axis must be symmetric to the meridian plane.
System aberration
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The phenomenon that the image formed by the lens (or lens group) is not exactly similar to the original appearance.It is caused by the fact that the angle of intersection between the light emitted from the object point and the main axis of the lens is too large, it is far away from the axis, or the refractive index of the lens material changes with the wavelength of the light.Aberration size reflects the quality of imaging.There are 7 kinds of aberrations;There are five kinds of monochromatic light, namelySpherical aberration、Coma、Astigmatism and image surface curvatureAnd distortion.There are two kinds of chromatic aberration for polychromatic light, namely axial chromatic aberration and vertical chromatic aberration.It is an important task to eliminate or reduce these aberrations as far as possible in designing optical system.
